CN106688288B - Synchronization process and resource control method and device for communication in D2D system - Google Patents

Abstract

The present disclosure relates to a fifth generation (5G) or quasi-5G communication system for supporting higher data rates than 4G communication systems, such as Long Term Evolution (LTE). A method is provided for a terminal to establish synchronization with another terminal in a network supporting device-to-device (D2D) communication. The method comprises the following steps: scanning, at the terminal, for a synchronization signal from at least one base station; acquiring synchronization with a base station based on a synchronization signal in case of receiving the synchronization signal from the base station; measuring a power of the synchronization signal received from the base station; and transmitting a synchronization signal as a synchronization relay terminal when data to be transmitted is generated in the idle mode and the reception signal power is less than the reception signal power.

Description

Synchronization process and resource control method and device for communication in D2D system

Technical Field

The present disclosure relates to a synchronization method including a synchronization resource structure and a synchronization procedure for device-to-device (D2D) communication, and a transmission resource structure and a resource control technique of a terminal in D2D discovery and communication.

Background

In order to meet the ever increasing demand for wireless data services since the deployment of 4G communication systems, efforts are constantly being made to develop improved 5G or quasi 5G communication systems. Accordingly, the 5G or quasi-5G communication system is also referred to as a "super 4G network" or a "post-LTE system".

The 5G communication system is considered to be implemented in a higher frequency (mmWave) band (e.g., 60GHz band) in order to achieve a higher data rate. In order to reduce propagation loss of radio waves and increase transmission distance, beamforming, massive Multiple Input Multiple Output (MIMO), full-dimensional MIMO (FD-MIMO), array antenna, analog beamforming, massive antenna technology are discussed in the 5G communication system.

Further, in the 5G communication system, development for system network improvement is underway based on advanced small cells, cloud Radio Access Network (RAN), ultra-dense network, device-to-device (D2D) communication, wireless backhaul, mobile network (moving network), cooperative communication (CoMP), reception side interference cancellation, and the like.

In 5G systems, hybrid FSK and QAM modulation (FQAM) and Sliding Window Superposition Coding (SWSC) as Advanced Coding Modulation (ACM) and filter bank multi-carrier (FBMC), non-orthogonal multiple access (NOMA) and Sparse Code Multiple Access (SCMA) as advanced access techniques have been developed.

With the popularity of smart phones, data traffic is increasing rapidly. An increasing number of smart phone users stimulate the consumption of smart phone-based application services, such as Social Network Services (SNS) and games, resulting in an unprecedented increase in data traffic. More specifically, in addition to human-to-human communication, if machine intelligent communication (such as human-to-machine communication and machine-to-machine communication) is activated as a new market area, the service set of the base station will exceed the existing limit.

A technique for solving these problems is therefore required, and direct communication between devices is attracting attention as one such technique. This so-called device-to-device (D2D) communication technique is promising for licensed band communication systems, such as cellular communication systems, and unlicensed band communication systems, such as wireless lan (wlan) systems.

In cellular communication systems, D2D communication is attractive in increasing traffic adaptation capability and reducing overload. For example, it is advantageous to reduce the number of communication links from 2 to 1 because D2D communications are implemented such that multiple User Equipments (UEs) located in the same or neighboring cells establish a D2D link and exchange data over the D2D link without involving any evolved node b (enb).

The Long Term Evolution (LTE) based D2D communication features D2D discovery and D2D communication. D2D discovery is the process by which a UE determines the identity or interest of other UEs located close to it or advertises the identity or interest of the UE to other UEs located nearby. At this time, the identity and interest may be represented by a UE Identifier (ID), an application identifier, or a service identifier, and can be configured differently depending on the D2D service and operation scenario.

As a distinguishing feature from unicast communication in cellular networks, the LTE-based D2D communication aims to support broadcast communication for common security scenarios and does not provide feedback functions such as channel measurement reporting, hybrid arq (harq), and acknowledgement ACK/Negative Acknowledgement (NACK). The biggest challenge of D2D broadcast communication is to support reliable and seamless D2D communication with high link quality in the absence of an eNB. More specifically, in a scenario where UEs operate in a distributed manner without the assistance of an eNB, there is a need to solve a resource conflict problem that occurs when different UEs occupy the same resource.

The main goal of D2D communication is to identify the communication needs that occur between humans, between humans and objects, and between objects, and to handle traffic occurring in local areas in an appropriate manner while preventing unnecessary waste of radio resources. Thus, current research on D2D communication is focused on a scheme for efficient D2D discovery that enables many proximate devices to broadcast and receive information about services and content items in an efficient manner.

Currently, the third generation partnership project (3GPP) standardization organization is continuously working on the LTE release 12 specification that provides D2D communication for public safety purposes.

In D2D communication for public safety purposes, LTE release 12 attempts to provide a broadcast service that supports push-to-talk (PTT) functionality. As D2D communication for public safety will be operable in the absence of an eNB, the UE must directly perform procedures related to synchronization and resource allocation between itself. In the presence of the eNB, the eNB may control the UE to broadcast within the resources allocated by the eNB for D2D. Furthermore, D2D communication for public safety must account for the case where the eNB part is unavailable.

Disclosure of Invention

Technical problem

It is necessary to establish synchronization between the devices to perform D2D communication. A device may establish synchronization with another device by using timing information received from a synchronizing eNB or through a Global Positioning System (GPS) reception module. For this, the device should be able to access the sync eNB or GPS module.

However, depending on the communication scheme the network operator may not support synchronized enbs. In this case, the device may fail to establish synchronization because the device is unable to obtain timing information from the synchronizing eNB. Additionally, when a device is placed in a GPS shadow area (e.g., between high-rise buildings, inside a tunnel, within a building, etc.), the device may fail to establish synchronization because the device is unable to obtain timing information from a GPS receiving module. In other words, the device may fail to initiate D2D communication when the connection to the sync eNB or GPS receiving module is not good.

The above information is presented merely as background information to aid in understanding the present disclosure. No determination is made as to whether any of the above is likely to be applicable as prior art with respect to the present disclosure, and no assertion is made.

Solution to the problem

Aspects of the present disclosure are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, one aspect of the present disclosure is to provide a method and apparatus to support device-to-device (D2D) synchronization for discovery and communication.

Another aspect of the present disclosure is to provide synchronization resource structure, synchronization establishment procedure, synchronization signal measurement and conditions to be synchronized terminals, synchronization and resource pool information carried in the synchronization signal, and synchronization procedure for D2D discovery and communication of terminals within network coverage, partial network coverage and out of network coverage.

Further, the present disclosure provides a transmission resource pool selection method for D2D discovery and communication.

In accordance with one aspect of the present disclosure, a method for establishing synchronization of a terminal with another terminal in a network supporting D2D communication is provided. The method comprises the following steps: scanning, at the terminal, for a synchronization signal from at least one base station; acquiring synchronization with a base station based on a synchronization signal when the synchronization signal is received from the base station; measuring a power of the synchronization signal received from the base station; and transmitting a synchronization signal as a synchronization relay terminal when data to be transmitted is generated in the idle mode and the received signal power is less than the received signal power.

In accordance with another aspect of the present disclosure, a terminal supporting device-to-device communication is provided. The terminal includes: a radio processing unit that transmits to and receives from the base station and other terminals; a memory storing control data; and a control unit that controls the radio processing unit, wherein the control unit controls scanning, at the terminal, for a synchronization signal from at least one base station, acquiring synchronization with the base station based on the synchronization signal when the synchronization signal is received from the base station, measuring power of the synchronization signal received from the base station, and transmitting the synchronization signal as a synchronization relay terminal when data to be transmitted is generated in an idle mode and the received signal power is less than a reception signal power.

In accordance with another aspect of the present disclosure, a method for establishing synchronization of a terminal with another terminal in a network supporting D2D communication is provided. The method comprises the following steps: scanning, at the terminal, for synchronization signals from a base station and other terminals; scanning for a synchronization signal from at least one synchronization relay terminal in case no synchronization signal is received from any base station; acquiring synchronization with the terminal transmitting the synchronization signal having the highest received signal power in the case where the synchronization signal is received from the at least one synchronization relay terminal; and transmitting a synchronization signal as a synchronization relay terminal when data to be transmitted is generated and the received signal power is less than the received signal power.

In accordance with yet another aspect of the present disclosure, a terminal supporting device-to-device communication is provided. The terminal includes: a radio processing unit that transmits to and receives from the base station and other terminals; a memory storing control data; and a control unit that controls the radio processing unit, wherein the control unit controls scanning synchronization signals from the base station and other terminals at the terminals, scanning a synchronization signal from at least one synchronization relay terminal without receiving a synchronization signal from any base station, acquiring synchronization with the terminal that transmitted the synchronization signal having the highest received signal power when receiving a synchronization signal from the at least one synchronization relay terminal, and transmitting a synchronization signal as a synchronization relay terminal when data to be transmitted is generated and the received signal power is smaller than the received signal power.

Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses various embodiments of the disclosure.

Advantageous effects of the invention

As disclosed herein, the D2D network of the present disclosure is able to provide efficient and stable synchronization, and is advantageous in mitigating interference between terminals and improving communication performance in D2D discovery, 1:1 communication, and 1: N communication.

Drawings

The above and other aspects, features and advantages of particular embodiments of the present disclosure will become more apparent from the following description taken in conjunction with the accompanying drawings, in which:

fig. 1 illustrates operating regions, processes, signals and messages for device-to-device (D2D) synchronization according to an embodiment of the present disclosure;

fig. 2 is a block diagram illustrating a configuration of a User Equipment (UE) according to an embodiment of the present disclosure;

fig. 3a and 3b illustrate resource structures for D2D synchronization according to an embodiment of the present disclosure;

fig. 4a is a timing diagram illustrating the timing of a primary system information broadcast in the Downlink (DL) and a system information broadcast for D2D transmission by a UE in the Uplink (UL) according to an embodiment of the present disclosure;

fig. 4b and 4c illustrate offset transmission schemes for an out-of-coverage (OOC) UE when an integrated circuit IC UE transmits a synchronization sequence and a synchronization channel to the OOC UE, according to embodiments of the present disclosure;

fig. 5a, 5b, 5c and 5d illustrate various transmission and reception resource pool allocation methods according to embodiments of the present disclosure;

6a, 6b, 6c, and 6D illustrate various resource pool structures and locations of synchronization resources for D2D discovery or communication, according to embodiments of the present disclosure;

7a, 7b and 7c illustrate a periodically synchronized resource utilization method according to an embodiment of the present disclosure;

fig. 8a and 8b illustrate a scenario for selecting a synchronization sequence between an IC UE and an out-of-coverage OOC UE according to an embodiment of the present disclosure;

fig. 9 is a flowchart illustrating a transmission timing synchronization process according to an embodiment of the present disclosure;

10a, 10b and 10c are flow diagrams illustrating a scanning process for reception according to embodiments of the present disclosure;

fig. 11 is a flowchart illustrating a procedure of determining a condition to become a synchronous relay UE according to an embodiment of the present disclosure;

fig. 12a and 12b are flowcharts illustrating a UE procedure for stopping synchronization signal transmission according to an embodiment of the present disclosure;

fig. 13a, 13b, 13c and 13d illustrate transmission of frame numbers and Master Information Blocks (MIB) or synchronization resources in a Long Term Evolution (LTE) system according to an embodiment of the present disclosure;

fig. 14 is a diagram illustrating a D2D frame number determination operation in both in-coverage and out-of-coverage network scenarios, according to an embodiment of the present disclosure;

fig. 15 illustrates D2D frame number determination in an OOC scene according to an embodiment of the present disclosure;

fig. 16 is a flowchart illustrating a process of aligning and selecting synchronization information based on an absolute time according to an embodiment of the present disclosure;

fig. 17 is a flowchart illustrating a process of aligning and selecting synchronization information without considering absolute time according to an embodiment of the present disclosure;

FIG. 18 is a schematic diagram illustrating a synchronization resource structure according to an embodiment of the present disclosure;

19a, 19b, 19c and 19D illustrate resource pool structures with synchronized resources for D2D discovery and D2D communication, according to embodiments of the present disclosure;

fig. 20a and 20b are flowcharts illustrating a transmission synchronization process according to an embodiment of the present disclosure;

21a, 21b and 21c are flow diagrams illustrating a process for scanning for reception according to embodiments of the present disclosure;

fig. 22 is a flowchart illustrating a process of becoming a relay synchronization source (R-SS) according to an embodiment of the present disclosure; and

fig. 23 is a flowchart illustrating a method of becoming an R-SS capable of compensating for a hierarchical synchronization procedure problem of an OOC UE according to an embodiment of the present disclosure.

Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures.

Detailed Description

The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the disclosure as defined by the claims and their equivalents. It includes various specific details to assist in this understanding, but these are to be considered exemplary only. Accordingly, one of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. Moreover, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are not limited to the bibliographic meanings, but, are used only by the inventors to enable a clear and consistent understanding of the disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of the various embodiments of the present disclosure is provided for illustration only and not for the purpose of limiting the disclosure as defined by the appended claims and their equivalents.

It is understood that the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "component surfaces" includes reference to one or more of these surfaces.

By the term "substantially" it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but may be subject to variation or variation in amounts that do not preclude the effect that the characteristic is intended to provide, including for example, tolerances, measurement error, measurement accuracy limitations, and other factors known to those of skill in the art.

First, a description is made of a device-to-device (D2D) communication technique that is the basis of the present disclosure.

Assume that the protocol layers of a User Equipment (UE) include a D2D application layer, a D2D management layer, and a D2D transport layer. The D2D application layer accommodates D2D service applications running on an Operating System (OS) of the UE, the D2D management layer is responsible for functions of converting discovery information generated by the D2D application into a format suitable for a transport layer, and the transport layer corresponds to a physical layer (PHY)/Medium Access Control (MAC) layer of the LTE or Wi-Fi wireless communication standard. D2D discovery was performed in the following process. If the user executes the D2D application, the application layer generates discovery information to the D2D management layer. The management layer converts the discovery information received from the application layer into a management layer message. The management layer message is transmitted through a transport layer of the UE, and the UE receiving the message operates in the reverse order of the transmission procedure.

D2D communication is a communication method for exchanging traffic between UEs without transmitting over any infrastructure, such as an evolved node b (enb) or Access Point (AP). D2D communication may be performed based on the results of the D2D discovery procedure (i.e., with multiple User Equipments (UEs) discovered or without the D2D discovery procedure). Whether the D2D discovery process is required before D2D communication depends on the D2D service and operational scenario.

The D2D service scenario may be classified as a business service or a non-public safety service and a public safety service. These services may include a myriad of examples, such as advertisements, Social Networking Services (SNS), games, public safety services, and the like.

A brief description of the types of the aforementioned services is made below.

1) Advertising: D2D-enabled communication network operators allow pre-registered stores, cafes, movie theaters and restaurants to announce their identities to D2D users located within a short distance using D2D discovery or D2D communications. At this time, the interest may include a promotion, event information, and a discount coupon of the advertiser. If the corresponding identity matches the user's interests, the user may pay for access to the corresponding store to gather more information through either traditional cellular communications or D2D communications. In another example, an individual user may discover taxis around him/her through D2D discovery and exchange data regarding destinations or fees through traditional cellular communications or D2D communications.

2) SNS: the user may send the user's applications and interests in the corresponding applications to other users located within a short distance. At this point, the identity or interest for D2D discovery may be a buddy list (buddy list) or an application identifier. The user may share content, such as photos and videos, with nearby users through D2D communications after D2D discovery.

3) And (3) playing: the user discovers other users and game applications for playing a mobile game with neighboring users through a D2D discovery process and performs D2D communication for transmitting data necessary for the game.

4) Public safety service: police and fire fighters may use D2D communication technology for public safety purposes. For example, in the event that cellular communication is unavailable due to cellular network disruption caused by emergency situations (such as fires and landslides or natural disasters, such as earthquakes, volcanic eruptions, and tsunamis), police and firefighters may use D2D communication technology to discover nearby colleagues or share emergency information with nearby users.

Current third generation partnership project (3GPP) Long Term Evolution (LTE) D2D standardization is dedicated to both D2D discovery and D2D communication but varies in the scope of standardization. Current D2D discovery techniques are targeted for commercial use and therefore must be designed to operate in the network service area of the eNB. For example, D2D discovery is not supported in the absence of an eNB (or outside the coverage area of an eNB). D2D communication, which is targeted for public safety and disaster network services (i.e., non-commercial use), must therefore be designed to support both inside and outside the eNB's network coverage and part of the eNB's network coverage (communication in the case where some UEs are located in the eNB's coverage area and other UEs are located outside the eNB's coverage area). Thus, public safety and disaster network services are provided by D2D communications that do not support D2D discovery.

It is characterized by both D2D discovery and D2D communication being performed in LTE uplink subframes. For example, the D2D transmitter transmits D2D discovery signals and data for D2D communications in an uplink subframe, and the D2D receiver receives them in an uplink subframe. In current LTE systems, the UE receives data and control information from the eNB through a downlink and transmits data and control information to the eNB through an uplink, but the operation of the current D2D transmitter/receiver is different from those in the conventional LTE. For example, a UE supporting D2D function has an Orthogonal Frequency Division Multiplexing (OFDM) -based receiver for receiving downlink data and control information from an eNB and a single carrier frequency division multiplexing (SC-FDM) -based transmitter for transmitting uplink data and control information to the eNB. However, because the D2D UE must support both cellular and D2D modes, the D2D UE must have an additional SC-FDM receiver for receiving D2D data and control information in the uplink as well as an OFDM-based receiver and an SC-FDM-based transmitter.

The current LTE D2D defines two types of D2D discovery schemes according to resource allocation schemes.

1) Type 1 finds that: the eNB broadcasts the uplink resource pool available for D2D discovery in System Information Blocks (SIBs) for all D2D UEs within the cell under its control. At this point, the resource size (e.g., x consecutive subframes) available for D2D and the periodicity of the resources (e.g., repeating every y seconds) are signaled. The transmitting D2D UE having received the information selects resources for transmitting the D2D discovery signal in a distributed manner. Meanwhile, the receiving D2D UE must receive all D2D discovery signals transmitted in the resource pool and including SIB information.

2) Type 2 discovery: the eNB uses the SIB to inform the receiving D2D UE of the discovery resource pool. The discovery signal resources for transmitting D2D UEs are scheduled by the eNB. At this time, the eNB may perform scheduling in a semi-persistent manner or a dynamic manner.

Similar to the D2D discovery method, the D2D communication method also operates in two modes according to a resource allocation scheme.

1) Mode 1: the eNB directly notifies the D2D transmitter of data transmission resources for D2D communication.

2) Mode 2: the eNB informs the D2D sender of the available resource pool and the UE selects resources among the resource pool for transmission in a distributed manner.

Prior to the description of the present disclosure, some useful explanation examples of terms used in the present specification are presented. However, note that they are not limited to the explanations set forth below.

A base station is an entity that communicates with UEs and may be referred to as a BS, a Base Transceiver Station (BTS), a node b (nb), an eNB, or an AP.

The user equipment is an entity that communicates with the base station and may be referred to as a UE, a device, a Mobile Station (MS), a Mobile Equipment (ME), or a terminal.

The D2D communication UE may be classified as one of a synchronization source UE and a synchronization destination UE according to its role. The sync source UE (also referred to as sync source) is a UE that transmits a D2D sync signal (D2DSS), a D2D sync channel (D2DSCH), a D2D broadcast channel (D2DBCH), or a D2D system information channel (D2D SICH), and may include a sync reference UE and a sync relay UE. Depending on the circumstances, the UE transmitting the D2D synchronization signal may be considered a "synchronization reference UE". A synchronization destination UE (also referred to as D2DUE) is a UE that receives (but does not transmit) a synchronization signal or a synchronization channel.

The synchronization signal means a signal including a synchronization sequence for use in identifying a cluster or a synchronization source UE, and the synchronization signal resource means a resource for use in transmitting the synchronization signal. The synchronization signal means a channel including a synchronization message or a system message, and the synchronization channel resource means a resource for use in transmitting the synchronization channel. The synchronization source UE may transmit a synchronization signal or a synchronization channel. It should be noted that the term "synchronization signal" is to be interpreted as a concept including a synchronization signal and a synchronization channel unless strictly stated otherwise.

Except in the case of being located within the eNB coverage area, the synchronization reference UE (alternatively referred to as an independent synchronization source (I-SS)) is the synchronization source UE responsible for creating the synchronization representative role of the cluster and can determine the time independently.

A synchronization relay UE (also referred to as a synchronization source (D-SS)) is a synchronization source UE that relays (i.e., receives and forwards) a synchronization signal or a synchronization channel received from a synchronization reference UE after acquiring timing synchronization with the synchronization reference UE. For example, when the synchronization relay UE relays synchronization from the eNB, the synchronization relay UE may function as a Synchronization Hub (SH). A special type of synchronous relay UE may function as a voluntarily synchronized UE to relay a synchronization reference between multiple synchronization reference UEs.

The in-coverage UE is in a Radio Resource Control (RRC) IDLE state or an RRC CONNECTED state.

RRC IDLE: a state of selecting an eNB (or cell), monitoring a paging channel, and acquiring System Information (SI) but not exchanging data with the eNB.

RRC CONNECTED (RRC CONNECTED): monitor a control channel and exchange a status of data with the eNB through a data channel. Reporting measurement results associated with the eNB and the neighboring enbs to assist status of scheduling of the eNB.

Fig. 1 illustrates operation regions, processes, signals and messages for D2D synchronization according to an embodiment of the present disclosure.

Referring to fig. 1, a first eNB 101 has a coverage area 11 (within network coverage) in which a first UE 121 and a second UE 122 are located. The second eNB 102 has a coverage area 12 (within network coverage) in which a third UE 123 is located. The fourth UE 124, the fifth UE 125, and the sixth UE 126 are located outside the coverage area of the first eNB 101 and the second eNB 102, but they may receive synchronization signals for D2D communication from the first UE 121 located within the coverage area 11 of the first eNB 101. The fourth UE 124 to the sixth UE 126 correspond to a case called partial network coverage.

In fig. 1, the seventh UE 127, the eighth UE 128, and the ninth UE 129 that are outside the coverage area 11 of the first eNB and the coverage area 12 of the second eNB and that do not receive synchronization signals from any UE located in the coverage area 11 of the first eNB and the coverage area 12 of the second eNB correspond to a case referred to as being outside of network coverage.

