Patent Publication Number: US-11026280-B2

Title: System and method for data communication

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/388,289, filed on Apr. 18, 2019, which is a continuation of U.S. application Ser. No. 15/551,167 filed on Aug. 15, 2017, now U.S. Pat. No. 10,314,094, issued on Jun. 4, 2019, which is a National Stage of International Application No. PCT/JP2016/000724, filed on Feb. 12, 2016, which claims priority from Australian Patent Application No. 2015900674, filed on Feb. 26, 2015, the contents of all of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to data communication. In particular, although not exclusively, the present invention relates to providing network relay services using device-to-device communication. 
     
       
         
           
               
             
               
                   
               
               
                 &lt;ABBREVIATIONS&gt; 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 3GPP 
                 3 rd  Generation Partnership Project 
               
               
                 D2D 
                 Device-to-Device Communication or Direct 
               
               
                 D2D-UE 
                 Cellular User equipment with direct communication 
               
               
                 DFN 
                 D2D Frame Number 
               
               
                 DRX 
                 Discontinuous reception 
               
               
                 DTX 
                 Discontinuous transmission 
               
               
                 eNB or eNodeB 
                 Evolved NodeB 
               
               
                 ePDCCH 
                 enhanced Physical Downlink Control Channel 
               
               
                 FDD 
                 Frequency Division Duplex 
               
               
                 HARQ 
                 Hybrid Automatic Repeat Request 
               
               
                 LTE 
                 Long Term Evolution 
               
               
                 MAC 
                 Medium Access Control 
               
               
                 MAC PDU 
                 Medium Access Control Protocol Data Unit 
               
               
                 OOC 
                 Out of coverage 
               
               
                 PDCCH 
                 Physical Downlink Control Channel 
               
               
                 PLMN 
                 Public Land Mobile Network 
               
               
                 PRB 
                 Physical Resource Block 
               
               
                 PS UE 
                 Public Safety User Equipment 
               
               
                 PTT 
                 Push to Talk or Press to Transmit 
               
               
                   
                 While one device transmits, the other(s) receive. 
               
               
                 RB 
                 Resource Block 
               
               
                 RLC 
                 Radio Link Control 
               
               
                 RX 
                 Receive 
               
               
                 SA 
                 Scheduling Assignment 
               
               
                 SFN 
                 System Frame Number 
               
               
                 SIB 
                 System Information Broadcast 
               
               
                 TB 
                 Transport Block 
               
               
                 TDD 
                 Time Division Duplex 
               
               
                 T-RPT 
                 Time Resource Pattern of Transmission 
               
               
                 TX 
                 Transmit 
               
               
                 UE 
                 User Equipment 
               
               
                   
               
            
           
         
       
     
