WO2016185285A1

WO2016185285A1 - Method for selecting a ue-to-network relay and synchronization method for relay discovery

Info

Publication number: WO2016185285A1
Application number: PCT/IB2016/000837
Authority: WO
Inventors: Dong Li; Yong Liu; Yun DENG
Original assignee: Alcatel Lucent
Priority date: 2015-05-15
Filing date: 2016-05-13
Publication date: 2016-11-24
Also published as: CN106304257A

Abstract

The present disclosure provides a method for selecting a UE-to-network relay and a synchronization method for relay discovery. The method for selecting a UE-to-network relay comprises steps at a remote UE outside network coverage: transmitting a relay selection request message during a D2D discovery period when the remote UE intends to communicate with the network; switching to a receiving mode after transmitting the relay selection request message; receiving, during the D2D discovery period, relay selection response messages from one or more candidate relay UEs, wherein the one or more candidate relay UEs are within the network coverage and have a relay capability; and selecting a first relay UE from the one or more candidate relay UEs based on the relay selection response messages.

Description

METHOD FOR SELECTING A UE-TO-NETWORK RELAY AND SYNCHRONIZATION METHOD FOR

RELAY DISCOVERY

FIELD OF THE INVENTION

[0001] The present disclosure generally relates to the field of wireless communication, and more specifically to a method for selecting a UE-to-network relay, a synchronization method for relay discovery, and associated apparatuses.

BACKGROUND OF THE INVENTION

[0002] A UE(User Equipment)-to-Network relay (hereinafter referred to as a UE-to-NW relay) is an important topic in long-term evolution (LIE) R13, for connecting a remote UE outside a network coverage with a cellular network, such that the remote UE can communicate with a relevant part (e.g., a rescue control center) in the network. This is particularly significant for public safety applications that may involve out-of-network coverage or partial network coverage, e.g., in some disaster rescue or other critical scenarios.

[0003] Before a relay station that performs the UE-to-NW relay takes effect in relaying data of a remote UE, the first step is to discover such a UE that is within network coverage and has a UE-to-NW relay capability as the relay UE for the remote UE. Additionally, a UE-to-NW relay discovery scheme shall take the mobility of the relay UE and the remote UE into account so as to support service continuity.

[0004] On the other hand, in LIE Rel-12, D2D (device-to-device) discovery is discussed only considering the in-coverage scenario, where the discovering UE is synchronized to network timing through a DL(downlink) reference signal from the base station (eNB). In this D2D discovery, the UE performing D2D discovery may transmit an SLSS (Sidelink Synchronization Signal). However, in Rel-12, the purpose for the discovering UE transmitting the SLSS is to enable inter-cell discovery in an asynchronous network, where a UE correctly receives the discovery signals from UEs in the neighbor cells only by estimating timing offsets of neighboring cells. Therefore, in Rel-12, if the SLSS transmission is trigged by the discovering UE, the discovering UE does not transmit PSBCH (Physical Sidelink Broadcast Channel). In addition, given a discovery resource pool where the discovery signals are transmitted, the UE only transmits the SLSS in one subframe of each discovery period.

SUMMARY OF THE INVENTION

[0005] It may be seen that the current D2D discovery mechanism only supports a D2D discovery with network coverage.

[0006] In view of the above problem, the present disclosure provides a selection/reselection mechanism for the UE-to-NW relay discovery and a synchronization mechanism so as to support relay discovery and synchronization with partial network coverage and without network coverage.

[0007] In accordance with one aspect of the present disclosure, there is provided a method for selecting a UE-to-network relay, comprising steps at a remote UE outside network coverage: transmitting a relay selection request message during a D2D discovery period when the remote UE intends to communicate with the network; switching to a receiving mode after transmitting the relay selection request message; receiving, during the D2D discovery period, relay selection response messages from one or more candidate relay UEs, wherein the one or more candidate relay UEs are within the network coverage and have a relay capability; and selecting a first relay UE from the one or more candidate relay UEs based on the relay selection response messages.

[0008] In accordance with another aspect of the present disclosure, there is provided a synchronization method for UE-to-network relay discovery, comprising steps at a UE within network coverage: receiving a reference signal from a serving base station of the UE; determining, based on the reference signal, a reference signal receiving power from the serving base station, and comparing the reference signal receiving power with a predefined first threshold; and when determining that the reference signal receiving power is lower than the first threshold and the UE has D2D data to transmit, transmitting an SLSS/PSBCH in a preconfigured synchronization resource during a D2D direct communication period, such that a remote UE outside the network coverage can select the UE as a synchronization reference.

[0009] In accordance with yet another aspect of the present disclosure, there is provided a synchronization method for UE-to-network relay discovery, comprising steps at a UE within network coverage, wherein the UE has a relay capability but is not selected as a relay UE by a remote UE outside network coverage, the synchronization method comprising: receiving a dedicated signaling from a serving base station of the UE, the dedicated signaling being for instructing the UE to transmit continuous SLSSs/PSBCHs; and transmitting the continuous SLSSs/PSBCHs according to the dedicated signaling such that the remote UE can select the UE as a synchronization reference.