In the coverage area (within network coverage) of the first eNB 101 and the second eNB 102, a reference time for transmission/reception is typically determined based on a synchronization signal received from the enbs. In the 3GPP LTE standard, a UE determines symbol and frame boundaries based on a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS) transmitted by an eNB. The frame number of the currently received frame may be determined based on a Broadcast Channel (BCH) received from the eNB. In the LTE standard, a System Frame Number (SFN) is used to indicate a currently received frame, and there are 1024 SFNs from SFN0 to SFN 1023. Typically, SFN0 is considered to be the reference frame number of the eNB.

Meanwhile, the network in-coverage operation includes inter-cell operation. In the embodiments of the present disclosure, it is assumed that although a UE performs a transmission/reception operation based on a reference time of a synchronization device (i.e., eNB or UE), the UE may perform a discovery and communication operation based on a resource structure configured based on a reference time received from another synchronization device for an additional reception operation. This assumption may apply to operations in inter-cell operation, partial network coverage operation, and out-of-network coverage operation.

Among the UEs illustrated in fig. 1, the first UE 121, the fourth UE 124, the sixth UE 126, the seventh UE 127, and the eighth UE 128 are UEs that transmit D2D signals for D2D discovery or D2D communication. In fig. 1 these UEs send D2D signals. All UEs may receive D2D signals for D2D discovery or D2D communication. The first UE 121 may transmit a D2D signal. These UEs may receive the D2D signal transmitted by the first UE 121 in various ways. This is described with reference to fig. 1. First, the D2D signal transmitted by the first UE 121 is received by the second UE 122 served by the same serving eNB. Second, the D2D signal transmitted by the first UE 121 is received by a third UE 123 served by a second eNB (eNB 2). Third, the D2D signal transmitted by the first UE 121 is received by a fourth UE 124 located in a partial network coverage area. At this time, in addition to the second UE 122 receiving the reference time provided by the first eNB 101, the first UE 121 transmits a synchronization signal and a synchronization channel to provide reference signal reception to the third UE 123 and the fourth UE 124.

If the UE transmitting the synchronization signal and the synchronization channel and the UE transmitting the D2D signal are different from each other, the receiving UE may receive the synchronization signal and the channel instead of the D2D signal (mismatch). Therefore, it is possible to consider whether the UE transmits/receives the D2D signal as a condition to become a UE that is allowed to transmit the synchronization signal and the channel. Such conditions are described below.

Although the partial network coverage UEs (i.e., the fourth UE 124, the fifth UE 125, and the sixth UE 126) are outside the coverage area, they acquire the same reference signal of the first eNB 101 via the first UE 121. If the hop count of the relay reference time increases, the reference time may be distorted. Therefore, the number of hops for relaying should be considered. In fig. 1, the first UE 121 is the first hop, the fourth UE 124 and the fifth UE 125 are the second hop, and the sixth UE 126 is the third hop. Since the synchronization signal reception error increases as the number of hops increases, it is necessary to limit the number of hops. The UE corresponding to the last hop may transmit a synchronization signal and a channel according to predetermined conditions. The UE that receives the synchronization signal and channel transmitted by the last hop UE no longer relays the synchronization signal and channel.

Since the out-of-network-coverage UEs (i.e., the seventh UE 127 to the ninth UE 129) cannot receive the reference time from any eNB, each of them may become a synchronized UE having an independent reference time or, if a synchronization signal is received from a neighboring independent synchronization source (I-SS) with a received signal power greater than or equal to a predetermined level, match the reference signal with the synchronization signal. The out-of-network coverage UE may operate in a synchronous relay mode or flat mode procedure (flat mode procedure). The seventh UE 127, the eighth UE 128, and the ninth UE 129 located outside the network coverage area are configured to transmit the synchronization signal and the channel only when they have the D2D signal to transmit to avoid mismatch of the synchronization signal transmission area. However, it may be necessary to transmit a synchronization signal and a channel even when the UE does not have a D2D signal to transmit depending on the synchronization procedure. Which is described in detail below.

The timing information for UE synchronization includes symbol timing, frame timing, and system frame number. The symbol timing and the frame timing may be acquired by receiving a synchronization sequence of a synchronization signal. SFN is the sequence number of frames per eNB in D2D discovery or communication and may be variously referred to as D2D frame number or Direct Frame Number (DFN). The DFN may be transmitted through a synchronization channel or a broadcast channel among the D2D synchronization signals transmitted by the synchronization transmission UE.

Fig. 2 is a block diagram illustrating a configuration of a UE according to an embodiment of the present disclosure.

Referring to fig. 2, the configuration illustrated in fig. 2 may be the configuration of the first to ninth UEs 121 to 129 illustrated in fig. 1, and if necessary, include additional components with or without any of the components depicted in fig. 2. The UE may be any of a smartphone, a handheld device, a laptop, a tablet, a relay node for D2D communication, and the like.

Referring to fig. 2, a radio communication unit 201 up-converts and amplifies a signal to be transmitted through an Antenna (ANT). The radio communication unit 201 also performs low noise amplification and down-conversion on a signal received through the ANT to output a baseband signal to the modem 203. The radio communication unit 201 may comprise LTE communication circuitry and further communication circuitry as well as D2D communication circuitry. For example, radio communication unit 201 may also include Wi-Fi communication circuitry and/or Global Positioning System (GPS) reception circuitry.

The modem 203 performs encoding and modulation on a signal to be transmitted, and, if necessary, converts a digital signal into an analog signal which is output to the radio communication unit 201. The modem 203 can also perform demodulation and decoding on data received by the radio communication unit 201 and output the resultant signal to the control unit 205. The modem 203 can convert an analog signal received by the radio communication unit 201 into a digital signal, if necessary.

The control unit 205 controls the overall operation of the UE. The control unit 205 may control operations performed according to embodiments of the present disclosure. If the modem 203 is implemented in the form of a communications processor, it may be configured to perform some of the operations of the control unit 205. In certain cases, the modem 203 and the control unit 205 may be integrated into a processor. In a particular case, the radio communication unit 201, the modem 203, the control unit 205, and the memory 207 may be integrated into a single chip. In the embodiments of the present disclosure, the UE can be implemented with various configurations without limitation as long as the UE can perform operations to be described below. For convenience of explanation, it is assumed in the following description that the control unit 205 controls the overall operation of the UE.

The memory 207 may store data according to the user's intention and may have an area for storing data required for the control operation of the control unit 205 and data generated under the control of the control unit 205. The memory 207 may be implemented in various types, such as Read Only Memory (ROM), Random Access Memory (RAM), a hard disk, a compact disc read only memory (CD-ROM), a Digital Versatile Disc (DVD), and so on.

The input unit 209 is a device for receiving commands or data input by a user and may be implemented with various input means such as keys, a touch screen, a voice recognition module, a text recognition module, and the like.

The display unit 211 is an apparatus for displaying a current operation state of the UE and may be implemented with various output devices such as a Liquid Crystal Display (LCD), a Light Emitting Diode (LED), a speaker, a vibration motor, and the like.

A description of two embodiments of the present disclosure is made below with reference to the accompanying drawings. A first embodiment of the present disclosure is directed to in-network coverage and out-of-network coverage UE operation for D2D communication. A second embodiment of the present disclosure is directed to D2D discovery and D2D communication.

[1]In-network coverage and out-of-network coverage operation

Synchronous resource structure

The synchronization resource may consist of a Synchronization Resource Unit (SRU) capable of carrying synchronization signals and channels. The SRU is a set of resources for a synchronization signal (D2DSS) and a synchronization channel (PD2DSCH) transmitted by a synchronized UE. The length of the synchronization resource unit may be equal to the length of the LTE subframe (1 ms). In order to transmit a synchronization signal or a synchronization channel, at least one physical symbol unit is required. Since the synchronization channel is a message, it is possible to insert a pilot pattern used by the receiver in accessing the physical channel quality into the synchronization channel resource.

The synchronization resource structure may be configured for various scenarios depending on whether the timing and transmission resource regions of an in-coverage (IC) UE and an out-of-coverage (OOC) UE match. The IC UE may receive an instruction to transmit a synchronization signal or transmit the synchronization signal of the IC UE under a condition determined by the eNB or a predetermined condition. These conditions are described below. If the IC UE transmits a synchronization signal, another IC UE or OOC UE served by the neighboring eNB may receive the synchronization signal. An OCC UE is referred to as a category 1OOC UE (OOC _ UE _ cat1) if the OOC UE receives a synchronization signal from an IC UE and follows the reference timing of the eNB. Otherwise, the OCC UE is referred to as a class 2OOC UE (OOC _ UE _ cat 2). Depending on the configuration of the synchronization operation, if the OOC UE receives a synchronization signal from the IC UE and complies with the reference timing of the eNB, there are three types of UEs, i.e., the IC UE, the category 1OOC UE, and the category 2OOC UE. Furthermore, if the OOC UE receives a synchronization signal from the IC UE and maintains the reference timing used in the out-of-coverage area without observing the reference timing of the eNB, there are two types of UEs, i.e., the IC UE and the category 2OOC UE. A class 1OOC UE may synchronize its transmission resource region with a transmission region notified by an IC UE only in a scenario where all three types of UEs exist.

In the case where the resource location is fixed (e.g., fixed to the DFN), the DFN in the IC (same as the SFN) and the DFN in the OCC may be different from each other, and there may or may not be a predetermined offset between the IC and the OCC synchronization resource locations. If there is a predetermined offset, this signal is distinguished from the synchronization signal sent by the OOC synchronization source UE because the synchronization source UE sending the signal on the IC synchronization resource uses an IC synchronization signal (identified by an ID in the sequence or message). The OOC UE selects one of a plurality of synchronization signals for the IC when discovering the IC synchronization signal (if a condition for becoming a synchronization source UE is satisfied), and selects a synchronization resource among a plurality of OOC synchronization resources having a predetermined offset to transmit the synchronization signal. In this way, the IC UE can receive synchronization signals of OOC UEs on OCC synchronization resources only at predetermined locations to discover the presence of any OOC UE at low power. If the IC UE does not transmit the synchronization signal first (e.g., if there is no data to transmit), the IC UE has difficulty receiving the signal of the OCC synchronization source UE and thus the eNB may command a specific IC UE to transmit the synchronization signal using a dedicated control signal or command all IC UEs to transmit the synchronization signal using a common control signal. At this time, the eNB may control to transmit the synchronization signal in a certain period or within a predetermined time duration, instruct a specific IC UE to stop transmitting the synchronization signal after the synchronization signal transmission command (i.e., the UE may ignore the command from the eNB if the UE is transmitting data), or instruct a UE that is not transmitting data among all IC UEs to stop transmitting the synchronization signal.

If the IC UE transmits a synchronization signal under the control of the eNB, the OOC UE changes the synchronization signal transmission timing immediately or just after the end of data transmission or at the time of becoming a new synchronization source UE or at a predetermined time. The synchronization timing includes all of the symbol, subframe, and system frame number. The synchronization signal transmission timing of the OOC UE is changed such that the synchronization signal is mapped to the OOC synchronization resource in the coverage area of the eNB, whereby the IC UE can receive the synchronization signal of the OOC synchronization source UE.

Fig. 3a and 3b illustrate resource structures for D2D synchronization according to an embodiment of the present disclosure.

Referring to fig. 3a and 3b, a brief description is made of a synchronization signal transmission case and method. Basically, a UE having a discovery signal or a control/data signal to be transmitted transmits a synchronization signal before transmitting the discovery signal or the communication control/data signal. Specifically, the synchronization source UE starts to transmit a synchronization signal in a synchronization resource, which is a synchronization resource located between a point of time that precedes a resource pool for a discovery signal or a communication control/data signal as much as at least one synchronization period and a start point of the resource pool, and the synchronization resource within the resource pool. At this time, the synchronization signal transmission is triggered by one of the following conditions.

The eNB commands the UE to transmit a synchronization signal.

The conditions configured by the eNB or preconfigured in the UE are met.

The synchronization resource structure is configured as shown in fig. 3a such that the IC UE and the OOC UE do not transmit their synchronization signals at the same time.

Fig. 3a is for the case where the synchronization period 310 is 40 ms. The synchronization source UE may send synchronization signals at different timings within the D2D Bandwidth (BW) during the synchronization period 310. For example, the IC UE may transmit a synchronization signal at SRU #0 using 6 synchronization PRBs for a predetermined duration at the beginning of a synchronization period as represented by reference numeral 311. The synchronization signal is followed by D2D control and data 312. The upper part of fig. 3a shows an exemplary case where the frame 301 has a length of 40 ms.

In contrast, the OOC UE may transmit a synchronization signal at SRU #1321a and/or SRU #2321b using 6 synchronization PRBs for a predetermined duration at a predetermined time offset from the start time of the synchronization period. The D2D control and data 322 is sent in a synchronization period other than SRU #1321 and/or SRB # 2. The time offset from the start point of the off-sync period may be set to 10ms and 30ms, respectively. At this time, the time offset for transmitting the synchronization signal of the OOC UE should be set such that SRU #1321a and SRU #2321b do not overlap with SRU # 0311. In this way, IC UEs can transmit signals in a frame format 301 as shown in the upper part of fig. 3a, while OOC UEs can transmit signals in a frame format as shown in the lower part of fig. 3a in a frame format 302.

The synchronization resource structure shown in fig. 3a is configured such that IC UEs and OOC UEs do not transmit signals at the same timing. For example, the IC UE transmits a synchronization signal using SRU #0 positioned at the beginning of the synchronization period allocated for the IC UE. In contrast, the OOC UE transmits a synchronization signal using SRU #1 having an offset of 10ms or SRU #2 having an offset of 30ms from the beginning of a synchronization period allocated for the OOC UE. According to this structure, SRU #0 for IC UEs carries synchronization signals transmitted through OOC UEs or IC UEs served by a neighbor eNB, not through IC UEs served by a serving eNB. Synchronization signals from the UEs are multiplexed onto one SRU #0, and the synchronization source UE uses the same sequence in order to contribute to the reception performance improvement.

Because neighboring synchronization source UEs are able to receive signals from each other to help maintain timing without drifting, OOC UEs use two resources, i.e., SRU #1321a and SRU #2321 b. The OOC UE can select the synchronization resource in various ways. After acquiring the transmission timing, the eNB selects an explicit or predetermined synchronization resource as follows.

a) Monitoring to use a particular resource selected from the unoccupied resources. In the case where the monitoring period is configured, the above operation is repeated in the next period.

b) Resources (e.g., SRU #1) allocated for the I-SS among the synchronization resources are monitored to use a specific resource selected from unoccupied resources. In the case where a non-I-SS UE transmits a synchronization signal (e.g., if the timing of neighboring synchronization source UEs causes their timing to be mismatched, or if the synchronization signal of a low priority synchronization source UE is mismatched when the priority is predetermined), resources that are not non-I-SS resources (e.g., SRU #2) are monitored to use some of the unoccupied resources.

c) The synchronization resources are randomly changed at certain intervals based on the probability p.

In the above method, the unoccupied resources are determined by comparing the received power of the received synchronization signal with a threshold value predetermined or determined by the eNB.

Fig. 3b is a schematic diagram illustrating a case where the synchronization signal transmission resource is configured such that the IC UE and the OCC UE transmit the synchronization signal at the same timing of one synchronization cycle. In the above method, the synchronization resource receiving the synchronization signal is not determined as an idle resource.

It should be noted that the same reference numerals are used in fig. 3a and 3 b. The upper part of fig. 3b shows a frame format 301 with a synchronization period 310. As described above, the synchronization period 310 is 40ms for the IC UE to transmit a synchronization signal at SRU #0 using 6 synchronization PRBs for a predetermined duration at the beginning of the synchronization period as indicated by reference numeral 311. The synchronization signal is followed by D2D control and data 312.

In contrast, the OOC UE may transmit a synchronization signal at SRU #1321a starting at the beginning of the synchronization period and/or SRU #2321b defined by a predetermined time offset from the start time of the synchronization period using 6 synchronization PRBs, respectively. The D2D control and data 322 is sent in a synchronization period other than SRU #1321 and/or SRB # 2. The time offset from the start point of the off-sync period may be set to 20ms, respectively.

Assume that an I-SS UE is allocated SRU #2321b and a non-I-SS UE is allocated SRU #1321a based on a condition that an OOC UE becomes an I-SS for transmitting a synchronization signal. At this time, the IC UE only needs to receive the synchronization signal of the I-SS. Accordingly, the number of resources on which the IC UE must receive signals from the OCC UE can be reduced by allowing a synchronization signal to be transmitted at SRU # 0311, which is a synchronization resource for IC, and SRU #1321a for a synchronization source UE that is not I-SS among the OOC UEs, in the same subframe, compared to the case of fig. 3 a. In case the OCC UE having received the synchronization signal of the IC UE becomes the synchronization source UE in the exemplary case of fig. 3b, it is apparent that SRU #2321b is selected to avoid collision with SRU #0 as a transmission resource for the IC UE. An OCC UE that has received a synchronization signal of another OCC UE at SRU #2 may select SRU #1321a to avoid collision.

Synchronous resource offset

If the synchronization resource locations of all enbs or all clusters are identical to each other, it is not necessary to exchange offset information indicating the synchronization resource locations. However, if the eNB wants to control the location of the synchronization resource location or if it is necessary to consider half-duplex or interference issues of the synchronization source UE on the synchronization resource, it may be configured such that the synchronization resource has an offset (0-39 ms) in one synchronization period (e.g., 40 ms). For this purpose, the eNB may use SIBs in the BCH, or the synchronization source UE may use the synchronization channel (or D2D broadcast channel) to broadcast the offset.

The synchronization resource starts from SFN #0 in the eNB or starts with an offset having a value less than one synchronization period under the control of the eNB. Assuming that the synchronization resource starts from SFN #0 in the serving (camping) eNB, the offset of the synchronization position of the neighboring eNB corresponds to the difference between SFN #0 of the serving eNB and SFN #0 of the neighboring eNB.

Assuming that the synchronization resource starts with an offset in the serving eNB, the offset of the synchronization resource position of the neighboring eNB corresponds to the difference between SFN #0 of the serving eNB and the starting position of the synchronization resource of the neighboring eNB. For example, it is equal to (difference between serving eNB SFN #0 and neighbor eNB SFN # 0) + (difference between neighbor eNB SFN #0 and the starting position of the neighbor eNB synchronization resource). Since there may be an offset of at least one subframe (1ms) between non-synchronized enbs, the offset may range from 0 to 39 under the assumption that the synchronization period is 40 ms.

The serving eNB may operate differently depending on whether the synchronization resources start from SFN #0 or start with offset. If an offset shorter than the synchronization period is necessary in the serving eNB, this information has to be provided over the PD2DSCH (synchronization channel). If this information is received, the UE estimates the SFN of the eNB by subtracting the offset from the D2D Frame Number (DFN). If the synchronization resources always start from SFN0 at the serving eNB, the synchronization resource offset from the neighboring eNB is equal to the difference between SFN0 of the serving eNB and SFN0 of the neighboring eNB. Thus, overhead can be reduced by expressing the resource region of the neighbor eNB as [ synchronization resource offset ] + [ offset between neighbor eNB SFN0 and the starting point of the resource region ]. The resource region of the neighboring UE may be a synchronization resource, a discovery resource, a control resource for communication, or a data resource for communication.

Further, the resource region may be located as follows depending on whether offset information is used:

SFN #0 position when offset is used

Referencing resource region locations for synchronization period start points when offsets are used

Referencing resource region locations for DFN #0 when offset is not used

Fig. 4a is a timing diagram illustrating the timing of a primary system information broadcast in the Downlink (DL) and a system information broadcast for D2D transmission by a UE in the Uplink (UL) according to an embodiment of the present disclosure.

Referring to fig. 4a, an eNB transmits a new Master Information Block (MIB) at an interval of 40ms according to a conventional normal method. The same MIB is copied and transmitted at 10ms intervals between new MIB periods. At this time, 8 bits of a 10-bit SFN are transmitted in the MIB and 4 identical MIBs are transmitted in a period of 40ms so as to be distinguished from each other by using a CRC in a Cyclic Redundancy Check (CRC) mask having a MIB identifying 2 bits of 10 ms. The UE determined to transmit the synchronization signal transmits the synchronization signal every 40ms, and is determined under the control of the serving eNB with respect to the transmission of SFN # 0. For this operation, the eNB broadcasts the SIB to inform all UEs within the eNB coverage area of the offset between SFN #0 and the first synchronization resource. Since the period of the synchronization signal is 40ms, the offset falls in the range from 0 to 39 ms. Here, it is assumed that one subframe spans 1 ms.

The eNB also notifies information about the location of the resource pool having an offset with respect to the SFN #0 in the same manner. The eNB may also provide information regarding neighboring eNB synchronization resources and discovery/communication resource regions. Since an offset with respect to the synchronization resource may be generally requested when notifying the positions of the resource regions of the plurality of neighboring enbs, the offset of the synchronization resource can be used as a reference point for marking the respective resource regions.

The UE transmits a synchronization channel including DFN #0 on a synchronization resource that first appears with reference to SFN #0 when transmitting the synchronization channel together with the synchronization sequence. In addition, the UE may transmit an offset from SFN #0 to the first synchronization resource location on the synchronization channel. Fig. 4a is depicted under the assumption that the UE additionally transmits an offset. How to express the DFN offset for notifying the same timing can be determined in various ways. For example, "DFN #0+ 14" is the same as "DFN #1+4 ms". This is because the DFN is initialized in units of 10 ms.

A description is made of the relationship between the transmission timings and offsets of the SFN and DFN with reference to fig. 4 a.

The upper part of fig. 4a shows MIB transmission timing in downlink within the coverage area of a specific eNB. The MIBs 401, 411 and 421 transmitted at the first transmission opportunity of each MIB transmission period arriving at every 40ms include different information. The MIBs 402, 403, 404, 412, 413, 414, 422, 423 and 424 transmitted at the second and subsequent transmission occasions include the same information as the corresponding MIBs transmitted at the first transmission occasion in the corresponding MIB transmission cycles.

The lower portion of fig. 4a shows DFN transmission timing when DFNs 405, 415, and 425 for D2D transmission are transmitted using uplink resources within the coverage area of a particular eNB. As shown in fig. 4a, the DFN transmission period may be the same as the MIB transmission period of 40ms within the coverage area of a specific eNB. As described above, the DFNs 405, 415, and 425 transmitted at the timing determined based on the offset value carried in the MIB have an offset from the SFN #0 in one cycle range of 40 ms. The lower part of fig. 4a is for an exemplary case where the offset is set to 14 ms.