     BACKGROUND ART 
     Recent advancements in the field of cellular communication include supporting direct or device-to-device (D2D) communication between two or more mobile devices operating in the licensed spectrum. Direct communication between mobile devices, with or without the coordination of the cellular communication network, provides a vast array of advantages over traditional cellular communication including improving local coverage, facilitating traffic offloading from cellular networks, providing service continuity for mobile devices that are moving away from a cellular network, supporting mission critical public safety communication and providing potential for various types of new services and applications. 
     Currently, the 3rd Generation Partnership Project (3GPP) is working towards standardising D2D communication in its existing long Term Evolution Advanced (LTE-A) cellular networks. It is envisioned that the upcoming Release-12 of the 3GPP standards will include D2D functionality to support public safety as well as non-public safety services. More specifically, support for D2D discovery in network coverage, D2D direct communication in and out-of network coverage and high layer support to enable group cast and unicast over the physical layer broadcast have already been standardized. 
     A work item for 3GPP Release 13 has been approved relating to more advanced features to support discovery in partial and outside network coverage, network coverage extension using UE-to-network relays, and discovery in the presence of multiple carriers and Public Land Mobile Networks (PLMNs). Of the objectives in the 3GPP Release-13 work item, UE-to-network relay is of particular interest in the public safety community as it may facilitate reliable public safety communication via service continuity and coverage extension. A UE-to-network relay feature is expected to be implemented in 3GPP Release-13 as Layer 3 packet forwarding based relaying supported by Release-12 D2D direct communication in the Physical layer. 
     3GPP Release-12 D2D direct communication is anticipated to include two operation modes, namely Mode-1 operation mode and Mode-2 operation mode. In Mode-1 operation mode, a base station (BS) schedules resources to be used by a UE to transmit direct control information followed by direct data. In particular, the BS uses a Physical Downlink Control Channel (PDCCH) or enhanced Physical Downlink Control Channel (ePDCCH) to allocate resources to a D2D transmitter for D2D control information or scheduling assignment (SA) transmission and D2D Data transmission, thus enabling contention and collision free multi-user access for Mode-1 communication. In contrast, in Mode-2 operation mode, a D2D-UE selects resources on its own from network configured or pre-configured D2D communication resource pools, for transmission of direct control information or SA and direct data. Mode-2 communication can be operated under network coverage or out-of-network coverage. However, in both scenarios, resource selection for SA and data transmission is performed by the UE on its own without a central coordination. 
       FIG. 1  illustrates a communication system  1  including UE-to-network relay operation. In particular, a device-to-device communication capable UE (D2D relay UE  11 ) that supports relay functionality, which is in the coverage of a cellular BS  12 , may act as a Layer 3 (i.e. Internet Protocol) IP packet relay to another out-of coverage D2D-UE (i.e. OOC-UE  13 ). In this scenario, the communication between the D2D relay UE  11  and the OOC-UE  13  may take place in the physical layer using Mode-2 communication, while the communication between the D2D relay UE  11  and the BS  12  may follow cellular uplink (UL) and downlink (DL) communication procedures. 
     In one use case, the OOC-UE  13  is a public safety UE (PS UE) that wishes to communicate with another PS UE via a public safety network, and the OOC-UE  13  may utilise the D2D relay UE  11  for this purpose. In this scenario, the spectrum (f2) used by the OOC-UE  13  to communicate with other OOC-UEs is same as the spectrum (f1) used by the public safety network. In another use case, the D2D relay UE  11  may be a PS UE who is roaming on a commercial network; and this UE may act as a relay node to another PS UE (OOC-UE) who is outside the coverage of the commercial network. In this scenario, the spectrum (f2) used by the OOC-UE  13  to communicate with other OOC-UEs is different than the spectrum (f1) used by the commercial network. 
     Such D2D communication is anticipated to utilise the cellular UL resources, and operate in half-duplex manner. As a result of this single carrier operation, cellular UL transmission and D2D transmission (TX) or D2D reception (RX) cannot occur simultaneously. Therefore, from a UE perspective, cellular UL (TX) and D2D (TX/RX) operation needs to be time multiplexed. 3GPP has further chosen to prioritise cellular UL transmissions over D2D (TX/RX) communication in case of a resource collision or limited UE TX/RX capability, and not to introduce any discontinuous transmission (DTX) period for the purpose of aiding UE TX/RX switching. 