[0010] In accordance with a further aspect of the present disclosure, there is provided a synchronization method for UE-to-network relay discovery at a UE within network coverage, wherein the UE has relay capability but is not selected as a relay UE by a remote UE outside the network coverage, the synchronization method comprising: detecting an SLSS/PSBCH from the remote UE and determining whether timing of the remote UE is aligned with timing of a serving base station of the UE; and when the UE determines that the timing of the remote UE is misaligned with the timing of the serving base station of the UE, transmitting continuous SLSSs/PSBCHs such that the remote UE can select the UE as a synchronization reference.

[0011] In accordance with a yet further aspect of the present disclosure, there is provided a synchronization method for UE-to-network relay discovery, comprising steps at a UE in network coverage, wherein the UE is selected by a remote UE outside the network coverage as a relay UE for the remote UE, the synchronization method comprising: reporting to a serving base station of the UE that the UE is selected by the remote UE as the relay UE; and transmitting to the remote UE a continuous SLSSs/PSBCHs as a synchronization reference for the remote UE.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0012] The present disclosure will be better understood and other objectives, details, features and advantages of the present disclosure will become more obvious after reading the depiction of the preferred embodiments of the present disclosure with reference to the accompanying drawings. In the accompanying drawings:

[0013] Fig. 1 illustrates a flow diagram of a method for selecting a UE-to-NW relay according to one aspect of the present disclosure;

[0014] Fig. 2 illustrates a schematic diagram of partitioning a discovery resource pool for D2D discovery; [0015] Fig. 3A illustrates a flow diagram of a synchronization method for UE-to-NW relay discovery according to one embodiment of the present disclosure, Fig. 3B illustrates a schematic diagram of a system corresponding to the embodiment as shown in Fig. 3A, and Fig. 3C illustrates a time sequence diagram corresponding to the embodiment as shown in Fig. 3A;

[0016] Fig. 4A illustrates a flow diagram of a synchronization method for UE-to-NW relay discovery according to another embodiment of the present disclosure, Fig. 4B illustrates a schematic diagram of a system corresponding to the embodiment as shown in Fig. 4A, and Fig. 4C illustrates a time sequence diagram corresponding to the embodiment as shown in Fig. 4A;

[0017] Fig. 5A illustrates a flow diagram of a synchronization method for UE-to-NW relay discovery according to yet another embodiment of the present disclosure, and Fig. 5B illustrates a schematic diagram of a system corresponding to the embodiment as shown in Fig. 5A;

[0018] Fig. 6A illustrates a flow diagram of a synchronization method for UE-to-NW relay discovery according to still yet another embodiment of the present disclosure, and Fig. 6B illustrates a schematic diagram of a system corresponding to the embodiment as shown in Fig. 6A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, the preferred embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings. Although the accompanying drawings show the preferred embodiments of the present disclosure, it should be noted that the present disclosure may be implemented in various manners, not limited to the embodiments illustrated here. Instead, these embodiments are provided to make the present disclosure more thorough and complete so as to be capable of conveying the scope of the present disclosure completely to those skilled in the art.

[0020] Relay Selection/Reselection Mechanism For Enhanced UE-To-NW Relay Discovery:

[0021] The proposed solution comprises two main parts: partitioning of a discovery resource pool and a relay selection/reselection procedure performed by a remote UE.

[0022] Fig. 1 illustrates a flow diagram of a method 100 for selecting a UE-to-NW relay according to one aspect of the present disclosure. The method 100 is performed by a remote UE. As illustrated in Fig. 1, it is supposed the remote UE is a UE outside network coverage or in partial network coverage (i.e., a UE outside the network or at an edge of the network), and UE1-UE4 are UEs in network coverage (i.e., UEs within the network).

[0023] In step 101, when a remote UE located outside the network coverage intends to communicate with the network, it transmits a relay selection request message.

[0024] In one implementation, the relay selection request message may include relevant information, such as one or more of a message type (i.e., a relay discovery request), an identifier of the remote UE (for indicating a source of the relay selection request), a group identifier (i.e., an identifier of the group to which the remote UE belongs), and etc.

[0025] In one implementation, a pre-configured discovery resource pool for D2D discovery is partitioned into two parts, referred to as a request zone and a response zone, respectively. In such an implementation, the relay selection request message is transmitted in the request zone. The response zone is for transmitting relay selection response messages, as stated below.

[0026] Fig. 2 shows a schematic diagram of partitioning a discovery resource pool for D2D discovery. As illustrated in Fig. 2, a discovery resource pool in a D2D discovery period is partitioned, in time domain, into a request zone and a response zone, where the request zone is for transmitting a message like a relay discovery request, while the response zone may be used for transmitting a message like a relay discovery response. Fig. 2 illustrates a time domain partitioning for the discovery resource pool. However, the present disclosure is not limited thereto. Those skilled in the art will appreciate that other manners of partitioning the discovery resource pool are also feasible.

