WO2015130034A1

WO2015130034A1 - Method and apparatus for acquiring time synchronization in wireless communication system

Info

Publication number: WO2015130034A1
Application number: PCT/KR2015/001279
Authority: WO
Inventors: Hyeyoung Choi; Eunjong Lee; Heejeong Cho; Hyunsoo Ko; Jaehoon Chung; Genebeck Hahn
Original assignee: Lg Electronics Inc.
Priority date: 2014-02-27
Filing date: 2015-02-09
Publication date: 2015-09-03

Abstract

A method and apparatus for acquiring time synchronization in a wireless communication system is provided. In a convergence system of a cellular system and a Wi-Fi system, when a user equipment (UE) can perform uplink (UL) transmission to a evolved NodeB (eNB) of the cellular system but cannot perform DL reception from the eNB, the eNB acquires a time synchronization based on a first timing advance (TA) between the eNB and an access point (AP) of the Wi-Fi system, and a second TA between the AP and the UE.

Description

METHOD AND APPARATUS FOR ACQUIRING TIME SYNCHRONIZATION IN WIRELESS COMMUNICATION SYSTEM

The present invention relates to wireless communications, and more particularly, to a method and apparatus for acquiring time synchronization in a wireless communication system.

Universal mobile telecommunications system (UMTS) is a 3rd generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.

From rel-8 of 3GPP LTE, access network discovery and selection functions (ANDSF) for detecting and selecting accessible access networks have been standardized while interworking with non-3GPP access (e.g., wireless local access network (WLAN)) is introduced. The ANDSF may carry detection information of access networks accessible in location of a user equipment (UE) (e.g., WLAN, WiMAX location information, etc), inter-system mobility policies (ISMP) which is able to reflect operator’s policies, and inter-system routing policy (ISRP). Based on the information described above, the UE may determine which IP traffic is transmitted through which access network. The ISMP may include network selection rules for the UE to select one active access network connection (e.g., WLAN or 3GPP). The ISRP may include network selection rules for the UE to select one or more potential active access network connection (e.g., both WLAN and 3GPP). The ISRP may include multiple access connectivity (MAPCON), Internet protocol (IP) flow mobility (IFOM) and non-seamless WLAN offloading. Open mobile alliance (OMA) device management (DM) may be used for dynamic provision between the ANDSF and the UE.

MAPCON is a standardization of a technology which enables configuring and maintaining multiple PDN connectivity simultaneously through 3GPP access and non-3GPP access using different access point name (APN), and enables a seamless traffic offloading in units of all active PDN connections. MAPCON is a protocol-independent technology, and accordingly, proxy mobile IPv6 (PMIPv6), general packet radio service (GPRS) tunneling protocol (GTP), dual stack mobile IPv6 (DSMIPv6) may be used. For MAPCON, an ANDSF server ma provide APN information for performing offloading, routing rule, time of day information, and validity area information, etc.

IFOM is a DSMIPv6-based 3GPP/WLAN seamless offloading technology in a unit of IP flow, which is more flexible and more segmented than MAPCON. DSMIPv6 supports both IPv4 and IPv6 in the UE and network. IFOM has adopted DSMIPv6, since adoption of DSMIPv6 has increased due to diversification of mobile communication networks and importance of mobility support has increased. Further, IFOM has not adopted PMIPv6 since management in a unit of IP flow is difficult. IFOM is also client-based mobile IP (MIP) technology in which the UE informs an agent of movement of itself. A home agent (HA) is an agent managing mobility of mobile nodes, and has a flow binding table and binding cache table. IFOM enables access to different access networks even when the UE is connected to a PDN using the same APN, which is different from MAPCON. IFOM also enables mobility in a unit of specific IP traffic flow, not a unit of PDN, for a unit of mobility or offloading, and accordingly, services may be provided flexibly. For this, an ANDSF server may provide IP flow information for performing offloading, routing rule, time of day information, and validity area information, etc.

The non-seamless WLAN offloading is a technology that offloads traffics completely so as not to go through an evolved packet core (EPC) as well as that changes a path of a specific IP traffic to WLAN. The offloaded IP traffic cannot be moved to 3GPP access seamlessly again since anchoring is not performed to the P-GW for mobility support. For this, an ANDSF server may provide information as similar as the information provided for IFOM.

3GPP/WLAN interworking may be performed in various scenarios. Among the various scenarios, when a UE connected with 3GPP LTE can perform only uplink (UL) transmission to an evolved nodeB (eNB) and has a difficulty in downlink (DL) reception from the eNB, 3GPP/WLAN interworking may be performed. In this case, there may be a problem in time synchronization acquisition between the UE and the eNB.

The present invention provides a method and apparatus for acquiring time synchronization in a wireless communication system. The present invention provides a method of acquiring time synchronization of a cellular system supporting only an uplink in a convergence system of a cellular system and a Wi-Fi system. For a case where a UE that can simultaneously transmit/receive data of a cellular network and a Wi-Fi network cannot use a downlink or uplink of the cellular network due to a location and/or a network situation, the present invention provides a method for performing initial access to the cellular network by using the Wi-Fi network and a method for acquiring time synchronization of the cellular network.