     Moreover, RX switching/re-tuning from one carrier to another (for instant from cellular DL to UL in FDD) may require at least one additional subframe for tuning purposes. As such, in subframes allocated for D2D, a D2D-UE with limited TX/RX capability may have to suspend D2D (TX/RX) communication in order to perform cellular communication. Since D2D transmission from the OOC-UE can be in any subframe within the Mode-2 resource pool that are also used by the relay UE in UL cellular communication (due to autonomous resource selection by the D2D-UE for Mode-2), the relay UE may miss reception of D2D data from the OOC-UE, resulting in packet loss or high latency. However, from a UE-to-network relay perspective, both cellular and D2D data is equally important to facilitate a reliable and low latency (for example VoIP) link between OOC-UE and the network. 
     Accordingly, there is a need for an improved method and system for data communication. 
     It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country. 
     SUMMARY OF INVENTION 
     The present invention is directed to a system and method for data communication, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice. 
     With the foregoing in view, the present invention in one form, resides broadly in a data communication method for use in a wireless communication system, the wireless communication system including a base station (BS), a relay user equipment (UE) and an out of coverage UE (OOC-UE), the method including: 
     receiving, at the relay UE and from the BS, a relay cycle configuration, the relay cycle configuration defining a relay cycle including a first time period for device to device (D2D) communication, and a second time period for cellular communication; and 
     receiving, at the relay UE by D2D communication, within the first time period and from the OOC-UE, relay data; and 
     providing, from the relay UE by cellular communication, within the second time period and to the BS, the relay data. 
     Advantageously, embodiments of the present invention enable coordination of cellular and D2D communication at the Relay-UE to achieve low latency, reliable UE-to-network relay functionality. 
     The relay cycle configuration may include a relay offset indicator, defining an offset of the relay cycle relative to a reference point. 
     The relay cycle configuration may include a relay cycle length, defining a length of the relay cycle, and a second time period length, defining a length of the second time period. 
     The method may further comprise providing, from the relay UE to the OOC-UE, an advertisement of network-relay services of the relay UE. 
     The step of providing the advertisement may comprise broadcasting, by the relay UE and to a plurality of OOC-UEs, the relay cycle configuration. 
     The relay UE may use a reserved or pre-determined Scheduling Assignment (SA) resource index to broadcast the relay cycle configuration. 
     The relay UE may use a pre-determined SA identifier to differentiate between Medium Access Control Protocol Data Units (MAC-PDUs) carrying the relay cycle configuration, and other MAC-PDUs including relayed MAC-PDUs. 
     The relay cycle configuration may be initially broadcast by the relay UE intensively, and subsequently broadcast periodically. A period of time that the relay UE performs intensive broadcasting of the relay cycle configuration may be configured by the BS. 
     The relay cycle configuration may include a relay cycle configuration window, defining when the relay UE may broadcast the relay cycle configuration in the relay cycle. The relay cycle configuration window may be defined by an integer number of repetitions of the relay cycle. A start of the relay cycle configuration window may be aligned with a start of the relay cycle. 
     The method may further comprise receiving, at the relay UE and from the BS, a relay cycle reconfiguration message, the relay cycle reconfiguration defining a new relay cycle, wherein subsequent relay data is transmitted between the relay UE and the OOC-UE, and between the relay UE and the BS according to the new relay cycle. 
     A plurality of relay cycles may be concatenated in a super-frame of 1024*10 ms. A first relay cycle of the plurality of relay cycles may be offset from a start of the super-frame by a relay offset indicator. The start of the super-frame may be defined by a first D2D Frame Number (DFN) or a first System Frame Number (SFN). 
     A last relay cycle of the plurality relay cycles may be truncated to fit into the super-frame. 
     The relay offset indicator may be equal to a scheduling assignment (SA) offset indicator. The relay offset indicator may be between 0 and 39 milliseconds. 
     The relay cycle may be configured to span over a single SA period, wherein the second time period is at least partially overlapping with a D2D data pool associated with the SA Period. 