[0027] Partitioning of the discovery resource pool can be implemented simply. In one implementation, several bits (e.g., 5 or 6 bits at most) may be used to indicate a number of subframes as the request zone. For example, 0 may be used to indicate that there is no resource pool partitioning, while 1 indicates that the first subframe of the configured discovery resource pool is used as the request zone (such that the remaining subframes are used as the response zone), etc. In another implementation, several resource pool partitioning modes may be predefined in a system specification, and indexes may be used to pre-configure the resource pool partitioning mode in use. For example, four resource pool partitioning modes {0, 8, 16, 32} may be defined, wherein the numbers represent the number of subframes of the request zone from the beginning of the resource pool, and it may use only two bits to index the size of the preconfigured request zone.

[0028] After the relay selection request message is transmitted in step 101 (e.g., before the beginning of the response zone) in step 101, the remote UE switches to a receiving mode.

[0029] In step 102, the relay selection request message of the remote UE is successfully received by some UEs (referred to as candidate relay UEs) having a relay capability within the network (e.g., it is supposed that UE1, UE2 and UE3 in Fig. 1 have received the message), and each of these candidate relay UEs transmits a relay selection response message to the remote UE to respond to the relay selection request message from the remote UE.

[0030] Here, from the perspective of the candidate relay UE, once it detects a relay selection request message and decides that it can take the relay role based on its own condition and capability, it will transmit the relay selection response message to the remote UE.

[0031] In one implementation, the relay selection response message includes information for indicating existence of the candidate relay UE and some information related to its relay functions (e.g., one of more of identifier of the candidate relay UE, identifier of the corresponding remote UE, backhaul link quality, remaining relay capacity, etc.).

[0032] In one implementation, the candidate relay UE transmits the relay selection response message in the response zone of the discovery resource pool.

[0033] By partitioning the discovery resource pool, in the relay discovery, the remote UE may configure its relay selection request message to be transmitted in the request zone, and switches to the receiving mode before the beginning of the response zone so as to receive and decode a relay selection response message transmitted by the potential relay UE. In this way, the following advantages may be achieved:

[0034] 1) The discovery response latency can be greatly reduced compared with a scheme that the discovery response message is scheduled in a discovery resource pool of a next discovery period or a scheme that the discovery response message is scheduled in the direct communication resource pool after the transmission of discovery request message (although the latency in the latter scheme is much better than that in the former scheme due to a large value of the configurable discovery period). Generally speaking, as small response latency as possible is highly desired, especially in public safety events.

[0035] 2) The half-duplex constraints for request/response messages can be avoided. For example, if the resource pool is not partitioned, it is needed to configure the discovery request message and the discovery response message into the discovery resource pool (i.e., the relay UE can transmit the discovery response message in the discovery resource pool of the same or the next discovery period depending on the specific timing), and if the remote UE does not select a relay UE, it may retransmit the discovery request message in the next discovery period. In this case, there may be collisions between the discovery request message retransmitted by the remote UE and the discovery response message(s) transmitted by some relay UE(s). Through partitioning of the resource pool, the issue can be avoided.

[0036] Furthermore, in one implementation, the relay selection request message and the relay selection response message may be transmitted using the selected or scheduled resources in the request zone or response zone in multicast, broadcast or unicast.

[0037] Next, in step 103, the remote UE selects a relay UE from the candidate relay UEs based on the received relay selection response messages from the candidate relay UEs.

[0038] In one implementation, the remote UE may select a relay UE based on a backhaul link quality between the candidate relay UE and its serving base station carried in the relay selection response message and/or a D2D link quality between the candidate relay UE and the remote UE carried in the relay selection response message or calculated by the remote UE.

[0039] More specifically, the remote UE may compare the D2D link quality between the candidate relay UE and the remote UE and/or the backhaul link quality between the candidate relay UE and the base station with respective predefined thresholds, list all candidate relay UEs that meet the selection standard to establish a candidate relay UE list, and select a best candidate relay UE as its relay UE. In Fig. 1, it is supposed that UE1 is selected as the relay UE by the remote UE.

[0040] On the other hand, if in step 103, the remote UE does not select a satisfactory relay UE, the remote UE will continuously transmit a relay selection request message in the request zone of the next discovery period (i.e., returning to step 101).

[0041] Next, in step 104, the remote UE establishes a D2D connection with the selected relay UE (i.e., UE1).

[0042] Creating the D2D connection comprises: first performing cross-authentication and D2D link security setting between the remote UE and the relay UE, and then modifying a PDN (Public Data Network) connection and/or EPS (Evolved Packet System) carrier between the relay UE and the core network based on the remote UE specific information (e.g., GUTI (Global Unique Temporary Identifier) of the remote UE). Then, the remote UE acknowledges the relay selection to the remote UE, and then a data traffic between the remote UE and the network may be started via relaying of the relay UE.

[0043] Here, the procedure of creating the D2D connection between the remote UE and the relay UE is similar to that in the prior art, and is thus not detailed here.

[0044] Now, a D2D connection is successfully established between the remote UE outside the network coverage and the relay UE within the network coverage, and thus the remote UE may communicate with the base station and the core network via the relay UE.