Fig. 4b and 4c illustrate offset transmission schemes of an OOC UE when an IC UE transmits a synchronization sequence and a synchronization channel to the OOC UE according to an embodiment of the present disclosure.

Referring to fig. 4b, a method for an IC UE to transmit an offset together with a DFN value on a synchronization channel is illustrated. In the embodiment of the present disclosure of fig. 4b, it is assumed that the IC UE is allocated a synchronization resource with an offset of 14ms in DFN #0 from the eNB. If the IC UE transmits DFN #0 and a value corresponding to 14ms on the synchronization channel, the OOC UE determines its synchronization resource location based on the corresponding value. If the OOC UE arbitrarily determines the synchronization resource location, the IC UE must frequently perform a scanning operation to receive a synchronization signal of the OOC UE, resulting in an excessive burden on the IC UE. Various embodiments of the present disclosure of fig. 4b and 4c aim to reduce unnecessary reception operations such that the OOC UE transmits synchronization signals at a location expected by the IC UE. The receiving operation can be simplified by two options in the above method.

Option 1: the OOC UE may determine a synchronization resource location according to offset _ SRU (offset _ SRU) from a time when the synchronization signal of the IC UE is received to a predetermined synchronization resource.

Option 2: the OOC UE may determine a synchronization resource position according to offset _ SRU from DFN #0 calculated based on an offset value transmitted by the IC UE when a synchronization signal of the IC UE is received to a predetermined synchronization resource.

The reason for informing the offset as in fig. 4b is to meet the necessity of even OOC UEs having different offsets between sync source UEs (e.g., to avoid half-duplex or interference problems) and to match the operation with IC UEs. However, option 1 calculates the offset based on the synchronization resource position of the IC UE on the assumption that the synchronization resource position of the IC UE is the reference timing of the synchronization period (refer to the synchronization resource structure of fig. 3). Further, option 2 calculates an offset based on DFN # 0.

Fig. 4b illustrates the case where the IC UE transmits the synchronization sequence and the synchronization channel to the OCC UE with an offset of 14 ms. For example, DFNs 405, 415, and 425 are transmitted at an offset of DFN0+14ms with a period of 40ms in the same manner as in fig. 4 a. In case of a class 1OOC UE receiving a reference signal from a UE located within the coverage area of the eNB, two options can be considered.

Referring to fig. 4b, DFNs 431a, 441a, and 451a indicating an offset value of 12ms may be transmitted based on DFN #0 transmitted by the IC UE as in option 1, or an offset value of DFN #0 transmitted by the IC UE may be calculated based on the offset value transmitted by the IC UE and the positions of synchronization resources 431b, 441b, and 451b may be calculated according to offset _ SRU of a predetermined synchronization resource starting from DFN # 0.

Meanwhile, it is necessary for the OOC UE to distinguish between at least two synchronization resources whose timings match (i.e., the UE is synchronized) in order to avoid the half-duplex problem. For this purpose, the synchronization resources may be distinguished from each other based on the DFN value, but the synchronization resource index can be used. For example, if a DFN value + offset value is received on the synchronization channel, the offset value may be removed and a modulo operation performed on the DFN value. For example, DFN #0, which depends on the result of the modulo-4 operation on the DFN value, may be an IC UE synchronization resource, DFN #1 is a first OOC UE synchronization resource, and DFN #3 is a second OOC UE synchronization resource. The modulo-4 operation may be performed on the MSB or LSB 8 of the DFN 10 bits. In another method, the MSB or LSB N bits of the field informing the offset are used. For example, an offset of [ 0-9 ] bits may indicate an IC UE synchronization resource, an offset of [ 10-19 ] ms indicates a first OOC UE synchronization resource, and an offset of [ 30-39 ] ms indicates a second OOC UE synchronization resource. For example, a 6 bit offset of 00 x may indicate an IC UE synchronization resource, a 6 bit offset of 01 x indicates a first OOC UE synchronization resource, and a 6 bit offset of 10 x indicates a second OOC UE synchronization resource.

According to fig. 4c, by noting the synchronization procedure between OOC UEs that are not able to receive eNB timing, it is assumed that the synchronization position is determined in association with the DFN and the relative offset between the synchronization resources is predetermined, since the offset is meaningless in the case where the network is not able to control the DFN #0 reference timing. In this scenario, although the IC UE transmits the offset, the offset is disregarded and synchronization is determined considering only the relative offset between the synchronization resources. In this case, it is necessary to indicate an index corresponding to the synchronization resource location. The synchronization resource index (of SRU1 and SRU 2) is signaled in N bits (e.g., 1 bit) on the synchronization channel using a CRC mask or a demodulation reference signal (DMRS). If there is no separate index information, the synchronization resource index can be obtained by modulo operation on the DFN value.

Referring to fig. 4c, the IC UE transmits DFN # 0405, DFN # 1415, and DFN # 8425 along with an offset value transmitted in one of the above-described methods according to a network reference time. However, the OOC UE may disregard the offset value and configure the offset value based on the received DFN according to a predetermined offset _ SRU value (the offset is set to 12ms in fig. 4 c). Further, the index of the DFN can be defined as described above. The offset value of DFN #0 transmitted by the IC UE may be calculated based on the offset value transmitted by the IC UE and the positions of the synchronization resources 431c, 441c, and 451c may be calculated according to offset _ SRU of a predetermined synchronization resource starting from DFN # 0.

Separately from the various embodiments of the present disclosure of fig. 4a, 4b, and 4c, the eNB may broadcast a synchronization resource offset configured for the OOC UE. Because the eNB can use the additional offset information to place the OOC UE synchronization resources at the expected location, interference can be controlled and power consumption of the IC UE reduced by attempting reception at the corresponding location based on this information.

Table 1 shows which resource pool must be notified to a receiving UE according to a transmission/reception relationship between UEs.

TABLE 1

[ Table 1]

	Tx resource pool	Rx resource pool
			Case 1: IC UE → OOC UE	0 (configuration) or X	0 (configuration)
Case 2: OOC UE → IC UE	X	0 (configuration or Pre-configuration)
			Case 3: OOC UE → OOC UE	X	0 (configuration or Pre-configuration)
Case 4: IC UE → IC UE	X	0 (configuration)

In case that the reception resource region is explicitly indicated, the UE having received the synchronization signal performs a reception operation at the reception resource region notified through the D2D synchronization channel. In case that the reception resource region is not explicitly indicated, the UE having received the synchronization signal performs a reception operation at a corresponding location on the assumption that the transmitting UE operates in a predetermined (pre-configured or fixed) transmission resource region. In case that the transmission resource region is explicitly indicated, only a case where the IC UE served by the eNB having a high priority notifies the OOC UE of the reception resource region is considered.

a) The transmission and reception operations are performed in a transmission resource region notified by the IC UE. The reception operation is performed not in all available resources but in the corresponding transmission resource region.

b) The transmission and reception operations are performed in the transmission resource region notified by the IC UE, and the reception operation is performed in all available resources other than the corresponding transmission resource region.

c) Only the transmission operation is performed in the transmission resource region notified by the IC UE, and the reception operation is performed in all available resources other than the corresponding transmission resource region.

d) Only the transmission operation is performed in the transmission resource region notified by the IC UE, and the reception operation is performed in all available resource regions without interfering with the transmission operation.

In order to configure various operations as above, the eNB may transmit 1-bit information for distinguishing cases where transmission and reception resource regions are identical to and different from each other through a synchronization channel. At this time, if the resource region information is shared, the eNB may be configured to preset a transmission resource region without separate notification.

A description of the transmission and reception resource pool configuration summarized in table 1 is made hereinafter.

-case 1: this is a case where the IC UE transmits a synchronization signal to the OOC UE and thus the OOC UE performs a D2D signal transmission operation in a transmission resource region (Tx resource pool) notified by the IC UE through a D2D synchronization channel (or D2D broadcast channel). Meanwhile, in case that the transmission resource region is not notified, the IC UE may perform a transmission operation at a predetermined resource location (e.g., a resource determined based on the D2D frame number).

To protect the IC terminal, the eNB may inform the OOC UE of the reception resource region, but the eNB does not instruct transmission of the resource region. This corresponds to a transmission resource region of the IC UE, and the OOC UE may perform an operation of receiving IC UE signals in a reception resource region notified from the eNB in a predetermined transmission resource region in priority and perform a transmission/reception operation with other OOC UEs on other resources.

Case 2: this is the case when the IC UE receives the synchronization signal of the OOC UE, and in this case, the OOC UE cannot configure any transmission or reception resource of the IC UE if it is assumed that the priority of the IC UE is higher. Because the OOC UE is not controlled by the eNB, the OOC UE uses a predetermined resource region. If a synchronization signal of an OOC UE is received, the IC UE performs a reception operation on the assumption that transmission will be performed in a predetermined resource region. Further, in case of an OOC UE directly controlled by an eNB or an OOC UE capable of being controlled to use a resource region predetermined according to a D2D communication group, the OOC UE may perform a reception operation based on a reception resource region included in a synchronization channel received by an IC UE.

-case 3: this is the case where the OOC UE receives a synchronization signal of another OOC terminal, and in this case, the OOC UE may configure a reception resource region and operate in a predetermined reception resource region for the same reason as described in case 2.

Case 4: this is the case where a UE served by a neighbor eNB establishes synchronization, and in this case, since it is impossible to control a transmission operation of the UE served by the neighbor eNB, it is possible to exercise control in a reception resource region.

Considering cases 1 to 4, the most economical configuration method applicable to various scenarios while using information of only one resource region is described as follows. A description of the two most economical deployment methods is made below. These two methods are selectively used.

a) If the IC UE notifies the transmission resource region through the synchronization channel, the OOC UE performs a transmission/reception operation in the corresponding transmission resource region. The OOC UE, having received the synchronization channel of the OOC UE, considers a transmission resource region as a reception resource region, so that only a reception operation, not a transmission operation, is performed on the corresponding reception resource region.

b) If the IC UC informs the reception resource region through the synchronization channel, the OOC UE performs only the reception operation and does not perform the transmission operation in the corresponding reception resource region. Of course, the OOC UE may perform a transmission/reception operation in its transmission resource region.

Fig. 5a, 5b, 5c and 5d illustrate various transmission and reception resource pool allocation methods according to embodiments of the present disclosure.

Fig. 5a, 5b, 5c and 5D illustrate exemplary scenarios in which the above principles apply when allocating D2D transmit and receive resource pools. In fig. 5a, 5b, 5c, and 5d, UEs are classified into three types.

The IC UE is a UE capable of receiving a synchronization signal of the eNB to connect to the network. OOC UEs are sub-classified into two types. First, a class 1OOC UE (OOC UE cat.1) is a UE that receives a synchronization signal relayed by an IC UE and follows the reference timing of an eNB. Second, category 2OOC UE (OOC UE cat.2) is a UE that does not follow the reference timing of the eNB.

Referring to fig. 5a, IC UEs are performing transmission/reception operations from a transmission resource pool indicated by an eNB, and a sync source UE among the IC UEs relays transmission resource pool information to an OOC UE. The class 1OOC UE can transmit/receive signals only in a transmission resource pool indicated from the eNB. Meanwhile, the class 2OOC UE can transmit/receive signals in the entire pre-configured resource pool.

Referring to fig. 5a, an IC UE may transmit and receive signals in D2D Tx resource pools 501, 502, and 503 allocated by an eNB. In fig. 5A, resource pools 501, 502, and 503 used by the IC UE to transmit and receive signals are D2D Tx resource pools allocated by the eNB. The category 1OOC UE receiving the synchronized Tx resource pool information from the IC UE transmits/receives signals only in the Tx resource pools 501, 502, and 503 indicated from the IC UE (i.e., indicated from the eNB). However, the class 2OOC UE may transmit/receive signals in the entire predetermined resource pool (i.e., D2D Tx pool 500). In fig. 5, an area marked by WAN means a resource pool for use in the LTE system.

Referring to fig. 5b, the IC UE performs a transmission/reception operation in a transmission resource region notified from the eNB, and a synchronization source UE among the IC UEs relays information on the same reception resource region as the transmission resource region and a transmission resource region for the OOC UE notified from the eNB to the OOC UE. The class 1OOC UE must perform only a reception operation in a reception resource region notified from the eNB and be able to perform a transmission/reception operation in a transmission resource region notified from the eNB. The reason for this configuration is because the D2D inter-UE interference is higher on the D2D link used to receive signals over short distances, as opposed to weak interference from OOC UEs on the uplink of the WAN. Meanwhile, the class 2OOC UE may perform a transmission/reception operation on the entire predetermined resource region.

Referring to fig. 5b, resources allocated to IC UEs are divided into a D2D resource region (i.e., D2D Tx pools 501, 502, and 503) and a WAN resource region. At this time, the sync Tx UE among the IC UEs configures the Rx pools 521, 522, and 523 for the category 1OOC UE in the same timing as its D2D Tx pools 501, 502, and 503 and notifies the category 1OOC UE about the Rx pools. In this way, the category 1OOC UE can configure the IC UE's D2D Tx pools 501, 502, and 503 as its Rx pool. The category 1OOC UE may also configure all or a portion of the other areas 531, 532, and 533 in addition to the Rx pool as a D2D Tx pool. The D2D Tx pools 531, 532, and 533 may follow the configuration of the eNB as described above. As described with reference to fig. 5A, the class 2OOC UE may perform transmission/reception in the entire predetermined resource region (i.e., the D2D Tx pool 500).

Referring to fig. 5c, the IC UE performs a transmission/reception operation in a transmission resource region notified from the eNB, and a synchronization source UE among the IC UEs relays information on the transmission resource region to the OOC UE. The class 1OOC UE must perform a transmission/reception operation in a transmission resource region notified from the eNB. Also, the category 2OOC UE performs only a reception operation in a transmission resource region notified from the eNB among predetermined resource regions and can perform a transmission/reception operation in the remaining resources. The reason for the configuration of fig. 9 is to prevent interference to IC UEs and class 1OOC UEs that adhere to eNB timing.

Referring to fig. 5c, resources allocated to IC UEs are divided into D2D resources (i.e., D2D Tx resource pools 501, 502, and 503) and WAN resources. At this time, the synchronized Tx UE among the IC UEs configures its Tx pools 501, 502, and 503 as Tx pools for the category 1OOC UEs and notifies the category 1OOC UEs about the Tx pools 501, 502, and 503. Thus, the category 1OOC UEs can configure the D2D Tx pools 501, 502, and 503 at the same timing as the D2D Tx pools of IC UEs. The category 1OOC UE also configures its Tx pools 501, 502 and 503 as Rx pools for the category 2OOC UE and informs the category 2OOC UE about these Rx pools. Accordingly, the class 2OOC UE can configure the D2D Rx pools 541, 542, and 543 at the same timing as the D2D Tx pool of the class 1OOC UE. The reason for this has been described above. Finally, the category 1OOC UE may configure all or a portion of the other areas 551, 552, and 553 except the Rx pool of the category 2OOC UE as a Tx pool. The D2D Tx pools 531, 532, and 533 may follow the configuration from the eNB as described above.

The embodiment of fig. 5d is similar to the embodiment of fig. 5b, except that the category 2OOC UE does not use the entire predetermined resource region and performs a reception operation in the reception resource region of the category 1OOC UE whose information is indicated from the synchronization source UE and performs a transmission/reception operation in the remaining resources of the predetermined resource region for similar reasons in fig. 5 c.

Referring to fig. 5D, the resources allocated to IC UEs are divided into D2D resource regions (i.e., D2D Tx pools 501, 502, and 503) and WAN resource regions 511, 512, and 513. At this time, the sync Tx UE among the IC UEs configures the Rx pools 521, 522, and 523 for the category 1OOC UE in the same timing as its D2D Tx pools 501, 502, and 503 and notifies the category 1OOC UE about the Rx pools. In this way, the category 1OOC UE can configure the IC UE's D2D Tx pools 501, 502, and 503 as its Rx pool. The IC UE also configures WAN resources as D2D Tx pools 531, 532, and 533 for category 1OOC UEs and informs the category 1OOC UEs about the configured D2D Tx pools. As a result, the class 1OOC UE is able to configure both the D2D Tx pool and the Rx pool.

A category 1OOC UE may instead configure its D2D Tx and Rx pools to category 2OOC UEs, i.e., as D2D Rx and Tx pools. For example, a category 1OOC UE may timely configure its Rx pools 521, 522, and 523 as Tx pools 541, 542, and 543 of category 2OOC UEs and its Tx pools 531, 532, and 533 as Rx pools 551, 552, and 553 of category 2OOC UEs and inform the category 2OOC UEs about the configured D2D Tx and Rx pools.

In the embodiments of fig. 5a, 5b, 5c and 5d, it is assumed that the synchronization timings of the IC UE and the class 1OOC UE match each other. However, OOC UEs may not adhere to the timing of IC UEs in a particular system. To reduce interference to IC UEs in such an environment, the OOC UE may transmit a synchronization signal as follows.

1. Synchronous resource frequency hopping: the location of the synchronization resource for transmitting the synchronization signal is changed according to a time variable. For example, the output of the function with an input of the DFN value is changed resource location information regarding the resource location in DFN # 0. The change position where the resource is increased by 1 according to the DFN value may be predetermined. In case there are multiple synchronized resources, the same change equation may be applied or a resource-specific change equation may be applied.

2. Offset randomization: although the period of the synchronization resource is fixed, the start position of the synchronization resource can be controlled as much as an offset value in one synchronization period. In this context, a new offset value may be applied to a predetermined period (scan period) of the scan duration that is longer than the synchronization period. In particular, when an OOC UE becomes a synchronization source UE, any synchronization resource offset other than a predetermined synchronization offset may be used. Once the UE becomes a synchronization source, the UE scans the surrounding environment to determine whether to function, and if the synchronization source UE reappears or is updated, the offset of the synchronization resource is changed to an arbitrary value in units of 1ms (i.e., 1 LTE subframe) within one synchronization period (e.g., 40 ms). At this time, if the offset configured by the neighbor eNB is known, the offset may be changed to any value among offsets that do not cause synchronization source collision to avoid interference.

As shown in the synchronization resource structure examples of fig. 3a and 3b, a plurality of Synchronization Resource Units (SRUs) can be configured in one synchronization cycle. In the embodiments of fig. 3a and 3b, the SRUs are connected to different types of synchronization source UEs, and the SRU positions may be determined according to the hop count or role (independent/auxiliary) of the synchronization source UEs.

The synchronization signal is transmitted per SRU, but the synchronization channel may be transmitted according to a specific condition. The condition for not transmitting the synchronization channel is as follows.

1. When the synchronization channel period is set to a value longer than the synchronization signal period,

2. when a particular resource region specified by the eNB overlaps with the SRU location (e.g., it is configured to use only synchronization signals (D2DSS) in a discovery resource region not controlled by the eNB),

3. when the synchronization channel reception performance is degraded due to too many participating UEs as a result of the synchronization channel monitoring, the UE may immediately change the synchronization channel cycle or offset according to the monitoring result or report the monitoring result to the eNB to change the synchronization channel cycle or offset according to the determination of the eNB. Alternatively, the UE may immediately change the transmission probability of the synchronization channel according to the monitoring result or report the monitoring result to the eNB to change the transmission probability of the synchronization channel according to the determination of the eNB.

Fig. 6a, 6b, 6c, and 6D illustrate various resource pool structures and locations of synchronization resources for D2D discovery or communication, according to embodiments of the present disclosure.

The structures illustrated in fig. 6a, 6b, 6c and 6d are characterized by a logical chronological arrangement and other resources may actually be added in addition to the displayed areas. For example, in case that the UE operates in TDD (time division duplex), the D2D resource region is configured in uplink and thus downlink resources may be located in time domain. However, in view of the D2D UE, the resources are logically configured as if there were contiguous D2D resources.

Referring to fig. 6a, a structure of resource regions, i.e., Scheduling Assignment (SA) regions (pools) 611 and 612 and communication data resource regions (data pools) 621 and 622, and a structure of synchronization resources 601a, 601b, and 601c for notifying reference timing of the resource regions, which are notified from an eNB for performing D2D communication, are illustrated. In the SA pools 611 and 612, the UE transmits an SA signal including information on data resources allocated or selected in advance in the SA pools 611 and 612 to the neighboring UEs before data transmission. The UEs having received the SA signals in the SA pools 611 and 612 receive the data signals transmitted by the intended transmitting UEs in the subsequent data areas 621 and 622. The IC UE determines the absolute time of the reference frame (SFN0) by receiving the BCH and acquires the relative location information of the resource region expressed based on the reference frame by receiving the SIB.

UEs served by neighboring enbs or located in partial or within network coverage areas receive the synchronization signals and synchronization channels transmitted in SRUs 601a, 601b, and 601c to first determine the absolute time of a reference frame (SFN0) and to obtain relative location information of the resource area indicated based on the reference frame.

The D2D UE may select one synchronization UE to synchronize for the transmitted reference timing according to a predetermined priority. If there are other synchronized UEs having a reference timing different from that of the selected synchronized UE, the transmission reference timing of the selected synchronized UE is used to synchronize the reference timing used for receiving synchronization signals from the other synchronized UEs during a predetermined period of time. At least one synchronization resource (i.e., SRU) may be arranged during one synchronization signal period and, if the reference timing for transmission is synchronized, may operate so as not to transmit a synchronization signal without an idle SRU when checking conditions for synchronization signal transmission, such as reception power and signal detection of eNB signals or other synchronized UEs. For example, although the condition for transmitting the synchronization signal is satisfied, the UE transmits the synchronization signal using one SRU selected among idle SRUs only when the idle SRUs exist. It may be possible to apply a relatively loose idle SRU determination condition. For example, if the number of synchronization signals received in a Synchronization Resource (SRU) at a higher received power than a predetermined received power threshold is less than N, the SRU may be determined to be an idle SRU.

Fig. 6b is a schematic diagram illustrating resource pools for synchronized resource construction for D2D discovery, according to an embodiment of the present disclosure.

Fig. 6b shows a resource region notified from the eNB for performing D2D discovery, i.e., a structure for discovery resource regions (discovery pools) 631 and 632 and a structure of synchronization resources 601a, 602b, 601c, and 602a for indicating reference timing of the resource pools. Similar to the case of fig. 6A, the UE receives a BCH from the eNB in the discovery resource area 1631 to determine the absolute time of the reference frame (SFN0) and acquires relative location information about the discovery area indicated by the reference frame by receiving the SIB.