     The relay cycle may be configured to span over several SA periods, wherein the second time period is at least partially overlapping with a D2D data pool associated with an SA period of the several SA periods. The several SA periods comprise an integer number of SA periods. 
     The relay cycle may be configured to span over a first integer number of SA periods, and the second time period is configured to span over a second integer number of SA periods, wherein the second integer number may be as large as the first integer number. In particular, the second integer number may be less than or equal to the first integer number. 
     In another form, the invention resides broadly in a data communication method for use in a wireless communication system, the wireless communication system including a base station (BS), a relay user equipment (UE) and an out of coverage UE (OOC-UE), the method including: 
     receiving, at the relay UE and from the BS, a relay cycle configuration, the relay cycle configuration defining a relay cycle including a first time period for device to device (D2D) communication, and a second time period for cellular communication; and 
     receiving, at the relay UE by cellular communication, within the second time period and from the BS, relay data; and 
     providing, by the relay UE by D2D communication, within the first time period and to the OOC-UE, the relay data. 
     In yet another form, the present invention resides broadly in a wireless communication system including: 
     a base station (BS); 
     an out of coverage UE (OOC-UE); and 
     a relay user equipment (UE), the relay UE configured to: 
     receive, from the BS, a relay cycle configuration, the relay cycle configuration defining a relay cycle including a first time period for device to device (D2D) communication, and a second time period for cellular communication; 
     receiving, by D2D communication, within the first time period and from the OOC-UE, relay data; and 
     provide, by cellular communication, within the second time period and to the BS, the relay data. 
     According to certain embodiments of the present invention, the relay cycle (relay_cycle) is configurable and periodically or repeatedly used in the BS, the relay UE and the OOC-UEs to realise D2D relay communication between the BS and OOC-UEs via the relay UE. A configurable relay cycle comprises a first time period (“cellular silence period”) and a second time period (“cellular active period”) that are multiplexed in time forming the relay cycle, wherein the first time period in the said relay cycle may be designated for D2D communication among the relay UE and OOC-UE(s), and the second time period in the said relay cycle may be designated for cellular uplink (UL) and downlink (DL) communication between the BS and relay UE in exchanging control data, user data and/or application data. The method further allows for configuration and reconfiguration of the first time period and second time period lengths forming the overall relay cycle length, such that their ratio may be further determined to adapt to the number of connected OOC-UEs, the number of connected OOC-UEs requiring network relay services, routing traffic volume and/or meet desired quality-of-service. 
     The relay cycle may be configured so that one relay cycle may fully overlap with the regionally configured or pre-configured SA Period (that is standardised in 3GPP Release 12) where the second time period length is further configured to span partially over the D2D data pool leaving sufficient number of T-RPTs for use in restricted D2D communication as part of the first time period length. The relay cycle may also be configured so that one relay cycle may fully overlap with an integer number of regionally configured or pre-configured SA Periods, where the second time period length is further configured to span partially over the D2D data pool of an SA Period. This may leave a sufficient number of T-RPTs for use in D2D communication or span over an integer number of the SA Periods. 
     The method may further comprise the BS configuring the relay UE with a relay cycle configuration or reconfiguration information elements (IEs) including but not being limited to relay cycle period, a second time period for deriving first time period being used in conjunction with a D2D communication pool. The relay UE may then at the implicitly signalled activation time forward the relay cycle configuration or reconfiguration IEs to OOC-UEs within its transmission range by means of intensive or rapid broadcasting (e.g. on every relay cycle) the relay cycle configuration information for a defined period of time to advertise its network-relay services and associated configuration IEs to the existing OOC-UEs in the proximity. Later, periodic broadcasting (e.g. on every configurable integer number of relay cycles) of the relay cycle configuration information and relay cycle configuration IEs may be used for the duration of a configuration, to maintain the current configured or reconfigured relay cycle configuration and allow late arrival of OOC-UE(s). In broadcasting relay cycle configuration IEs on a D2D communication pool, the relay-UE may use a reserved or pre-determined scheduling assignment (SA) resource index, to allow OOC-UEs to search for SA guiding to the reception of relay cycle configuration, and/or a pre-determined SA-ID (i.e. SA Identifier) in a transmitted SA to allow OOC-UEs to differentiate the associated MAC-PDUs carrying relay cycle configuration IEs with other MAC-PDUs including relayed MAC-PDUs. 