In an aspect, a method for acquiring, by an evolved NodeB (eNB) of a first system, time synchronization in a wireless communication system is provided. The method includes transmitting a downlink (DL) data to an access point (AP) of a second system, acquiring a first timing advance (TA) which is a timing difference between a timing at which the eNB transmits the DL data and a timing at which the AP receives the DL data, acquiring a second TA which is a timing difference between a timing at which the AP transmits a DL frame to a user equipment (UE) and a timing at which the UE receives the DL frame, receiving a preamble from the UE, and acquiring a third TA based on the first TA, the second TA, and a timing difference between a timing at which the eNB transmits the DL data and a timing at which the eNB receives the preamble.

In another aspect, a method for acquiring, by an access point (AP) of a second system, time synchronization in a wireless communication system is provided. The method includes receiving a downlink (DL) data from an evolved NodeB (eNB) of a first system, transmitting a DL frame at a timing that the AP receives the DL data to a user equipment (UE), receiving a uplink (UL) frame from the UE, acquiring a timing advance (TA) which is a timing difference between a timing at which the AP transmits the DL frame and a timing at which the UE transmits the UL frame, and transmitting the acquired TA to the eNB.

In a convergence system of a cellular system and a Wi-Fi system, time synchronization of the cellular system supporting only an uplink can be acquired.

FIG. 1 shows a cellular system.

FIG. 2 shows a wireless local area network system.

FIG. 3 shows an example of an RRC connection establishment procedure.

FIG. 4 shows an example of a contention based random access procedure of 3GPP LTE.

FIG. 5 shows an example of a network structure for 3GPP LTE/Wi-Fi interworking.

FIG. 6 shows an example of scenarios of a convergence network of 3GPP LTE and Wi-Fi.

FIG. 7 shows an example of a network structure of 3GPP LTE/Wi-Fi interworking according to an embodiment of the present invention.

FIG. 8 shows an example of an RRC connection establishment procedure according to an embodiment of the present invention.

FIG. 9 shows an example of acquiring time synchronization according to an embodiment of the present invention.

FIG. 10 shows another example of a method for acquiring time synchronization according to an embodiment of the present invention.

FIG. 11 shows another example of a method for acquiring time synchronization according to an embodiment of the present invention.

FIG. 12 shows a wireless communication system to implement an embodiment of the present invention.

A technology below can be used in a variety of wireless communication systems, such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), and single carrier frequency division multiple access (SC-FDMA). CDMA can be implemented using radio technology, such as universal terrestrial radio access (UTRA) or CDMA2000. TDMA can be implemented using radio technology, such as global system for mobile communications (GSM)/general packet radio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMA can be implemented using radio technology, such as IEEE 802.11(Wi-Fi), IEEE 802.16(WiMAX), IEEE 802-20, or Evolved UTRA (E-UTRA). IEEE 802.16m is the evolution of IEEE 802.16e, and it provides a backward compatibility with an IEEE 802.16e-based system. UTRA is part of a universal mobile telecommunications system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is part of evolved UMTS (E-UMTS) using evolved-UMTS terrestrial radio access (E-UTRA), and it adopts OFDMA in downlink (DL) and SC-FDMA in uplink (UL). LTE-A (advanced) is the evolution of 3GPP LTE.

3GPP LTE(-A) and IEEE 802.11 are chiefly described as an example in order to clarify the description, but the technical spirit of the present invention is not limited to 3GPP LTE(-A) and IEEE 802.11.

FIG. 1 shows a cellular system. Referring to FIG. 1, the cellular system 10 includes one or more base stations (BSs) 11. The BSs 11 provide communication services to respective geographical areas (in general called ‘cells’) 15a, 15b, and 15c. Each of the cells can be divided into a number of areas (called ‘sectors’). A user equipment (UE) 12 can be fixed or mobile and may be referred to as another terminology, such as a mobile station (MS), a mobile terminal (MT), a user terminal (UT), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), a wireless modem, or a handheld device. In general, the BS 11 refers to a fixed station that communicates with the UEs 12, and it may be referred to as another terminology, such as an evolved-NodeB (eNB), a base transceiver system (BTS), or an access point.

The UE generally belongs to one cell. A cell to which a UE belongs is called a serving cell. A BS providing the serving cell with communication services is called a serving BS. The cellular system includes other cells neighboring a serving cell. Other cells neighboring the serving cell are called neighbor cells. A BS providing the neighbor cells with communication services is called as a neighbor BS. The serving cell and the neighbor cells are relatively determined on the basis of a UE.

This technology can be used in the downlink (DL) or the uplink (UL). In general, DL refers to communication from the BS 11 to the UE 12, and UL refers to communication from the UE 12 to the BS 11. In the DL, a transmitter may be part of the BS 11 and a receiver may be part of the UE 12. In the UL, a transmitter may be part of the UE 12 and a receiver may be part of the BS 11.

FIG. 2 shows a wireless local area network system. The wireless local area network (WLAN) system may also be referred to as Wi-Fi. Referring to FIG. 2, the WLAN system includes one access point (AP) 20 and a plurality of stations (STAs) 31, 32, 33, 34, and 40. The AP 20 may be linked to each

STA

31, 32, 33, 34, and 40 and may communicate therewith. The WLAN system includes one or more basic service sets (BSSs). The BSS is a set of STAs that may be successfully synchronized with each other and may communicate with each other, and does not mean a specific region.