UEs served by neighboring enbs or located in partial or within network coverage areas receive the synchronization signals and synchronization channels transmitted in SRUs 601a, 601b, and 601c to first determine the absolute time of a reference frame (SFN0) and to obtain relative location information of the resource area indicated based on the reference frame. For example, if the synchronization channel received from the UE served by the neighboring eNB indicates SFN10, reference frame SFN #0 is positioned 10 frames before.

Meanwhile, UEs served by the neighboring eNB receive a BCH and a SIB from the serving eNB in the discovery resource area 2632 to acquire information about the location relative to the reference time (SFN0) of the serving eNB for the reference resource area 2632 of at least one other neighboring eNB. When the starting point of the resource region of another eNB is reached, the UE may perform a transmission/reception operation using the WAN, and if the UE is not transmitting a synchronization signal or other D2D signal, the UE receives a synchronization signal and a channel expected at the starting time of the corresponding resource region.

The synchronization signal is transmitted according to a specific condition, but the synchronization channel may not be transmitted. A terminal served by a neighbor eNB receives a synchronization signal in order to acquire an accurate received synchronization reference timing with respect to the start of a corresponding discovery resource region. Such operations may be applied to the D2D communication resource region, but are not limited to this example.

Meanwhile, the discovery resource region 1631 and the discovery resource region 2632 are simultaneously used, and a field indicating whether the first duration of the resource region (e.g., subframe) is used for the SRU should be included in the BCH or SIB from the eNB. In case of a synchronization source UE, whether there is a resource for transmitting a synchronization signal should be determined based on a field informing availability of an SRU, and in case of a receiving UE served by a neighbor eNB, whether to receive a synchronization signal and a channel in a corresponding resource region or whether to receive a discovery signal may be determined according to the field informing availability of the SRU. This field is referred to as a periodic synchronous transmission field in embodiments of the present disclosure. In case of using the periodic SRUs 601a, 601b, and 601c, the eNB opens the periodic sync transmission field to indicate it through BCH or SIB. In case of using the temporary (one-time use) SRU 602a, the periodic synchronization transmission field is turned off to indicate it through BCH or SIB.

If the additional SRU 602a is not used in the resource region 2632, the UE located in the partial network coverage region or outside the network coverage region receives the synchronization signal and the synchronization channel transmitted by the UE in the periodically transmitted SRUs 601a, 601b, and 601c to determine the absolute time of the reference frame (SFN0) and to acquire relative position information on both the resource region 1631 and the resource region 2632 indicated compared to the reference frame. The relative location information can be obtained based on information relayed by the neighboring eNB and information received from the serving eNB. After acquiring the relative location information of the resource region, the UE may wake up at least one synchronization period earlier than the starting point of the resource region from the idle state to receive a synchronization signal at the periodic SRUs 601a, 601b, and 601c to compensate for synchronization.

Meanwhile, in case of using the additional SRU 602a in the resource region 2632, the UE may receive the periodic SRUs 601a, 601b, and 601c and acquire relative position information from the reference time (SFN0) of the serving eNB for the resource region of the neighboring eNB and may wake up from an idle state just before the start time of the resource region 2632 to receive a synchronization signal in the additional SRU 602a to compensate for synchronization. In the case where no periodic SRUs 601a, 601b, and 601c use only the additional SRU 602a in the resource region 2632, if the eNB has not provided information on the start point of the resource region in advance or if the UE is an out-of-coverage UE, the UE may inefficiently operate to perform a synchronization procedure for receiving the aperiodic SRU 602a for a long time.

Fig. 6c is a schematic diagram illustrating a resource pool structure having a synchronization resource for D2D discovery and communication according to an embodiment of the present disclosure, and fig. 6D is a schematic diagram illustrating a resource pool structure having a synchronization resource for D2D discovery and communication according to an embodiment of the present disclosure. The difference between the embodiments of fig. 6c and fig. 6d is that in the embodiment of fig. 6B the aperiodic SRU 602a is used in the discovery resources. The periodic SRUs 601a, 601B, and 601c and the aperiodic SRU 602a of the discovery pool 630 are characterized as described with reference to fig. 6A and 6B.

The periodic synchronization resources 601a, 601b and 601c are used in a synchronous transmission mode suitable for D2D communication. In order to provide a control channel (scheduling assignment (SA)) region 610 and a data channel (data) resource region 620 for D2D communication, UEs satisfying a condition among D2D UEs may become synchronization source UEs. If a UE that is not transmitting data becomes a synchronization source UE, the receiving UE may not receive data even after acquiring synchronization. Therefore, the presence of data to be transmitted is a necessary condition for becoming the synchronization source UE. The conditions for the presence of data to be transmitted can be embodied in various ways. This is described with reference to fig. 7a, 7b and 7 c.

Fig. 7a, 7b and 7c illustrate a periodically synchronized resource utilization method according to an embodiment of the present disclosure.

Referring to fig. 7a, non-D2D communication resources 701 and 702, SA resources 711 and 712, and D2D communication resources 721 and 722 coexist. Since the case where these resources coexist has been described above, a detailed description thereof is omitted here. SRU locations for transmitting synchronization signals may be configured in these resources. Fig. 7a is directed to a method for transmitting SA signals 711 and 712 and data in synchronization resources 721a, 721b, 721c, 722a, 722b, and 722c included in SA or data regions 721 and 722. For example, SRUs 701a, 701b, 702a, 702b, and 702c located in non-D2D communication resource regions 701 and 702 are unused. However, this method has a disadvantage in that synchronization resources just before the SA resource regions 711 and 712 cannot be used. Since the synchronization signal is not transmitted in a case just before the SA resource region, the Rx UE can receive the synchronization signal after the start of the SA resource region.

To solve this problem, if there is an SA or data resource region between synchronization resources whose utilization is to be determined and synchronization resources following a synchronization period, a UE having an SA signal or data is allowed to transmit a synchronization signal. For example, the UE can be configured to use SRUs 701b and 702b that are not used in the case of fig. 7 a. At this time, the SRUs 701b and 702c before the SA regions 711 and 712 may be SRUs at timings different from those of the SRUs 701b and 702c of fig. 7 a. For example, the SRUs 701b and 702c preceding the SA areas 711 and 712 in fig. 7b may be SRUs different in timing from the SRUs 701b and 702c of fig. 7a because it is required to have a position configured as long as the preceding sync period matches the sync period existing in the data period. Depending on the circumstances, the SRUs 701b and 702c before the SA regions 711 and 712 of fig. 7b may be identical in timing to the SRUs 701b and 702c of fig. 7 a.

Referring to fig. 7b, a minimum synchronization operation is possible in a practical implementation but it is necessary to receive a synchronization signal within a predetermined duration to guarantee performance to some extent. For example, the perceptual performance on the physical layer can be improved by periodically receiving the synchronization signal. To compensate for the offset of the oscillator controlling the timing, it may be possible to attempt to receive the signal before a known period. To receive signals on the resource region of the neighboring cell, it may be possible to attempt early reception based on as many gaps as the timing difference between enbs.

To guarantee performance in several scenarios, the eNB may control the UE to perform signal reception w earlier than the expected synchronization signal transmission timing by configuring the reception window. This may apply to resource regions of the neighboring eNB as well as the serving eNB. The eNB may transmit the parameter w as common information through a BCH or a SIB or using a UE-specific control signal. By paying attention to the UE performing a reception operation based on the value of w, the UE operating as a synchronization source transmits a synchronization signal on a synchronization resource as much as w before the predicted starting point t of the resource region.

Referring to fig. 7c, receive window operations may be additionally considered in the discovery and OOC regions. In a non-synchronized cell environment, synchronization signals for the corresponding resource region may be identified before and after the start time of the D2D resource region (resource pool) of the neighboring eNB, taking into account as many offsets as the size of the reception window. A common window value or a UE-specific window value may be used in a specific cell. For synchronization resources of the serving eNB, a synchronization signal for D2D may also be identified using a window value at the start time of the D2D resource region in the same manner. The window values for the serving eNB and the neighbor enbs are provided separately.

For example, if the synchronization signal is identified before the window w 750 as shown in fig. 7c, it is possible to receive the synchronization signal transmitted at the SRUs 701a, 701b, 702b, and 702c before the SA areas 711 and 712. This means that keneng receives at least one synchronization signal transmitted in the regions 701 and 702 before the SRUs 721a, 721b, 721c, 722a, 722b and 722c in the data regions 721 and 722, and thus it is possible to accurately acquire the start time points of the SA regions 711 and 712.

Furthermore, since the eNB cannot inform the out-of-coverage UE of the starting point of the D2D resource, the out-of-coverage UE uses a predetermined D2D resource region. The synchronization signal of the synchronization resource UE includes a synchronization sequence and a synchronization channel, and the D2D frame number is transmitted on the synchronization channel. The location of the D2D resource region may be predetermined based on the D2D frame numbers. Outside the coverage area, a synchronous reception operation may be performed at the start point of the D2D resource region according to a predetermined window value. The resource region-specific window values and the window values of OOC UE Cat1 and OOC UE Cat2 may be different from each other.

Although fig. 7a, 7b, and 7c do not show any case where the synchronization and SA resource regions overlap, the SA resource region and the synchronization resource may overlap in the same subframe under the control of the eNB. Since the SA resource region is small compared to the data resource region (e.g., the data resource region is composed of 64 subframes whereas the SA resource region is composed of 2), using synchronization resources overlapping the SA resource region may significantly reduce the SA resource utilization efficiency. Exceptionally, it is configured not to transmit a synchronization signal on a synchronization resource overlapping with the SA resource region. Alternatively, as shown in fig. 16 and 17, it is necessary to apply a method capable of transmitting a synchronization signal before the SA resource region.

Synchronization sequence selection method

It is necessary for a UE that has received a synchronization signal to determine its transmission timing based on the timing derived from the received synchronization signal, and to determine a synchronization sequence to be used for transmitting the synchronization signal in advance if a condition for becoming a synchronization source UE is satisfied. The operation to be considered is whether to use a synchronization sequence or another sequence when operating with the timing of the sequence as a synchronization source UE per transmission reference. It is assumed that the synchronization sequence transmitted by the IC UE is different from the synchronization sequence transmitted by the OOC UE.

Fig. 8a and 8b illustrate scenarios for selecting a synchronization sequence between an IC UE and an OOC UE, according to embodiments of the present disclosure.

Referring to fig. 8a and 8b, an eNB 801 has a predetermined coverage area 810 in which an IC UE 811 is located. Further, the second UE 821 is a category 1OOC UE that receives signals from the IC UE 811, and the third UE 831 is a category 2OOC UE that is not capable of receiving signals from the second UE 821 and the network.

In fig. 8a, when the OOC UE (UE2)821 receives a synchronization signal transmitted by the IC UE (UE1)811, since the UE served by the serving eNB has a high priority, the UE 2821 changes its transmission timing to the timing received from the UE 1. In case of a condition for the UE 2821 to become a synchronization source UE, for example, if transmission data occurs, the UE 2821 selects a resource among OCC synchronization resources to transmit an OOC synchronization sequence (D2DSSue _00n) based on the timing received from the UE 1811. In this case, the timing of the eNB is limited to the range of receiving the IC synchronization sequence (D2DSSue _ net). The UE 3831 receiving the OOC synchronization sequence from the UE 2821 determines whether to comply with the timing of the UE 2821 according to a determination condition it has. Such determination conditions may be based on the synchronization signal reception power and variables included in the synchronization channel (D2D frame number, time left until scanning period, aging of the synchronization signal, expiration of synchronization source expiration, number of delayed hops, etc.).

In fig. 8b, when the OOC UE (UE2)821 receives the synchronization signal transmitted by the IC UE (UE1)811, since the UE served by the eNB has high priority, the UE 2821 changes its transmission timing to the timing received from the UE 1811. In the case of the condition for the UE 2821 to become the synchronization source UE, for example, if transmission data occurs, the UE 2821 selects a resource among OCC synchronization resources to transmit an IC synchronization sequence based on the timing received from the UE 1811. In this case, the timing of the eNB may be extended indefinitely for receiving the IC synchronization sequence (D2DSSue _ net). Thus, it may be possible to determine whether to relay the IC synchronization sequence to limit the relay to a predetermined range based on variables included in the synchronization channel (i.e., relay hop count, synchronization signal accuracy, etc.).

In case of transmitting/receiving a synchronization signal between OOC UEs, there are advantages and disadvantages depending on a synchronization sequence matching of a sequence and a synchronization signal assumed to be complied with. If the sequence is identical to the sequence of the adhered synchronization signal, the reference timing is used to transmit the synchronization signal, and since the synchronized UEs use the same sequence while the unsynchronized UEs use different sequences, it is easy to identify synchronization mismatch if different sequences are detected during scanning. However, there is a disadvantage in that it is impossible to correct timing offset occurring due to clock drift when synchronized UEs join after dispersion.

The synchronization source UE may also use different synchronization sequences in different ways and in this case it is difficult to detect the non-synchronization situation due to synchronization sequence mismatch if even synchronized UEs use different synchronization sequences and therefore it is advantageous to match all synchronization resource locations to each other. For example, UEs cannot listen to each other in the same resource due to a half-duplex problem, and thus, if there is a synchronization signal received at a point of time other than the synchronization transmission timing, it may be determined that the signal is received from an unsynchronized UE.

To solve the above disadvantages, the following method can be considered.

Upon receiving the synchronization sequences transmitted by the IC UE, the OOC UE transmits one of the synchronization sequences or an in-use sequence for the OOC. If the OOC UE has received the synchronization signal of the OOC UE and synchronized the Tx timing, the OOC UE transmits the signal with the same sequence as the received synchronization sequence. However, since there may be a synchronization offset even between synchronized UEs in a dynamic environment, the synchronization sequence may change periodically. For example, in order to perform scanning in a scanning cycle, the previous synchronization sequence may be changed. Although the scan periods of the synchronized UEs match each other (e.g., the scan periods are configured based on the DFN), if the synchronized UEs use different sequences in the scan periods, it is possible to determine the synchronization offset based only on the sequence difference without calculating the timing offset. The above operation may be performed using the ID of the synchronization source UE instead of the synchronization sequence.

Similar to the method of changing the sequence or synchronizing the source UE ID at each scanning period, if it becomes a new I-SS after scanning (because there is no person around), the following operations can be performed while maintaining the timing of the D2DSS (i.e., symbol and frame boundary timing).

1) If the scan timing is determined based on the D2D frame number, the DFN is changed.

2) If there is a control signal informing an additional scan timing (scan time, TTS), the DFN is maintained but the TTS value is changed.

Synchronous signal measuring method

Upon receiving the synchronization signal, the D2D UE may measure other synchronization signals to determine whether it becomes a synchronization source UE or report to the eNB according to the UE's connection status. Meanwhile, the synchronization measurement method may be changed depending on the situation in a case where a resource region requiring a periodic synchronization signal and a resource region of a synchronization signal sufficient for one-time use are divided. Synchronized UEs, either under control of the eNB or according to predetermined conditions, must transmit a valid synchronization signal once using the same SRU.

At this time, the synchronization signal transmission is limited to a method of transmitting the same signal on the same resource in a single frequency network manner. At this time, since one or more synchronization signals are received in an overlapping manner, the received signal power may be high compared to a case where one synchronization UE transmits a synchronization signal and the synchronization signal may propagate data farther. In case that the eNB has allocated resources for the periodic synchronization signal, the synchronization UE selects an idle synchronization resource to transmit the synchronization signal under the control of the eNB or according to a predetermined condition. At this time, since the synchronization signals from the synchronization UEs overlap in number as little as possible, the received signal power at the receiving UE is relatively low compared to the case of one-time-use synchronization signal transmission and its propagation distance is not long compared to the data propagation distance.

Due to these environmental differences, it is problematic to use one-time-use synchronization signal transmission as a measurement target for selecting a synchronous relay UE for an out-of-coverage UE. Therefore, it is natural to select only the periodic synchronization signal as a measurement target for selecting the synchronous relay UE. For this purpose, the synchronized UE located in the eNB coverage area must first inform the synchronization resource location, i.e., SRU period and offset information, through a synchronization signal and a synchronization channel. The synchronizing UE may also relay information (i.e., offset, bitmap, etc.) about resource regions that are not used as periodic synchronization resources.

The UE that has received the synchronization signal and the synchronization channel from the synchronization UE first measures the synchronization signal from the neighboring synchronization UE to become a synchronization relay UE. The synchronization signal received from the UE served by the eNB in the corresponding resource region acquired from the information on the resource region not used as the periodic synchronization resource may be disregarded based on the information on the resource regions other than the periodic synchronization resource region (this can be determined based on the synchronization signal transmitted through the synchronization signal and the ID included in the synchronization signal and the synchronization channel). For example, in this embodiment of the present disclosure the UE measures synchronization signals transmitted over IC and class 1OOC UEs served only by the neighbor enbs or class 2OOC UEs not served by any eNB.

In another example, the synchronizing UE may relay information (i.e., an offset, a bitmap, etc.) regarding the resource region for the periodic synchronization resource. The UE that has received the synchronization signal and the synchronization channel from the synchronization UE first measures the synchronization signal from the neighboring synchronization UE to become a synchronization relay UE. Synchronization signals received from UEs served by the same eNB in the corresponding resource region according to only information on the resource region using the periodic synchronization resource are used for measurement (this can be determined based on the ID included in the synchronization sequence and the synchronization channel).

Meanwhile, it may be determined to periodically transmit a synchronization signal in a resource region not using the periodic synchronization resource, based on information on a resource region using the periodic synchronization resource and a relative resource region and a relay hop number from the eNB. For example, in case of relay hop count of 2 (i.e., eNB (0 hop) → UE1(1 hop) → UE2(2 hops)), UE2 may refrain from the synchronization transmission so as to avoid being affected by signal transmission, such as interference when other UEs served by the eNB operate in a resource region that does not use the periodic synchronization resource. However, in the case where the relay hop count is 3 (i.e., eNB (0 hop) → UE1(1 hop) → UE2(2 hop) → UE3(3 hop)), the signal of UE3 does not reach the in-coverage UE and thus cannot transmit the synchronization signal in the resource region not using the periodic synchronization resource.

Meanwhile, in the out-of-coverage operation, if the synchronization resources are divided into I-SS resources and V-SS resources or resources for UEs having data and resources for UEs having no data, the synchronization signal measurement is limited to the resources for I-SS or resources for UEs having data. Therefore, when the UE measures the received signal power of the synchronization signal to determine the condition for becoming the synchronization source UE, the UE cannot become the synchronization source UE if the received signal power strength of the synchronization signal from the I-SS or the UE having data is greater than the predetermined X dBm. According to this method, although the UE receives synchronization from neighboring V-SSs or UEs having no data, the UE may become a synchronization source UE because no closely located V-SS or UEs having data receive a synchronization signal (greater than or equal to X dBm).

Synchronization for Tx

A description of the synchronization process for D2D discovery and communication is made below. The synchronization process is constituted by a transmission synchronization process of reference timing for signal transmission and a reception synchronization process of reference timing for signal reception. Although the reference timing for default transmission is not possible under the assumption of perfect synchronization under conditions outside the network coverage area, the purpose is to allow reception of D2D discovery or communication signals in accordance with another synchronization signal. Therefore, the entire process is constituted by the processing of the transmission synchronization acquisition and action determination process after scanning so as to acquire the periodic transmission synchronization and the processing of the reception synchronization acquisition after scanning so as to simultaneously acquire the reception synchronization. This is described with reference to fig. 9.

Fig. 9 is a flowchart illustrating a transmission timing synchronization procedure according to an embodiment of the present disclosure.

Referring to fig. 9, the UE turns on at operation 900, i.e., D2D mode is enabled, and starts scanning for enbs. This operation may be performed by the radio communication unit 201 or the modem 203 of fig. 2 under the control of the control unit 205. If the UE is in a turned on state, operation 900 may be omitted. It should be noted that the UE operates under the control of the control unit 205 in the procedure of fig. 9.

The UE determines whether the current time is a timing of a scan transmission at operation 902. Scanning may be performed to search for enbs and UEs or enbs only. The UE first scans to search for an eNB at operation 904. However, in case of scanning to search for a UE, the operation 904 of scanning to search for an eNB may be omitted and a jump is made to the operation 920 of scanning to search for a UE. As a result of the scanning operation at operation 904, at least one eNB synchronization signal (PSS/SSS) is detected, the control unit 205 selects the synchronization signal of the at least one eNB to acquire synchronization with the eNB at operation 908 and determines whether to operate as a relay synchronization source (R-SS) at operation 910. The operation of selecting one of the at least one eNB synchronization signals can be performed generally based on received signal power strength as specified in the conventional LTE standard. The process of becoming R-SS is described below with reference to the accompanying drawings. After determining to become the R-SS, the control unit 205 returns the process to operation 902 to periodically scan for enbs.

If an eNB synchronization signal is not detected at operation 906, the UE scans for UE synchronization signals transmitted by other synchronization source UEs at operation 920. After finding the UE synchronization signal, the control unit 205 determines whether at least one UE synchronization signal (D2DSS) or UE broadcast/synchronization channel is detected in operation 922. If at least one D2DSS or D2DBCH/D2DSCH is detected, the control unit 205 selects one of the at least one D2DSS to acquire synchronization with the corresponding UE in operation 924 and determines whether to become a synchronization relay UE in operation 926. The operation of selecting one of the at least one UE synchronization signals may be performed based on at least one of a received signal power strength, a D2D frame number, a relay hop count, a synchronization UE validity duration, aging of the synchronization UE, and synchronization stage information of the device. At this time, if the eNB synchronization signal and the D2DSS are not detected at both operations 906 and 922, the UE having the discovery signal, the communication control signal, and the communication data signal autonomously becomes an independent synchronization source (I-SS).

Unlike a UE synchronized with an eNB, a UE synchronized with a sync source UE must periodically perform a scanning operation. This is because unlike an eNB, a UE is mobile so that it is likely to change networks frequently. The synchronization source UE must always determine the condition for stopping the synchronization signal transmission. The synchronization signal transmission stop process is described below with reference to the drawings.

The UE determines whether it is time to scan for transmission at operation 940, and if so, returns the process to operation 902 to scan for the eNB or UE again. If it is not the time of the scan transmission, the UE determines whether it is the time of the scan reception at operation 930. The process of scanning reception is described below with reference to the accompanying drawings.

The scanning for transmission and the scanning for reception may differ from each other in transmission scheme in view of the synchronization source UE. The scanning for transmission is performed to predict the change of synchronization backward, but the transmission timing is maintained even after the scanning for reception. Therefore, the synchronization source UE stops the synchronization signal transmission during the scanning for transmission but continues to transmit the synchronization signal during the scanning for reception.