     According to certain embodiments of the present invention, multiple configurable relay cycles are time-multiplexed without gap, forming a timing structure having a time period of 1024*10 ms. The start of a first relay cycle may align with the start of a first SA period, and a last relay cycle may be truncated to fit into a window of 1024*10 ms. An activation time of the initial relay cycle configuration may be the start of the newly configured relay cycle that is immediately after the completion of a relay cycle configuration at the relay UE. The activation time of a relay cycle reconfiguration may be start of the newly configured relay cycle that is immediately after the end of the currently configured relay cycle during which the completion of relay cycle reconfiguration has occurred at the Relay-UE. The period of time that the relay UE performs intensive/rapid broadcasting of a relay cycle configuration/reconfiguration may be implicitly signalled by the BS and defined as the time from the activation time to the time where the first periodic broadcast of a relay cycle configuration or reconfiguration occurs. The BS may further configure the relay UE with a “relay cycle configuration window”, where the start of the first “relay cycle config window” may align with the start of the first relay cycle in the unique time-frame of 1024*10 ms, and the relay UE uses the relay cycle configuration window (“relay_cycle_config_window”) in periodically broadcasting (i.e. on every configurable integer number of relay cycles) relay cycle configuration IEs for the duration of a configuration. 
     Embodiments of the present invention provide methods to support D2D relay communication between BS or network access node and out of coverage D2D-UEs via a network relay capable D2D-UE (the relay UE) by structurally coordinating/configuring cellular communication and D2D communication operations and periodicity at the relay UE such that low latency or predicted latency, and/or reliable relay functionality is achieved. Furthermore, mandatory TX/RX requirements are eliminated, and as such the present invention may be used for single TX/RX UEs. Further again, power consumption due to frequent RX retuning from one frequency to the other is eliminated. 
     The network relay functionality may be built on top of 3GPP Rel-12 D2D framework, where Rel-12 D2D broadcast communication is used as baseline communication by the relay UE to communicate with the other D2D-UEs that are requesting the relay services. 
     Certain embodiments of the present invention include a SuperFrame, in which the D2D relay cycle is repeated contiguously without a gap, where SuperFrame starts from D2D Frame Number (DFN) zero and ends when the DFN is reset to a zero. 
     Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention. 
     The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Various embodiments of the invention will be described with reference to the following drawings, in which: 
         FIG. 1  illustrates a communication system including UE-to-network relay operation; 
         FIG. 2  illustrates an advanced wireless communication system, according to an embodiment of the present invention; 
         FIG. 3  illustrates a data structure of the system of  FIG. 2 , according to an embodiment of the present invention; 
         FIG. 4  illustrates a relay cycle to saPeriod mapping of the system of  FIG. 2 , according to an embodiment of the present invention; 
         FIG. 5  illustrates a relay cycle to saPeriod mapping of the system of  FIG. 2 , according to a further embodiment of the present invention; 
         FIG. 6  illustrates a relay cycle to saPeriod mapping of the system of  FIG. 2 , according to yet a further embodiment of the present invention; 
         FIG. 7  illustrates a timing diagram of the system of  FIG. 2 , according to an embodiment of the present invention; 
         FIG. 8  illustrates a timing diagram of an exemplary relay reconfiguration, according to an embodiment of the present invention; 
         FIG. 9  illustrates a timing diagram of a further exemplary relay reconfiguration activation time, according to an embodiment of the present invention; 
         FIG. 10A  illustrates relay configuration diagram, according to an embodiment of the present invention; 
         FIG. 10B  illustrates relay configuration diagram, according to an embodiment of the present invention; and 