An infrastructure BSS includes one or more non-AP stations, APs that provide a distribution service (DS), and a DS that links a plurality of APs with each other. In the infrastructure BSS, an AP manages non-AP STAs of the BSS. Accordingly, the WLAN system shown in FIG. 2 may include an infrastructure BSS. In contrast, an independent BSS (IBSS) is a BSS that operates in ad-hoc mode. The IBSS does not include an AP and thus lacks a centralized management entity. That is, in the IBSS, the non-AP STAs are managed in a distributed manner. The IBSS may have all the STAs constituted of mobile STAs and is not allowed to access the distribution system, thus achieving a self-contained network.

The STA is random functional medium that includes a physical layer interface for a wireless medium and an media access control (MAC)) observing IEEE 802.11 standards, and in its broader concepts, it includes both the AP and non-AP station.

The non-AP STA is an STA, not an AP. The non-AP STA may also be referred to as a mobile terminal, a wireless device, a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station (MS), a mobile subscriber unit or simply as a user. Hereinafter, for ease of description, the non-AP STA denotes an STA.

The AP is a functional entity that provides access to a distribution system via a wireless medium for an STA associated with the AP. In the infrastructure BSS including an AP, communication between STAs is basically done via an AP, but in case a direct link is established, direct communication may be achieved between STAs. The AP may also be referred to as a central controller, a base station (BS), a NodeB, a base transceiver system (BTS), or a site controller.

A plurality of infrastructure BSSs may be linked with each another through a distribution system. The plurality of BSSs linked with each another is referred to as an extended service set (ESS). The APs and/or STAs included in the ESS may communicate with each other, and in the same ESS, an STA may move from one BSS to another, while in seamless communication.

A radio resource control (RRC) connection establishment procedure of 3GPP LTE is described.

FIG. 3 shows an example of an RRC connection establishment procedure. In step S50, a non-access stratum (NAS) layer of the UE transmits a NAS service request to an RRC layer of the UE. In step S51, the RRC layer of the UE performs access barring check, and if not barred, in step S52, the RRC layer of the UE transmits an RRC connection request to a lower layer of the UE (i.e., packet data convergence protocol (PDCP) layer, radio link control (RLC) layer, media access control (MAC) layer, L1). In step S53, the lower layer of the UE performs a random access procedure with an eNB. The random access procedure is described below in detail. In step S54, the lower layer of the UE transmits an RRC connection setup to the RRC layer of the UE. In step S55, the RRC layer of the UE performs signaling radio bearer (SRB)-1 setup. In step S56, the RRC layer of the UE transmits an RRC connection setup complete to the lower layer of the UE. In step S57, the lower layer of the UE transmits an RRC connection setup complete to the eNB.

Random access procedure of 3GPP LTE is described in detail. It may be referred to Section 10.1.5 of 3GPP TS 36.300 V11.4.0 (2012-12). The purpose of the random access procedures of 3GPP LTE is a UL synchronization and RRC connection establishment.

The random access procedure is characterized by:

- Common procedure for frequency division duplex (FDD) and time division duplex (TDD);

- One procedure irrespective of cell size and the number of serving cells when carrier aggregation (CA) is configured.

The random access procedure is performed for the following events related to the primary cell (PCell):

- Initial access from RRC idle mode (RRC_IDLE);

- RRC connection re-establishment procedure;

- Handover;

- DL data arrival during RRC connected mode (RRC_CONNECTED) requiring random access procedure (e.g., when UL synchronization status is “non-synchronized”);

- UL data arrival during RRC_CONNECTED requiring random access procedure (e.g., when UL synchronization status is "non-synchronized" or there are no physical uplink control channel (PUCCH) resources for scheduling request (SR) available);

- For positioning purpose during RRC_CONNECTED requiring random access procedure (e.g., when TA is needed for UE positioning);

The random access procedure is also performed on a secondary cell (SCell) to establish time alignment for the corresponding secondary timing advance group (sTAG).

Furthermore, the random access procedure takes two distinct forms:

- Contention based (applicable to first five events);

- Non-contention based (applicable to only handover, DL data arrival, positioning and obtaining timing advance alignment for a sTAG).

Normal DL/UL transmission can take place after the random access procedure.

A relay node (RN) supports both contention-based and non-contention-based random access. When an RN performs the random access procedure, it suspends any current RN subframe configuration, meaning it temporarily disregards the RN subframe configuration. The RN subframe configuration is resumed at successful random access procedure completion.

FIG. 4 shows an example of a contention based random access procedure of 3GPP LTE. The UE obtains a random access channel (RACH) preamble configuration by receiving a system information block (SIB)-2. Specifically, the SIB2 includes the radioResourceConfigCommonSIB information element (IE), the radioResourceConfigCommonSIB includes the PRACH-ConfigSIB IE, and the PRACH-ConfigSIB IE includes the PRACH-ConfigInfo IE, which indicates the RACH preamble configuration. The layer 1 procedure is triggered upon request of a preamble transmission by a higher layer. A preamble index, a target preamble received power (PREAMBLE_RECEIVED_TARGET_POWER), a corresponding random access radio network temporary identity (RA-RNTI) and a physical random access channel (PRACH) resource are indicated by a higher layer as part of the request.