Synchronization for Rx

Fig. 10a, 10b and 10c are flowcharts illustrating a scanning process for reception according to an embodiment of the present disclosure.

Referring to fig. 10a, it should be noted that the UE operates under the control of the control unit 205 in the processes of fig. 10a, 10b and 10c, and the terms "UE" and "control unit 205" may be used interchangeably. This operation is described with reference to the block diagram of fig. 2, if necessary.

The UE determines whether it is transmitting or receiving a D2D signal at operation 1000. For example, if the UE is transmitting control or data signals for discovery or communication, it may be difficult to receive other synchronization signals before the Tx/Rx operations are completed. If the UE is transmitting/receiving the D2D signal, the control unit 205 determines whether it is time to scan for reception at operation 1002. This operation may be omitted if it is configured to always perform scanning for reception. If it is time to scan for reception, the control unit 205 performs scanning for eNB synchronization signals at operation 1004 and determines whether any eNB synchronization signal is listened to, i.e., whether any eNB synchronization signal is received at operation 1006. If any eNB synchronization signal is received at operation 1006, the control unit 205 establishes synchronization with the eNB at operation 1010 and determines whether it is operating in the D2D signal reception mode at operation 1014. If it is determined in operation 1014 that the operation is in the D2D signal reception mode, the operation proceeds to operation 1022 and an operation for the signal reception mode 1 is performed. Otherwise, if it is determined in operation 1014 that the operation is not in the D2D signal reception mode, the operation proceeds to operation 1020 and an operation for the signal reception mode 2 is performed.

Otherwise, if the eNB synchronization signal is not sensed at operation 1006, the control unit 205 determines whether any D2DSS is sensed at operation 1008. If the synchronization signal is not listened to at operation 1008, the control unit 205 ends the routine of fig. 10 a. Otherwise, if any UE synchronization signal is sensed at operation 1008, the control unit 205 configures Rx reference time at operation 1012 and determines whether the UE is operating in D2D signal reception mode at operation 1014.

As described with reference to fig. 10a, the D2D signal may be received in one of the first reception synchronization pattern (Rx synchronization pattern 1) and the second reception synchronization pattern (Rx synchronization pattern 2). Since Tx timing can be used for a reception operation in a resource region of a serving cell in an Rx mode, a detailed description thereof is omitted herein.

A description of the difference between the Rx sync pattern 1 and the Rx sync pattern 2 is made hereinafter.

Rx sync pattern 1: assume that the serving eNB informs resource region information such as an offset from a neighboring eNB using an offset from the reference frame timing (e.g., SFN 0). Accordingly, the UE performs D2D reception in the notified resource region, and, if a synchronization signal from a UE served by a neighbor eNB is detected, acquires a symbol and frame boundary to perform a predetermined D2D operation.

An out-of-coverage UE that acquires eNB reference timing from a synchronization signal relayed by another UE may perform a receive operation based on an offset from a neighboring eNB and resource region information of the neighboring eNB as it is served by the eNB relayed by the synchronized UE through a synchronization channel. At this time, the receiving UE must determine the ID of the eNB with which the synchronized UE is associated or camped on by synchronizing the synchronization signal of the UE.

Rx synchronization mode 1 is not easily used for out-of-coverage UEs that acquire independent reference timing not derived from the receiving network. This is because there is no case where the synchronized relay UE is served by the eNB so the synchronized UEs appear independently. If complexity is not critical, the following method can be used. The UE served by the eNB may receive the reference timing of the independent out-of-coverage UE and report the reference timing of the independent out-of-coverage UE to the eNB, and the eNB may notify the UE served by the eNB of the offset from the independent out-of-coverage UE based on the report from the at least one UE. However, the complexity is critical, and Rx sync mode 2 seems to be more efficient.

Rx sync mode 2: assume that a neighboring eNB or a synchronized UE served by the neighboring eNB informs resource region information, such as an offset from the neighboring eNB (cell) based on a reference timing (e.g., SFN0) of the corresponding neighboring eNB. The resource region information may include information on synchronization resources as well as discovery resources, communication control resources, and communication data resources. Therefore, the UE must receive each synchronization signal and channel to acquire symbol and frame boundaries from the synchronization signal and acquire the current SFN and location of the resource region from the synchronization channel. If the current timing matches the determined D2D resource region, the UE performs D2D operations in that resource region, otherwise, may stay in an idle state until the D2D resource region is reached without WAN-associated operations and D2D operations.

In Rx synchronization mode 2, the out-of-coverage UE acquiring eNB reference timing based on synchronization signals relayed by the UE or not acquiring eNB reference timing based on synchronization signals relayed by the UE may perform D2D reception operation by receiving SFN and resource region information of a neighboring eNB transmitting a synchronization channel by the synchronized UE.

Referring to fig. 10b, compared to fig. 10a, an operation in Rx sync mode 1 and Rx sync mode 2 is illustrated. In the Rx synchronization mode 1, that is, at operation 1030, the UE acquires information on a start point of a resource region of a neighbor eNB based on a SIB received from the eNB and updates an information list for determining a start point of scanning for receiving a signal based thereon. In the Rx synchronization mode 2, that is, at operation 1032, the UE acquires information on a start point of a resource area of a synchronization source UE and a neighboring eNB through a D2DBCH or a D2DSCH and updates an information list for determining a start point of scanning for receiving a signal based thereon.

In contrast to fig. 10b, fig. 10c is a flowchart for explaining a method of individually managing scan cycles for reception from an eNB and a UE.

Referring to fig. 10c, the control unit 205 determines whether it is transmitting or receiving a D2D signal at operation 1000. If it is neither transmitting nor receiving any D2D signal, the control unit 205 determines whether it is time to scan for reception from the eNB at operation 1040. This operation may be omitted if configured to always scan reception. If it is time to scan for the receiving enbs, the eNB 205 scans for eNB synchronization signals and determines whether any eNB synchronization is listened to, i.e., any eNB synchronization signals are received, at operation 1042. If any eNB synchronization signal is received at operation 1044, the UE acquires synchronization with the eNB for reception at operation 1046 and determines whether the UE is operating in D2D synchronization mode 1 at operation 1048.

If it is not time to scan for the receiving eNB in operation 1040, the control unit 205 determines whether it is time to scan for the receiving UE in operation 1050. If it is time to scan the UE for reception at operation 1050, the UE performs scanning the UE for reception at operation 1052 and determines whether any synchronization signal is received from at least one UE at operation 1054. If any synchronization signal is detected at operation 1054, the control unit 205 acquires synchronization with the UE for reception at operation 1056. At operation 1048 the control unit 205 determines whether it is operating in Rx synchronization mode 1 and, if so, the process goes to operation 1060, otherwise to operation 1062.

The control unit 205 updates a scan time list for receiving signals through the D2D channel in the Rx synchronization mode 2 at operation 1060, and updates a scan time list for receiving signals through the SIB in the Rx synchronization mode 1 at operation 1062.

In fig. 10c, a plurality of reception timings are managed in the form of a list at operations 1060 and 1062, but it is impossible to manage all reception timings due to communication chip performance and memory constraints. Therefore, the reception operation must be performed at a predetermined number of reception timings. For example, if 5 reception timings need to be selected among the reception timings acquired from 30 received synchronization signals, a condition for determining priority for selection is required. For example, when tracking N Rx syncs, N sync signals are selected based on their priorities.

The reception operation is performed in priority and in order of the serving eNB transmission timing, the in-coverage UE transmission timing, and the out-of-coverage UE transmission timing based on the synchronization signal of the serving eN or the synchronization source UE (i.e., its own transmission timing). At this time, if N is 1, the order is exceptionally adjusted so that the transmission timing of the eNB has the highest priority. In another example, the eNB's transmission timing has the highest priority and is followed by its own transmission timing, in-coverage UE transmission timing, and out-of-coverage UE transmission timing. In the case of a system using absolute time (such as GPS time), the transmit timing of a UE using GPS-based absolute time may be located between the in-coverage UE transmit timing and the out-of-coverage UE transmit timing. In the above example, if the priorities are equal to each other, the priorities are determined based on the received signal power of the synchronization signal or the D2D frame numbers.

In the case of synchronization resources used by a UE having a discovery signal, a communication control signal, and a communication data signal to be transmitted and synchronization resources used by a UE transmitting a synchronization signal (although the UE has no signal to be transmitted) in order to support a synchronization procedure, synchronization resources of a synchronization source UE supported by a tracked synchronization target may be excluded. Although the synchronization signals are different from each other, if the synchronization signals of the UE transmitting the discovery/communication signal and the UE not transmitting the discovery/communication signal are distinguished from each other, the synchronization signals supporting the UE may be excluded also when selecting the tracked reception synchronization target.

There may be a cluster of synchronization source UEs if no suitable synchronization procedure is provided in the out-of-coverage environment. In this environment, it is difficult to match transmission/reception areas of neighboring clusters, and it is necessary to simultaneously perform transmission/reception operations in the neighboring clusters. Since it is difficult to transmit signals at the timing of the neighboring cluster in the inter-cluster synchronization mismatch situation, transmission is performed at the timing of the synchronization source UE while reception is performed according to the plurality of timings of the neighboring synchronization source UE as described above. In order to perform D2D discovery and communication with multiple reception timings of, for example, OOC UEs at the same priority, a list may be created based on the collision of resource regions as follows.

a) The UE determines the location of a resource region obtained from the receive timing at the top of the receive time sequence table.

b) The reception-time sequence table that conflicts in time with the determined location of the resource region is marked as unused or deleted.

The above procedure may be applied differently depending on the type of the resource region, and in particular, the collision reception timing is not used for the communication control resource region, but collision is allowed in the communication data resource region.

Along with the problem of collisions between the synchronization timing and resource regions, it is necessary to consider the problem of collisions between different resource regions. When tracking multiple timings and acquiring resource region information from a synchronization channel, there may be conflicts between operations in one or more resource regions. For example, the data resource regions received from the IC UE and the OOC UE may be configured simultaneously. To avoid complexity, the UE may operate according to the following principles.

Priority is given to the control resource region if the control resource region and the data resource region for D2D communication exist in the same subframe. If resource regions indicated from the IC UE and the OOC UE exist in the same subframe, priority is given to a reception resource region indicated from the IC UE. In case this problem is not solved using two principles, priority is given to the resource region indicated from the UE with high received signal power of the synchronization signal.

Fig. 11 is a flowchart illustrating a procedure of determining a condition to become a synchronous relay UE according to an embodiment of the present disclosure. The flowchart of fig. 11 illustrates the detailed operation of the UE at operation 910 of fig. 9.

Referring to fig. 11, the control unit 205 determines whether the UE has a discovery/control/data signal to transmit at operation 1100. In embodiments of the present disclosure, the control unit of a UE in connected mode may not determine whether the control unit of the UE has a discovery/control/data signal to transmit. If it is determined at operation 1100 that the UE has a discovery/control/data signal to transmit, the control unit 205 determines at operation 1102 whether the UE is in a connected mode and at operation 1104 whether it is time to scan for transmission.

If it is determined at operation 1102 that the UE is in the connected mode, the control unit 205 becomes a synchronous relay UE under the control of the eNB in one of the following four methods.

The first method is a command-reporting process 1110 of determining, using the control unit 205, whether the UE has received a synchronization signal measurement command from the eNB at operation 1112, performing measurement at operation 1114 upon receiving the synchronization signal measurement command, and reporting the measurement result to the eNB at operation 1116, and starting synchronization signal transmission at operation 1160. The eNB controls the UE to operate as a relay UE based on the measurement results or other information.

The second method is to use a request-response process 1120 in which the UE requests the eNB in order to become a synchronization source (relay) UE at operation 1122 and replies back with a request to receive a response message from the eNB at operation 1124. The UE may transmit additional measurement results to the eNB at operation 1126. If the response message includes information indicating to become the synchronous relay UE, the UE becomes the synchronous relay UE in operation 1128. In case the measurement result is transmitted to the eNB, the eNB transmits a control signal to the UE to control the UE to operate as a synchronous relay UE.

The third method is a BSR-grant process 1130 of transmitting a Buffer Status Report (BSR) to request resource allocation for data transmission using the UE at operation 1132 and receiving a resource grant from the eNB at operation 1134. The eNB may send information indicating acceptance/rejection to become a synchronous relay UE to the UE together with the resource grant. The UE may also report the measurement results to the eNB at operation 1136. Depending on the situation, the eNB may request the UE for additional measurements. If the eNB allows the UE to operate as a synchronous relay UE, the UE becomes a relay-synchronization source (R-SS) at operation 1138.

A fourth method is a blind configuration process 1140 of controlling a UE to operate as a synchronous relay UE using the eNB without any information. However, if the UE becomes a synchronous relay UE unconditionally, the UE unnecessarily performs a synchronous signal transmission operation. Accordingly, if the UE operates under the control of the eNB at operation 1142, the UE measures a synchronization signal to acquire a received signal power from the eNB and other UEs and determines whether the UE has a discovery/control/data signal to transmit in order to become R-SS based thereon at operation 1144.

The UE may measure synchronization signals of UEs served by the same eNB or synchronization signals of OOC UEs. If the synchronization signal of a UE served by the same eNB is lower in received signal power than the synchronization signal of an OOC UE, the UE may determine that there is no close synchronization source UE and transmit the synchronization signal. If the synchronization signal of the OOC UE is higher than a predetermined value in terms of received signal power, it is necessary to transmit timing of the eNB to mitigate interference caused by subsequently transmitted discovery/control/data signals, and thus the synchronization signal is transmitted even if the IC UE does not find/control/data signals to transmit.

The UE in idle mode at operation 1104 determines a scanning period for transmission to autonomously determine whether the UE can become a sync source UE (this may be omitted if the scanning period is not configured). If the UE is in the scanning period, the UE measures synchronization signals from the serving eNB and the neighboring synchronization source UE in operation 1152. If the predetermined condition is satisfied, the UE operates as a synchronous relay UE in operation 1154. For example, the UE determines at least one of whether the received signal power from the eNB is less than X dBm, whether the received signal power of the synchronization signal from the neighboring synchronization relay UE is less than Y dBm, whether the received signal power of the I-SS from among the neighboring OOC UEs is greater than Z dBm, whether there are any discovery/control/data signals to transmit, whether it becomes a synchronization relay UE. If the synchronization signal of a UE served by the same eNB is higher than a predetermined value in terms of received signal power, the UE determines that there is no close synchronization source UE and thus transmits the synchronization signal. If a synchronization signal of an I-SS UE among OOC UEs is greater than a predetermined value in terms of received signal power, it is necessary to transmit timing of an eNB to mitigate interference caused by a subsequently transmitted discovery/control/data signal and thus the UE can transmit the synchronization signal even if the IC UE is a UE that has no discovery/control/data signal to transmit.

Condition for becoming a synchronized relay or supporting a UE in OOC

According to the procedure of becoming a synchronous relay UE in IC or OOC, the UE becomes a relay source UE when it is necessary to transmit a discovery signal, a control signal for communication, and a data signal. This serves to synchronize the transmission areas of the synchronization signal and the control and data signals as much as possible. However, there may be a condition for becoming a synchronization source UE to transmit a synchronization signal in order to deliver the synchronization signal to an out-of-coverage UE or to deliver the synchronization signal to an non-synchronization source UE without transmitting a discovery/control/data signal other than the synchronization signal. It is advantageous in receiving a UE because the reception operation does not require a distinction between a UE having a discovery/control/data signal and a UE having no discovery/control/data signal. For convenience of explanation, a UE having a discovery/control/data signal and transmitting a synchronization signal is referred to as a synchronization reference UE (I-SS), and a UE not having a discovery/control/data signal but transmitting a synchronization signal as necessary is referred to as a synchronization relay/support (R-SS/V-SS) UE. The conditions for becoming I-SS at operation 928 of fig. 9 and R-SS at operation 926 of fig. 9 based on this classification are described below.

a) Conditions for becoming I-SS: if a discovery/control/data signal to be transmitted occurs, the UE may determine whether to transmit a synchronization signal based on at least one of a reception power of the synchronization signal transmitted by the neighboring I-SS and a priority value of the synchronization channel before the receiving UE transmits the synchronization signal at a time point as much as a specific window before a starting point of the resource region. For example, if the received signal power of the synchronization signal is greater than a predetermined value due to a close proximity I-SS, the UE does not become a synchronization source UE. If the neighboring I-SS has a high priority (e.g., DFN) and timing compliance is required, the UE does not become a synchronization source UE (e.g., if the neighboring I-SS has a low priority value, the UE operates as a synchronization source UE). Further, if the neighboring I-SS has a high priority and is located nearby and thus the received signal power of the synchronization signal is high, the UE does not become a synchronization source UE (i.e., if the neighboring I-SS has a low priority or is located far and thus the received signal power of the synchronization signal is low, the UE operates as a synchronization source UE).

b) Conditions for changing to R-SS/V-SS: if a plurality of I-SSs (including the I-SS to which it belongs) are found in the scanning process, when it belongs to the I-SS in addition, the UE can become a synchronization source UE even if the UE does not find/control/data signals to transmit in the case where there is no other neighboring I-SS having a priority higher than or equal to that of the serving I-SS (i.e., there is only an I-SS having a priority lower than that of the serving I-SS).

It is advantageous in terms of receiving the UE because it is not necessary to perform unnecessary reception operations that require distinction between the synchronization reference UE having discovery/control/data signals and the synchronization relay/support UE without these signals. The UE can distinguish between the synchronization reference UE and the synchronization relay/support UE in various ways as follows.

a) The synchronization reference UE and the synchronization relay/support UE configure the synchronization sequence or the synchronization source UE ID independently. For example, a particular sequence or set of sequences may be assigned to the synchronization relay/support UE. Furthermore, the synchronization relay/support UE may be assigned a portion of the synchronization source UE ID range.

b) The synchronization reference UE and the synchronization relay/support UE are configured to use different synchronization resources. For example, the synchronization resource at the first location is used by the synchronization reference UE, while the synchronization resource at the second location is used by the synchronization relay/support UE. For this distinction, it is necessary to inform the synchronization resource index of the synchronization channel.

c) The synchronization reference UE and the synchronization relay/support UE are matched with different D2D Frame Numbers (DFNs). For example, the synchronization reference UE is matched with DFN #0, DFN #4, DFN #8, etc. and the synchronization relay/support UE is matched with DFN #2, DFN #6, DFN #10, etc. Thus, if a modulo-4 operation on the received DNF yields 0, this means synchronizing the reference UE, and if a modulo-4 operation on the received DFN yields 2, this means synchronizing the relay/support UE.

The receiving UE distinguishes between the synchronization reference UE and the synchronization relay/support UE based on at least one of the methods a), b) and c), and, if the UE is not a synchronization reference UE, the UE considers only a synchronization signal from the synchronization reference UE as a synchronization source UE. If the UE is a synchronization reference UE, the UE regards all synchronization source UEs as receiving target synchronization source UEs without distinguishing the synchronization reference UE from the synchronization relay/support UE.

The distinguishing method is described hereinafter. For example, the synchronization reference UE uses one synchronization sequence selected among sequences for OOC while the synchronization relay/support UE may be distinguished from the synchronization reference UE in various methods. A first method is characterized in that a separate common synchronization sequence (or set of sequences) may be used by the synchronization relay/supporting UE. The second method is characterized in that the synchronization relay/support UE is able to transmit information indicating that the UE uses the same sequence as the serving synchronization reference UE and is a synchronization relay/support UE through a synchronization channel. The information indicating the synchronization supporting UE may be synchronization source UE role information (1 bit), synchronization resource index/location information (1-2 bits), or D2D frame number (8-14 bits) indicating the type of the synchronized UE.

In the case of information indicating the synchronization index/position, the role of the position of the synchronization resource should be determined in advance. For example, a first location synchronization resource of one synchronization period may be used by a synchronization reference UE, and a second location synchronization resource may be used by a synchronization relay/support UE. In case of using D2D frame numbers, the role (synchronization reference and relay/support) should be predetermined in association with the D2D frame numbers. For example, odd-coded D2D frame numbers may be used by synchronous reference UEs, and even-coded D2D frame numbers may be used by synchronous relay/support UEs. By distinguishing the synchronization reference UE and the synchronization relay/support UE, various effects can be expected. For example, it is necessary to consider only the synchronization signal transmitted by the neighboring synchronization reference UE for controlling the reception timing. Further, the synchronization reference UE may consider only the synchronization signal transmitted by the synchronization relay/support UE to determine the transmission timing. For example, if two synchronization reference UEs enter a state of a specific received signal power range, a synchronization reference UE with a low priority depends on a synchronization reference UE with a high priority.

Although an operation of scanning for a synchronization signal transmitted by a neighboring synchronization reference UE may be always performed to control reception timing, the synchronization signal scanning operation may be periodically performed or may not be performed any more after a predetermined number of reception timings are determined and stored. In case of counting the number of reception timings, a synchronization source UE served by the eNB and relaying the eNB timing may be further included. The operation of scanning for the synchronization signal transmitted by the neighboring synchronization relay/support UE may be always performed at either an interval determined by the synchronization reference UE or a predetermined interval. The UE may set a period for scanning for the synchronization signal transmitted by the neighboring reference UE to a value greater than a period for scanning for the synchronization signal transmitted by the neighboring synchronization relay/support UE, or may determine such a relationship in advance.

Procedure for stopping the transmission of synchronization signals

Fig. 12a and 12b are flowcharts illustrating a UE procedure for stopping synchronization signal transmission according to an embodiment of the present disclosure.

Referring to fig. 12a, if there are no previously recorded synchronization signal measurement values, the synchronization source UE measures synchronization signals of the serving eNB and/or the neighboring UEs at operation 1200. The synchronization source UE determines whether a synchronization signal transmission stop command is received from the eNB in operation 1202. If it is determined at operation 1202 that the synchronization signal transmission stop command is received, the UE immediately determines at operation 1206 whether a timer has expired to relinquish the role as a synchronization source UE.

If it is determined at operation 1202 that a synchronization signal transmission stop command (e.g., idle mode UE) is not received, the sync source UE determines whether the latest condition that triggers the UE to successfully become the sync source UE is maintained at operation 1204. If so, the UE continues to transmit the synchronization signal, otherwise, stops transmitting the synchronization signal immediately at operation 1208 or when the condition of operation 1206 is satisfied. Although the condition is no longer satisfied, the UE may stop transmitting the synchronization signal under the control of the eNB or when the duration for which the UE has operated as a synchronization source UE is greater than a timer value.

