KR20170035328A - A method for selecting a relay ue in a wireless communication system and a ue using the same - Google Patents

Abstract

A method of selecting relay user equipment (UE) performed by UE in a wireless communication system, and UE using the method are provided. The method includes determining whether a measurement of current relay UE is smaller than a threshold and selecting candidate relay UE satisfying a specific condition as relay UE when the measurement of the current relay UE is smaller than the threshold. Here, the candidate relay UE is selected as relay UE only when a measurement of the candidate relay UE is greater than a minimum hysteresis value (minHyst) based on the threshold.

Description

TECHNICAL FIELD [0001] The present invention relates to a method of selecting a relay UE performed by a terminal in a wireless communication system and a terminal using the method.

The present invention relates to wireless communication, and more particularly, to a relay terminal selection method performed by a terminal in a wireless communication system and a terminal using the relay terminal selection method.

In the International Telecommunication Union Radio Communication Sector (ITU-R), standardization of International Mobile Telecommunication (IMT) -Advanced, a next generation mobile communication system after 3rd generation, is under way. IMT-Advanced aims to support IP (Internet Protocol) based multimedia service at data rates of 1Gbps in a stationary and low-speed moving state and 100Mbps in a high-speed moving state.

The 3rd Generation Partnership Project (3GPP) is a system standard that meets the requirements of IMT-Advanced. It is based on Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA) / Single Carrier-Frequency Division Multiple Access (SC- LTE-Advanced (LTE-A) is being prepared. LTE-A is one of the strong candidates for IMT-Advanced.

Recently, there is a growing interest in D2D (Device-to-Device) technology for direct communication between devices. In particular, D2D is attracting attention as a communication technology for the public safety network. Commercial telecommunication networks are rapidly changing to LTE, but current public safety networks are mainly based on 2G technology in terms of conflicts with existing telecommunications standards and cost. These technological gaps and demands for improved services have led to efforts to improve public safety networks.

Public safety networks have higher service requirements (reliability and security) than commercial communication networks, and require direct signal transmission and reception, or D2D operation, between devices, particularly when the coverage of cellular communication is insufficient or unavailable .

The D2D operation is called ProSe (Proximity Service) operation in that it transmits and receives signals between adjacent devices, and may have various advantages. For example, the D2D terminal has high data rate and low delay and is capable of data communication. In addition, the D2D operation can distribute traffic to the base station, and can expand the coverage of the base station if the D2D terminal functions as a repeater.

The D2D terminal may also function as a terminal acting as a repeater for connecting a sidelink and a cellular link. That is, the D2D terminal can operate as a relay terminal.

The relay terminal can serve as a repeater between a specific terminal and a network, and a specific terminal can be referred to as a remote terminal at this time. The remote terminal can select the relay terminal, and it is necessary to specify how to select the relay terminal.

Particularly, when the remote terminal selects the relay terminal by using only the process of comparing the measured value of the link between the remote terminal and the candidate relay terminal to the predetermined threshold value, the network can not intervene in the relay terminal selection process Not much. In other words, it will be difficult for the network to control the relay terminal selection process of the remote terminal.

SUMMARY OF THE INVENTION The present invention provides a relay terminal selection method performed by a terminal in a wireless communication system and a terminal using the relay terminal selection method.

 In one aspect, there is provided a method of selecting a relay UE performed by a remote UE in a wireless communication system. The method includes determining whether a measured value for a current relay UE is less than a threshold value, and if the measured value for the current relay terminal is less than a threshold value, selecting a candidate relay terminal satisfying a specific condition as a relay terminal And selects the candidate relay terminal as the relay terminal only when the measured value for the candidate relay terminal is larger than the minimum history value minHyst based on the threshold value.

If the upper layer of the remote terminal instructs the relay terminal to reselect, the candidate relay terminal can be selected as the relay terminal only when the measured value for the candidate relay terminal is larger than the minimum history value (minHyst) have.

The measured value for the current relay UE may be a value obtained by measuring a reference signal received power (RSRP) of the reference signal received from the current relay terminal.

The candidate relay terminal may not be selected as the relay terminal when the measured value for the candidate relay terminal is larger than the threshold value and equal to or less than the sum of the threshold value and the minimum history value minHyst.

The threshold may be provided by the network.

The relay terminal may be a device for providing a relay service between the remote terminal and the network.

A terminal provided in another aspect includes: a radio frequency (RF) unit for transmitting and receiving a radio signal; and a processor operating in conjunction with the RF unit, wherein the processor measures a current relay UE And if the measured value for the current relay terminal is less than the threshold value, the candidate relay terminal satisfying the specified condition is selected as the relay terminal. In this case, the candidate relay terminal is selected as the relay terminal only when the measured value for the candidate relay terminal is larger than the minimum history value (minHyst) based on the threshold value.

According to the present invention, it is possible to prevent unnecessary change / re-selection of the relay terminal despite the absence of a large difference in channel quality. Also, the necessity of the relay terminal reselection may be different for each remote terminal, and the relay terminal reselection process can be controlled in consideration of this.

1 shows a wireless communication system to which the present invention is applied.
2 is a block diagram illustrating a radio protocol architecture for a user plane.
3 is a block diagram illustrating a wireless protocol structure for a control plane.
4 is a flowchart showing the operation of the UE in the RRC idle state.
5 is a flowchart illustrating a process of establishing an RRC connection.
6 is a flowchart showing an RRC connection resetting process.
7 is a diagram showing a procedure of RRC connection re-establishment.
FIG. 8 illustrates sub-state transitions and substates that the UE can have in the RRC_IDLE state.
Figure 9 shows a reference structure for ProSe.
10 shows examples of arrangement of UEs and cell coverage that perform ProSe direct communication.
11 shows a user plane protocol stack for ProSe direct communication.
Figure 12 shows a PC 5 interface for D2D discovery.
13 illustrates a relay terminal.
14 illustrates the relationship between a relay terminal and a remote terminal.
15 illustrates a method of selecting a relay terminal by a remote terminal.
FIG. 16 is a specific example of applying the method of FIG.
FIG. 17 illustrates a method of selecting a relay terminal by using two history values.
18 illustrates a method by which a network transmits bias information.
19 is a block diagram illustrating a terminal in which an embodiment of the present invention is implemented.

1 shows a wireless communication system to which the present invention is applied. This may be referred to as Evolved-UMTS Terrestrial Radio Access Network (E-UTRAN) or Long Term Evolution (LTE) / LTE-A system.

The E-UTRAN includes a base station (BS) 20 that provides a user plane (UE) with a control plane and a user plane. The terminal 10 may be fixed or mobile and may be referred to by other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a mobile terminal (MT) . The base station 20 is a fixed station that communicates with the terminal 10 and may be referred to as another term such as an evolved NodeB (eNB), a base transceiver system (BTS), an access point, or the like.

The base stations 20 may be interconnected via an X2 interface. The base station 20 is connected to an S-GW (Serving Gateway) through an MME (Mobility Management Entity) and an S1-U through an EPC (Evolved Packet Core) 30, more specifically, an S1-MME through an S1 interface.

The EPC 30 is composed of an MME, an S-GW, and a P-GW (Packet Data Network-Gateway). The MME has information on the access information of the terminal or the capability of the terminal, and this information is mainly used for managing the mobility of the terminal. The S-GW is a gateway having an E-UTRAN as an end point, and the P-GW is a gateway having a PDN as an end point.

The layers of the radio interface protocol between the UE and the network are classified into L1 (first layer), L1 (second layer), and the like based on the lower three layers of the Open System Interconnection (OSI) A physical layer belonging to a first layer provides an information transfer service using a physical channel, and a physical layer (physical layer) An RRC (Radio Resource Control) layer located at Layer 3 controls the radio resources between the UE and the network. To this end, the RRC layer exchanges RRC messages between the UE and the BS.

2 is a block diagram illustrating a radio protocol architecture for a user plane. 3 is a block diagram illustrating a wireless protocol structure for a control plane. The user plane is a protocol stack for transmitting user data, and the control plane is a protocol stack for transmitting control signals.

Referring to FIGS. 2 and 3, a physical layer (PHY) provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to a MAC (Medium Access Control) layer, which is an upper layer, through a transport channel. Data is transferred between the MAC layer and the physical layer through the transport channel. The transport channel is classified according to how the data is transmitted through the air interface.

Data moves between different physical layers, i. E., Between the transmitter and the physical layer of the receiver, over the physical channel. The physical channel can be modulated by an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and uses time and frequency as radio resources.

The function of the MAC layer includes a mapping between a logical channel and a transport channel and a multiplexing / demultiplexing into a transport block provided as a physical channel on a transport channel of a MAC SDU (service data unit) belonging to a logical channel. The MAC layer provides a service to a Radio Link Control (RLC) layer through a logical channel.

The function of the RLC layer includes concatenation, segmentation and reassembly of the RLC SDUs. The RLC layer includes a Transparent Mode (TM), an Unacknowledged Mode (UM), and an Acknowledged Mode (RB) in order to guarantee various QoSs required by a radio bearer (RB) , And AM). AM RLC provides error correction via automatic repeat request (ARQ).