Referring to FIG. 4, in step S60, the UE transmits the PRACH preamble (message 1) carrying the RA-RNTI to the eNB through the RRC layer. The RA-RNTI may be computed as (1+t_id+10*f_id), where t_id is the index of the first subframe of the specified PRACH (0≤ t_id <10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0≤ f_id< 6). The eNB decodes the PRACH preamble and obtains the RA-RNTI.

Upon transmitting the PRACH preamble, the UE monitors a response message with the RA-RNTI during (3+windows size) superframe. In step S61, the eNB transmits the PRACH response (message 2) with the RA-RNTI, which the eNB decodes from the PRACH preamble, to the UE through the RRC layer. The UE decodes the PRACH response and obtains a resource block assignment, a modulation and coding scheme (MCS) configuration, and a temporary cell RNTI (C-RNTI). With this information, the eNB configures itself to receive the RRC connection request message via the downlink control information (DCI) format 0.

In step S62, the UE transmits the RRC connection request message using the temporary C-RNTI obtained from the PRACH response to the eNB through the RRC layer.

In step S63, the eNB transmits the RRC connection setup message using the temporary C-RNTI to the UE through the RRC layer. The RRC connection setup message carries the C-RNTI. From this point, the UE and network exchanges messages with the C-RNTI.

Beacon of Wi-Fi is described. It may be referred to Section 4.3.8.2 of IEEE Std 802.11-2012. The beacon request/report pair enables a STA to request from another STA a list of APs whose beacons it can receive on a specified channel or channels. This measurement may be done by active mode (like active scan), passive mode (like passive scan), or beacon table modes. If the measurement request is accepted and is in passive mode, a duration timer is set. Then the measuring STA monitors the requested channel, measures beacon, probe response, and measurement pilot power levels (received channel power indicator (RCPI)), and logs all beacons, probe responses, and measurement pilots received within the measurement duration. If the measurement request is in active mode, the measuring STA sends a probe request on the requested channel at the beginning of the measurement duration, then monitors the requested channel, measures beacon, probe response, and measurement pilot power levels (RCPI), and logs all beacons, probe responses, and measurement pilots received within the measurement duration. If the request is beacon table mode, then the measuring STA returns a beacon report containing the current contents of any stored beacon information for any supported channel with the requested service set identifier (SSID) and basic service set identifier (BSSID) without performing additional measurements.

Cellular/WLAN interworking is described. One of requirements for 5^th generation mobile communication technologies is the interworking between heterogeneous wireless communication systems. The 5^th generation mobile communication system may adopt a plurality of radio access technologies (RATs) for always gaining easy access and maintaining efficient performance in any place. The 5^th generation mobile communication system may use multiple RATs in a converging manner through the interoperation between heterogeneous wireless communication systems. By interworking between heterogeneous wireless communication systems, peak throughput can be increased, and data traffic can be off-loaded. Each entity in the plurality of RATs constituting the 5^th generation mobile communication system may exchange information therebetween, and accordingly, the optimal communication system may be provided to a user in the 5^th generation mobile communication system.

Among the plurality of RATs constituting the 5^th generation mobile communication system, a specific RAT may operate as a primary RAT system, and another specific RAT may operate as a secondary RAT system. That is, the primary RAT system may mainly play a role to provide a communication system and control information transmission to a user in the 5^th generation mobile communication system, while the secondary RAT system may assist the primary RAT system and may be used for data transmission. In general, a cellular system with relatively wider coverage may be a primary RAT system. The cellular system may be one of 3GPP LTE, 3GPP LTE-A, and IEEE 802.16 system (e.g., WiMax, WiBro). A WLAN system with relatively smaller coverage may be a secondary RAT system. The WLAN system may be Wi-Fi. In particular, WLAN is a wireless communication system that is commonly used for various user equipments, and thus, the cellular/WLAN interoperation is a high-priority convergence technique. Offloading by the cellular-WLAN interoperation may increase the coverage and capacity of the cellular system.

FIG. 5 shows an example of a network structure for 3GPP LTE/Wi-Fi interworking. The interworking of 3GPP LTE and Wi-Fi is a practical model of interworking of a cellular system and a WLAN system. Referring to FIG. 5, a multi-RAT UE having a capability of accessing two or more RATs is connected to an eNB in E-UTRAN, and also is connected to an AP in Wi-Fi. The eNB is connected to each of a mobility management entity (MME) and a serving gateway (S-GW) in an evolved packet core (EPC) through an S1-AP and a GPRS tunneling protocol user plane (GTP-U), respectively. The AP is connected to an evolved packet data gateway (ePDG) and a dynamic host configuration protocol (DHCP) in the EPC. The S-GW and the ePDG are connected to a packet data network (PDN) gateway (P-GW) connected to the Internet. A backhaul control connection may exist between the AP and the eNB through a backbone network passing through the P-GW or the EPC or the like. Alternatively, a wireless control connection may exist between the eNB and the AP. In addition thereto, the AP is connected to an authentication, authorization and accounting (AAA) server in the EPC. The AAA server is connected to the P-GW, and also is connected to a home subscriber server (HSS) in the EPC. The HSS is connected to the MME.