[0045] Further, the method 100 may also comprise a step 105 in which the relay UE may periodically transmit a relay monitoring message after the remote UE establishes the D2D connection with the remote UE (e.g., during the course when the relay UE serves the remote UE).

[0046] Here, transmission of the relay monitoring message may be provided in the request zone or response zone of the discovery resource pool. In other words, the resource for transmitting the relay monitoring message may be selected in the entire discovery resource pool, regardless the partitioning of the resource pool. In this way, severe interference on some specific resource zones may be avoided.

[0047] Next, in step 106, the remote UE may determine the D2D link quality between the relay UE and the remote UE and/or the backhaul link quality between the relay UE and the base station based on the relay monitoring message.

[0048] In one implementation, the backhaul link quality between the relay UE and the base station may be measured by the relay UE and carried in the relay monitoring message.

[0049] In one implementation, the D2D link quality between the relay UE and the remote UE may be derived by measuring the reference signal in the relay monitoring message by the remote UE, for example.

[0050] Furthermore, in order to eliminate the effect of small scale fading on the measurement result, high layer filtering may also be used on the physical layer measurement results in step 106.

[0051] Next, in step 107, the remote UE determines whether the D2D link quality and/or backhaul link quality determined in step 106 is lower than respective predefined thresholds.

[0052] For example, due to mobility of the remote UE and/or the relay UE, the D2D link quality between the remote UE and the relay UE or the backhaul link quality between the relay UE and the base station becomes worse and lower than the corresponding predefined threshold (it is determined to be yes in step 107), the remote UE determines that the previously selected relay UE (i.e., UE1) is not suitable for acting as the relay UE of the remote UE. In this case, the remote UE will trigger the following relay reselection procedure.

[0053] However, the present disclosure is not limited thereto. Those skilled in the art may understand that even neither of the D2D link quality and the backhaul link quality is decreased, the remote UE may still initiate the following relay reselection procedure so as to reselect a better or more suitable relay UE.

[0054] The relay reselection procedure is similar to the relay selection procedure as depicted above in conjunction with steps 101-104, except that the relay reselection request message should further include the identifier of the previously selected relay UE, and the relay reselection response message may include the D2D link quality between the previously selected relay UE and the new candidate relay UE(s), so as to be available for potential data traffic migration, thereby supporting service continuity, as will be detailed hereinafter.

[0055] On the other hand, if neither of the D2D link quality and the backhaul link quality is deteriorated (it is determined to be no in step 107), the remote UE may continue periodically receiving the relay monitoring message from the relay UE.

[0056] In step 108, when the remote UE determines that the D2D link quality and/or backhaul link quality determined in step 106 is lower than respective predefined thresholds (it is determined to be yes in step 107), the remote UE transmits a relay reselection request message to request for reselecting a relay UE.

[0057] Similar to the relay selection request message, the relay reselection request message may include relevant information, such as one or more of a message type (i.e., a relay discovery request), an identifier of the remote UE (for indicating a source of the relay reselection request), an identifier of the group (i.e., an identifier of the group to which the remote UE belongs), and etc.

[0058] The relay reselection request message may indicate quality degradation of the D2D link and/or backhaul link, and therefore, it is required to reselect another relay UE.

[0059] Furthermore, in one implementation, the relay reselection request message may also include an identifier of the previously selected relay UE (i.e., UE1) such that the D2D link quality between the previously selected relay UE and a potential new relay UE may be considered in relay reselection, so as to be available for potential transmission of data from the previous relay UE to the new relay UE, thereby supporting service continuity.

[0060] Similar to the relay selection request message, the relay reselection request message may be configured in the request zone of the discovery resource pool for transmission.

[0061] Similarly, after the remote UE transmits the relay reselection request message, in more general, before the beginning of the response zone, the remote UE switches to the receiving mode.

[0062] Next, in step 109, the relay reselection request message of the remote UE is successfully received by candidate relay UEs within the network (e.g., it is supposed that UE1, UE2, and UE3 in Fig. 1 have received the message), and each of these candidate relay UEs transmits a relay reselection response message to the remote UE so as to respond to the relay reselection request message from the remote UE.

[0063] Here, the relay reselection response message is similar to the relay selection response message, except that the relay reselection response message may also include D2D link quality between other candidate relay UEs (i.e., other candidate relay UEs than the current relay UE, e.g., UE2 and UE3 in Fig. 1) and the current relay UE (e.g., UE1). The reason for including this information is for possible data traffic migration between the new relay UE and the old relay UE, so as to support service continuity.

[0064] Similar to the relay selection response message, the relay reselection response message may be configured in the response zone of the discovery resource pool for transmission.

[0065] Next, in step 110, the remote UE reselects a new relay UE from the candidate relay UEs based on the received relay reselection response messages from the candidate relay UEs.

[0066] In one implementation, the remote UE may perform relay reselection based on the D2D link quality between the remote UE and the new relay UE, the backhaul link quality between the new relay UE and its serving base station, and the D2D link quality between the previous relay UE and the new relay UE. The remote UE will update a list of candidate relay UEs based on the detected relay reselection response messages and relevant measurement values, and selects a best candidate relay UE as the reselected relay UE. In Fig. 1, it is supposed that UE3 is reselected by the remote UE as the relay UE.