Unlike the procedure of fig. 12a, if the latest condition for becoming the synchronization source UE is not used, it is possible to determine whether to operate as the synchronization source UE by sequentially determining the conditions as shown in the procedure of fig. 12 b.

Referring to fig. 12b, if there are no previously recorded synchronization signal measurement values, the synchronization source UE measures a synchronization signal from the serving eNB and/or the neighboring UE at operation 1200. The sync source UE determines whether the sync source UE is in a connected state with the eNB in operation 1220. If so, the synchronization source UE determines whether a synchronization signal transmission stop command is received from the eNB in operation 1222. If the synchronization signal transmission stop command is received, the UE immediately stops transmitting the synchronization signal at operation 1250. Otherwise, if the synchronization signal transmission stop command is not received from the eNB at operation 1222, the synchronization source UE determines whether it has data to transmit at operation 1224. If the synchronization source UE has data to transmit at operation 1224, the UE compares the received signal quality from the eNB to a predetermined threshold at operation 1226. If the received signal quality from the eNB is less than the predetermined threshold, the synchronization source UE immediately stops transmitting the synchronization signal at operation 1250 or stops transmitting the synchronization signal based on the timer expiration determination result at operation 1230. Otherwise, if the received signal quality from the eNB is greater than or equal to the predetermined threshold, the synchronization source UE continues to transmit the synchronization signal.

If the synchronization source UE has no data to transmit at operation 1224, the synchronization source UE determines whether it satisfies a condition for becoming a synchronization relay UE (R-SS) at operation 1228. If the condition for changing to R-SS is met, the process moves to operation 1226, otherwise moves to operation 1250 to immediately stop sending synchronization signals.

If it is determined at operation 1240 that the synchronization source UE is operating in the idle state, the process goes to operation 1224, otherwise it is determined at operation 1242 whether the UE is a synchronization reference UE (I-SS) and has no data to transmit. If both conditions are satisfied, the synchronization source UE continues to transmit the synchronization signal. Otherwise, if at least one of the two conditions is not satisfied, the synchronization source UE determines whether it is an R-SS and satisfies the R-SS condition in operation 1244. If it is determined that the UE is an R-SS and the R-SS condition is satisfied at operation 1244, the UE continues to transmit the synchronization signal, and otherwise, in case at least one condition is not satisfied, stops transmitting the synchronization signal at operation 1250.

Relationship between eNB ID and UE synchronization signals

The receiving UE must have the capability to determine the ID of the eNB serving the synchronizing UE by synchronizing the synchronization signal (including the synchronization channel in a broad sense) of the UE. For this purpose, the eNB ID is transmitted as it is by the synchronization signal, or a UE synchronization signal ID or a UE synchronization signal set ID having a predetermined relationship with the eNB ID is transmitted. If the UE having received this synchronization signal can determine the ID of the eNB serving the UE transmitting the synchronization signal or on which the UE has camped. In embodiments of the present disclosure, the IDs of the set of enbs may be transmitted on a synchronization signal transmitted by the UE.

D2D frame number priority

The D2D frame numbers are assigned in the synchronization process for communication because although frame boundary synchronization has been completed based on the received synchronization signal, it is necessary to know that the respective devices map as necessary to define the frames of the operational process differently at the respective sub-frames. Accordingly, communication systems are designed such that frames are assigned a (system or) frame number or index to facilitate distinguishing between them. For example, in the LTE system, SFNs are notified to devices through Master Information Blocks (MIBs) 1301, 1302, 1303, 1311, 1312, and 1313 transmitted via BCH as shown in the example of fig. 13 a. The SFN may be interchangeably referred to as a radio frame number.

Fig. 13a, 13b, 13c, and 13d illustrate transmission of frame numbers and a Master Information Block (MIB) or synchronization resource in the LTE system according to an embodiment of the present disclosure.

Referring to fig. 13a, 13b, 13c, and 13d, examples of a frame structure of an LTE system in which MIBs 1301, 1302, 1303, 1311, 1312, and 1313 are transmitted at a first subframe (SFN0) of a frame (e.g., 4 identical MIBs are transmitted at a period of 10 ms) are illustrated. In the LTE system, the SFN is 10-bit information capable of expressing system frame numbers 0 to 1023. The BCH is masked with a 2-bit check CRC and thus, if the BCH is received, 20-bit information can be determined. The MIB transmitted through the BCH includes 8 bits of partial SFN information. By combining the 2-bit information received through the BCH and the 8-bit partial SFN information of the MIB, the original 10-bit SFN can be obtained. Since one frame spans 10ms, the SFN can indicate a time of up to 10 seconds. For example, assuming that a broadcast cycle for D2D discovery needs to occur every 10 seconds, the eNB can notify information by transmitting an SIB on BCH which is a control broadcast channel using SFN0 indicating a D2D broadcast frame.

Fig. 13a, 13b, 13c, and 13d illustrate frame structures of an LTE system in which MIBs 1301, 1302, 1303, 1311, 1312, and 1313 are transmitted at a first subframe (SFN0) of a frame (e.g., 4 identical MIBs are transmitted over a period of 10 ms). In the LTE system, the SFN is 10-bit information capable of expressing system frame numbers 0 to 1023. The BCH is masked with a 2-bit check CRC and thus, if the BCH is received, 20-bit information can be determined. The MIB transmitted through the BCH includes 8 bits of partial SFN information. By combining the 2-bit information received through the BCH and the 8-bit partial SFN information of the MIB, the original 10-bit SFN can be obtained. Since one frame spans 10ms, the SFN can indicate a time of up to 10 seconds. For example, assuming that a broadcast cycle for D2D discovery needs to occur every 10 seconds, the eNB can notify information by transmitting an SIB on BCH which is a control broadcast channel using SFN0 indicating a D2D broadcast frame.

The D2D frame numbers are frame numbers used in D2D communications and correspond to system frame numbers of enbs. In case of allocating D2D synchronization resources every 10ms as shown in fig. 13b, one synchronization source UE can transmit signals every 10ms as when the eNB transmits BCH. However, since the transmission distance of one synchronization source UE is short compared to that of the eNB, it is not preferable that the synchronization source UE uses each synchronization resource by focusing attention on a synchronous relay or a distributed synchronization scheme.

Therefore, it is preferable to increase the synchronization period from 10ms to 40ms and use the remaining synchronization resources 1302, 1303, 1304, 1312, 1313 and 1314 as shown in fig. 13c in view of D2D discovery and communication operations and application diversity. For example, it may be configured such that the D2D synchronization signal is transmitted only at the first frames 1301 and 1311 of the respective synchronization periods of 40 ms.

Since the UE is mobile unlike the eNB, a plurality of UEs may transmit a synchronization signal in a specific area. At this time, if there is only one synchronization resource, signals transmitted by the synchronization source UEs interfere with each other, and when the synchronization source UEs attempt to receive the synchronization signals, they cannot receive any signal in a state where signals are transmitted on the same resource due to a half-duplex problem. Therefore, it is advantageous for the neighboring synchronization source UE to use resources divided in time in view of performance.

As shown in fig. 13d, it may be configured such that 4 synchronization source UEs transmit synchronization signals at different offsets from the reference timing (DFN 0). As shown in fig. 13d, the synchronization source UE1 transmits synchronization signals in the first synchronization resources 1301 and 1311 of the reference timing (DFN0), the synchronization source UE2 transmits synchronization signals in the second synchronization resources 1302 and 1312 of the reference timing (DFN0), the synchronization source UE3 transmits synchronization signals in the third synchronization resources 1303 and 1313 of the reference timing (DFN0), and the synchronization source UE4 transmits synchronization signals in the fourth synchronization resources 1304 and 1314 of the reference timing (DFN 0).

The synchronization source UE may be a synchronization relay UE located at1 hop, 2 hop, 3 hop, and 4 hop distances from the eNB (0 hop), respectively, according to the relay hop count. In another example, each synchronization source UE may determine whether there is an available synchronization resource and select the available synchronization resource to transmit a synchronization signal. In another example, synchronization source UE1 is an IC UE, while synchronization sources UE2, UE3, and UE4 are OOC UEs. In another example, synchronization sources UE1 and UE2 are in-coverage UEs, while synchronization sources UE3 and UE4 are out-of-coverage UEs. In the embodiment of the present disclosure, the Synchronization Source UE1 is a Synchronization reference Source as a Cluster Head (Cluster Head), and the Synchronization sources UE2, UE3, and UE4 are volunteer Synchronization sources (volmenting Synchronization sources) that assist in Synchronization between Cluster heads.

The DFN may provide a role of a synchronization source UE according to an allocation method assumed in the corresponding example. In case of selecting the synchronization resource according to the number of hops, a modulo 4 operation is performed on the DFN received through the synchronization channel, and if DFN | mod4 is 0, it means that the signal is transmitted through the 1-hop synchronization source UE. In the same way, if DFN | mod4 is 3, this means that the resource is used by the 3-hop synchronization source UE. In another example, if DFN | mod4 is 0, this means that the synchronization signal is transmitted by the in-coverage UE, and if DFN | mod4 is 2, this means that the signal is transmitted on the second of the resources for the out-of-coverage UE. In another example, if DFN | mod4 is 1, this means that the signal is transmitted on the second of the resources for the in-coverage UE, and if DFN | mod4 is 2, this means that the signal is transmitted on the first of the resources for the out-of-coverage UE. In another example, if DFN | mod4 is 0, this means that the synchronization signal is transmitted by the cluster head, and if DFN | mod4 is 2, this means that the signal is transmitted on a second resource among the resources for synchronizing the supporting UEs. Although these descriptions are for the case where the modulo-4 operation is performed for four resources, other modulo operations can be applied depending on the number of synchronized resources. In the case where the synchronization resources are not periodically allocated, other types of operations can be applied.

Fig. 14 is a schematic diagram illustrating a D2D frame number determination operation in both in-coverage and out-of-coverage network scenarios, according to an embodiment of the present disclosure.

Referring to fig. 14, eNB 11401 has its coverage area 1400 and eNB 21411 has its coverage area 1410. The UE 11431 is located in an overlapping region of the coverage area 1400 of eNB 11401 and the coverage area 1410 of eNB 21411. There are multiple synchronization source UEs, namely, synchronization source UE1 (sse 1)1441 located in the coverage area 1400 of eNB 11401, synchronization source UE2 (sse 2)1442 located in the coverage area 1410 of eNB 21411, synchronization source UE3 (sse 3)1443 autonomously generating and transmitting a synchronization signal, and synchronization source UE4 (sse 4)1444 receiving absolute time from a satellite and transmitting a synchronization signal based thereon. Further, there are UEs that do not serve the UE, i.e., UE 21432, UE 31433, and UE 41434.

In fig. 14, UE 11431 receives eNB synchronization signals (PSS/SSS) and eNB BCH from eNB 11401 and eNB 21411 and a UE broadcast channel (D2DBCH) (or UE synchronization channel (PD2DSCH)) from synchronization source UE2 (sse 2) 1442. In general, D2DSS and D2DBCH from the ssee 21442 are ignored because the priority of the eNB synchronization signal is higher than the priority of the UE synchronization signal. The UE 11431 selects an eNB based on the received signal power of PSS/SSS from eNB 11401 and eNB 21411. In certain circumstances, if a UE1 served by eNB 21411 fails to receive PSS/SS and receives D2DSS from sync source UE 21442, it may exceptionally follow the UE synchronization signal from sync source UE 21442 instead of any eNB signal.

In fig. 14, UE 31433 receives a UE synchronization signal and a UE broadcast channel from a synchronization source UE 11441 and a UE 21442. Excluding the exceptional condition, the UE 31433 selects one of a plurality of UE synchronization signals according to a priority rule of the UE synchronization signals and receives the UE broadcast channel at a position determined by the selected UE synchronization signal. If the UE broadcast channel is successfully received, the UE acquires the DFN and complies with it. In the above example, in case that information on enbs synchronized with absolute time (e.g., GPS time) is notified, if there is an eNB using absolute time after receiving a UE broadcast channel together with a UE synchronization signal, the UE complies with a reference signal of a corresponding eNB, and otherwise, if there is no eNB using absolute time, the eNB complies with a reference time of a selected UE synchronization signal according to a priority rule and a DFN of the UE broadcast channel.

In fig. 14, the UE 21432 receives a UE synchronization signal and a UE broadcast channel from the in-coverage synchronization source UE 11441 and the out-of-coverage synchronization source UE 31443. The synchronization signal or the synchronization channel should include information indicating whether the UE is an in-coverage UE or an out-of-coverage UE in order for the UE to distinguish. UE 31433 preferentially selects an in-coverage synchronization source UE over an out-of-coverage synchronization source UE. If there are multiple in-coverage synchronization source UEs, the UE operates in the same manner as the UE 21442 selects an in-coverage synchronization source UE. In fig. 14, UE 41434 is similar to UE 31433, but the out-of-coverage synchronization source UE has absolute time. Basically, 2 rules can be applied.

1) Absolute time has the highest priority.

2) The eNB has the highest priority.

However, both rules may have problems, respectively. If absolute time has the highest priority, the UE4 must always adhere to absolute time. However, signals transmitted by UEs 4 that are not synchronized with the eNB may cause interference to in-coverage UEs. Further, if the eNB has the highest priority, the UE4 always has to obey the reference time of the synchronization source UE 2. If so, the UE4 may receive and cause interference from/to the absolute time compliant UE. Although the eNB may transmit additional priority rule information to freely use the two rules, the eNB may also use information on whether absolute time is used to indicate the two rules for using legacy signals.

For example, if the eNB uses absolute time, it may be configured to preferentially use absolute time. Otherwise, if the eNB does not use absolute time, it may be configured to preferentially use the eNB. For example, when receiving a synchronization signal and a synchronization channel from an out-of-coverage UE and an in-coverage UE having an absolute time, the UE determines a synchronization source UE whose transmission time reference is to be used based on information on whether to use the absolute time received when the UE has connected to/resides on the eNB. If a value indicating whether to use absolute time is not received or expires, the UE operates according to a predetermined determination rule.

Fig. 15 illustrates D2D frame number determination in an OOC scene according to embodiments of the present disclosure.

Referring to fig. 15, a UE 11501 receives synchronization signals and synchronization channels from an out-of-coverage synchronization source UE 11511, a UE 21512, and a UE 31513. Unlike the scenario where there is an in-coverage synchronization source UE, the priority of the in-coverage synchronization source UE is excluded in this scenario. Therefore, the synchronization source UE having absolute time has the highest priority. UE 11501 adheres to the reference time and DFN from synchronization source UE 11511 with absolute time. Meanwhile, the UE 21502 having received the UE synchronization signal and the UE broadcast channel from the sync source UE 21512 and the sync source UE 31513 can select one of the two sync source UEs by comparing various parameter values. For example, UE 21502 may compare the synchronization source UE ID, validity time, aging, and hop count for the selected synchronization source UE. Since the illustrated parameters require additional information, the simplest parameter is the DFN value currently used by the UE. For example, the synchronization source UE may be selected by comparing the D2D frame numbers. Since the UE uses the DFN value that has been used to determine the location of the resource region, the synchronization source UE transmits this DFN through the UE broadcast channel. In fig. 15, if the UEs exchange synchronization signals and synchronization channels with each other like the synchronization source UE 31513 and the synchronization source UE 41514, the UEs determine the priority of the synchronization source UE having transmitted the DFN by comparing the received DFN with the maintained DFN. If the DFN value is used by the synchronization source UE (including the current UE), no change is needed to the synchronization source UE.

According to the embodiments of fig. 14 and 15, this operation can be configured for two different cases: one case where absolute time is considered in the system and another case where absolute time is not considered. In case of considering absolute time in the system, the absolute time usage information should be transmitted through an eNB BCH or a UE broadcast channel (D2DBCH or PD2 DSCH). Under this assumption, the UE can determine to use absolute time after receiving multiple eNB/UE synchronization signals and eNB/UE broadcast channels.

Fig. 16 is a flowchart illustrating a process of aligning and selecting synchronization information based on an absolute time according to an embodiment of the present disclosure.

Referring to fig. 16, the UE starts scanning at operation 1600 and scans for a synchronization signal at operation 1602. Here, the start scanning is an operation of turning on the radio communication unit 201 and the modem 203 to receive a synchronization signal, and the control unit 205 controls the radio communication unit 201 and the modem 203 to receive a broadcast channel for at least one detected synchronization signal. The UE scans for synchronization signals at operation 1602 and determines whether any synchronization signals are detected at operation 1604. If any synchronization signals are detected, the process moves to operation 1606, otherwise, to operation 1620 where the UE determines whether a re-attempt to scan is necessary. If a retry of the scan is necessary, the process flows to operation 1602, otherwise, to operation 1612.

If the process transitions from operation 1604 to operation 1606, the UE attempts to receive the broadcast channel and determines whether the broadcast channel was successfully received at operation 1608. If the UE fails to receive the broadcast channel at operation 1608, the process goes to operation 1620, and the UE successfully receives the broadcast channel, the UE receives the synchronization signal and the broadcast channel and stores the information acquired therefrom at operation 1610. Thereafter, the UE stops scanning for the synchronization signal at operation 1612 and aligns the acquired synchronization signal and the broadcast channel information at operation 1614. The information alignment is made in the order of information on whether absolute time is used, information on whether eNB reference time is used, a DFN value, a (relay hop count), and received signal power strength. If the alignment is completed, the UE makes a final determination to synchronize the reference time and DFN of the source UE using the highest priority at operation 1616.

Fig. 17 is a flowchart illustrating a process of aligning and selecting synchronization information regardless of absolute time according to an embodiment of the present disclosure.

Referring to fig. 17, the UE starts scanning at operation 1700 and scans for a synchronization signal at operation 1702. Here, the start scanning is an operation of turning on the radio communication unit 201 and the modem 203 to receive a synchronization signal, and the control unit 205 controls the radio communication unit 201 and the modem 203 to receive a broadcast channel for at least one detected synchronization signal. The UE scans for synchronization signals at operation 1702 and determines whether any synchronization signals are detected at operation 1704. If any synchronization signals are detected, the process moves to operation 1706, otherwise, to operation 1730 where the UE determines whether a re-attempt to scan is necessary. If a retry of the scan is necessary, the process moves to operation 1702, otherwise, to operation 1708.

If the process transitions from operation 1704 to operation 1706, the UE stores the synchronization information and stops scanning for the synchronization signal at operation 1708. Thereafter, the UE aligns synchronization signal information at operation 1710. The information alignment is made in order of information on whether absolute time is used, (relay hop count), and received signal power strength. The UE selects the highest synchronization source UE at operation 1712 and determines whether the highest synchronization source UE uses the eNB reference time at operation 1714. If it is determined at operation 1714 that the UE does not use the eNB reference time, the process moves to operation 1718, otherwise to operation 1716 where the UE makes a final determination to operate as the highest synchronization source UE.

At operation 1718, the UE attempts to receive the broadcast channel for all detected synchronization signals or some having a high priority among the detected synchronization signals. The UE stores information on the successfully received broadcast channel at operation 1720 and stops receiving the broadcast channel at operation 1722 when the scan period ends. The UE aligns the received synchronization signal and the broadcast channel information at operation 1724. The information alignment is made in order of DFN, (relay hop count) and received signal power strength. If the alignment is complete, the UE makes a final determination to synchronize the reference time and DFN of the source UE using the highest priority at operation 1726.

Transmission period and resource selection control method for type 1 discovery

The basic procedure for the D2D discovery operation is as follows. The UE determines a discovery resource region configured by the eNB and a discovery pattern at the discovery resource region. Type 1 discovery mode is characterized by the UE autonomously selecting resources, while type 2 discovery mode is characterized by the UE requesting resource selection from the eNB. The description will be limited to the case where the resource region corresponds to the type 1 discovery mode. The discovery resource zone is periodically allocated every 10 seconds. It is assumed that one UE can select one discovery resource block in one period.

The UE selects N arbitrary resources among N discovery resource blocks within the discovery zone to perform detection thereof, determines this as a congestion situation in the absence of a resource block whose energy level is greater than or equal to X dBm, and increases the discovery signal transmission period (or decreases the transmission probability) in the current resource block. If there are any resource blocks whose energy level is greater than or equal to X dBm, the UE determines this as an idle resource, selects one of these idle resource blocks to transmit a signal, and reduces the discovery signal transmission period (or increases the transmission probability). The threshold X dBm for determining the amount of received energy can be controlled by the eNB and, if X dBm is a very large value (infinite), this means that any resource block is selected among all resource blocks and is therefore the same as any resource selection method. The discovery signal transmission period may be increased from an initial minimum period value to a maximum period value. The minimum period value is equal to the period (e.g., 10 seconds) in which the resource region is found. The minimum period value may not be the period of the real discovery resource region, but logically increases in the order of 1, 2, 3, 4. In the case of adjusting the transmission rate based on the transmission probability, if the probability is low, this may result in exceeding the maximum period value, and transmission may be immediately performed up to the maximum period regardless of the transmission probability in the case of transmission failure to avoid it. If there are likely to be multiple UEs that fail transmission up to the maximum period, it is configured that in another example, the transmission probability increases to a (p < a <1) when it reaches X% of the maximum period and increases to b (p < a < b <1) when it reaches Y% of the maximum period. In an example, if the UE fails to transmit up to the maximum period, it may request the eNB to adjust the transmission probability. The UE may transmit the discovery signal more frequently based on the transmission probability increased according to the transmission probability readjustment of the eNB. Although the above example is directed to the discovery signal, the method can be applied to a communication control area or a data transmission/reception area. In this case, the maximum period may be a maximum delay constraint (maximum latency constraint) of the corresponding traffic.

There may be various examples of the determination conditions for the above operations.

1) Case where there are any resource blocks with energy less than or equal to X dBm

2) There is a case where any resource block having an energy less than or equal to X dBm exists in addition to a subframe of the resource block having an energy less than or equal to Y dBm

3) There are cases where there are resource blocks having a received power ratio of less than A% than the minimum received power + alpha x (maximum received power-minimum received power)

It is to be understood that the above UE operations are performed under the control of the control unit 205. Although the control unit 205, the radio communication unit 201, and the modem 203 are configured as separate components in the above description, it is apparent to those skilled in the art that the corresponding components can be integrated into one component (e.g., a signal chip).

The above-described operations of the eNB or UE can be implemented with a memory device storing corresponding program codes at a portion of the eNB or UE. For example, the controller of the eNB or UE can perform the above-described operations in such a manner that program codes stored in a memory device are read out by means of a processor or a Central Processing Unit (CPU).