Interworking of 3GPP LTE and Wi-Fi may be performed on the basis of a multi-RAT UE by using the structure described in FIG. 5. In order for the multi-RAT UE to access a specific RAT, the multi-RAT UE may request the specific RAT to establish a connection, and may transmit/receive data through the RAT. This is a UE-initiated multi-RAT access technique based on a core network. In addition, information may be exchanged among a plurality of RATs by using an access network discovery and selection function (ANDSF) server.

FIG. 6 shows an example of scenarios of a convergence network of 3GPP LTE and Wi-Fi. Referring to FIG. 6, the scenarios of the convergence network of 3GPP LTE and Wi-Fi may be classified as follows.

1) Cellular only access: A UE has access to only the 3GPP LTE. The UE transmits/receives control information (C-plane) or data (U-plane) through the 3GPP LTE. In this scenario, prior arts required for Wi-Fi automatic switching/simultaneous transmission may be defined. For example, AP information management for 3GPP LTE/Wi-Fi interworking is achieved at a network level. Wi-Fi discovery or Wi-Fi access are achieved at a device level.

2-1) Cellular/Wi-Fi simultaneous access and U-plane automatic switching: The UE simultaneously accesses 3GPP LTE and Wi-Fi. The UE transmits/receives control information through the 3GPP LTE. After the U-plane automatic switching, all data is transmitted/received only through the Wi-Fi.

2-2) Cellular/Wi-Fi simultaneous access and U-plane simultaneous transmission: The UE simultaneously accesses the 3GPP LTE and the Wi-Fi. The UE transmits/receives control information and data through the 3GPP LTE. Further, the UE transmits/receives data through the Wi-Fi. For the U-plane simultaneous transmission, bearer/flow/data automatic switching may be performed. After the bearer/flow/data automatic switching, data may be simultaneously transmitted/received through the 3GPP LTE/Wi-Fi by using bandwidth segregation/aggregation. The bandwidth segregation is automatic switching for each flow, and is a technique by which different flows can be transmitted through different RATs. In this case, the automatic switching for each flow may be automatic switching for one or each of at least one service/IP flow. That is, the bandwidth segregation may be automatic switching for each data radio bearer (or evolved packet system (EPS) bearer)). The bandwidth aggregation is a technique by which even the same flow can be transmitted through different RATs in unit of data.

3) Wi-Fi only access: The UE accesses only the Wi-Fi. The UE transmits/receives control information or data through the Wi-Fi. This scenario may occur in a special case after the scenario 2-1) or 2-2), and a technique for controlling a link of 3GPP LTE based on the Wi-Fi may be defined. For example, control information for paging or radio link failure may be received through the Wi-Fi among links of the 3GPP LTE.

4-1) Cellular/Wi-Fi duplex and cellular DL only: The UE simultaneously accesses the 3GPP LTE and the Wi-Fi. However, in this case, it is difficult to perform UL transmission through the 3GPP LTE. That is, the UL transmission must be performed through the Wi-Fi.

4-2) Cellular/Wi-Fi duplex and cellular UL only: The UE simultaneously accesses the 3GPP LTE and the Wi-Fi. However, in this case, it is difficult to perform DL reception through the 3GPP LTE. That is, the DL reception must be performed through the Wi-Fi.

As described above, the conventional cellular/WLAN interworking technique is designed based on a UE request, and thus interworking between a cellular network and a WLAN network is not required. Accordingly, a specific network server manages WLAN information, and an inter-RAT handover is possible at the UE request. Further, since only flow mobility/IP-flow mapping is supported at a network level in the absence of a control at a radio level, the UE can access a plurality of RATs. For such a reason, the conventional cellular/WLAN interworking technique does not require any control connection between the cellular network and the WLAN network. However, since the cellular/WLAN interworking technique based on the UE request cannot correctly recognize a network situation and an RAT is selected based on the UE, there is a limitation in the increase in an overall network efficiency. In order to improve quality of service (QoS) of the UE and to increase the overall network efficiency through the cellular/WLAN interworking, there is a need to provide a tightly-coupled multi-RAT management technique based on a network instead of the cellular/WLAN interworking based on the UE request. In the tightly-coupled multi-RAT management technique, a direct control connection needs to be configured between different RATs at a network level to perform more efficient and rapid interworking, and data of the UE needs to be transmitted through an optimal RAT by an entity of corresponding interworking.

As the number of small cells is increased and a UE capability is improved, the UE can sufficiently transmit a UL signal to an eNB. However, due to insufficient transmit power or capability of the eNB, the UE may not be able to sufficiently receive a DL signal. Alternatively, it may be difficult to receive the DL signal from the eNB due to an influence of interference from a neighboring cell. This corresponds to the scenario 4-2) described with reference to FIG. 6. In this case, the UE must be able to recognize that a corresponding region is a region in which only UL transmission is possible, and accordingly, must be able to receive DL information of a cellular network through a WLAN network.

In addition, a UE which has a difficulty in DL reception must be able to receive DL system information through the WLAN and transmit an RACH preamble to the cellular network in a UL. The eNB must be able to acquire a timing advance (TA) of the UE and transmit it to an AP. Thereafter, the AP must transmit the received TA to the UE so that the UE can use the acquired TA when performing UL transmission to the cellular network.