The Radio Resource Control (RRC) layer is defined only in the control plane. The RRC layer is responsible for the control of logical channels, transport channels and physical channels in connection with the configuration, re-configuration and release of radio bearers. RB means a logical path provided by a first layer (PHY layer) and a second layer (MAC layer, RLC layer, PDCP layer) for data transmission between a UE and a network.

The functions of the Packet Data Convergence Protocol (PDCP) layer in the user plane include transmission of user data, header compression and ciphering. The functions of the Packet Data Convergence Protocol (PDCP) layer in the control plane include transmission of control plane data and encryption / integrity protection.

The setting of the RB means a process of defining characteristics of a radio protocol layer and a channel to provide a specific service, and setting each specific parameter and an operation method. RB can be divided into SRB (Signaling RB) and DRB (Data RB). The SRB is used as a path for transmitting the RRC message in the control plane, and the DRB is used as a path for transmitting the user data in the user plane.

When an RRC connection is established between the RRC layer of the UE and the RRC layer of the E-UTRAN, the UE is in the RRC connected state, and if not, the UE is in the RRC idle state.

The downlink transmission channel for transmitting data from the network to the terminal includes a BCH (Broadcast Channel) for transmitting system information and a downlink SCH (Shared Channel) for transmitting user traffic or control messages. In case of a traffic or control message of a downlink multicast or broadcast service, it may be transmitted through a downlink SCH, or may be transmitted via a separate downlink MCH (Multicast Channel). Meanwhile, the uplink transmission channel for transmitting data from the UE to the network includes a random access channel (RACH) for transmitting an initial control message and an uplink SCH (Shared Channel) for transmitting user traffic or control messages.

A logical channel mapped to a transport channel is a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), a multicast traffic Channel).

A physical channel is composed of several OFDM symbols in the time domain and a plurality of sub-carriers in the frequency domain. One sub-frame is composed of a plurality of OFDM symbols in the time domain. A resource block is a resource allocation unit, and is composed of a plurality of OFDM symbols and a plurality of sub-carriers. In addition, each subframe can use specific subcarriers of specific OFDM symbols (e.g., first OFDM symbol) of a corresponding subframe for a physical downlink control channel (PDCCH), i.e., an L1 / L2 control channel. The TTI (Transmission Time Interval) is the unit time of the subframe transmission.

Hereinafter, the RRC state (RRC state) and the RRC connection method of the UE will be described in detail.

The RRC state refers to whether or not the RRC layer of the UE is in logical connection with the RRC layer of the E-UTRAN. If the RRC layer is connected, the RRC connection state (RRC_CONNECTED) RRC_IDLE). Since the RRC-connected terminal has an RRC connection, the E-UTRAN can grasp the existence of the corresponding terminal on a cell-by-cell basis, thereby effectively controlling the terminal. On the other hand, the terminal in the RRC idle state can not be grasped by the E-UTRAN, and is managed by the CN (core network) in units of a tracking area (tracking area) larger than the cell. That is, the UEs in the RRC idle state are only detected on the basis of a large area, and must move to the RRC connection state in order to receive normal mobile communication services such as voice and data.

When the user turns on the terminal for the first time, the terminal searches for the appropriate cell first and then stays in the RRC idle state in the corresponding cell. The UE in the RRC idle state establishes the RRC connection with the E-UTRAN through the RRC connection procedure when the UE needs to establish the RRC connection, and transitions to the RRC connection state. There are a number of cases where the UE in the RRC idle state needs to make an RRC connection. For example, if uplink data transmission is required due to a user's call attempt or the like, or a paging message is received from the E-UTRAN And transmission of a response message to the received message.

The non-access stratum (NAS) layer located at the top of the RRC layer performs functions such as session management and mobility management.

In order to manage the mobility of the terminal in the NAS layer, two states of EMM-REGISTERED (EPS Mobility Management-REGISTERED) and EMM-DEREGISTERED are defined, and these two states are applied to the terminal and the MME. The initial terminal is in the EMM-DEREGISTERED state, and the terminal performs a process of registering with the network through an initial attach procedure to access the network. When the Attach procedure is successfully performed, the UE and the MME enter the EMM-REGISTERED state.

In order to manage the signaling connection between the terminal and the EPC, two states of ECM (EPS Connection Management) -IDLE state and ECM-CONNECTED state are defined, and these states are applied to the terminal and the MME. When the UE in the ECM-IDLE state establishes the RRC connection with the E-UTRAN, the UE enters the ECM-CONNECTED state. The MME in the ECM-IDLE state enters the ECM-CONNECTED state when it makes an S1 connection with the E-UTRAN. When the UE is in the ECM-IDLE state, the E-UTRAN does not have context information of the UE. Therefore, the terminal in the ECM-IDLE state performs terminal-based mobility-related procedures such as cell selection or cell reselection without receiving commands from the network. On the other hand, when the terminal is in the ECM-CONNECTED state, the mobility of the terminal is managed by the command of the network. If the location of the terminal differs from the location known by the network in the ECM-IDLE state, the terminal informs the network of the corresponding location of the terminal through a tracking area update procedure.

The following describes the system information.

The system information includes essential information that the terminal needs to know in order to access the base station. Therefore, the terminal must receive all the system information before connecting to the base station, and always have the latest system information. Since the system information is information that must be known by all terminals in a cell, the base station periodically transmits the system information. System information is divided into MIB (Master Information Block) and SIB (System Information Block).

The MIB may contain a limited number of parameters that are most essential and most frequently transmitted that are required to be obtained for other information from the cell. The terminal searches for the MIB first after downlink synchronization. The MIB may include information such as downlink channel bandwidth, PHICH setting, SFN that supports synchronization and operates as a timing reference, and eNB transmit antenna settings. The MIB may be broadcast transmitted over a broadcase channel (BCH).

Among the included SIBs, SIB1 (SystemInformationBlockType1) is included in the message "SystemInformationBlockType1 ", and other SIBs other than SIB1 are included in the system information message and transmitted. The mapping of the SIBs to the system information message can be flexibly set by the scheduling information list parameter included in SIB1. However, each SIB is included in a single system information message, and only SIBs with the same scheduling requirement (e.g., cycle) can be mapped to the same system information message. Also, SIB2 (SystemInformationBlockType2) is always mapped to a system information message corresponding to the first entry in the system information message list of the scheduling information list. A plurality of system information messages can be transmitted in the same period. SIB1 and all system information messages are transmitted on the DL-SCH.

In addition to the broadcast transmission, the E-UTRAN can be dedicated signaled with SIB1 including parameters set equal to the previously set value, in which case SIB1 can be included in the RRC connection reset message and transmitted.

SIB1 includes information related to UE cell access and defines the scheduling of other SIBs. The SIB1 includes PLMN identifiers of the network, Tracking Area Code (TAC) and cell ID, a cell barring status indicating whether the cell is a cell capable of camping on, The lowest receive level, and the transmission time and period of other SIBs.

The SIB2 may include radio resource setting information common to all terminals. SIB2 includes an uplink carrier frequency and an uplink channel bandwidth, a RACH setting, a paging configuration, a normal link power control setting, a sounding reference signal configuration, a PUCCH setting supporting ACK / NACK transmission, And may include information related to the PUSCH setting.

The UE can apply the system information acquisition and change detection procedure only to the primary cell (PCell). In a secondary cell (SCell), the E-UTRAN can provide all system information related to the RRC connection state operation through dedicated signaling when the corresponding SCell is added. Upon a change of the relevant system information of the established SCell, the E-UTRAN can release and add the SCell under consideration, which can be performed with a single RRC connection reset message. The E-UTRAN may set the parameter values that are broadcast in the considered SCell and other parameter values through dedicated signaling.

The terminal must guarantee the validity of a specific type of system information, and such system information is called required system information. Required system information can be defined as follows.

- If the terminal is in RRC idle state: The terminal shall ensure that it has a valid version of MIB and SIB1 as well as SIB2 to SIB8, which may be subject to the support of the radio access technology (RAT) considered.

- If the terminal is in RRC connection state: The terminal shall ensure that it has a valid version of MIB, SIB1 and SIB2.

In general, system information can be validated up to 3 hours after acquisition.

Generally, the service provided by the network to the terminal can be classified into the following three types. Also, the terminal recognizes the type of the cell differently depending on what service can be provided. In the following, the service type is first described, and the type of the following cell is described.

1) Limited service: This service provides Emergency call and Earthquake and Tsunami Warning System (ETWS) and can be provided in an acceptable cell.

2) Normal service: This service is a general purpose general service, and can be provided in a regular cell.

3) Operator service: This service refers to a service for a network operator. This cell can only be used by a network operator and can not be used by a general user.

With respect to the service type provided by the cell, the type of the cell can be divided as follows.

1) Acceptable cell: A cell in which a terminal can receive a limited service. This cell is not barred for the terminal, but is a cell that satisfies the cell selection criterion of the terminal.

2) Suitable cell: A cell where the terminal can receive regular service. This cell satisfies the conditions of the acceptable cell and satisfies the additional conditions at the same time. As an additional condition, this cell must be a PLMN (Public Land Mobile Network) belonging to the terminal, and the cell should not be prohibited from performing the tracking area update procedure of the terminal. If the corresponding cell is a CSG cell, the terminal must be a cell capable of connecting to this cell as a CSG member.