The various components and modules making up the entities, enbs, and UEs as described in this specification can be operable with hardware circuitry (such as complementary metal oxide semiconductor-based logic circuitry), firmware, software, and/or a combination of hardware and firmware and/or software stored in a machine-readable medium. For example, the various electrical structures and methods can be implemented with circuits (such as transistors, logic gates, application specific integrated circuits, etc.).

[2]D2D discovery and communication operations

A second embodiment of the present disclosure is directed to D2D discovery and communication operations.

Synchronous resource structure

The synchronization resources may include Synchronization Resource Units (SRUs) capable of carrying synchronization signals and channels. The SRU is a set of resources for a synchronization signal (D2DSS) and a synchronization channel (PD2DSCH) transmitted by a synchronized UE. The length of the synchronization resource unit may be equal to the length of the LTE subframe (1 ms). In order to transmit a synchronization signal or a synchronization channel, at least one physical symbol unit is required. Since the synchronization channel is a message, a pilot pattern for use by the receiver in accessing the physical channel quality can be inserted into the synchronization channel resource.

In 3GPP, DMRS is used for data demodulation, and an existing DMRS pattern (DMRS pattern) and a modified version thereof may be used. A sequence pattern of a base sequence, a symbol-level shifted version (symbol-level shifted version), or a scrambled version of the base sequence (scrambled version) may be used as the DMRS pattern. Based on the case where multiple synchronization sources transmit the same message, UEs at the same logical location may use the same DMRS pattern. For example, a synchronization source at n hops away from the eNB may use a DMRS pattern corresponding to the n hops. In this case, information on the number of hops need not be transmitted via a synchronization signal or a synchronization channel. In an embodiment of the present disclosure, in addition to information on a DMRS pattern corresponding to a hop count, information on UE support of an absolute reference timing device, such as a GPS receiver, use of time division duplex/frequency division duplex (TDD/FDD), a required transmission distance level, a power level, the number of retransmissions, and a battery level may be delivered.

Fig. 18 is a schematic diagram illustrating a synchronization resource structure according to an embodiment of the present disclosure.

Referring to FIG. 18, synchronization resources 1821, 1822, 1823, and 1824 are distributed in synchronization period 1810 without overlapping with D2D control and data resources 1831, 1832, 1833, and 1834. As shown in fig. 18, a plurality of SRUs 1821, 1822, 1823, and 1824 are configured in a synchronization period 1810. The SRUs are linked to different types of synchronization source UEs and the location of the SRUs may be determined from the relay hop count.

The synchronization signal is transmitted per SRU, but the synchronization channel may be transmitted according to a specific condition. The condition for not transmitting the synchronization channel is described as follows.

a) When the synchronization channel period is set to a value longer than the synchronization signal period,

b) when a particular resource region specified by the eNB overlaps with the SRU location (e.g., it is configured to use only synchronization signals (D2DSS) in a discovery resource region not controlled by the eNB),

c) when the synchronization channel reception performance is degraded by too many participating UEs as a result of the synchronization channel monitoring, the UE may immediately change the synchronization channel period and offset according to the monitoring result or report the monitoring result to the eNB to change the synchronization period and offset according to the determination of the eNB. Alternatively, the UE may immediately change the transmission probability of the synchronization channel according to the monitoring result or report the monitoring result to the eNB to change the transmission probability of the synchronization channel according to the determination of the eNB.

Fig. 19a, 19b, 19c and 19D illustrate resource pool structures with synchronized resources for D2D discovery and D2D communication according to embodiments of the present disclosure.

Referring to fig. 19a, 19b, 19c, and 19d, a resource structure in which synchronization resources are arranged and other resources may be added is illustrated. For example, in case that the UE operates in TDD (time division duplex), the D2D resource region is configured in uplink and thus downlink resources may be located in time domain. However, in view of the D2D UE, the resources are logically configured as if there were contiguous D2D resources.

Fig. 19a shows the structure of resource regions, i.e., Scheduling Assignment (SA) regions (pools) (1911, 1912) and communication data resource regions (data pools) (1921, 1922), and the structure of synchronization resources (1901, 1902, 1903) for notifying a reference timing of a resource region notified from an eNB for performing D2D communication. In the SA area, the UE transmits an SA signal including information on data resources allocated or selected in the SA area in advance before data transmission to neighboring UEs. The UE having received the SA signal in the SA area receives a data signal transmitted from an intended transmitting UE in a subsequent data area. The IC UE determines the absolute time of the reference frame (SFN0) by receiving the BCH and acquires the relative location information of the resource region expressed based on the reference frame by receiving the SIB. UEs served by neighboring enbs or located in partial or within a network coverage area receive synchronization signals and synchronization channels transmitted in SRUs to first determine absolute times of reference frames (SFN0) and obtain relative location information of resource areas indicated based on the reference frames.

The D2D UE may select one synchronization UE to synchronize for the transmitted reference timing according to a predetermined priority. If there are other synchronized UEs having a reference timing different from that of the selected synchronized UE, the transmission reference timing of the selected synchronized UE is used during a predetermined period of time to synchronize the reference timing for receiving a synchronization signal from another synchronized UE. At least one synchronization source (i.e., SRU) may be arranged during one synchronization signal period, and if a reference timing for transmission is synchronized, it may operate not to transmit a synchronization signal without an idle SRU when checking conditions for synchronization signal transmission, such as reception power and signal detection of an eNB signal or other synchronized UEs. For example, although the condition for transmitting the synchronization signal is satisfied, the UE transmits the synchronization signal using one SRU selected among idle SRUs only when the idle SRUs exist. A relatively loose idle SRU determination condition may be applied. For example, if the number of synchronization signals received in an SRU at a higher received power than a predetermined received power threshold is less than N, the SRU may be determined to be an idle SRU.

Fig. 19b is a schematic diagram illustrating a resource pool structure including synchronization resources for D2D discovery, according to an embodiment of the present disclosure.

Similar to the case of fig. 19a, the UE receives a BCH from the eNB in the discovery resource area to determine the absolute time of the reference frame (SFN0) and acquires relative location information on the discovery area indicated by the reference frame by receiving the SIB. UEs served by neighboring enbs or located in partial or within network coverage areas receive synchronization signals and synchronization channels transmitted in SRUs to first determine absolute times of reference frames (SFN0) and to obtain relative location information of resource areas indicated based on the reference frames.

For example, if the synchronization channel received from the UE served by the neighboring eNB indicates SFN10, the reference frame SFN0 would be located 10 SFN cycles before. Meanwhile, the UE served by the neighbor eNB in the discovery resource region 2(1942) receives a BCH and a SIB from the serving eNB to acquire information on a position with respect to a reference time (SFN0) of the serving eNB for a reference resource region 2(1942) of at least one other neighbor eNB. When the starting point of the resource region of another eNB is reached, the UE may perform a transmission/reception operation using the WAN, and if the UE is not transmitting a synchronization signal or other D2D signal, the UE receives a communication signal and channel expected at the starting time of the corresponding resource region. The synchronization signal is transmitted according to a specific condition, but the synchronization channel may not be transmitted. A terminal served by a neighbor eNB receives a synchronization signal in order to acquire an accurate received synchronization reference timing with respect to the start of a corresponding discovery resource region. Such operations may be applied to the D2D communication resource region, without limitation to this example.

Meanwhile, the discovery resource region 1(1941) and the discovery resource region 2(1942) are used simultaneously, and a field for indicating whether the first duration of the resource region (e.g., subframe) is used for SRU should be included in BCH or SIB from eNB. In case of a synchronization source UE, whether there is a resource for transmitting a synchronization signal should be determined based on a field informing availability of an SRU, and in case of a receiving UE served by a neighbor eNB, whether to receive a synchronization signal and a channel in a corresponding resource region or whether to receive a discovery signal may be determined according to the field informing availability of the SRU. This field is referred to as a periodic synchronous transmission field in embodiments of the present disclosure.

In case of using the periodic (blue) SRU (1931, 1932, 1933), the eNB opens the periodic sync transmission field to inform it through BCH or SIB. In case of using the temporary (one-time use) SRU (1934), the periodic synchronization transmission field is turned off to inform it through BCH or SIB.

If no additional (pink) SRU is used in resource area 2 (1934), then a UE located in a partial or out of network coverage area receives the synchronization signals and synchronization channels transmitted by the UE in periodically transmitted (blue) SRUs (1931, 1932, 1933) to determine the absolute time of the reference frame (SFN0) and to obtain relative location information for both the indicated resource area 1(1941) and resource area 2(1942) compared to the reference frame. The relative location information can be obtained based on information relayed by the neighboring eNB and information received from the serving eNB. After acquiring the relative location information of the resource region, the UE may wake up from the idle state at least one synchronization period earlier than the start point of the resource region to receive a synchronization signal at the period SRU to compensate for synchronization.

Further, where additional SRUs are used in resource area 2(1942), the UE may receive the periodic SRUs and obtain relative location information from the reference time (SFN0) of the serving eNB for the resource areas of the neighboring enbs and may wake up from the idle state just prior to the start time of resource area 2(1942) to receive synchronization signals in the additional SRUs to compensate for synchronization. In the case where only additional SRUs are used without periodic SRUs (1931, 1932, 1933) in the resource area 2(1942), if the eNB has not provided information about the starting point of the resource area in advance or if the UE is an out-of-coverage UE, the UE may inefficiently operate to perform a synchronization procedure for receiving aperiodic SRUs for a long time.

Fig. 19c is a schematic view illustrating a resource pool structure including synchronization resources for D2D discovery and communication according to an embodiment of the present disclosure, and fig. 19D is a schematic view illustrating a resource pool structure including synchronization resources for D2D discovery and communication according to another embodiment of the present disclosure.

Referring to fig. 19c, SRUs 1931, 1932, and 1933 are arranged periodically, and an SA pool 1911 for resource allocation and a data pool 1921 for communication are arranged. Further, it is found that the wells 1941 are arranged together. The resource structure of fig. 19d is different from that of fig. 19c in that an aperiodic SRU 1934 is included in the first transmission region of the discovery pool.

For each resource pool information, the eNB may explicitly or implicitly inform the UE whether to listen for a periodic synchronization signal or a one-time-use synchronization signal. The SA pool (1911) and the data pool (1921) are used for communication and require synchronization using a periodic synchronization signal, but the discovery pool allows synchronization using a periodic synchronization signal or a one-time use synchronization signal.

Accordingly, the eNB may explicitly inform 1-bit information indicating one of the two synchronization setting patterns together with resource pool information via a broadcast channel (BCH and SIB) or UE-specific signaling. On the other hand, the implicit notification is described below. The eNB uses two pieces of offset information to inform the resource pool. The first offset information indicates a gap between reference frames (SFN0) of the serving eNB and the neighbor eNB. In LTE, this offset may be indicated by 10ms (fdd) or 20ms (tdd) as a frame unit. The second offset information indicates a starting point of a resource pool used by the neighboring eNB with respect to a reference frame (SFN0) of the neighboring eNB. In LTE, this offset may be indicated by 1ms as a subframe unit. The eNB receives information indicating whether a resource pool of a neighboring eNB utilizes the one-time-use synchronization signal from the network, and does not transmit the first offset information (or transmits null information) if the one-time-use synchronization signal is used. The eNB transmits both the first offset information and the second offset information if the periodic synchronization signal is used. The UE performs a synchronous reception operation to identify a start point of a desired resource pool by using the number of offset information, first offset information set to be empty, or an offset information delimiter (delimiter) received from the eNB.

In fig. 19a and 19b, it is assumed that the eNB notifies resource pool information via BCH and SIB. However, in some cases, the resource pool information may be notified via a UE-specific control signal (dedicated signaling). In case of allocating the first frame of the resource pool for aperiodic one-time use of the synchronization signal resource, all UEs in the coverage area of the eNB may transmit the synchronization signal at the first frame of the resource pool when the resource pool is notified via the BCH and the SIB. When the resource pool is notified via dedicated signaling, only UEs that have received the dedicated signaling from the eNB may transmit a synchronization signal.

Synchronization signal measurement

When the D2D UE receives the synchronization signal, the D2D UE may measure another synchronization signal in order to determine whether to become a synchronization source UE or report the measurement result to the eNB, depending on the status of the connection with the eNB. Further, when a resource pool requiring use of a periodic synchronization signal and a resource pool granted use of a one-time-use synchronization signal are distinguished, a scheme for measuring a synchronization signal may vary depending on a situation. In case of using the synchronization signal at one time, all UEs determined as synchronization sources according to control of the eNB or preset conditions must transmit by using a single SRU. Therefore, the synchronization signal is transmitted at the same resource and through the same transmission scheme as in the case of the single frequency network.

Here, when one or more synchronization signals are received in an overlapping manner, the reception power may be higher than a case where one synchronization UE transmits the synchronization signal at one resource, and the transmission distance of the synchronization signal may be greater than that of data. In case of resource allocation for a periodic synchronization signal from the eNB, all UEs determined as synchronization sources according to control of the eNB or preset conditions may transmit a synchronization signal by selecting an unused synchronization resource. Here, when synchronization signals from a small number of synchronized UEs may overlap at one resource, the reception power at the receiving UE may be lower than that of the synchronization signal used at one time, and the difference between the transmission distances of the synchronization signal and data is not large. Due to these environmental differences, it may not be appropriate to measure the one-time-use synchronization signal in order to select a synchronous relay UE for an out-of-coverage UE. To select a synchronous relay UE, it is more appropriate to measure only the periodic synchronization signal. For this, the synchronized UE within the coverage should inform the location of the synchronization resource (i.e., SRU period and offset) via the synchronization signal and the synchronization channel.

The synchronizing UE may relay information (i.e., an offset, bitmap, etc.) about a resource pool that does not utilize periodic synchronization resources. To become a synchronous relay UE, a UE that has received a synchronization signal and a synchronization channel from a synchronous UE measures a synchronization signal from a neighboring synchronous UE. Based on the information on the resource pool not utilizing the periodic synchronization resource, the synchronization signal transmitted by the UE belonging to the same eNB and received at the resource pool can be disregarded (this can be known from the ID contained in the synchronization signal and the synchronization channel).

As another example, the synchronizing UE may relay information (i.e., an offset, bitmap, etc.) about a resource pool that utilizes periodic synchronization resources. To become a synchronous relay UE, a UE that has received a synchronization signal and a synchronization channel from a synchronous UE measures a synchronization signal from a neighboring synchronous UE. Based on the information on the resource pool utilizing the periodic synchronization resource, only the synchronization signal transmitted by the UE belonging to the same eNB and received at the resource pool is measured (this can be known from the ID contained in the synchronization signal and the synchronization channel).

Meanwhile, based on the information on the resource pool using the periodic synchronization resource or the resource pool not using the periodic synchronization resource and the information on the relay hop count from the eNB, it is still able to determine whether to periodically transmit the synchronization signal at the resource pool not using the periodic synchronization resource. For example, for a relay hop count of 2, i.e., eNB (0 hop) > UE1(1 hop) > UE2(2 hop), the synchronization transmission of UE2 may be limited so as not to cause interference to or impact other in-coverage UEs operating at a resource pool that does not utilize periodic synchronization resources. On the other hand, for a relay hop count of 3, i.e., eNB (0 hop) - > UE1(1 hop) - > UE2(2 hop) - > UE3(3 hop), UE3 may perform synchronized transmission at a resource pool that does not utilize periodic synchronization resources because the signal of UE3 does not reach an in-coverage UE.

Synchronization for Tx

A description is given of the overall synchronization process for D2D discovery and communication. The overall synchronization process includes a transmission synchronization process for synchronizing a reference time for transmission and a reception synchronization process for synchronizing a reference time for reception. The reason is as follows. While reference times for transmission have been synchronized under the assumption that it is extremely difficult to achieve accurate synchronization in out-of-coverage situations, it is desirable to allow the UE to perform D2D discovery or receive communication signals in accordance with different synchronization signals when different synchronization signals are received. The overall process includes scanning to acquire transmit synchronization, and acquiring transmit synchronization and determining the effect while scanning to acquire receive synchronization and acquire receive synchronization.

Fig. 20a and 20b are flowcharts illustrating a transmission synchronization process according to an embodiment of the present disclosure.

Referring to fig. 20a, when the UE is turned on in operation 2000, for example, when the wireless communication unit (201) for D2D operation and the modem (203) for D2D operation are turned on by the controller (205), the UE scans for enbs in operation 2002. If synchronization signals (PSS/SSS) of at least one eNB are detected in operation 2004, the UE selects one of the detected synchronization signals in operation 2004, synchronizes with a reference time of the selected synchronization signal in operation 2006, and determines whether to become a relay synchronization source (R-SS) in operation 2008. Selecting one of the synchronization signals of the multiple enbs is based on the received power level as in the case of the existing LTE standard. Determining whether to become a relay synchronization source is described below with reference to the accompanying drawings. After the determination, the process returns to operation 2002 to scan for enbs.

If the synchronization signal of the eNB is not detected in operation 2004, the UE scans for a synchronization source UE by moving to operation 2010. If a synchronization signal (D2DSS, D2DBCH, or D2DSCH) of at least one synchronization source UE is detected in operation 2012, the UE selects one of the detected synchronization signals, moves to operation 2014 to synchronize with a reference time of the selected synchronization signal, and moves to operation 2016 to determine whether to become a relay synchronization source. Selecting one of the synchronization signals for the UE may be based on at least one of a received power level, a D2D frame number, a relay hop count, a UE active duration, a UE aging, and a synchronization phase. If the UE fails to detect the eNB synchronization signal or the D2D synchronization signal, the UE may act as an independent synchronization source (I-SS), that is, when moved from operation 2012 to operation 2018.

Further, unlike the case of synchronization with the eNB, a UE synchronized with a synchronization source UE must periodically perform scanning. This is because network variability is high due to UE mobility. Thereafter, the UE determines the expiration of the time for the scanning transmission in operation 2020, and the process returns to scanning for enbs in operation 2002 if the time for the scanning transmission has not expired. If the time of the scan transmission has expired, the process continues to determine the arrival of the time of the scan reception in operation 2022. Scanning for reception is described below with reference to the following figures.

From the standpoint of the synchronization source UE, the transmission scheme may differ between scanning for transmission and scanning for reception. For example, while a change in synchronization is expected in the scan for transmission, the existing transmission timing is maintained in the scan for reception. Therefore, although the synchronization source UE stops transmitting the synchronization signal during the scanning period for transmission, the synchronization source UE continues to transmit the synchronization signal during the scanning period for reception.

Referring to fig. 20b, it further includes an operation 2030, compared to fig. 20 a. In operation 2030, the time of the scan transmission is different for the eNB and the UE. Specifically, the eNB scan may be performed in a short period after matching with the reference time of the synchronization source UE. After scanning for the eNB, UE scanning may be performed at a long period.

Synchronization for Rx

Fig. 21a, 21b and 21c are flowcharts illustrating a process of scanning for reception according to an embodiment of the present disclosure.

Referring to fig. 21a, the UE determines whether D2D transmission or reception is in progress in operation 2100. If a control or data signal is being transmitted or received for D2D discovery or communication, the UE may have difficulty receiving another synchronization signal before completing the transmit and receive operations. If D2D transmission or reception is not in progress during the search result of operation 2100, the UE determines expiration of time for scan reception in operation 2102. If the time for scanning for reception has not expired, the UE scans for an eNB for a synchronization signal in operation 2104, synchronizes with a detected reference time for the received synchronization signal if the synchronization signal is detected in operation 2106, and receives a D2D signal in an Rx mode in operation 2110. However, if the synchronization signal is not detected from the eNB, the UE scans for the synchronization signal in operation 2108, synchronizes with a reference time of the detected synchronization signal for reception in operation 2112 if the synchronization signal is detected, and receives a D2D signal in the Rx mode.

It is possible to synchronize between two Rx modes: one of the Rx sync mode 1 and the Rx sync mode 2 receives the D2D signal. The UE determines an Rx synchronization mode at operation 2114. Here, a reception operation in a resource pool for a serving cell may be omitted in the Rx mode. If the current operation mode is Rx sync mode 1, the process goes to operation 2118, otherwise, if the current operation mode is Rx sync mode 2, the process goes to operation 2116.

Difference between Rx sync pattern 1 and Rx sync pattern 2:

rx sync pattern 1: it is assumed that the serving eNB informs the resource pool information including an offset related to the neighbor eNB as a difference from the reference frame timing (e.g., SFN 0). The UE performs D2D reception only at the notified resource pool, and, if a synchronization signal from a UE belonging to a neighboring eNB is detected, obtains a symbol and frame boundary from the detected synchronization signal and performs a desired D2D operation.

An out-of-coverage UE, which has obtained an eNB reference time via a synchronization signal relayed by the UE, may receive information on an offset and a resource pool of a neighboring eNB relayed by the synchronized UE through a synchronization channel and perform a reception operation as in the case of an in-coverage UE. Here, via a synchronization signal from the synchronizing UE, an ID that the receiving UE should be able to identify an eNB with which the synchronizing UE is associated or on which the synchronizing UE resides is received.

It is not easy to apply Rx synchronization mode 1 to out-of-coverage UEs that have acquired independent reference times that do not originate from the receiving network. This corresponds to the situation where a synchronized UE is generated due to the lack of a synchronized relay UE in the network coverage. When complexity is not critical, the following is possible. An in-coverage UE may receive an independent reference time for an out-of-coverage UE and report the reference time to a corresponding eNB. Upon receiving such a report from at least one UE, the eNB may notify UEs in the cell of an offset related to the reference time of the out-of-coverage UE. The use of Rx sync mode 2 may be more efficient when complexity is critical.

Rx sync mode 2: it is assumed that the neighboring eNB or a synchronized UE belonging to the neighboring eNB informs resource pool information including an offset related to the neighboring eNB as a difference from a reference timing (e.g., SFN0) of the neighboring eNB. The UE must receive the synchronization signal and channel, obtain symbol and frame boundaries from the synchronization signal, and identify the current SFN and resource pool location from the synchronization channel. When the current timing corresponds to the identified D2D resource pool, the UE performs D2D operations at the D2D resource pool. When the current timing does not correspond to the identified D2D resource pool, the UE may enter an idle state waiting for the arrival of the D2D resource pool in the absence of WAN operations other than D2D operations.

In Rx synchronization mode 2, an out-of-coverage UE, which has obtained an eNB reference time via a synchronization signal relayed by the UE or other out-of-coverage UEs, may receive information about the SFN and resource pool of a neighboring eNB relayed by the synchronized UE through a synchronization channel and perform D2D reception operation.