Hereinafter, it is assumed that an entity for controlling interworking is an entity of 3GPP LTE. That is, it is assumed that an interworking function is implemented by using any one of the conventional eNB, MME, or an interworking management entity (IWME) newly defined for interworking. The interworking function relates to an interworking related procedure which may occur between eNB-UE or eNB-AP, and the entity for controlling the interworking may store/manage AP information. In addition, the entity for controlling the interworking may also store/manage information of multi-RAT UEs located in its coverage.

Further, it is assumed that the AP of Wi-Fi, the 3GPP LTE entity such as the eNB, MME, or the IWME, etc., can share information with each other. The information sharing may be performed through a wired control connection described with reference to FIG. 5. That is, the information may be shared through a new interface configured through a backbone network. Alternatively, the information sharing may be performed through a wireless control connection between the eNB and the AP described with reference to FIG. 5. The AP having an air interface with the eNB may be called an eAP (enhanced AP). The eAP must support not only MAC/PHY of Wi-Fi but also a 3GPP LTE protocol stack, and may play a role of the UE from a perspective of the eNB to communicate with the eNB. Alternatively, the sharing information may be performed through a conventional server located out of the network, such as ANDSF. Meanwhile, it is assumed in an embodiment of the present invention described hereinafter that the eNB knows in advance information regarding a region in which UL transmission of the UE is impossible in a cell according to channel information collected from the UE in a previous cell or a configuration of an operator.

FIG. 7 shows an example of a network structure of 3GPP LTE/Wi-Fi interworking according to an embodiment of the present invention. In order to allow a multi-RAT UE to effectively use a 3GPP LTE/Wi-Fi convergence network, as described above, a 3GPP LTE entity such as an eNB, an MME, or an IWME, etc., may manage information of an AP or the multi-RAT UE by using the following four methods.

1) Use of an air interface between the eNB and the AP: The eNB may control the AP similarly to the UE by using a wireless control connection with the AP.

2) Use of a backhaul interface between the eNB and the AP: The eNB may control the AP by using a wired control connection with the AP.

3) Use of a control interface between the MME and the AP: The MME may control the AP by using a control connection with the AP. In this case, the control connection may use both air/backhaul interfaces.

4) Use of a control interface between the IWME and the AP: The IWME may control the AP by using a control connection with the AP. In this case, the control connection may use both air/backhaul interfaces.

FIG. 8 shows an example of an RRC connection establishment procedure according to an embodiment of the present invention. In step S100, the NAS layer of the UE transmits a NAS service request to the RRC layer of the UE. In step S110, the RRC layer of the UE performs access barring check, and if not barred, in step S120, the RRC layer of the UE transmits an RRC connection request to the lower layer of the UE (i.e., PDCP/RLC/MAC layer, L1). In step S130, the UE detects that only UL transmission is available in 3GPP LTE. Accordingly, in step S131, the UE performs a random access procedure with an eNB through AP. In step S140, the lower layer of the UE transmits an RRC connection setup to the RRC layer of the UE. In step S150, the RRC layer of the UE performs SRB1 setup. In step S160, the RRC layer of the UE transmits an RRC connection setup complete to the lower layer of the UE. In step S170, the lower layer of the UE transmits an RRC connection setup complete to the eNB. In step S180, the eNB transmits a DL dedicated message addressed to the C-RNTI to the UE.

Hereinafter, a method of acquiring time synchronization of 3GPP LTE supporting only a UL in a convergence system of 3GPP LTE and Wi-Fi is described according to an embodiment of the present invention. According to the embodiment of the present invention, an eNB may instruct an AP to transmit a beacon of the AP or a transmission timing of specific DL data of the AP according to transmission/reception timing of the DL data. Accordingly, a UE connected to 3GPP LTE in which only UL transmission is possible can predict a TA for adjusting UL synchronization with the 3GPP LTE. Hereinafter, it is assumed that an operator or an eNB configures a corresponding AP as an AP which substitutes the transmission of DL data of the eNB. It is also assumed that the AP can report that the AP is an AP which supports the UE to communicate with a 3GPP LTE cell through a beacon or a probe response message. It is also assumed that the AP can receive a signal of 3GPP LTE. It is also assumed that a TA between an eNB and a UE in a coverage of the AP can be approximated to a TA between the AP and the eNB, and an error between the two values is within a TA error tolerance range.

1) First, when the eNB recognizes that UL transmission of the UE is impossible, the eNB may report a specific preamble (set) and PRACH resource allocation for an AP to the AP capable of substituting the transmission of its DL signal. The AP may transmit the specific preamble (set) for the AP to the eNB through the PRACH resource, aligned with a DL reception timing. Accordingly, TA_a which is a TA between the eNB and the AP may be obtained, and the eNB may transmit the obtained TA_a to the AP. The AP may transmit the obtained TA_a to a UE which performs initial attach to the eNB through a beacon or a probe response message. Alternatively, the AP may transmit the obtained TA_a to the UE through a random access response message.