3) Barred cell: It is a cell that broadcasts information that a cell is prohibited through system information.

4) Reserved cell: It is a cell that broadcasts information that a cell is a reserved cell through system information.

4 is a flowchart showing the operation of the UE in the RRC idle state. FIG. 4 shows a procedure in which a terminal with an initial power on is registered in a network via a cell selection process, and then, when necessary, performs cell reselection.

Referring to FIG. 4, the MS selects a radio access technology (RAT) for communicating with a public land mobile network (PLMN), which is a network to which the MS desires to receive services (S410). Information on the PLMN and the RAT may be selected by a user of the UE or may be stored in a universal subscriber identity module (USIM).

In step S420, the UE selects the cell having the largest value among the cells whose measured signal strength or quality is greater than a specific value (Cell Selection). This may be referred to as an initial cell selection in which a terminal that is powered on performs cell selection. The cell selection procedure will be described later in detail. After the cell selection, the terminal receives the system information periodically transmitted by the base station. The above-mentioned specific value refers to a value defined in the system in order to guarantee the quality of a physical signal in data transmission / reception. Therefore, the value may vary depending on the applied RAT.

If the terminal needs to register the network, the terminal performs a network registration procedure (S430). The terminal registers its information (eg, IMSI) to receive a service (eg, Paging) from the network. The terminal does not register in the network to be connected every time the cell is selected. When the information of the network (e.g., Tracking Area Identity) (TAI) received from the system information is different from the information of the network it knows, .

The UE performs cell re-selection based on the service environment provided in the cell or the environment of the UE (S440). If the strength or quality of a signal measured from a serving base station (serving base station) is lower than a value measured from a base station of an adjacent cell, the terminal provides better signal characteristics than the cell of the base station to which the terminal is currently connected Select one of the other cells. This process is called cell re-selection by distinguishing the initial cell selection from the initial cell selection in step 2. At this time, a time constraint is set in order to prevent the cell from being reselected frequently according to the change of the signal characteristics. The cell reselection procedure will be described in detail later.

5 is a flowchart illustrating a process of establishing an RRC connection.

The MS sends an RRC Connection Request message to the network requesting an RRC connection (S510). The network sends an RRC Connection Setup message in response to the RRC connection request (S520). After receiving the RRC connection setup message, the UE enters the RRC connection mode.

The MS sends an RRC Connection Setup Complete message to the network to confirm successful completion of RRC connection establishment in operation S530.

6 is a flowchart showing an RRC connection resetting process. RRC connection reconfiguration is used to modify the RRC connection. This is used to establish / modify / release RB, perform handover, and setup / modify / release measurements.

The network sends an RRC Connection Reconfiguration message for modifying the RRC connection to the terminal (S610). In response to the RRC connection re-establishment, the MS sends an RRC Connection Reconfiguration Complete message to the network (S620), which is used to confirm the successful completion of the RRC connection re-establishment.

A public land mobile network (PLMN) will be described below.

A PLMN is a network deployed and operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified as a Mobile Country Code (MCC) and a Mobile Network Code (MNC). The PLMN information of the cell is included in the system information and broadcasted.

In PLMN selection, cell selection and cell reselection, various types of PLMNs may be considered by the terminal.

HPLMN (Home PLMN): PLMN having MCC and MNC matching with MCC and MNC of terminal IMSI.

EHPLMN (Equivalent HPLMN): A PLMN that is treated as equivalent to HPLMN.

RPLMN (Registered PLMN): The PLMN where the location registration was successfully completed.

EPLMN (Equivalent PLMN): A PLMN that is treated as equivalent to RPLMN.

Each mobile service consumer subscribes to HPLMN. When a general service is provided to a terminal by HPLMN or EHPLMN, the terminal is not in a roaming state. On the other hand, when a service is provided to a terminal by a PLMN other than HPLMN / EHPLMN, the terminal is in a roaming state, and the PLMN is called VPLMN (Visited PLMN).

The terminal initially searches for available public land mobile networks (PLMNs) when it is powered on and selects the appropriate PLMNs to receive services. A PLMN is a network deployed and operated by a mobile network operator. Each mobile network operator runs one or more PLMNs. Each PLMN may be identified by a mobile country code (MCC) and a mobile network code (MNC). The PLMN information of the cell is included in the system information and broadcasted. The terminal attempts to register the selected PLMN. If registration is successful, the selected PLMN becomes a registered PLMN (RPLMN). The network may signal the PLMN list to the UE, which may consider PLMNs included in the PLMN list as PLMNs such as RPLMN. A terminal registered in the network must be reachable by the always-on network. If the terminal is in the ECM-CONNECTED state (or RRC connection state equally), the network recognizes that the terminal is receiving service. However, if the terminal is in the ECM-IDLE state (or the RRC idle state), the state of the terminal is not valid in the eNB but is stored in the MME. In this case, the location of the UE in the ECM-IDLE state is only known to the MME with the granularity of the list of TA (tracking Area). A single TA is identified by a tracking area identity (TAI) consisting of the PLMN identifier to which the TA belongs and a tracking area code (TAC) uniquely representing the TA within the PLMN.

Then, among the cells provided by the selected PLMN, a cell having a signal quality and characteristics such that the UE can receive an appropriate service is selected.

Next, in the prior art, a procedure for selecting a cell by the terminal will be described in detail.

When the power is turned on or remains in the cell, the terminal performs procedures for selecting and reselecting an appropriate quality cell to receive the service.

The terminal in the RRC idle state should always select a cell of appropriate quality and prepare to receive the service through this cell. For example, a powered down terminal must select a cell of the appropriate quality to register with the network. When the UE in the RRC connection state enters the RRC idle state, the UE must select a cell in the RRC idle state. In this manner, a process of selecting a cell satisfying a certain condition in order to stay in a service waiting state such as the RRC idle state is called a cell selection. It is important to select the cell as quickly as possible because the cell selection is performed in a state in which the UE does not currently determine a cell to remain in the RRC idle state. Therefore, even if the cell provides the best radio signal quality to the UE, it can be selected in the cell selection process of the UE.

Now, with reference to 3GPP TS 36.304 V8.5.0 (2009-03) "User Equipment (UE) procedures in idle mode (Release 8)", a method and a procedure for a UE to select a cell in 3GPP LTE will be described in detail.

The cell selection process is roughly classified into two types.

First, an initial cell selection process is performed. In this process, the UE does not have prior information on a wireless channel. Therefore, the terminal searches all wireless channels to find an appropriate cell. In each channel, the terminal finds the strongest cell. Thereafter, when the terminal finds a suitable cell satisfying the cell selection criterion, it selects the corresponding cell.

Next, the terminal can utilize the stored information or select the cell using the information broadcast in the cell. Therefore, the cell selection can be quicker than the initial cell selection process. If the terminal finds a cell satisfying the cell selection criterion, the cell is selected. If an appropriate cell satisfying the cell selection criterion is not found through this process, the UE performs an initial cell selection process.

The cell selection criterion can be defined as follows.

[Formula 1]

Here, each variable of the above Equation 1 can be defined as shown in Table 1 below.

[Table 1]

The signaled values Q _{rxlevminoffset} and Q _{qualminoffset} may be applied only if the cell selection is evaluated as a result of a periodic search for a higher priority PLMN while the UE is camping in the regular cell in the VPLMN. During a periodic search for a higher priority PLMN as described above, the terminal may perform cell selection evaluation using stored parameter values from other cells of such higher priority PLMN.

Once the UE has selected a cell through the cell selection process, the strength or quality of the signal between the UE and the BS may be changed due to mobility of the UE or change of the radio environment. Thus, if the quality of the selected cell degrades, the terminal may select another cell that provides better quality. When the cell is reselected in this way, a cell is selected that generally provides better signal quality than the currently selected cell. This process is called cell reselection. The cell reselection process is basically aimed at selecting a cell that provides the best quality to the UE in terms of the quality of the radio signal.

In addition to the quality of the radio signal, the network can determine the priority for each frequency and inform the terminal of the priority. The MS receiving the priority order takes priority over the radio signal quality reference in the cell reselection process.

In order to select a cell for reselection in the cell reselection, there is a method of selecting or reselecting a cell according to the signal characteristics of the wireless environment, There may be a choice.

- Intra-frequency cell reselection: reselects cells with the same center-frequency as the RAT such as the cell in which the terminal is camping (camping)

Inter-frequency cell reselection: reselects a cell with a center frequency different from the RAT of the cell the terminal is camping on

- Inter-RAT (Inter-RAT) cell reselection: Reselect a cell that uses a different RAT than the RAT the terminal is camping on

The principle of the cell reselection process is as follows

First, the UE measures the quality of a serving cell and a neighboring cell for cell reselection.

Second, cell reselection is performed based on cell reselection criteria. The cell reselection criterion has the following characteristics with respect to the serving cell and the neighbor cell measurement.

Intra-frequency cell reselection is basically based on ranking. Ranking is the task of defining the index values for the cell reselection evaluation and ordering the cells by the index value size using the index values. A cell with the best indicator is often called the highest ranked cell. The cell index value is a value obtained by applying a frequency offset or a cell offset as necessary based on a value measured by the terminal for the corresponding cell.