Referring to fig. 21b, operations corresponding to Rx sync mode 1 and Rx sync mode 2 are illustrated. If the UE is in Rx synchronization mode 1, the UE moves to operation 2124, and if the UE is in Rx synchronization mode 2, the UE moves to operation 2122. In Rx synchronization mode 1, the UE uses the SIB from the eNB to obtain information about the starting point of the resource pool of the neighboring eNB and updates a list of information on timing for starting scan reception accordingly. In Rx synchronization mode 2, the UE uses D2DBCH (broadcast channel) or D2DSCH (synchronization channel) to obtain information on the starting point of the resource pool of the neighbor eNB and the sync source UE, and updates a list of information on the timing for starting scanning reception accordingly.

Referring to fig. 21c, unlike the case of fig. 21b, the time of scan reception is different for the eNB and the UE.

At operation 2100, the UE determines whether it is transmitting or receiving a D2D signal. For example, if the UE is transmitting or receiving control or data signals for discovery or communication, it may be difficult to receive other synchronization signals before the transmission/reception operation is completed. If the UE is not transmitting or receiving a D2D signal at operation 2100, the UE determines whether it is time to scan for reception from the eNB at operation 2102. If it is time to scan for reception, the UE scans for eNB synchronization signals at operation 2104, and if any eNB synchronization signals are listened to at operation 2130, configures an Rx time reference to receive a D2D signal at operation 2132. Otherwise, if no eNB synchronization signal is listened to, the UE ends the routine of fig. 21 c.

If it is not time to scan for reception from the eNB at operation 2102, the UE determines whether it is time to scan for reception from other UEs at operation 2140. If it is time to scan for reception from other UEs, the UE scans for synchronization signals from the other UEs at operation 2142 and detects any synchronization signals from the other UEs at operation 2144, configures a reception time reference to receive a D2D signal at operation 2146. If a synchronization signal from another UE is not detected at operation 2144, the UE ends the routine of FIG. 21 c.

Meanwhile, if the process goes to operation 2150, the UE determines whether it is in Rx synchronization mode 1 or Rx synchronization mode 2, and if it is in Rx synchronization mode 1, the process goes to operation 2154, otherwise, if it is in Rx synchronization mode 2, the process goes to operation 2152.

To R-SS

Fig. 22 is a flowchart illustrating a process of becoming a relay synchronization source according to an embodiment of the present disclosure.

Referring to fig. 22, the UE determines whether a connection mode exists in an eNB area in operation 2200. Based on the results of operation 2200, an in-coverage UE in connected mode may operate as a relay synchronization source under control of the eNB according to one of the following three options.

The first option is a command-reporting scheme as in operation 2210, in which when a command for synchronization signal measurement is received from the eNB in operation 2212, the UE performs corresponding measurement and reports the measurement result to the eNB in operation 2214.

The second option is a request-response scheme as in operation 2220, in which when the UE transmits a request to become a relay synchronization source to the eNB in operation 2222, the eNB checks the request and transmits a response message to the UE. The UE receives the response message in operation 2224 and may also transmit the measurement result to the eNB in operation 2226. When the response message indicates ready agreement, the UE operates as a relay synchronization source. When the measurement result is transmitted, the eNB may permit the UE to operate as a relay synchronization source via a separate control signal.

The third option is a BSR-grant scheme as in operation 2230, in which the UE transmits additional request information when transmitting a Buffer Status Report (BSR) to request resource allocation for data transmission in operation 2232, and the eNB checks the request and may transmit an R-SS indication when transmitting a grant control signal to the UE. The UE may receive a resource grant message indicating whether to operate as an R-SS at operation 2234. Depending on the situation, the eNB may request the UE to perform additional measurements. The UE may perform additional measurement reporting at operation 2236.

The UE may become an R-SS at operation 2250 by one of three methods as described above.

If the UE is in idle mode operation 2200, it is determined whether there is any data to send (b) at operation 2202. If there is no data to send, the UE does not attempt to become a relay synchronization source.

In another embodiment of the present disclosure, the UE may become a relay synchronization source regardless of the data (a) to be transmitted. To check for the possibility of becoming a relay synchronization source, the UE determines the expiration of the time of the scanning transmission in operation 2240. If the time for the scanning transmission has not expired, the UE measures synchronization signals from the camped and neighboring enbs in operation 2242. When the measurement result satisfies the preset condition, the UE operates as a relay synchronization source in operation 2244. Satisfaction of this condition may be related to determining whether the received power level of the signal from the eNB is less than a threshold of X dBm, whether the received power level of the synchronization signal from the neighboring synchronous relay UE is less than a threshold of Y dBm, or whether the received power level of the signal from the independent neighboring synchronous relay UE is less than a threshold of Z dBm.

Relationship between eNB ID and D2D synchronization signals

The receiving UE should be able to identify the ID of the eNB with which the synchronizing UE is associated or on which the synchronizing UE resides via a synchronization signal (including a synchronization channel) from the synchronizing UE. To this end, the eNB ID may be directly transmitted as part of the synchronization signal, or the IDs of the UE-specific synchronization signal or the IDs of the set of UE-specific synchronization signals whose relationship with the eNB ID is predefined may be transmitted. Accordingly, a UE that has received a synchronization signal may identify an ID of an eNB associated with or camped on by the UE that has transmitted the synchronization signal based on the ID of the synchronization signal. Alternatively, the UE may transmit a synchronization signal with the ID of the set of enbs.

Area ID (zone ID)

A "region" may include one or more enbs. The zones may be used interchangeably with clusters, groups, or common configurations.

When receiving zone information from higher layers via eNB, it may be necessary to identify serving cell information (resource location and ID of synchronization signal of serving eNB has been informed by eNB).

-SFN need not be obtained from the synchronization channel (PD2DSCH) when the reference frame (SFN0) of the cell that has sent the SIB has already been obtained from the current serving eNB.

The region will enable multiple enbs to collectively use some or all of the RRC parameters for D2D communication between enbs. For example, bitmap information representing resource pool allocation may be commonly used, and a start point of an eNB-specific resource pool may be represented by different offsets. Alternatively, the allocation bitmap information and the offset may be the same and the location of the eNB-specific D2D frequency resources may be different. For this purpose, the region-specific information may be separated from the cell-specific information in the SIB. The region specific information may be notified together with the region ID and may be implicitly known from SIB reception as to the serving eNB. It is necessary to explicitly indicate the region to which the serving eNB belongs. For example, in a SIB, the eNB must separate resource pool information for the region that includes the serving eNB from resource pool information for the region that does not include the serving eNB.

A scheme that does not explicitly indicate a region may be used if separation of resource pool information for a region including a serving eNB from resource pool information for a region not including a serving eNB requires additional signaling or causes difficulties in supporting UEs in idle mode. For example, both resource pool information for a region and a list of IDs of synchronization signals available in the region may be transmitted. Here, the synchronization signal may include PSS/SSS from the eNB and D2DSS from the UE.

In a scenario where the regional IDs are managed separately for different PLMNs, the PLMN IDs will be sent through SIBs or PD2DSCH in addition to the regional IDs.

No need to send and receive SFN information via the PD2DSCH for operation between enbs when the offset information between the areas is signaled by the network. However, SFN information (or D2D frame number) needs to be sent via the PD2DSCH for out-of-coverage operation.

Although the region ID may be configured separately, existing paging area IDs or timing group (TAG) IDs (cells in one TAG have the same Timing Advance (TA) for coordinated multipoint (CoMP) operations) may be reused to configure the region ID. On the other hand, in case of not using the region ID, common parameters may be applied to all enbs in one Public Land Mobile Network (PLMN) area.

The UE may receive the region ID and the region-specific information associated therewith from a D2D server (e.g., a proximity service server, a Mobility Management Entity (MME), etc.) in advance at initial attach. Otherwise, the eNB should inform the UE of all or some regions via BCH and SIB when the UE is in or camped on the eNB. When the zone ID is not separately configured, the D2D server or MME informs the UE of common parameters of all enbs at initial attach.

Hierarchical enhancement function (Hierarchical enhancement)

Fig. 23 is a flowchart illustrating a method of becoming an R-SS capable of compensating for a hierarchical synchronization procedure problem of an OOC UE according to an embodiment of the present disclosure.

Referring to fig. 23, the UE turns on the D2D function at operation 2300 and scans for signals from the eNB at operation 2302. The UE determines whether a synchronization signal from the eNB is detected in operation 2304. If a synchronization signal is received from any eNB, the UE acquires synchronization with the eNB at operation 2306 to become R-SS at operation 2308.

If the eNB synchronization signal is not received at operation 2304, the UE scans for signals from the UE at operation 2310 and determines whether any synchronization signal is received from at least one UE synchronized with the eNB at operation 2312. If any synchronization signal is received from at least one UE synchronized with the eNB, the UE acquires synchronization with the UE having the highest priority in operation 2314 and becomes R-SS if a condition for becoming R-SS is satisfied in operation 2316.

If the process goes from 2312 to 2320, the UE determines whether the UE with the highest priority is detected. If the UE having the highest priority is detected at operation 2320, the UE acquires synchronization with the UE having the highest priority at operation 2322 and becomes R-SS if a condition for becoming R-SS is satisfied at operation 2324. If the UE having the highest priority is not detected at operation 2320, the UE becomes an independent synchronization source UE (I-SS).

The UE determines at operation 2330 whether it is time to scan for transmission and, if so, it returns the process to operation 2302, otherwise, returns to operation 2332 to scan for reception at operation 2332.

In the existing hierarchical synchronization process, synchronization is achieved from an independent synchronization source (I-SS) by repeating the relay until the maximum hop count. However, synchronization mismatch between clusters formed by different independent synchronization sources may cause interference or inconsistency of action timing between UEs of different clusters. In fig. 23, high synchronization performance is achieved by combining the periodic changes of the independent synchronization sources and converging the previous reference time to a new reference time as the I-SS of operation 2326 changes. The I-SS is determined based on a priority value transmitted by the UE via the D2DBCH or D2 DSCH. The UE desiring to become I-SS transmits its priority value to the approaching UE, and the approaching UE finds the UE having the highest priority value and establishes transmission synchronization with the UE having the highest priority value. The UE having established transmission synchronization determines whether to become an R-SS.

The condition to become R-SS may correspond to the reception power level of D2DSS, the reception power level of a synchronization signal from a synchronous relay UE belonging to the same nearby cluster and having the same hop count, and whether to transmit a D2D discovery/communication signal. When the UE receives synchronization signals from a plurality of synchronization source UEs during the time of scan transmission and the time of scan reception, the UE updates the reference time to be used for the next I-SS. The reference time to be used for operation as an I-SS is determined based on timings obtained from different synchronization signals according to a preset rule. The rules are designed to enable convergence of the reference times for all UEs. Examples of such rules may include average timing based determination, earliest timing based determination, and pulse coupled oscillator modeling. If it becomes I-SS, the UE transmits a synchronization signal as ISS during an operation period of a synchronization source UE determined according to a rule randomly preconfigured or determined or predetermined by the UE under the control of the eNB and performs a scanning procedure. If the condition for becoming the I-SS is satisfied as a result of the scanning, the UE repeats the above operations to determine the transmission timing based on the synchronization signal reception timing received during the I-SS operation period.

D2D frame number priority

In synchronization for communication, although synchronization with a frame boundary is achieved based on a received synchronization signal, each device needs to identify a frame associated with the device when different operation procedures are defined for different frames. Thus, in a communication system, the (system) frame numbers or indices are designed and assigned to enable the different frames to be distinguished. For example, in the LTE system, an SFN (system frame number) is transmitted to a device by transmitting an MIB (master information block) via a control BCH. The SFN may be referred to as a radio frame number.

Since the frame number configuration process has been described in the first embodiment with reference to fig. 13a, 13b, 13c, and 13d, a description thereof is omitted herein.

A description of D2D frame number determination in both the in-coverage and partial-coverage network scenarios is made with reference to fig. 14 for the first embodiment of the present disclosure.

In fig. 14, UE 11431 receives PSS/SSS and BCH from eNB1 and eNB2 and D2DSS and D2DBCH (or PD2DSCH) from ssee 2 (synchronization source UE 2). Because the synchronization signal from the eNB is generally prioritized over the synchronization signal from the UE, the D2DSS and D2DBCH from the ssee 2 are ignored. UE1 selects one of eNB1 and eNB2 according to the received power level of PSS/SSS from it. In a special case, for example, where UE1 belonging to eNB2 fails to receive PSS/SSS and successfully receives D2DSS from ssee 2 belonging to eNB2, UE1 may synchronize with D2DSS from ssee 2 (except for the synchronization signal from eNB) as an exception.

In fig. 14, the UE 31433 receives a UE synchronization signal and a UE broadcast channel from a synchronization source UE1 (sse 1)1441 and a UE2 (sse 2) 1442. Excluding the abnormality, the UE 31433 selects one of a plurality of UE synchronization signals according to a priority rule for the UE synchronization signal and receives a UE broadcast channel at a position determined based on the selected UE synchronization signal. The priority rule may be determined based on at least one of a received signal power strength or quality, an eNB ID or an SSUE ID indicated in the synchronization signal, and an SFN or DFN. If the UE broadcast channel is successfully received, the UE acquires and follows the DFN. The UE operating in the DFN becomes the R-SS, and the UE calculates a DFN associated with the synchronization signal based on the acquired DFN and transmits the calculated DFN in the synchronization signal or the UE broadcast channel. In the aforementioned exceptional condition, in case the eNB informs it to use absolute time (e.g., GPS provided time), if there is an eNB using absolute time according to information included in the UE broadcast channel, the UE follows the reference time of the corresponding eNB, otherwise, if there is no eNB using absolute time, the UE follows the reference time of the UE synchronization signal selected according to the priority rule for the UE synchronization signal and the DFN of the UE broadcast channel.

In fig. 14, the UE 21432 receives UE synchronization signals and UE broadcast channels from the in-coverage sse 11441 and the out-of-coverage sse 31443. In order to make it possible to identify the UE 21432, the synchronization signal or synchronization channel/broadcast channel must include information indicating in-coverage/out-of-coverage scenarios. The UE 21432 preferentially selects the in-coverage sse 1 over the out-of-coverage ssee 31443. If there are multiple in-coverage SSUEs, UE selection is performed according to the priority rule as in the case where the UE 21432 selects an in-coverage SSUE. In fig. 14, although it is similar to UE 31433, UE 41343 is an out-of-coverage UE with absolute time. Basically, there may be two rules as described with reference to fig. 14.

Further, the out-of-coverage D2D frame number determination scenario described with reference to fig. 15 in the first embodiment may be applied to the second embodiment with appropriate modifications. In the second embodiment of the present disclosure, a detailed description about the out-of-coverage D2D frame number determination scene is omitted.

Similarly, the absolute time based synchronization information alignment and selection process has been described in the first embodiment with reference to fig. 16. For example, operation 2022 of fig. 20a or 20b, which shows the entire operation of the second embodiment, may be applied without modification to the process of fig. 16 or 17, which illustrates the first embodiment. Therefore, detailed description thereof is omitted herein.

inter-PLMN support

There are two schemes for supporting D2D discovery and communication between UEs belonging to different PLMNs. For example, assuming that UE1 belongs to PLMN 1 and UE2 belongs to PLMN 2, in a first scheme, each UE transmits in their serving PLMN and receives in another PLMN. In a second scheme, each UE transmits in another PLMN and receives in their serving PLMN. In a first scheme, the UE may operate normally in the same PLMN based on the resource pool information of the serving eNB and the neighbor enbs notified by the network, but may have difficulty in notifying another PLMN of dynamic exchange of resource pool information between enbs. Further, when the UE2 first operates in its serving PLMN 2, the UE2 may not perform reception in another PLMN while performing transmission and reception in the serving PLMN. Therefore, it is uncertain that the signal transmitted by UE1 in PLMN 1 is received by UE2 in PLMN 2. In a second scheme, the UE1 may receive information indicating D2D support in PLMN 2 from an eNB while remaining in PLMN 1, detect an eNB synchronization signal in a frequency band used by PLMN 2, receive system information for D2D communication from a corresponding eNB if accessible, and connect to the corresponding eNB of PLMN 2 to perform D2D discovery and communication as necessary. Based on both schemes, a UE wishing to transmit (UE1) may desire to know whether UEs belonging to different PLMNs are configured to receive from the PLMN of UE 1. For this reason, it is not practical to individually inform whether a particular UE is configured to receive from a given PLMN due to more overhead. Alternatively, the eNB may control the D2D UE via a broadcast channel, SIB, or UE-specific control signaling to receive D2D signals in the operating frequency band of a particular PLMN. Further, the eNB may inform the D2D UE of the PLMN frequency band and time resource pool (temporal resource pool) information indicating the time interval of reception. UEs (UE1) belonging to PLMN 1 who wish to transmit D2D signals to UEs (UE2) belonging to PLMN 2 first switch to the frequency band of PLMN 2, detect eNB sync signals, receive system information (SIB or UE-specific control signals) for D2D communication from the corresponding eNB if accessible, and determine to transmit D2D signals in PLMN 1 according to eNB control or UEs if the information obtained from the corresponding eNB indicates that their UEs are configured to receive from PLMN 1. Otherwise (i.e., without information obtained from the corresponding eNB indicating that their UE is configured to receive from PLMN 1 or information obtained from the corresponding eNB indicating that their UE is configured not to receive from PLMN 1), UE1 switches to PLMN 2, connects to the corresponding eNB, and determines to transmit a D2D signal according to eNB control or the UE. In embodiments of the present disclosure, the PLMN reception control indication transmitted via the SIB may also be transmitted by the UE relaying the SIB from the eNB over a synchronization channel (PD2 DSCH).

Control of transmission periodicity and resource selection for type 1 discovery

The basic procedure of the D2D communication operation is also the same as that of the first embodiment of the present disclosure. In an environment where performance is not guaranteed according to the above-described 3 determination conditions, the eNB may configure a period value and a transmission probability to the UE, but it is almost impossible to control the UE adaptively to the situation of the UE in the case where the UE has a large coverage area. In the above embodiments of the present disclosure, the initial transmission period or transmission probability is determined under the control of the eNB and may be changed by the UE according to environmental situations. In an embodiment of the present disclosure, the eNB may transmit the minimum and maximum values of the transmission period or probability, and the UE may change the values depending on environmental situations in a predetermined range configured by the eNB.

Rollback operation

When the eNB synchronization signal is not detected while T310 is operating, the UE may receive a D2DSS from a terminal belonging to the serving eNB to establish a reference time. The UE may initiate a handover or reselection procedure if the UE fails to receive synchronization signals from both the eNB and the terminal. The UE establishes a reference time based on the eNB or D2D synchronization signal, switches to mode 2, and communicates through preset fallback resources. To reduce overhead caused by notifying fallback resources via a separate resource pool, the eNB may notify some of the resource allocation patterns in mode 1 or mode 2 as fallback resources in advance via a broadcast channel or a UE-specific control channel.

While the disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims and their equivalents.

Industrial applicability

The present disclosure is applicable to D2D wireless communication systems.

Claims (12)

1. A method for a first user equipment, UE, to establish synchronization with a second user equipment, UE, in a network supporting device-to-device, D2D, communications, the method comprising:

identifying whether the first UE is located within a coverage of a base station based on a first synchronization signal;

measuring power of a signal received from the base station if the first UE is within coverage of the base station and when the first UE in a radio resource control, RRC, idle state requires D2D communication;

transmitting a second synchronization signal to the second UE in at least one subframe for D2D synchronization signal transmission if the received signal power is less than a first threshold; and

transmitting D2D data for D2D communication to the second UE over a subframe.

2. The method of claim 1, further comprising:

identifying whether the first synchronization signal is from at least one UE if the first UE is located outside the coverage of the base station; and

transmitting the second synchronization signal in at least one subframe for D2D synchronization signal transmission if the first synchronization signal is not received from any terminal.

3. The method of claim 1, further comprising:

measuring power of a signal received from at least one third UE if the first UE is located outside the coverage of the base station and the first synchronization signal is received from the third UE; and

transmitting the second synchronization signal in at least one subframe for D2D synchronization signal transmission if the received signal power from the third UE is less than a predetermined threshold.

4. The method of claim 3, further comprising:

selecting a UE having a highest priority if the first synchronization signal is received from two or more UEs.

5. The method of claim 4, wherein each of the synchronization signals received from the two or more UEs comprises information identifying in-coverage or out-of-coverage.

6. The method of claim 1, further comprising:

selecting a subframe for D2D synchronization based on synchronization offset information if a system information block SIB includes the synchronization offset information; and

transmitting the D2D synchronization signal to the second UE through the selected subframe.

7. A user equipment, UE, supporting device-to-device, D2D, communications, the UE comprising:

a radio processing unit configured to transmit to and receive from a base station and other UEs;

a memory configured to store control data; and

a control unit configured to control the radio processing unit,

wherein the control unit is further configured to control:

identifying whether the UE is located within a coverage of the base station based on a first synchronization signal,

measuring a power of a signal received from the base station if the UE is located within a coverage of the base station and when the UE in a radio resource control, RRC, idle state requires D2D communication, transmitting a second synchronization signal to the other UEs in at least one subframe for D2D synchronization signal transmission if the power of the received signal is less than a threshold, and

transmitting D2D data to the other UEs over subframes used for D2D communication.

8. The UE of claim 7, wherein the control unit is further configured to:

identifying whether the first synchronization signal is received from at least one UE if the UE is located outside the coverage of the base station, and

transmitting the second synchronization signal in at least one subframe for D2D synchronization signal transmission if the first synchronization signal is not received from any terminal.

9. The UE of claim 7, wherein the control unit is further configured to:

if the UE is located outside the coverage of the base station and the first synchronization signal is received from at least one third UE, measuring the power of the signal received from the third UE, and

transmitting the second synchronization signal in at least one subframe for D2D signaling if the power of the signal received from the third UE is less than a predetermined threshold.

10. The UE of claim 8, wherein the control unit is further configured to: selecting a UE having a highest priority if the first synchronization signal is received from two or more UEs.

11. The UE of claim 10, wherein each of the first synchronization signals received from the two or more UEs includes information identifying within-coverage or outside-coverage.

12. The UE of claim 7, wherein the control unit is further configured to:

selecting a subframe for D2D synchronization based on synchronization offset information if a system information block SIB includes the synchronization offset information; and transmitting the D2D synchronization signal to the other UEs through the selected subframe.

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