2) The AP may transmit a beacon or specific DL frame for the UE to the UE at a timing at which DL data of the eNB is received. The beacon or the specific DL frame may include an indicator indicating that the beacon or the specific DL frame is being transmitted aligned with the timing at which the DL data of the eNB is received and/or a subframe index, system frame number, etc., of the DL data. Alternatively, a beacon or specific DL frame transmitted before a beacon or specific DL frame to be transmitted aligned with the timing at which the DL data of the eNB is received may include information indicating a specific time interval after which the beacon or specific DL frame to be transmitted aligned with the timing at which the DL data of the eNB is received will be transmitted. Alternatively, if the beacon or specific DL frame for the UE is transmitted at the timing at which the DL data of the eNB is received, it may imply that the beacon or specific DL frame for the UE is transmitted to the UE after a specific time (e.g., N subframe intervals or M slot intervals) predetermined between the eNB and the UE at the timing at which the DL data of the eNB is received. In this case, the AP may inform the eNB of information regarding a specific time interval after which the beacon or the specific DL frame will be transmitted after receiving the DL data.

The UE which receives the beacon or the specific DL frame from the AP may transmit a UL frame to the AP at a timing at which the beacon or the specific DL frame is received (or after a predetermined specific time). In this case, the beacon or specific DL frame may be a beacon or specific DL frame transmitted aligned with the timing at which the DL data of the eNB is received, or may be a beacon or specific DL frame for calculating TA_b. Information thereof may be indicated. If data cannot be transmitted at a timing agreed by using a distributed coordination function (DCF), the UE may transmit a UL frame by including both of a timing at which DL data is received and a timing at which the UL frame is transmitted. The AP may calculate the TA_b through the timing at which the DL data is received from the eNB and the timing at which the UL frame is received from the UE, and may report the acquired TA_b to the eNB.

3) The UE which receives the beacon or the specific DL frame from the AP may transmit the preamble to the eNB aligned with the timing at which the beacon or the specific DL frame is received under the assumption that an error of the timing at which the beacon or the specific DL frame is received and the timing at which the DL data is received from the eNB can be neglected. If the UE transmits the preamble to the eNB at the timing at which the beacon or the specific DL frame is received, it may imply that the UE transmits the preamble to the eNB after a specific time (e.g., N subframe intervals or M slot intervals) predetermined between the eNB and the UE at the timing at which the beacon or the specific DL frame is received. The specific time predetermined between the eNB and the UE may vary depending on a subframe configuration of 3GPP LTE. For this, the UE may acquire information regarding a resource for transmitting a specific subframe and preamble by using 3GPP LTE SIB information acquired from the AP. Meanwhile, if the UE performs initial attach by using Wi-Fi (in particular, C-RNTI may be acquired by being identified through an IP address or the like of the UE in Wi-Fi), a procedure of transmitting the preamble by the UE to the eNB may be omitted. In this case, the UE may perform UL transmission to the eNB at a timing earlier by TA_a than the timing at which the beacon or the specific DL frame is received.

Various methods may be used to decrease the error between the timing at which the beacon or the specific DL frame is received from the AP and the timing at which the DL data is received from the eNB. For example, the AP may request the UE to transmit a TA between the UE and the eNB, and the UE may transmit a response signal for this to the AP. The AP which obtains the TA between the UE and the eNB may transmit a TA having a small error statistically to the UE. Alternatively, the AP may transmit a more accurate TA to the UE by using UE’s measurement report information (i.e., measurement information of an AP to which the UE is connected, measurement information of a neighbor AP), location information of the neighbor AP, a TA between the AP and the UE, etc.

The eNB which receives the preamble from the UE may calculate TA_d which is a TA of the UE on the basis of the TA_a, the TA_b, and a difference Y between the timing at which the eNB transmits the DL data to the AP and the timing at which the preamble is received from the UE. That is, TA_d = Y – TA_a – TA_b. The eNB may report the calculated TA_d to the UE.

The timing or the timing difference described above may be represented by a subframe index, a system frame number, etc., or may be represented by an absolute time.

FIG. 9 shows an example of acquiring time synchronization according to an embodiment of the present invention. Referring to FIG. 9, an eNB transmits DL data to an AP, and the AP receives it after TA_a. The AP transmits a beacon at a timing at which the DL data is received, and the UE receives it after TA_b. The TA_b may be calculated through a UL frame transmitted to the AP at a timing at which the UE receives the beacon. Under the assumption that an error of the timing at which the UE receives the beacon and the timing at which the DL data is received from the eNB can be neglected, the UE transmits a preamble to the eNB at the timing at which the beacon is received. The eNB may receive the preamble, and may finally calculate TA_d which is a TA between the eNB and the UE on the basis of the TA_a, the TA_b, and a difference Y between the timing at which the eNB transmits the DL data and the timing at which the preamble is received.

FIG. 10 shows another example of a method for acquiring time synchronization according to an embodiment of the present invention. In step S100, the eNB of 3GPP LTE transmits a DL data to the AP of Wi-Fi. In step S110, the eNB acquires a first TA which is a timing difference between a timing at which the eNB transmits the DL data and a timing at which the AP receives the DL data. That is, the first TA may be TA_a described above. In step S120, the eNB acquires a second TA which is a timing difference between a timing at which the AP transmits a DL frame to a multi-RAT UE and a timing at which the multi-RAT UE receives the DL frame. That is, the second TA may be TA_b described above. In step S130, the eNB receives a preamble from the multi-RAT UE. In step S140, the eNB acquires a third TA based on the first TA, the second TA, and a timing difference between a timing at which the eNB transmits the DL data and a timing at which the eNB receives the preamble. That is, the third TA may be TA_d described above. The third TA is acquired by subtracting the first TA and the second TA from the timing difference between the timing at which the eNB transmits the DL data and the timing at which the eNB receives the preamble. The eNB may inform the multi-RAT UE of the third TA.