[0067] On the other hand, if the remote UE fails to reselect a satisfactory relay UE in step 110, then the remote UE will continue transmitting the relay reselection request message in the request zone of the next discovery period (i.e., returning to step 108).

[0068] Next, in step 111, the remote UE establishes a D2D connection with the reselected relay UE (i.e., UE3), similar to step 104.

[0069] Synchronization Mechanism For The UE-To-NW Relay Discovery:

[0070] As stated in the Background, the existing D2D discovery only considers the scenario with network coverage, where the discovering UE is synchronized to the network always through a downlink reference signal of the base station, and in such a D2D discovery, the discovery UE only transmits an SLSS, rather than a PSBCH, in one subframe of each discovery period.

[0071] However, for the UE-to-NW relay, it is desired that the continuous transmission of the SLSSs/PSBCHs of the discovering UE is as less as possible, so as to minimize the interference with the cellular network and the energy consumption. However, once a UE having a relay capability starts relaying for one or more remote UEs, it continuously transmits SLSSs/PSBCHs based on the system configurations for synchronization, so as to provide a permanent and reliable synchronization reference to the remote UEs.

[0072] In view of the above, the present disclosure devises a scheme below for a synchronization mechanism for UE-to-NW relay discovery.

[0073] Fig. 3A illustrates a flow diagram of a synchronization method 300 for UE-to-NW relay discovery according to one embodiment of the present disclosure. Fig. 3B illustrates a schematic diagram of a system corresponding to the embodiment as illustrated in Fig. 3A. Fig. 3C illustrates a time sequence diagram corresponding to the embodiment of Fig. 3A. Herein, in Fig. 3C, blocks labeled with 1, 2, 3 indicate a PSCCH (Physical Sidelink Control Channel) resource pool, a PSSCH (Physical Sidelink Shared Channel) resource pool and a PSDCH (Physical Sidelink Discovery Channel) resource pool, respectively.

[0074] As shown in Fig. 3B, for the synchronizing method 300, it is supposed that UE1 and UE2 are within network coverage, while the remote UE is outside network coverage; and UE1 is only involved in D2D discovery, while UE2 is involved in D2D direct communication.

[0075] The method 300 comprises step 310, wherein the UE2 receives a RS (Reference Signal) from its serving base station.

[0076] Next, in step 320, UE2 determines a RSRP (Reference Signal Receiving Power) from its serving base station based on the RS, and compares it with a predefined threshold.

[0077] In step 330, when it is determined that the RSRP is lower than the threshold and UE2 has D2D data to transmit, UE2 transmits the SLSS/PSBCH in the configured synchronization resource during a D2D direct communication period.

[0078] Here, if the UE2 has no data to transmit during some D2D direct communication periods, the UE2 might not transmit the SLSS/PSBCH during the direct communication periods.

[0079] Next, in step 340, the remote UE determines a S-RSRP (Sidelink Reference Signal Receiving Power) from UE2 based on the received PSBCH of the UE2, and when the determined S-RSRP exceeds a predefined threshold, UE2 is selected as a synchronization reference (SyncRef).

[0080] In the method 300, because UE1 is only involved in D2D discovery and SLSS is only transmitted in the configured discovery resource pool (as shown in Fig. 3C), the remote UE cannot measure the S-RSRP of UE1, such that UE1 cannot be selected as the synchronization source by the remote UE.

[0081] Fig. 4A illustrates a flow diagram of a synchronization method 400 for UE-to-NW relay discovery according to another embodiment of the present disclosure. Fig. 4B illustrates a schematic diagram of a system corresponding to the embodiment of Fig. 4A. Fig. 4C illustrates a time sequence diagram corresponding to the embodiment of Fig. 4A. Herein, in Fig. 4C, blocks labeled with 1, 2, 3 indicate a PSCCH resource pool, a PSSCH resource pool and a PSDCH resource pool, respectively.

[0082] As illustrated in Fig. 4B, as far as the synchronization method 400 is concerned, similar to the method 300, it is supposed that UE1 and UE2 are within network coverage, while the remote UE is outside network coverage; and UE1 is only involved in D2D discovery, while UE2 is involved in D2D direct communication.

[0083] The method 400 comprises step 410, in which UE1 transmits an SLSS in the configured discovery resource pool. Here, UE1 has a UE-to-NW relay capability but is not selected by the remote UE as an actual relay UE. In this case, UE1 will follow the D2D discovery synchronization mechanism in LIE R12, i.e., only transmitting an SLSS once in each discovery period.

[0084] In one implementation, UE1 determines an RSRP from its serving base station based on a RS from its serving cell and compares it with a predefined threshold. When the measured RSRP is lower than the threshold, UE1 transmits an SLSS once in the first subframe of the configured discovery resource pool (if the first subframe belongs to the synchronization resource). Otherwise, the SLSS is transmitted in the last synchronization subframe before the configured discovery resource pool.