Inter-frequency cell reselection is based on the frequency priorities provided by the network. The terminal attempts to camp on the frequency with the highest frequency priority (hereinafter referred to as camp on). The network may provide frequency priority for each UE through signaling (dedicated signaling), or may provide common frequency priority or frequency priority to UEs in a cell through broadcast signaling. The cell reselection priority provided through broadcast signaling may be referred to as a common priority, and the cell reselection priority set by the network for each terminal may be referred to as a dedicated priority. When the terminal receives the dedicated priority, it can receive the validity time associated with the dedicated priority. Upon receiving the dedicated priority, the UE starts a validity timer set to the validity time received together. The terminal applies the dedicated priority in the RRC idle mode while the validity timer is operating. When the validity timer expires, the terminal discards the dedicated priority and applies the public priority again.

For inter-frequency cell reselection, the network may provide the terminal with parameters (e.g., frequency-specific offset) used for cell reselection on a frequency-by-frequency basis.

For intra-frequency cell reselection or inter-frequency cell reselection, the network may provide the terminal with a Neighboring Cell List (NCL) used for cell reselection. This NCL includes cell-specific parameters (e.g., cell-specific offsets) used for cell reselection

For intra-frequency or inter-frequency cell reselection, the network may provide the terminal with a cell blacklist, which is used for cell reselection. The UE does not perform cell reselection for cells included in the forbidden list.

Next, the ranking performed in the cell reselection evaluation process will be described.

The ranking criterion used to prioritize the cell is defined as:

[Formula 2]

R _s = Q _{meas, s} + Q _hyst , R _n = Q _{meas, n} - Q _offset

Here, R _s is the UE is currently camping and have ranking index of the serving cell, R _n is a ranking index of the neighboring cell, Q _{meas, s} is a terminal is measured for the serving cell quality value, Q _{meas, n} is the terminal Q _hyst is the hysteresis value for the ranking, and Q _offset is the offset between the two cells.

At the intra-frequency, Q _offset = Q _{offsets, n} when the UE receives the offset (Q _{offsets, n} ) between the serving cell and the neighboring cell _, and Q _offset = 0 if the UE has not received Q _{offsets, n} .

Inter-case in the frequency, the UE and the case of receiving an offset (Q _{offsets, n)} for that cell Q _offset = Q _{offsets, n} + Q _frequency, the UE does not receive the Q _{offsets, n} Q _offset = Q _frequency to be.

If the ranking indicator R _s of the serving cell and the ranking indicator R _n of the neighboring cell fluctuate in a similar state, the ranking of the fluctuation result ranking is reversed so that the UE can reselect the cells alternately. Q _hyst is hysteresis in cell reselection and is a parameter to prevent the UE from alternating between two cells.

The UE measures R _s of the serving cell and R _n of the neighboring cell according to the above equation, and regards the cell having the highest ranking index value as the highest ranked cell and reselects this cell.

According to the above criterion, it can be confirmed that the quality of the cell is the most important criterion in cell reselection. If the reselected cell is not a suitable cell, the terminal excludes the corresponding frequency or the corresponding cell from the cell reselection target.

The radio link failure will now be described.

The UE continuously measures the quality of the radio link with the serving cell receiving the service. The terminal determines whether communication is impossible in the current situation due to deterioration of the quality of the radio link with the serving cell. If the quality of the serving cell is so low that communication is almost impossible, the terminal determines the current situation as a failure of the wireless connection.

If the radio link failure is determined, the UE abandons maintenance of communication with the current serving cell, selects a new cell through a cell selection (or cell reselection) procedure, re-establishes an RRC connection to the new cell -establishment.

The 3GPP LTE specification does not allow normal communication.

- If the terminal determines that there is a serious problem with the downlink communication link quality based on the measurement result of the radio quality of the physical layer of the terminal (when it determines that the quality of the PCell is low during RLM)

- The MAC sublayer has determined that there is a problem with the uplink transmission due to the continuous failure of the random access procedure.

- In case that the uplink data transmission in the RLC sublayer continues to fail and there is a problem in uplink transmission.

- When handover is judged to have failed.

- The message received by the terminal does not pass the integrity check.

Hereinafter, the RRC connection re-establishment procedure will be described in more detail.

7 is a diagram showing a procedure of RRC connection re-establishment.

Referring to FIG. 7, in step S710, the UE stops using all radio bearers except for Signaling Radio Bearer # 0 (SRB0) and initializes various sub layers of an Access Stratum (AS). In addition, each of the sub-layers and the physical layer is set as a default configuration. During this process, the terminal maintains the RRC connection state.

The MS performs a cell selection procedure to perform the RRC connection re-establishment procedure (S720). During the RRC connection re-establishment procedure, the cell selection procedure may be performed in the same manner as the cell selection procedure performed by the UE in the RRC idle state even though the UE remains in the RRC connection state.

After performing the cell selection procedure, the UE checks system information of the corresponding cell and determines whether the corresponding cell is a suitable cell (S730). If it is determined that the selected cell is an appropriate E-UTRAN cell, the UE transmits an RRC connection reestablishment request message to the cell (S740).

On the other hand, if it is determined through the cell selection procedure for performing the RRC connection re-establishment procedure that the selected cell is a cell using another RAT other than E-UTRAN, the RRC connection re-establishment procedure is stopped, and the UE enters the RRC idle state (S750).

The UE can be configured to complete the cell validation within a limited time through the cell selection procedure and the system information reception of the selected cell. To do this, the UE can start the timer by starting the RRC connection re-establishment procedure. The timer can be stopped if it is determined that the terminal has selected a suitable cell. If the timer expires, the terminal may assume that the RRC connection re-establishment procedure has failed and enter the RRC idle state. This timer is hereinafter referred to as a radio link failure timer. LTE Specification In TS 36.331, a timer named T311 can be used as a radio link failure timer. The UE can acquire the set value of this timer from the system information of the serving cell.

Upon receiving the RRC connection re-establishment request message from the terminal and accepting the request, the cell transmits an RRC connection reestablishment message to the terminal.

Upon receiving the RRC connection re-establishment message from the cell, the UE reconfigures the PDCP sublayer and the RLC sublayer for SRB1. Also, various key values related to the security setting are recalculated, and the PDCP sublayer responsible for security is reconfigured with newly calculated security key values. Accordingly, the UE-cell SRB 1 is opened and the RRC control message can be exchanged. The MS completes the resumption of the SRB 1 and transmits an RRC connection reestablishment complete message indicating that the RRC connection re-establishment procedure to the cell has been completed (S760).

On the other hand, if the UE receives the RRC connection re-establishment request message and does not accept the request, the cell transmits an RRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfully performed, the cell and the terminal perform the RRC connection re-establishment procedure. Through this, the UE recovers the state before performing the RRC connection re-establishment procedure, and ensures the continuity of the service to the maximum.

FIG. 8 illustrates sub-state transitions and substates that the UE can have in the RRC_IDLE state.

Referring to FIG. 8, the UE performs an initial cell selection process (S801). The initial cell selection process can be performed when there is no stored cell information for the PLMN or when a suitable cell is not found.

If the regular cell can not be found in the initial cell selection process, the state transitions to the arbitrary cell selection state (S802). The arbitrary cell selection state is a state in which the terminal can not camp on an acceptable cell in the normal cell and attempts to find an acceptable cell of any PLMN capable of camping. If the UE does not find any cells that can camp, the UE remains in a random cell selection state until it finds an acceptable cell.

If the regular cell is found in the initial cell selection process, the state transits to the regular camp state (S803). The regular camp state refers to a state where a camp is camped on a regular cell, and a paging channel can be selected and monitored according to given information through the system information, and an evaluation process for cell reselection is performed .

If the cell reselection evaluation process S804 is caused in the regular camp state S803, the cell reselection evaluation process S804 is performed. In the cell reselection evaluation process (S804), if a suitable cell is found, the state transits to the regular camp state (S803) again.

If an acceptable cell is found in the arbitrary cell selection state (S802), the state transits to the arbitrary cell camp state (S805). An arbitrary cell camp state is a camp on state of an acceptable cell.

In the arbitrary cell camp state (S805), the terminal can select and monitor the paging channel according to the given information through the system information, and perform the evaluation process (S806) for cell reselection. If an acceptable cell is not found in the evaluation process for the cell reselection (S806), the process transitions to the arbitrary cell selection state (S802).

The D2D operation will now be described. In 3GPP LTE-A, a service related to D2D operation is called Proximity based Services (ProSe). Hereinafter, ProSe is equivalent to D2D operation, and ProSe can be mixed with D2D operation. We will now describe ProSe.

ProSe has ProSe direct communication and ProSe direct discovery. ProSe direct communication refers to communication performed between two or more adjacent terminals. The terminals can perform communication using a user plane protocol. A ProSe-enabled UE (ProSe-enabled UE) refers to a terminal that supports procedures related to ProSe's requirements. Unless otherwise stated, ProSe capable terminals include both public safety UEs and non-public safety UEs. The common safety terminal is a terminal that supports both the common safety-related function and the ProSe process. The non-common safety terminal supports the ProSe process but does not support the common safety-related function.