FIG. 11 shows another example of a method for acquiring time synchronization according to an embodiment of the present invention. In step S200, the AP receives a DL data from the eNB. In step S210, the AP transmits a DL frame at a timing that the AP receives the DL data to the multi-RAT UE. In step S220, the AP receives a UL frame from the multi-RAT UE. In step S230, the AP acquires a TA which is a timing difference between a timing at which the AP transmits the DL frame and a timing at which the multi-RAT UE transmits the UL frame. That is, the TA may be TA_b described above. In step S240, the AP transmits the acquired TA to the eNB.

An eNB 800 includes a processor 810, a memory 820, and a radio frequency (RF) unit 830. The processor 810 may be configured to implement proposed functions, procedures, and/or methods in this description. Layers of the radio interface protocol may be implemented in the processor 810. The memory 820 is operatively coupled with the processor 810 and stores a variety of information to operate the processor 810. The RF unit 830 is operatively coupled with the processor 810, and transmits and/or receives a radio signal.

An AP or UE 900 may include a processor 910, a memory 920 and an RF unit 930. The processor 910 may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor 910. The memory 920 is operatively coupled with the processor 910 and stores a variety of information to operate the processor 910. The RF unit 930 is operatively coupled with the processor 910, and transmits and/or receives a radio signal.

The

processors

810, 910 may include application-specific integrated circuit (ASIC), other chipset, logic circuit and/or data processing device. The

memories

820, 920 may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and/or other storage device. The

RF units

830, 930 may include baseband circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in

memories

820, 920 and executed by

processors

810, 910. The

memories

820, 920 can be implemented within the

processors

810, 910 or external to the

processors

810, 910 in which case those can be communicatively coupled to the

processors

810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure.

Claims

A method for acquiring, by an evolved NodeB (eNB) of a first system, time synchronization in a wireless communication system, the method comprising:
transmitting, by the eNB of the first system, a downlink (DL) data to an access point (AP) of a second system;
acquiring, by the eNB of the first system, a first timing advance (TA) which is a timing difference between a timing at which the eNB transmits the DL data and a timing at which the AP receives the DL data;
acquiring, by the eNB of the first system, a second TA which is a timing difference between a timing at which the AP transmits a DL frame to a user equipment (UE) and a timing at which the UE receives the DL frame;
receiving, by the eNB of the first system, a preamble from the UE; and
acquiring, by the eNB of the first system, a third TA based on the first TA, the second TA, and a timing difference between a timing at which the eNB transmits the DL data and a timing at which the eNB receives the preamble.
The method of claim 1, wherein the third TA is acquired by subtracting the first TA and the second TA from the timing difference between the timing at which the eNB transmits the DL data and the timing at which the eNB receives the preamble.
The method of claim 1, further comprising, by the eNB of the first system, informing the UE of the third TA.
The method of claim 1, wherein the UE can perform uplink (UL) transmission to the eNB, but cannot perform DL reception from the eNB.
The method of claim 1, wherein acquiring the first TA comprises:
transmitting a specific preamble for the AP and allocating a physical random access channel (PRACH) resource to the AP; and
receiving the specific preamble through the PRACH resource from the AP.
The method of claim 1, wherein the first TA and the second TA are represented by one of a subframe index, a system frame number, or absolute time.
The method of claim 1, wherein the eNB and the AP have a wired control connection through a backbone network, or have a wireless control connection.
The method of claim 1, wherein the first system is 3rd generation partnership project (3GPP) long-term evolution (LTE), and
wherein the second system is Wi-Fi.
The method of claim 1, wherein the UE is a multi radio access technology (RAT) UE which can support both the first system and the second system.
A method for acquiring, by an access point (AP) of a second system, time synchronization in a wireless communication system, the method comprising:
receiving, by the AP of the second system, a downlink (DL) data from an evolved NodeB (eNB) of a first system;
transmitting, by the AP of the second system, a DL frame at a timing that the AP receives the DL data to a user equipment (UE);
receiving, by the AP of the second system, a uplink (UL) frame from the UE;
acquiring, by the AP of the second system, a timing advance (TA) which is a timing difference between a timing at which the AP transmits the DL frame and a timing at which the UE transmits the UL frame; and
transmitting, by the AP of the second system, the acquired TA to the eNB.
The method of claim 10, wherein the DL frame is a beacon frame or a specific DL frame.
The method of claim 10, wherein the DL frame includes at least one of an indicator indicating that the DL frame is transmitted at the timing that the AP receives the DL data, or a subframe index or a system frame number indicating the timing that the AP receives the DL data.
The method of claim 10, wherein transmitting the DL frame at the timing that the AP receives the DL data comprises:
transmitting the DL frame to the UE after a specific time interval from the timing at which the AP receives the DL data.
The method of claim 13, further comprising transmitting, by the AP of the second system, information on the specific time interval to the eNB.
The method of claim 10, wherein the TA is represented by one of a subframe index, a system frame number, or absolute time.