[0085] Next, in step 420, UE1 receives a dedicated signaling from its serving base station. The dedicated signaling is used for instructing the UE to transmit continuous SLSSs/PSBCHs.

[0086] For example, when the serving base station of the UE1 knows that its neighboring base station crashes, it may explicitly transmit the dedicated signaling to UE1. For example, the base station knows that UE1 has a relay capability and is located at an edge of network coverage, such that transmitting the SLSSs/PSBCHs by the UE1 can provide a reliable synchronization reference to UEs in partial network coverage and outside network coverage, without causing serious interference with other transmissions in the base station.

[0087] Next, in step 430, UE1 transmits continuous SLSSs/PSBCHs based on the received dedicated signaling.

[0088] In step 440, the remote UE selects UE1 as the synchronization reference (SyncRef), similar to step 340.

[0089] Note that because UE1 and UE2 use the same synchronization configuration, the SLSSs/PSBCHs transmitted thereby are identical and naturally combined over the air interface (for the direct communication period that UE2 has data to transmit).

[0090] Fig. 5A illustrates a flow diagram of a synchronization method 500 for UE-to-NW relay discovery according to yet another embodiment of the present disclosure. Fig. 5B illustrates a schematic diagram of a system corresponding to the embodiment of Fig. 5A.

[0091] As shown in Fig. 5B, for the synchronization method 500, it is supposed that UE1 is located within network coverage, the remote UE is outside network coverage, and UE1 is only involved in D2D discovery.

[0092] The method 500 comprises step 510, wherein UE1 transmits an SLSS in the configured discovery resource pool. Here, UE1 has a UE-to-NW relay capability but is not selected by the remote UE as an actual relay UE. In this case, UE1 will follow the D2D discovery synchronization mechanism in LTE R12, i.e., only transmitting an SLSS once in each discovery period.

[0093] In one implementation, UE1 determines an RSRP from its serving base station based on a RS from its serving cell and compares it with a predefined threshold. When the measured RSRP is lower than the threshold, UE1 transmits an SLSS once in the first subframe of the configured discovery resource pool (if the first subframe belongs to the synchronization resource). Otherwise, the SLSS is transmitted in the last synchronization subframe before the configured discovery resource pool. [0094] Next, in step 520, UEl detects the SLSS/PSBCH from the remote UE, and determines whether timing of the remote UE is aligned with timing of the serving base station of UEl.

[0095] Here, for example, it is supposed that the remote UE has some data to transmit (e.g., it intends to discover the UE-to-NW relay UE by transmitting the relay discovery request message), before its data transmission, the remote UE first searches for an existing synchronization reference. If a certain SyncRef UE is discovered, the remote UE adjusts its timing so as to synchronize to the synchronization reference. Otherwise, it will take a role of an independent SS (Synchronization Source) to transmit the SLSS/PSBCH and data traffic.

[0096] Next, in step 530, when UEl determines that the timing of the remote UE is misaligned with the timing of the serving base station of UEl, UEl transmits continuous SLSSs/PSBCHs to the remote UE.

[0097] In other words, in this case, the remote UE adopts a synchronization reference different from the network to which UEl belongs or acts as an independent synchronization source.

[0098] UEl detecting an SLSS/PSBCH of the UE outside network coverage indicates that the UE transmitting the SLSS/PSBCH is located outside the network coverage but a synchronization reference UE within cellular network coverage has not been found yet, or a synchronization reference UE located outside the network coverage but having an SLSS sequence within the network (i.e., a synchronization reference UE with partial network coverage) has not been found yet.

[0099] In step 540, the remote UE selects UEl as the synchronization reference (SyncRef).

[0100] After step 520 but before step 530, the method may also comprise: UEl transmits an SLSS/PSBCH transmission request to its serving base station and receives an acceptance response message from its serving base station, the acceptance response message being for allowing the UEl to transmit continuous SLSSs/PSBCHs.

[0101] Fig. 6 illustrates a flow diagram of a synchronization method 600 for UE-to-NW relay discovery according to still yet another embodiment of the present disclosure. Fig. 6B illustrates a schematic diagram of a system corresponding to the embodiment of Fig. 6A.

[0102] As illustrated in Fig. 6B, for the synchronization method 600, it is supposed that UEl is within network coverage, remote UE is outside network coverage, and UEl has a UE-to-NW relay capability.

[0103] The method 600 comprises step 610, wherein UEl is selected by the remote UE as a relay UE for the remote UE.

[0104] Next, in step 620, UEl reports to its serving base station that it is selected by the remote UE as a relay UE. In this way, the base station can know periodical SLSS/PSBCH transmission of the relay UE (UEl), thereby avoiding scheduling the relay UE to the resource occupied by the SLSS/PSBCH transmission.

[0105] Next, in step 630, UEl transmits continuous SLSSs/PSBCHs to the remote UE so as to act as a synchronization reference of the remote UE.