ProSe direct discovery is a process in which a ProSe capable terminal finds other ProSe capable terminals adjacent to each other, and uses only the capabilities of the two ProSe capable terminals. EPC-level ProSe discovery (EPC-level ProSe discovery) refers to a process in which an EPC determines whether two ProSe capable terminals are close to each other, and informs the two ProSe capable terminals of their proximity to each other.

Hereinafter, for convenience, ProSe direct communication may be referred to as D2D communication, and ProSe direct discovery may be referred to as D2D discovery.

Figure 9 shows a reference structure for ProSe.

Referring to FIG. 9, the reference structure for ProSe includes a plurality of terminals including an E-UTRAN, an EPC, and a ProSe application program, a ProSe APP server, and a ProSe function.

The EPC represents the E-UTRAN core network architecture. The EPC may include an MME, an S-GW, a P-GW, a policy and charging rules function (PCRF), a home subscriber server (HSS)

ProSe Application Server is a user of ProSe capability to create application functions. The ProSe application server can communicate with application programs in the terminal. The application program in the terminal can use the ProSe capability to create the request function.

The ProSe function may include, but is not necessarily limited to, at least one of the following.

Interworking via a reference point towards a third party application (towards a third reference point towards the third party applications)

- Authorization and configuration for the UE for discovery and direct communication.

- The ability of ProSe discovery at the EPC level (Enable the EPC level ProSe discovery)

- ProSe-related new subscriber data and data storage coordination, ProSe ID related adjustment (ProSe related new subscriber data and handling of data storage, and also handling of ProSe identities)

- Security related functionality

- Provide control towards EPC for policy related functions (Provide control towards policy related functionality)

- Provide functionality for charging (via or outside of EPC, eg, offline charging)

In the following, reference points and reference interfaces are described in the reference structure for ProSe.

- PC1: This is the reference point between the ProSe application program in the terminal and the ProSe application program in the ProSe application server. This is used to define the signaling requirements at the application level.

- PC2: It is a reference point between ProSe application server and ProSe function. It is used to define the interaction between the ProSe application server and ProSe functionality. Application data update of the ProSe database of the ProSe function can be an example of the above interaction.

- PC3: It is the reference point between the terminal and the ProSe function. It is used to define the interaction between the terminal and the ProSe function. A setting for ProSe discovery and communication may be an example of such an interaction.

- PC4: It is a reference point between EPC and ProSe functions. It is used to define the interaction between EPC and ProSe functions. The interaction may illustrate when establishing a path for 1: 1 communication between terminals or when authenticating ProSe services for real-time session management or mobility management.

- PC5: It is a reference point for using control / user plane for discovery and communication, relay, and 1: 1 communication between terminals.

- PC6: A reference point for using functions such as ProSe discovery among users belonging to different PLMNs.

- SGi: can be used for application data and application-level control information exchange.

<ProSe direct communication (D2D communication): ProSe Direct Communication>.

ProSe direct communication is a communication mode in which two public safety terminals can communicate directly via the PC 5 interface. This communication mode can be supported both when the UE is serviced within the coverage of the E-UTRAN or when it is out of coverage of the E-UTRAN.

10 shows examples of arrangement of UEs and cell coverage that perform ProSe direct communication.

Referring to FIG. 10 (a), the terminals A and B may be located outside the cell coverage. Referring to FIG. 10 (b), the terminal A may be located within the cell coverage, and the terminal B may be located outside the cell coverage. Referring to FIG. 10 (c), terminals A and B may all be located within a single cell coverage. Referring to FIG. 10 (d), terminal A may be located within the coverage of the first cell and terminal B may be located within the coverage of the second cell.

ProSe direct communication can be performed between terminals at various positions as shown in FIG.

On the other hand, the following IDs can be used for ProSe direct communication.

Source Layer-2 ID: This ID identifies the sender of the packet at the PC 5 interface.

Object Layer-2 ID: This ID identifies the target of the packet at the PC 5 interface.

SA L1 ID: This ID is the ID in the scheduling assignment (SA) at the PC 5 interface.

11 shows a user plane protocol stack for ProSe direct communication.

Referring to FIG. 11, the PC 5 interface includes PDCH, RLC, MAC, and PHY layers.

ProSe direct communication may not have HARQ feedback. The MAC header may include a source layer-2 ID and a destination layer-2 ID.

<Wireless Resource Allocation for ProSe Direct Communication>.

ProSe capable terminals can use the following two modes for resource allocation for ProSe direct communication.

1. Mode 1

Mode 1 is a mode for scheduling resources for ProSe direct communication from a base station. In order to transmit data by mode 1, the terminal must be in RRC_CONNECTED state. The MS requests a transmission resource to the BS, and the BS schedules resources for scheduling assignment and data transmission. The MS can send a scheduling request to the BS and send a ProSe BSR (Buffer Status Report). Based on the ProSe BSR, the BS determines that the UE has data for direct communication with ProSe, and resources for the transmission are required.

2. Mode 2

Mode 2 is a mode in which a terminal directly selects a resource. The terminal selects a resource for direct ProSe direct communication from a resource pool. The resource pool can be set or predefined by the network.

On the other hand, if the UE has a serving cell, that is, if the UE is in the RRC_CONNECTED state or the RRC_IDLE state, the UE is considered to be within the coverage of the BS.

If the terminal is out of coverage, only Mode 2 above can be applied. If the terminal is in coverage, mode 1 or mode 2 may be used, depending on the configuration of the base station.

If there is no other exceptional condition, the terminal can change the mode from mode 1 to mode 2 or from mode 2 to mode 1 only when the base station is set.

<ProSe direct discovery (D2D discovery): ProSe direct discovery>

ProSe direct discovery refers to the procedure used by a ProSe capable terminal to discover other ProSe capable terminals close to it, and may be referred to as D2D direct discovery or D2D discovery. At this time, an E-UTRA radio signal via the PC 5 interface may be used. The information used for ProSe direct discovery is hereinafter referred to as discovery information.

Figure 12 shows a PC 5 interface for D2D discovery.

Referring to FIG. 12, the PC 5 interface includes a MAC layer, a PHY layer, and a ProSe Protocol layer, which is an upper layer. In the upper layer (ProSe Protocol), an authorization for discovery information and monitoring is handled. The content of the discovery information is transparent to the AS (access stratum) )Do. The ProSe Protocol ensures that only valid discovery information is delivered to the AS for the announcement.

The MAC layer receives discovery information from an upper layer (ProSe Protocol). The IP layer is not used for discovery information transmission. The MAC layer determines which resources are used to announce discovery information received from higher layers. The MAC layer creates a MAC PDU (protocol data unit) carrying the discovery information and sends it to the physical layer. The MAC header is not added.

There are two types of resource allocation for discovery information announcement.

1. Type 1

The base station provides the resource pool configuration for discovery information announcement to the terminals in such a way that resources for announcement of discovery information are allocated not to be terminal specific. This setting can be included in the system information block (SIB) and signaled in a broadcast manner. Alternatively, the setting may be provided in the UE-specific RRC message. Alternatively, the setting may be broadcast signaling of a layer other than the RRC message or UE-specific positive signaling.

The terminal selects the resource itself from the indicated resource pool and announces the discovery information using the selected resource. The terminal can annotate discovery information through randomly selected resources during each discovery period.

2. Type 2

And the resources for the announcement of discovery information are allocated in a UE-specific manner. A terminal in RRC_CONNECTED state can request resources for discovery announcement through RRC signal. The base station can allocate resources for discovery signal announcements with the RRC signal. Resources for monitoring the discovery signal can be allocated within the resource pool set for the UEs.

For a terminal in the RRC_IDLE state, the base station may 1) inform the SIB of a type 1 resource pool for discovery information announcement. The terminals that are allowed to directly discover ProSe use a type 1 resource pool for discovery information announcement in RRC_IDLE state. Or the base station 2) notifies the base station through the SIB that ProSe direct discovery is supported but does not provide resources for discovery information announcement. In this case, the terminal shall enter the RRC_CONNECTED state for discovery information announcement.

For the terminal in the RRC_CONNECTED state, the base station can set whether to use the type 1 resource pool or the type 2 resource for the discovery information announcement through the RRC signal.

Hereinafter, the sidelink may refer to a terminal-to-terminal interface for D2D communication and D2D discovery. The side link corresponds to the above-described PC5 interface. The control channel for broadcasting the most basic system information for D2D communication is a Physical Sidelink Broadcast Channel (PSBCH), a Physical Sidelink Control Channel (PSCCH) for channels defined / used in the side link, . The channel through which the D2D discovery signal is transmitted may be defined as a Physical Sidelink Discovery Channel (PSDCH). The D2D synchronization signal may be referred to as a SideLink Synchronization Signal (SLSS) or a D2D Synchronization Signal (D2DSS).