         FIG. 11  illustrates relay configuration diagram, according to an embodiment of the present invention. Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Various embodiments of the present invention relate to device-to-device (D2D) communication. D2D communication is also commonly referred to as or peer to peer (P2P) communication or direct mode communication, and relates to direct communication between communications devices. 
     As described in further detail below, various embodiments of the present invention provide a UE-to-network relay, enabling network connectivity to out-of-coverage UE(s). The network relay may be provided in licensed spectrum that is allocated for public safety communication and cellular communication, such as long term evolution (LTE) communication specified by the 3rd Generation Partnership Project (3GPP). 
       FIG. 2  illustrates an advanced wireless communication system  10 , according to an embodiment of the present invention. The advanced wireless communication system  10  comprises a cellular network access node in the form of a base station (BS)  20  that provides wireless cellular coverage  21  to a D2D relay user equipment (UE)  30  on a paired or unpaired carrier frequency f1. The wireless communication system  10  further comprises one or more out-of-coverage UEs (OOC-UEs)  40 , which are out-of-coverage of the BS  20 , but are within transmission range of the D2D relay UE  30 . 
     The D2D relay UE  30  is configured as a relay UE node that is capable of relaying control data, protocol data, user data, and application data between OOC-UEs  40  and the BS  20  using D2D links  34  on carrier frequency f2, and a cellular link  22  on an uplink (UL) component of carrier frequency f1, in a coordinated manner. The frequency f2 can be a public-safety frequency and f2 may be the same as the UL component of f1. 
     The D2D relay UE  30  is configured by the BS  20  to operate as a relay UE node when it is in RRC-connected mode. The D2D relay UE  30  may also be requested by one or more OOC-UEs  40  in a D2D group to operate as a relay UE node, providing UE-to-network relay service to the OOC-UEs  40  in its direct transmission range. When being requested by an OOC-UE  40  to operate as a relay UE node, the D2D relay UE  30  may first establish an RRC connection with its servicing BS  20  and further request a relay configuration from the BS  20 . 
     As discussed in further detail below, a relay cycle (“Relay_cycle”) structure for cellular and D2D communication is then configured by the BS  20 , for use by the BS  20 , the D2D relay UE  30 , and OOC-UEs  40 . In particular, the relay cycle structure is employed at the D2D relay UE  30  to multiplex a D2D communication link  34  and cellular communication link  22  in the time domain, to enabling timely delivery of delay sensitive data and/or error intolerant data between the BS  20  and OOC-UEs  40 . 
       FIG. 3  illustrates a data structure  100  of the system  10 , according to an embodiment of the present invention. The data structure  100  includes a relay cycle  200  comprising first and second configurable communications regions or periods that are time-multiplexed. In particular, the relay cycle  200  includes a “cellular silence period (CSP)”  201 , which is reserved for D2D communication between the D2D relay UE  30  and the OOC-UEs  40 , and a “cellular active period (CAP)”  202 , which is reserved for cellular communication between the BS  20  and the Relay-UE  30 . Multiple relay cycles  132 ,  133 ,  139  are concatenated without gap to form the data structure  100 , having a standard time frame  131  that is 1024*10 milliseconds long. 
     The start of the first relay cycle  132  is offset by a relay offset indicator (relayOffsetIndicator)  130  from the start  120  of a frame (SuperFrame)  121 , i.e. offset from D2D frame number (DFN) 0 or System Frame Number (SFN) 0. The relayOffsetIndicator  130  is configured such that the start of the first relay cycle  132  and the first saPeriod  141  are aligned. As a result, the relayOffsetIndicator  130  is equal to the standardised saOffsetIndicator  140 , the configured value of which is selected from the range of [0, 1, 2, . . . , 38, 39] milliseconds. 
     Each configured relay cycle  132 ,  133 ,  139  of a configuration in the standard time frame  131  has a configurable length selected from the range of [40, 80, 160, 320, 640] milliseconds. The last relay cycle  139  may be truncated to fit in the length of the standard time frame  131 , i.e. 1024*10 milliseconds. 
     Furthermore, the CAP  202  within a configured relay cycle  132 ,  133 ,  139  is configurable to have a duration in the range of [20:200] milliseconds with a granularity of 1 ms. The CSP  201  of the relay cycle  132 ,  133 ,  139  can then be derived from the configured relay cycle and CAP durations. 
     Once configured, the data structure  100  is repeated until a relay cycle reconfiguration is received, termination of the relay cycle configuration is triggered by the BS  20 , or the D2D relay UE  30  has departed from the region and no longer available to provide the UE-to-Network relay service. 