[0106] In the embodiment, as illustrated in Fig. 6B, it is supposed that initially, the remote UE uses UE2 as its synchronization reference and transmits a relay discovery request message. After the relay selection procedure, the remote UE selects the UEl as the relay UE. In this case, the relay UE may implicitly continuously transmit the SLSSs/PSBCHs or transmit after the explicit command signaling from the eNB. In this case, the reason why the relay UE continuously transmits SLSSs/PSBCHs is that although the remote UE has a synchronization reference (e.g., UE2 involved in direct data communication), transmission of the SLSS/PSBCH of the synchronization reference is not continuous (i.e., it transmits the SLSSs/PSBCHs not in the direct communication period without data to be transmitted); therefore, by continuously transmitting SLSSs/PSBCHs using the relay UE (UE1) of the remote UE, a permanent and reliable synchronization reference may be provided to the remote UE.

[0107] The present disclosure suggests an enhanced UE-to-NW relay discovery solution, mainly comprising procedures of discovery resource pool partitioning and enhanced relay selection and reselection. With the discovery resource pool partitioning, half-duplex constraints of the relay discovery request and response messages can be avoided, and meanwhile the discovery response latency can be extremely reduced. With the enhanced relay selection/ reselection procedure, the relay reselection is triggered by the remote UE event, and periodical signaling transmission (discovery announcement messages) from all candidate relay UEs, which may lead to energy waste and potential interference to other transmissions, are avoided.

[0108] The present disclosure also suggests an effective synchronization control mechanism for public safety in LTE R13, particularly for UE-to-NW relay UE. With the suggested solution, on one hand, continuous transmission of the SLSS/PSBCH can be made to affect the cellular network as small as possible; on the other hand, continuous transmission of the (relay) UE can provide a permanent and reliable synchronization reference to the remote UE.

[0109] In one or more exemplary designs, functions of the present application may be implemented by hardware, software, firmware, or any combination thereof. In the case of being implemented by software, the functions may be stored on a computer-readable medium as one or more instructions or codes, or transmitted as one or more instructions or odes on the computer-readable medium. The computer-readable medium includes a computer storage medium and a communication medium, wherein the communication medium includes any medium facilitating the computer program to be transmitted from one place to another place. The storage medium may be any available medium that is accessible to a general or dedicated computer. Such computer-readable medium may comprise, for example, but not limited to, RAM, ROM, EEPROM, CD-ROM or other optical disk storage device, magnetic disk storage device, or other magnetic storage device, or any other medium available for carrying or storing desired program code modules in a form of instructions or data structure that is accessible by a general or dedicated computer or by a general or dedicated processor. Moreover, any connection may also be referred to as a computer readable medium. For example, if software is transmitted from a website, a server or other remote source using a co-axial cable, an optical fiber cable, a twisted pair cable, a DSL (Digital Subscriber Line) or radio technologies such as infrared, radio, microwave and the like, then the co-axial cable, optical fiber cable, twisted pair cable, DSL or radio technologies such as infrared, radio, microwave and the like are also included in the definition of the medium.

[0110] Various kinds of exemplary logical blocks, modules and circuits as described in conjunction with the present disclosure may be implemented or executed using a general processor, a DSP (Digital Signal Processor), an ASIC (Application Specific Integrated Circuit), a FPGA (Field Programmable Gate Array) or other programmable logic device, a discrete gate, or a transistor logic, discrete hardware components, or any combinations thereof for performing the functions of the present disclosure. The general-purpose processor may be a microprocessor; or, the processor may also be any common processor, controller, micro-controller or state machine. The processor may also be implemented as a combination of computing devices, e.g., a combination of DSP and microprocessor, a combination of microprocessors, or a combination of one or more microprocessors and DSP kernel, or any other kind of such structure.

[0111] A person of normal skill in the art should understand that various kinds of exemplary logic blocks, modules, circuits and algorithm steps described in conjunction with the embodiments of the present application may also be implemented as electronic hardware, computer software or a combination thereof. In order to clearly illustrate such exchangeability between hardware and software, various exemplary components, blocks, modules, circuits, and steps have been described above in general around their functions. As to whether this function is implemented into hardware or software, it depends on a specific application and a design constraint condition applied onto the entire system. Those skilled in the art may implement the described functions in a flexible manner for each specific application. However, such implementation decision should not be construed as departing from the protection scope of the present disclosure.

[0112] The description above of the present disclosure is to enable any person of normal skill in the art to implement or use the present disclosure. For a person of normal skill in the art, various modifications of the present disclosure are obvious; moreover, a general principle of the definition herein may also be applied to other variations without departing from the spirit and protection scope of the present disclosure. Therefore, the present disclosure is not limited to the instances and design as described here; instead, it is consistent with the broadest scope of the principle and novelty features of the present disclosure.

Claims

1. A method for selecting a UE-to-network relay, comprising steps at a remote UE outside network coverage:

transmitting a relay selection request message during a D2D discovery period when the remote UE intends to communicate with the network;

switching to a receiving mode after transmitting the relay selection request message;

receiving, during the D2D discovery period, relay selection response messages from one or more candidate relay UEs, wherein the one or more candidate relay UEs are within the network coverage and have a relay capability; and

selecting a first relay UE from the one or more candidate relay UEs based on the relay selection response messages.