In the LTE-A system (Rel-12, 13 or higher), the D2D communication terminal is set to transmit with the PSBCH and the SLSS or to transmit the SLSS. In addition, the LTE-A system newly defines S-RSRP (Sidelink RSRP) for synchronizing with other terminals in D2D communication. That is, when the terminals attempt to perform D2D communication, it is possible to measure the S-RSRP and perform D2D communication only by synchronizing the terminals that are above a specific value. At this time, the S-RSRP can be measured from a demodulation reference signal (DM-RS) on the PSBCH. However, for the D2D relay operation, the S-RSRP may be measured from the DM-RS on the PSDCH. Meanwhile, a SD-RSRP (Sidelink Discovery reference signal received power) may be used for the D2D relay operation. The SD-RSRP may be defined as a linear average of the power contribution of the resource elements carrying the demodulation reference signal associated with the PSDCH with which the cyclic redundancy check (CRC) is authenticated. The reference point of the SD-RSRP may be an antenna connection of the terminal. If reception diversity is used by the terminal, a lower value than SD-RSPR by an individual diversity branch may not be reported.

In addition, the out-coverage terminal measures the S-RSRP / SD-RSRP based on the DM-RS of the SLSS and / or PSBCH / PSCCH / PSSCH, a synchronization source can be determined.

Hereinafter, the D2D relaying operation may be referred to simply as a relaying operation, and the terminal performing the D2D relaying operation is referred to as a relay terminal. The relay terminal is located between the first terminal and the second terminal and can relay the signals between the first and second terminals. Alternatively, the relay terminal may be located between the other terminal and the network (cell / base station) and relay the signal between the other terminal and the network. Hereinafter, it is assumed that the relay terminal relays signals between other terminals and the network.

13 illustrates a relay terminal.

The relay terminal 132 is a terminal that provides network connectivity to the remote terminal 133. The relay terminal 132 is connected to the remote terminal 133 and the net.

And relay the signals between the works 131. The remote terminal 133 may be a UE that is difficult to communicate directly with the base station even if it is located in an out-of-coverage area of the base station or in a coverage area.

The relay terminal can transmit information received from the base station to the general terminal or transmit information received from the general terminal to the base station while maintaining the link with the base station as well as the link with the general terminal (e.g., remote UE) have. At this time, the link between the base station and the relay terminal may be referred to as a backhaul link, and the link between the relay terminal and the remote terminal may be referred to as an access link. In addition, a link for performing direct communication between terminals without involvement of a base station can be defined as a D2D link or a side link.

14 illustrates the relationship between a relay terminal and a remote terminal.

In FIG. 14, UE1 and UE3 are terminals out of coverage, UE2 and UE4 are terminals in coverage, and rUE is a relay terminal configured to perform a relay operation. Here, the UE 2 corresponds to the UE within the coverage for the second base station eNB2, but may correspond to the UE outside the coverage for the first base station. The first base station eNB1 may be a serving cell for rUE.

The rUE may be a UE set to rUE by an instruction of the first base station eNB1 or a coordination between rUEs, and a rUE may broadcast a discovery signal or the like so that neighboring UEs may know existence of a rUE. The UE may receive the D2D signal from the UE UE 4 in the serving cell, UE UE 2 in the neighboring cell, and UEs UE1 and UE3 outside the coverage, for uplink transmission.

Hereinafter, a process of selecting a relay terminal by a remote terminal will be described in detail. In addition, when the remote terminal selects the relay terminal, the operation / process performed by the protocol layers of the remote terminal will be described.

The process of selecting a relay terminal by a remote terminal may include three steps, and RAN auxiliary information and control information of different levels may be provided for each step. The remote terminal may be within the coverage of the base station or outside the coverage, which may affect the level controlled by the base station depending on where the remote terminal is located.

The steps of selecting the relay terminal among the candidate relay terminals will be described.

The relay terminal is set up between the candidate relay terminal and the network. This can be referred to as Step 1: setting of the relay terminal.

In order for the candidate relay terminal to participate in the discovery operation and to perform the relay operation between the remote terminal and the network, it may be necessary for the candidate relay terminal to be authenticated as a relay terminal from the remote terminal to the network. Therefore, it may be necessary for the candidate relay terminal to enter the RRC connection state and be allowed to operate as a relay terminal from the network (base station).

In addition, there are two applicable methods for the discovery (referred to as relay discovery) transmitted by the relay terminal. That is, there may be a relay discovery transmission initiated from a relay terminal and a relay discovery transmission starting from a remote terminal. Which of the two methods is used can be set / controlled by the base station.

That is, in order for the relay terminal to participate in the relay discovery and serve as a relay device between the remote terminal and the network, it may be necessary for the relay terminal to enter the RRC connection state and receive permission from the base station.

Next, the candidate relay terminal transmits a relay discovery signal to the remote terminal. This can be referred to as Step 2: Relay discovery that is assisted by the network.

In step 2, if the remote terminal is outside the cell coverage, the remote terminal performs evaluation on the candidate relay terminals. If the remote terminal is within the cell coverage, the selection of the relay terminal among the candidate relay terminals can be performed by the serving cell of the remote terminal based on the measurement report received from the remote terminal or the candidate relay terminals. Here, it is assumed that the remote terminal is located outside the cell coverage, and the relay terminal selection is assumed to be performed by the remote terminal.

The selection criterion of the relay terminal may include parameters for the connectivity of the candidate relay terminal (e.g., APN information) and measurement results (e.g., RSRP / RSRQ of the side link). The criterion that the remote terminal selects the relay terminal is an upper hierarchy reference and a lower hierarchy reference. For a remote terminal in the cell coverage, the base station may set the relay discovery to be started by the remote terminal.

When the remote terminal selects the specific candidate relay terminal as the relay terminal, a side link is established between the specific candidate relay terminal and the remote terminal. This can be referred to as step 3: establishment of a secure Layer-2 link over the PC5 interface.

In step 3, a unicast connection is established between the remote terminal and the relay terminal through the PC5 interface. This process can include authentication and security configuration processes. The security aspects of this process can be handled by SA3.

In terms of RAN, there are four cases related to the mobility of the remote terminal.

Case 1: A remote terminal in a cell establishes a connection with a relay terminal.

The intra-E-UTRAN-Access Mobility Support (E-UTRAN-Access Mobility Support) in the ECM-connected state for the UEs can be configured as a process related to the handover process and the dual connectivity. During the handover procedure, the source eNB determines whether the UE uses the radio resource provided by the target base station. Similarly, in the process related to the dual connection, the decision / command to allow the UE to use the radio resource provided by the Secondary eNB (SeNB) is issued by the MeNB (Master eNB).

The mobility between RATs is controlled by the network and this control is performed by the source base station. That is, the decision / instruction to use the radio resource of the target RAT for the UE is done by the source base station.

Therefore, the mobility for the RRC connected terminal is based on the handover of the network under the assistance of the terminal. The base station can control this using a combination of system information and a dedicated message (RRC connection reset).

The relay terminal relaying the transmission from the remote terminal to the network can be seen as another target in the case of the remote terminal. The source base station determines to use a relay terminal that performs relaying to the network from the remote terminal to the inter-RAT handover to the remote terminal in the RRC connection state, and the source base station may require the measurement result for this determination.

The UE in the cell coverage may be in the RRC idle state or the RRC connected state depending on the activation level of the UE. When the terminal is in the RRC connection state, it is possible to transmit and receive data while maintaining service continuity without data loss through handover under control of the network. In this respect, the remote terminal in the RRC connection state can be connected to the relay terminal under the control of the network. The terminal in the RRC idle state does not transmit or receive data, so it is not necessary to establish a connection with the relay terminal. Therefore, a terminal that is in the RRC idle state will first need to enter the RRC connection state.

The extent to which the base station participates in the control in selecting the relay terminal within the cell coverage may vary. The level of involvement in control by the base station can be divided into levels 0, 1, 2, and 3.

Level 0: The remote terminal can select the relay terminal according to its implementation. If data is no longer transmitted at the Uu interface between the base station and the terminal, the inactivity timer releases the remote terminal.

Level 1: The remote terminal can select the relay terminal based on the parameters included in the system information provided from the base station. If data is no longer transmitted at the Uu interface between the base station and the terminal, the inactivity timer releases the remote terminal.

Level 2: The remote terminal can select the relay terminal based on the parameters included in the dedicated signal provided from the base station. If data is no longer transmitted at the Uu interface between the base station and the terminal, the inactivity timer releases the remote terminal.

Level 3: The remote terminal reports to the base station the measurement result for the candidate relay terminal satisfying the minimum condition. The base station selects the relay terminal among the candidate relay terminals and hands over the remote terminal to the selected relay terminal. The handover can be indicated using an RRC connection reset message or an RRC release message.

According to the level 3, since the relay terminal is selected by the base station, the maximum flexibility and the consistency with the conventional operation can be maximally maintained.

The minimum criterion is that 1. the connectivity provided by the relay terminal satisfies the requirement provided by the upper layer, 2. the side link measurement quality, which is obtained by measuring the detection signal received from the relay terminal, (eg sidelink RSRQ) may be above a set threshold.

Case 2: The remote terminal connected to the relay terminal selects and connects to the other relay terminal.

In this case, the remote terminal may be a terminal outside the cell coverage. The remote terminal can perform the relay terminal reselection process. The relay terminal reselection process can be composed of the following three processes.

1. Maintain a set of relay terminals (called a candidate set) that satisfy the minimum conditions such that connectivity and side link measurements exceed a certain threshold. To this end, the remote terminal can use the discovery operation.