     Relay cycle configuration elements or reconfiguration information elements (IEs), including the relay cycle length and CAP length, may be appropriately determined by the servicing BS  20  provided that saPeriod configuration for the corresponding D2D communication link is known to the BS  20 . 
       FIG. 4  illustrates a relay cycle to saPeriod mapping  210  of the system  10 , according to an embodiment of the present invention. As discussed above, the mapping  210  is configured by the BS  20  to enable time multiplexing of communication from the D2D relay UE  30  to the BS  20 , and from the D2D relay UE  30  to the OOC-UE(s)  40 . 
     The relay cycle to SA period mapping  210  has been configured such that a relay cycle  211  can be mapped entirely to an SA period (saPeriod)  212 . A CAP length  208  of the relay cycle  211  has also been configured such that a sufficient number of Time-Resource pattern of transmissions (T-RPTs) are available for MAC-PDU transmission and reception are available within a D2D restricted region  209 . 
       FIG. 5  illustrates a relay cycle to SA period mapping  220  of the system  10 , according to a further embodiment of the present invention. 
     The relay cycle to SA period mapping  220  has been configured such that a relay cycle  221  can be mapped to an integer number (K) SA periods  222 . Furthermore, a CAP length  208  is configured to partially overlap with a D2D data pool of an SA period  223 , or  224  such that a sufficient number of T-RPT(s) are available for MAC-PDUs transmission and reception within the D2D restricted region  209 . 
       FIG. 6  illustrates a relay cycle to SA period mapping  230  of the system  10 , according to yet a further embodiment of the present invention. 
     The relay cycle to SA period mapping  230  has been configured such that a relay cycle  231  can be mapped entirely into an integer number (K) SA periods  232 . Furthermore, a CAP length  233  is configured to span over an integer number Y SA periods  234 . 
     Further embodiments of the present invention relate to a “Relay cycle configuration activation time” and “Relay cycle reconfiguration activation time”, as discussed below. 
       FIG. 7  illustrates a timing diagram  300  of the system  10 , according to an embodiment of the present invention. 
     A D2D relay configuration  301  is initially provided to a relay UE by a base station, as discussed above. The D2D relay configuration  301  defines an activation time  302  of a relay cycle that immediately succeeds it, namely a start of relay cycle n (Relay_cycle(n))  303 . 
     At the relay cycle configuration activation time  302 , the relay UE performs switching  311  from cellular (TX/RX) communication operation on an UL component of carrier frequency f1  315  to direct (TX/RX) communication operation on carrier frequency f2  316 . 
     In relay cycle n  303 , the relay UE performs switching  312  from direct communication (TX/RX) on carrier frequency f2  316  to cellular communication (TX/RX) on UL component of carrier frequency f1  315  when a CSP  314  of relay cycle n  303  ends. Furthermore, at the end of a relay cycle (and thus at the start of another relay cycle), the relay UE will again perform switching  313  from cellular communication (TX/RX) on the UL component of carrier frequency f1  315  to direct communication (TX/RX) on the carrier frequency f2  316 . 
     Once being configured and activated, a relay cycle configuration is valid until a relay cycle reconfiguration or a new relay cycle configuration is configured by the servicing BS and becomes activated, or the current configuration is terminated by the servicing BS. A relay cycle reconfiguration or new relay cycle configuration can occur anywhere in a relay cycle in a super-frame (1024*10 ms). 
       FIG. 8  illustrates a timing diagram  350  of an exemplary relay reconfiguration, according to an embodiment of the present invention. In particular, a relay UE initially operates at a first relay cycle configuration  351 , and then is configured to switch to a second relay cycle configuration  352 . 
     A relay cycle reconfiguration is provided, which is completed  354  during a relay cycle  353  of the first relay cycle configuration  351 . The relay cycle reconfiguration is activated at a start  355  of a relay cycle of the second relay cycle configuration  352  which is immediately after an end  357  of the relay cycle  353 . 
     In particular, the second relay cycle configuration  352  has an activation time  355  corresponding to the start of the first relay cycle of the second relay cycle configuration  352  that is after the end  357  of the relay cycle  353 . 
     An overlapping relay cycle  356  overlaps with the activation time  355 . The first relay cycle configuration  351  is used for all relay cycles before the overlapping relay cycle  356 , including any intermediate relay cycles, as discussed below with reference to  FIG. 9 . 