2. The method according to claim 1, wherein a discovery resource pool for D2D discovery in the D2D discovery period is partitioned into a request zone and a response zone, and wherein the relay selection request message is transmitted in the request zone, while the relay selection response messages are received in the response zone.

3. The method according to claim 1, wherein the relay selection request message includes one or more of a message type, an identifier of the remote UE and a group identifier of a group to which the remote UE belongs, and

wherein a relay selection response message of each candidate relay UE includes information for indicating existence of the candidate relay UE, and one of more of an identifier of the candidate relay UE, the identifier of the remote UE, backhaul link quality of the candidate relay UE and remaining relay capability of the candidate relay UE.

4. The method according to claim 1, wherein the remote UE selects the first relay UE based on a backhaul link quality between the candidate relay UE and its serving base station carried in each relay selection response message and/or a D2D link quality between the candidate relay UE and the remote UE carried in the relay selection response message or calculated by the remote UE.

5. The method according to claim 1, further comprising: periodically receiving a relay monitoring message from the first relay UE.

6. The method according to claim 5, further comprising:

determining a D2D link quality between the first relay UE and the remote UE and/or a backhaul link quality between the first relay UE and the serving base station based on the relay monitoring message.

7. The method according to claim 6, further comprising:

determining whether the determined D2D link quality and/or backhaul link quality is lower than a corresponding predefined threshold.

8. The method according to claim 7, further comprising:

when the determined D2D link quality and/or backhaul link quality is lower than the corresponding predefined threshold, transmitting a relay reselection request message to request for reselecting a relay UE;

receiving relay reselection response messages from one or more candidate relay UEs; and reselecting a second relay UE from the one or more candidate relay UEs based on the relay reselection response messages.

9. The method according to claim 8, wherein the relay reselection request message is transmitted in a request zone of the discovery resource pool, and the relay reselection response messages are received in a response zone of the discovery resource pool.

10. The method according to claim 8, wherein the relay reselection request message includes one or more of a message type, an identifier of the remote UE, a group identifier of a group to which the remote UE belongs and an identifier of the first relay UE; and

wherein a relay reselection response message of each candidate relay UE includes information for indicating existence of the candidate relay UE, one or more of an identifier of the candidate relay UE, the identifier of the remote UE, backhaul link quality of the candidate relay UE and remaining relay capability of the candidate relay UE, and information of D2D link quality between the first relay UE and the candidate relay UE.

11. An apparatus for selecting a UE-to-network relay, the apparatus being at a remote UE outside network coverage, the apparatus comprising:

a unit configured to transmit a relay selection request message during a D2D discovery period when the remote UE intends to communicate with the network;

a unit configured to switch to a receiving mode after transmitting the relay selection request message;

a unit configured to receive, during the D2D discovery period, relay selection response messages from one or more candidate relay UEs, wherein the one or more candidate relay UEs are within the network coverage and have a relay capability; and

a unit configured to select a first relay UE from the one or more candidate relay UEs based on the relay selection response messages.

12. A synchronization method for UE-to-network relay discovery, comprising steps at a UE within network coverage:

receiving a reference signal from a serving base station of the UE;

determining, based on the reference signal, a reference signal receiving power from the serving base station, and comparing the reference signal receiving power with a predefined first threshold; and

when determining that the reference signal receiving power is lower than the first threshold and the UE has D2D data to transmit, transmitting an SLSS/PSBCH in a preconfigured synchronization resource during a D2D direct communication period, such that a remote UE outside the network coverage can select the UE as a synchronization reference.

13. A synchronization method for UE-to-network relay discovery, comprising steps at a UE within network coverage, wherein the UE has a relay capability but is not selected as a relay UE by a remote UE outside the network coverage, the synchronization method comprising:

receiving a dedicated signaling from a serving base station of the UE, the dedicated signaling being for instructing the UE to transmit continuous SLSSs/PSBCHs; and

transmitting the continuous SLSSs/PSBCHs according to the dedicated signaling such that the remote UE can select the UE as a synchronization reference.

14. A synchronization method for UE-to-network relay discovery, comprising steps at a UE within network coverage, wherein the UE has relay capability but is not selected as a relay UE by a remote UE outside the network coverage, the synchronization method comprising:

detecting an SLSS/PSBCH from the remote UE and determining whether timing of the remote UE is aligned with timing of a serving base station of the UE; and

when the UE determines that the timing of the remote UE is misaligned with the timing of the serving base station of the UE, transmitting continuous SLSSs/PSBCHs such that the remote UE can select the UE as a synchronization reference.

15. A synchronization method for UE-to-network relay discovery, comprising steps at a UE in network coverage, wherein the UE is selected by a remote UE outside the network coverage as a relay UE for the remote UE, the synchronization method comprising:

reporting to a serving base station of the UE that the UE is selected by the remote UE as the relay UE; and

transmitting to the remote UE continuous SLSSs/PSBCHs as a synchronization reference for the remote UE.