2. If the currently connected relay terminal does not satisfy the minimum condition, it excludes the currently connected relay terminal in the candidate set, and triggers a re-selection process of selecting another relay terminal. A timer and / or a hysteresis may be used before excluding the currently connected relay terminal from the candidate set.

3. Relay terminal reselection may be performed as any of the following.

a. The remote terminal can rank the candidate relay terminals included in the candidate set and select the candidate relay terminal having the highest ranking as the relay terminal. The ranking may be based on side link RSRP measurements between each candidate relay terminal and the remote terminal.

b. The remote terminal can select the candidate relay terminal randomly selected from the candidate relay terminals included in the candidate set as the relay terminal.

c. The remote terminal selects a relay terminal among the candidate relay terminals included in the candidate set based on the terminal implementation. That is, the remote terminal selects the relay terminal according to its implementation method.

The parameters used in the minimum condition for determining the candidate relay terminal included in the candidate set can be preconfigured or set by the network. Among methods described in a, b, and c, a method, that is, a method of ranking candidate relay terminals, is advantageous in that a parameter used for ranking can be set in advance or can be controlled by a network. Which of the methods described in a, b, c above is used can be set or predetermined by the network.

Case 3: The remote terminal connected to the relay terminal enters cell coverage.

In this case, the remote terminal performs cell selection for selecting the E-UTRAN cell. Depending on the EMM state, the terminal may trigger an RRC connection establishment procedure. For example, the RRC connection establishment procedure may be triggered to perform a tracking area (TA) update.

When the remote terminal connected to the relay terminal enters cell coverage of a specific base station outside the cell coverage and detects it, the remote terminal changes its state from receiving the service from the specific base station to receiving service by the relay terminal.

Case 4: The remote terminal outside the cell coverage is connected to the relay terminal.

In this case, the remote terminal can perform the initial relay terminal selection process composed of the following two steps.

1. Create a set of relay terminals (called candidate sets) that satisfy the minimum conditions such that connectivity and side link measurements exceed a certain threshold. To this end, the remote terminal can use the procedure of side link discovery operation, that is, a procedure in which the remote terminal attempts to receive the side link discovery signal transmitted by the relay terminal.

2. Select the relay terminal. The relay terminal selection process can be performed by any one of the following methods.

a. The remote terminal can rank the candidate relay terminals included in the candidate set and select the candidate relay terminal having the highest ranking as the relay terminal. The ranking may be based on side link RSRP (e.g., SD-RSRP) measurements between each candidate relay terminal and the remote terminal.

b. The remote terminal can select the candidate relay terminal randomly selected from the candidate relay terminals included in the candidate set as the relay terminal.

c. The remote terminal selects a relay terminal among the candidate relay terminals included in the candidate set based on the terminal implementation. That is, the remote terminal selects the relay terminal according to its implementation method.

The parameters used in the minimum condition for determining the candidate relay terminal included in the candidate set can be preconfigured or set by the network. Among methods described in a, b, and c, a method, that is, a method of ranking candidate relay terminals, is advantageous in that a parameter used for ranking can be set in advance or can be controlled by a network. Which of the methods described in a, b, c above is used can be set or predetermined by the network.

The present invention will now be described.

If the remote terminal selects the relay terminal by selecting only the PC5 measurement, that is, comparing the measured value of the link between the remote terminal and the candidate relay terminal to a predetermined threshold value, the network intervenes in the relay terminal selection process There will not be much room. In other words, it will be difficult for the network to control the relay terminal selection process of the remote terminal.

The network may attempt to control the relay terminal selection process by applying a specific bias value, offset value, or hysteresis to the relay terminal selection process of the remote terminal according to knowledge or preference of the network itself.

15 illustrates a method of selecting a relay terminal by a remote terminal.

Referring to FIG. 15, the remote terminal determines whether the measured value for the current relay UE is less than a threshold value (S1510). The measured value may be a value obtained by measuring a reference signal received power (RSRP) of a reference signal received from the current relay terminal.

If the measured value for the current relay terminal is less than the threshold value, the remote terminal selects the candidate relay terminal that satisfies the condition that the measurement value for the candidate relay terminal is larger than the minimum history value (minHyst) based on the threshold value, as the relay terminal (S1520).

That is, instead of directly selecting a relay terminal from the candidate relay terminals that the measured value (for example, the side link RSRP) is larger than the threshold value, the candidate relay terminal satisfying the condition larger than the minimum history value is relayed And selects it as a terminal. The candidate relay terminal is not selected as the relay terminal when the measured value for the candidate relay terminal is larger than the threshold value and equal to or less than the sum of the threshold value and the minimum history value minHyst.

According to this method, it is possible to prevent the relay terminal from being changed / re-selected unnecessarily even though there is no great difference in channel quality. Also, the necessity of the relay terminal reselection may be different for each remote terminal, and the relay terminal reselection process can be controlled in consideration of this. For example, the first remote terminal may attempt to reselect the relay terminal to transmit or receive a signal for the current common safety or emergency situation, and the second remote terminal may merely reselect the relay terminal to transmit and receive user data. have. If a reliable relay terminal reselection is important even if frequent relay terminal changes are made and the second remote terminal is not desirable to change the relay terminal frequently, the minimum history value for the first remote terminal is relatively The minimum history value for the second remote terminal can be set relatively small.

The remote terminal changes (reselects) the relay terminal from the current relay terminal to the selected candidate relay terminal (S1530).

FIG. 16 is a specific example of applying the method of FIG.

Referring to FIG. 16, the remote terminal determines whether the measured value for the current relay UE is less than a threshold value (S161). The measured value may be an RSRP value for the current relay terminal.

If the measured value for the current relay terminal is less than the threshold value, the remote terminal selects a candidate relay terminal whose measurement value is larger than the minimum history value minHyst with the threshold value as a new relay terminal among the candidate relay terminals (S163).

If the measured value of the current relay terminal is not less than the threshold value, the remote terminal determines whether the upper layer of its own has instructed not to use the current relay terminal (S162).

If the remote terminal has instructed not to use the current relay terminal in its upper layer, the remote terminal transmits the candidate relay terminal whose measurement value is larger than the minimum history value (minHyst) based on the threshold to the new relay terminal (S163). That is, even if the measured value for the current relay terminal is larger than the threshold value, it may be necessary to change the relay terminal for another reason. In this case, the higher layer of the remote terminal can instruct the relay terminal change, Can be selected.

If the upper layer of the remote terminal has not instructed not to use the current relay terminal, the relay terminal is currently maintained (S164).

On the other hand, two history values can be introduced into the relay terminal selection process. The first hysteresis value (referred to as MinHyst) may be a hysteresis value applied to the suitability condition, and the second hysteresis value (DiffHyst) may be a hysteresis value applied to reselection triggering.

The remote terminal can regard the current relay terminal as inappropriate as the relay terminal when the measured value of the PC5 (RSRP) of the currently used relay terminal (current relay terminal) is lower than the threshold value.

The remote terminal determines that the candidate relay terminal exceeds the threshold value by the first hysteresis value MinHyst and the PC5 measurement value for the candidate relay terminal exceeds the PC5 measurement value for the current relay terminal by the second hysteresis value DiffHyst , The candidate relay terminal can be selected as a new relay terminal.

When evaluating the PC5 measurement values of the detected one or more relay terminals, the layer 3 filtering may be applied using the preset field coefficients before using the PC5 measurement results.

FIG. 17 illustrates a method of selecting a relay terminal by using two history values.

Referring to FIG. 17, the remote terminal receives the first history value and the second history value (S1710).

The remote terminal detects a candidate relay terminal whose measured value is larger than the sum of the threshold and the first hysteresis value among the candidate relay terminals (S1720). That is, if the measured value for the candidate relay terminal is M, the threshold value is T, and the first hysteresis value is H, M > T + H is detected as the candidate relay terminal.

In operation S1730, a candidate relay terminal having a measured value higher than the current relay terminal by a second hysteresis value is selected as a new relay terminal among the detected candidate relay terminals.

Alternatively, when the remote terminal does not have a current relay terminal or if it is considered that the current relay terminal is inappropriate, if the PC5 measurement value for the candidate relay terminal that exceeds the threshold by the first hysteresis value is detected, the relay offset (relayOffset) , The terminal having the highest PC5 measurement result can be selected. The relay offset may be an offset specified for each candidate relay terminal or an offset specified for a relay group to which a plurality of candidate relay terminals belong.

If there is more than one suitable candidate relay terminal, it is possible to force the remote terminal to select the candidate relay terminal having the highest rank, that is, the candidate relay terminal having the highest PC5 measurement result.

Or when there is at least one suitable candidate relay terminal, the remote terminal may arbitrarily select one of the candidate relay terminals satisfying the adequacy condition. A candidate relay terminal having a better PC5 measurement result does not guarantee a better quality in a combined link, that is, a link combining a PC5 link (link between a remote terminal and the candidate relay terminal) and a link between the network and the candidate relay terminal . If this method is applied, it may be necessary to introduce a mechanism to prevent the remote terminal from reselecting the relay terminal too often. For example, if the remote terminal once selects the relay terminal (i.e., the remote terminal establishes the L2 link with the relay terminal for 1: 1 side link communication), if at least one of the following reselection conditions is satisfied, The candidate relay terminal can be selected.