       FIG. 9  illustrates a timing diagram  360  of a further exemplary relay reconfiguration activation time, according to an embodiment of the present invention. The timing diagram  360  is similar to the timing diagram  350  of  FIG. 8 . In particular, a relay UE initially operates at a first relay cycle configuration  361 , and then is configured to switch to a second relay cycle configuration  362 . 
     A relay cycle reconfiguration is provided, which is completed  364  during a relay cycle  363  of the first relay cycle configuration  361 . The relay cycle reconfiguration is activated at a start  365  of a relay cycle of the second relay cycle configuration  352  which is immediately after an end  367  of the relay cycle  363 . 
     However, an intermediate relay cycle occurs between the end  367  of the relay cycle and the activation time  355 . As a result, an overlapping relay cycle  366 , which overlaps with the activation time  355 , is after the intermediate relay cycle. The first relay cycle configuration  351  is used for all relay cycles before the overlapping relay cycle  356 , including the intermediate relay cycle. 
     A further aspect of the present invention relates to a method for being used by a relay UE in advertising, and maintaining UE-to-network relay service to existing OOC-UE(s), and allowing new arrival OOC-UE(s) to utilise existing UE-to-network relay service when a relay UE node has been configured, established and operating. 
       FIG. 10A  and  FIG. 10B  illustrates relay configuration diagram  400 , according to an embodiment of the present invention. 
     Once a relay UE has been configured and is in service, the relay UE periodically, on a carrier frequency allocated/pre-configured for direct communication in a selected SA period  410 , broadcasts an SA  421  and associated MAC-PDUs  433  carrying a relay cycle configuration. The relay cycle configuration comprises a relay cycle length, a configured CAP length and a configured relay cycle configuration window (Relay_cycle_config_window)  402 ,  406 . 
     The relay cycle configuration window  402 ,  406  comprises an integer number of repetitions of the relay cycle length and the start of a first relay cycle configuration window  402  within a timing window of 1024*10 ms aligns with the start of a first relay cycle  407 . The last relay cycle configuration window may be truncated to fit into a super-frame or timing window of 1024*10 ms. 
     The relay cycles  405  and the relay cycle configuration window  402 ,  406  define a structure  408  that is repeated in every timing window of 1024*10 ms. 
     According to certain embodiments of the present invention, the relay UE selects an SA period  401 ,  403 ,  404  in a configured relay cycle configuration window  402 ,  406 , for broadcasting the SA  421  in an SA resource pool  420  and associated MAC-PDUs  433  in the data pool  430 . The relay UE preferably selects the first SA period  401 ,  403  of the relay cycle configuration window  402 ,  406 . The selected SA period  401 ,  403 ,  404  is used to carry relay cycle configuration IEs. When broadcasting the SA  421 , the relay UE may select special SA resource index preferably SA resource index 0  421 . 
     In a transmitted SA, the relay UE may further use a special SA-ID  422  that may be selected from (but not limited to) the set of {1111 1111; 0000 0000; 1111 0000; 0000 1111; 1010 1010; 0101 0101}, to enable the OOC-UE to determine if the associated MAC-PDUs  433  carry relay cycle configuration IEs. 
       FIG. 11  illustrates relay configuration diagram  500 , according to an embodiment of the present invention. 
     In order to enable an OOC-UE to acquire broadcasted relay cycle configuration or reconfiguration information with desirable confidence, the servicing BS may configure the relay UE to broadcast relay cycle configuration to OOC-UE(s) in addition to the periodically scheduled broadcast of relay cycle configuration as described above with reference to  FIG. 8  and  FIG. 9 . In particular, the BS may configure the relay UE to broadcast relay cycle configuration on every relay cycle  511 ,  512  that is immediately after a relay cycle configuration or reconfiguration activation time  510 , and within a relay cycle configuration window  530 . 
     As a result an OOC-UE may be able to utilise network relay functionality of the relay UE, even if the periodically scheduled broadcast of relay cycle configuration is missed. 
     In the present specification and claims (if any), the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers. 
     Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations. 
     In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art. 
     This application is based upon and claims the benefit of priority from Australian provisional patent application No. 2015900674, filed on Feb. 26, 2015, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           10  advanced wireless communication system 
           20  base station 
           30  D2D relay UE 
           40  out-of-coverage UE (OOC-UE)