1) the upper layer of the remote terminal triggers the reselection of the relay terminal,

2) the upper layer of the remote terminal provides a list of candidate relay terminals,

3) If the PC5 measurement result for the serving relay terminal does not exceed the threshold,

4) If the PC5 measurement result for the other candidate relay terminal exceeds the threshold value

5) If the PC5 measurement result for the serving relay terminal exceeds the threshold value and the time connected to the serving relay terminal exceeds the threshold after selecting the serving relay terminal.

&Lt; Relay terminal selection offset applied to relay terminal >

The network may wish to set a bias to the candidate relay terminal. For example, the network may want the remote terminal to select candidate relay terminals with higher Uu quality (quality on the link between the network and the candidate relay terminal). Or that the remote terminal does not select candidate relay terminals having a higher traffic load. Alternatively, the network may want the remote terminal to select candidate relay terminals in the same cell as the remote terminal.

In this case, a bias can be applied to the candidate relay terminal by introducing a terminal-specific offset parameter. Since the relay terminal can be identified by the identifier of the relay terminal, the offset can be associated with the terminal identifier. For the case where the remote terminal is within network coverage, it is possible for the network to transmit the offset parameter. For example, it is possible to transmit the offset parameter to a remote terminal through a broadcast channel or a terminal specific channel.

It is possible to introduce a common offset commonly applied to a plurality of relay terminals in addition to the relay terminal specific offset. For example, an offset commonly applied to relay terminals connected to a specific cell, so-called cell-specific offset, may be set in the remote terminal. At this time, an offset value for one or more cells may be set to the remote terminal. The relay terminal can include the identifier of the cell to which the relay terminal belongs in a message informing the remote terminal of its existence. Accordingly, the remote terminal can confirm the serving cell of the relay terminal through the cell identifier included in the message transmitted by the relay terminal. When the remote terminal confirms the serving cell of the relay terminal, the remote terminal can apply the offset corresponding to the cell to the relay terminal.

18 illustrates a method by which a network transmits bias information.

Referring to FIG. 18, the network determines a preferred relay terminal for the remote terminal (S1810), and transmits the bias information including the pair of the ID and the offset value of the candidate relay terminal to the remote terminal (S1820).

The network (base station) can signal the candidate relay terminal specific offset value. The offset value may be used to rank the candidate relay terminals.

The base station may broadcast the offset value or inform it of a dedicated signal for a specific terminal. The base station can notify each candidate relay terminal as a pair of {the L2 ID of the candidate relay terminal and the offset value of the candidate relay terminal}. Then, the remote terminal can apply the offset value when performing the ranking for the candidate relay terminals.

The default value of the offset may be a predetermined value (e.g., 0) and may be applied if the offset value is not signaled separately.

Alternatively, the default value may be applied when the remote terminal is within cell coverage. A predetermined configuration relating to the D2D operation is applied to the remote terminal outside the cell coverage. The parameters (relayRefMinHyst, relayRefDiffHyst, relayOffset) described above can be provided through the predetermined setting.

<AS-upper layer interaction>

There are several layers in one terminal. It may be a matter of which layer among these various layers ultimately selects the relay terminal. This may be a problem affecting the standard specification.

In selecting a relay terminal, there can be two kinds of criteria. The first is related to the radio environment, for example, suitability, re-selection triggering criteria, and so on. Second, for example, a service code may correspond to a criterion not related to a radio environment. More specifically, when the appropriateness of the relay terminal selection is determined based on the RSRP (for example, S-RSRP / SD-RSRP) of the side link between the remote terminal and the relay terminal, This is a relevant standard. A criterion not related to the radio environment may be, for example, a relay service code. The relay service code can be determined by the content / use of the relay service, and in this case, it can not be regarded as a standard related to the radio environment.

A minimum criterion for selection of relay terminals based on the AS layer is the conformance condition. Further, when selecting a relay terminal, it may be required to select a candidate relay terminal of the best rank.

Finally, the selected relay terminal shall satisfy both the upper layer reference and the AS layer reference. In this case, one of the following two methods can be applied as the interaction between the upper layer and the AS layer.

First, the upper layer finally selects the relay terminal.

The AS layer of the remote terminal receives the relay discovery message from the candidate relay terminal.

The AS layer of the remote terminal checks the conformance condition of the detected candidate relay terminal based on the PC5 measurement.

The AS layer of the remote terminal decodes the received discovery message when the conformance condition is satisfied, and delivers the decoded message to the upper layer (ProSe upper layer). When delivering the decoded message, the AS layer of the remote terminal can transmit the corresponding PC5 measurement result to the upper layer together.

The upper layer of the remote terminal evaluates both the reference related to the radio environment and the reference not related to the radio environment, and finally selects the relay terminal.

Next, the AS layer finally selects a relay terminal.

The AS layer of the remote terminal receives the relay discovery message from the candidate relay terminal.

The AS layer of the remote terminal checks the conformance condition of the detected candidate relay terminal based on the PC5 measurement.

The AS layer of the remote terminal decodes the received discovery message when the conformance condition is satisfied, and delivers the decoded message to the upper layer (ProSe upper layer).

The upper layer of the remote terminal evaluates for relay terminal selection / reselection only considering criteria not related to the radio environment.

The upper layer of the remote terminal provides a list of candidate relay terminals satisfying a criterion not related to the radio environment.

The AS layer of the remote terminal selects the relay terminal by performing evaluation for relay terminal selection / reselection considering only the reference related to the radio environment.

The first option is somewhat less interactive between the AS layer and the higher layer, and the second option makes it more clear that the functional partition between the wireless environment reference and the non-wireless related reference. To minimize interactions between the AS layer and the higher layer in the standard, the first option is preferred and a second option is desirable to further clarify the functional partitioning between the radio-related criteria and the non-radio-related criteria.

19 is a block diagram illustrating a terminal in which an embodiment of the present invention is implemented.

Referring to FIG. 19, a UE 1100 includes a processor 1110, a memory 1120, and a radio frequency unit (RF) unit 1130. Processor 1110 implements the proposed functionality, process and / or method. The RF unit 1130 is connected to the processor 1110 to transmit and receive radio signals.

The processor may comprise an application-specific integrated circuit (ASIC), other chipset, logic circuitry and / or a data processing device. The memory may include read-only memory (ROM), random access memory (RAM), flash memory, memory cards, storage media, and / or other storage devices. The RF unit may include a baseband circuit for processing the radio signal. When the embodiment is implemented in software, the above-described techniques may be implemented with modules (processes, functions, and so on) that perform the functions described above. The module is stored in memory and can be executed by the processor. The memory may be internal or external to the processor and may be coupled to the processor by any of a variety of well known means.

1100: terminal, 1110: processor, 1120: memory, 1130: RF section

Claims (12)

A method for selecting a relay UE performed by a remote UE in a wireless communication system,
Determine whether the measured value for the current relay UE is below a threshold, and
If the measured value for the current relay terminal is less than the threshold value, the candidate relay terminal satisfying the specific condition is selected as the relay terminal,
Wherein the candidate relay terminal is selected as the relay terminal only when the measured value for the candidate relay terminal is larger than the minimum history value (minHyst) based on the threshold value. 2. The method according to claim 1, wherein, when the upper layer of the remote terminal instructs the relay terminal to reselect, only when the measured value for the candidate relay terminal is larger than the minimum history value (minHyst) Is selected as a relay terminal. The method according to claim 1,
Wherein the measured value for the current relay UE is a value obtained by measuring a reference signal received power (RSRP) of the reference signal received from the current relay terminal. The method according to claim 1,
Wherein the candidate relay terminal is not selected as a relay terminal when the measured value for the candidate relay terminal is larger than the threshold value and equal to or less than the sum of the threshold value and the minimum history value minHyst. The method of claim 1, wherein
Characterized in that the threshold is provided by a network. The method of claim 1, wherein
Wherein the relay terminal provides a relay service between the remote terminal and the network. The terminal,
A radio frequency (RF) unit for transmitting and receiving a radio signal; And
A processor coupled to the RF unit; The processor comprising:
Determining whether a measured value for a current relay UE is less than a threshold value and selecting a candidate relay terminal that satisfies a specific condition as a relay terminal when the measured value for the current relay terminal is less than a threshold,
And selects the candidate relay terminal as the relay terminal only when the measured value for the candidate relay terminal is larger than the minimum history value (minHyst) based on the threshold value. The method as claimed in claim 7, further comprising: when the upper layer of the terminal instructs the reselection of the relay terminal, only when the measured value for the candidate relay terminal is larger than the minimum history value (minHyst) And selects the relay terminal as the relay terminal. 8. The method of claim 7,
Wherein the measured value of the current relay UE is a value obtained by measuring a reference signal received power (RSRP) of a reference signal received from the current relay terminal. 8. The method of claim 7,
Wherein the candidate relay terminal is not selected as a relay terminal when the measured value for the candidate relay terminal is larger than the threshold value and equal to or smaller than the sum of the threshold value and the minimum history value minHyst. The method of claim 7, wherein
Wherein the threshold is provided by a network. The method of claim 7, wherein
Wherein the relay terminal is a device for providing a relay service between the terminal and the network.

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