Patent Publication Number: US-2016242218-A1

Title: Communications method, user equipment, and wireless communications system for supporting device to device communications

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate generally to wireless communications systems, methods and apparatus, and more particularly to methods, apparatus and techniques for so called “device to device” (or “peer to peer”) communication in systems which support both device to device communication (a.k.a direct communication or D2D communication) and also network controlled communication such as cellular network communication. 
     BACKGROUND ART 
     The following abbreviations are used herein: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 3GPP 
                 third generation partnership project 
               
               
                 CP 
                 cyclic prefix 
               
               
                 D2D 
                 device to device 
               
               
                 D2D-UE 
                 cellular user equipment (UE) with direct communication 
               
               
                   
                 capability 
               
               
                 DL 
                 downlink 
               
               
                 DTX 
                 discontinuous transmission 
               
               
                 FDD 
                 frequency division duplex 
               
               
                 GP 
                 guard period 
               
               
                 OFDM 
                 orthogonal frequency division multiplexing 
               
               
                 RX 
                 receive 
               
               
                 SC-FDMA 
                 single carrier frequency division multiple access 
               
               
                 SF 
                 subframe 
               
               
                 TA 
                 timing advance 
               
               
                 TDD 
                 time division duplex 
               
               
                 TX 
                 transmit 
               
               
                 UE 
                 user equipment 
               
               
                 UL 
                 uplink 
               
               
                 UpPTS 
                 uplink pilot time slot 
               
               
                   
               
            
           
         
       
     
     Wireless communications networks and systems have been widely deployed and utilised for the past decade or more, and will likely continue to evolve in the future to provide communication content such as voice, video, packet data, messages, multimedia, broadcast, multicast, etc. These networks may be multiple-access networks including, for example, Time Division Multiple Access (TDMA) networks, Code Division Multiple Access (CDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks and/or Single Carrier FDMA (SC-FDMA) networks. A wireless communications network may also be referred to as a wide area network (WAN). 
     A wireless communications network may include a number of base stations that can provide wireless connectivity supporting communication by a number of mobile devices (mobile devices are also referred to as ‘user equipments’ or UEs). A mobile device (UE) may communicate with a base station via uplink and downlink channels. Downlink (also called the forward link) refers to the communication link from a base station to a mobile device (UE), and uplink (also called the reverse link) refers to the communication link from a mobile device (UE) to a base station. 
     Recently in the field of wireless communications, there has been a trend to make available and utilise licensed spectrum which is allocated for cellular networks for direct communication between one mobile device (UE) and another mobile device (UE), or among a group of mobile devices (UEs) within a local vicinity. This is referred to as direct or device to device (D2D) communication. Direct (D2D) communication may have advantages over cellular network communication, or it may coexist with the overlaid cellular network, thus improving overall spectral efficiency. For example, direct communication may be well suited for communicating small amounts of payload information directly and with low overhead compared with cellular network communication. Additionally, direct communication may be well suited to efficient communications in small local regions where channel conditions between the various devices may be good or better than the channel condition between the devices and base station(s). Furthermore, direct communication may allow an overlaid cellular network to offload traffic going through its base stations by allowing multiple paired devices to perform direct communication sharing the same allocated block of cellular network resources. 
     Very recently, 3GPP has recognised the potential importance of device to device communication operating on cellular licensed spectrum, and has endorsed a study item (SI) on device to device (D2D) communication. 3GPP SA (3GPP&#39;s Services and Systems Aspects technical specification group) and 3GPP RAN (3GPP&#39;s Radio Access Network technical specification group) have agreed that D2D communication may happen between D2D capable UEs (i.e. between so called D2D-UEs) which form a pair (or group) for D2D communication. Paired (or grouped) D2D-UEs may belong to the same or different cells/base stations. 3GPP RAN, and in particular 3GPP RAN-WG1 (one of the 3GPP RAN working groups), has agreed that D2D may operate in UL spectrum (in the case of FDD) or in UL subframes (in case of TDD) of the cell. 3GPP RAN-WG1 has also agreed that D2D transmission/reception should not use full duplex on a given carrier. Accordingly, D2D communication and cellular UL transmission are to share UL resource(s). 
     SUMMARY OF INVENTION 
     Technical Problem 
     Sharing the same resources for different kinds of communications (i.e. for cellular communication and D2D communication) may result in various types of interferences including, for example, timing misalignment of transmitted and received subframes, timing misalignment due to different synchronisation sources such as paired D2D-UEs belonging to different base stations, imperfect synchronisation among base stations, RX-to-TX or TX-to-TX transition switching, etc. Timing misalignments may result in intra-UE collision and/or inter-UE interference. 
     It is thought that resolving or managing intra-UE collision and/or inter-UE interference may be achieved through careful D2D subframe design. Therefore, in this document, carefully selected scenarios are chosen for timing analysis to illustrate the bad or “worst cases” of intra-UE collision and/or inter-UE interference, and based on this subframe designs for D2D communication are proposed for use in D2D communication between or among D2D-UEs belonging to the same or different servicing base stations. 
     Solution to Problem 
     It is to be clearly understood that mere reference herein to previous or existing apparatus, systems, methods, practices, standards (or information in any way relating to standards), publications or other information, or to any problems or issues, does not constitute an acknowledgement or admission that any of those things individually or in any combination formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art. 
     The present invention, at least in one form, is directed broadly to a signalling method for use in an advanced wireless communications system, wherein the system includes: 
     at least one servicing base station; and
 
at least a first and a second direct (D2D) communication capable user equipment (D2D-UE), wherein each D2D-UE is in cellular communication with a servicing base station, and the first D2D-UE and second D2D-UE, at least, are paired or grouped for D2D communication with one another,
 
the method comprising, on a given subframe,
 
performing D2D communication by transmitting D2D channel(s) and/or D2D signal(s) between one D2D-UE and another using a D2D subframe type which allows RX-TX switching time and/or prevents subframe collision at the receiving D2D-UE.
 
     This method may therefore assist in managing and/or resolving intra-UE collision and/or inter-UE interference due to, for example, timing misalignment, and it may hence help to enable direct communication in order to achieve higher end-user data rates in comparison to traditional cellular communication (alone). The method may also help to allow D2D communication sharing the same UL resource, or UL subframes dynamically, on a subframe by subframe basis. 
     In many embodiments, there may be a plurality of different D2D subframe types which can allow RX-TX switching time and/or prevent subframe collision at the receiving D2D-UE, depending on the subframe type of a preceding and/or subsequent subframe. In such embodiments, the above method may further comprise: 
     selecting an appropriate D2D subframe type from among the plurality of different D2D subframe types for use on a given subframe; and
 
transmitting D2D channel(s) and/or D2D signal(s) from one D2D-UE to another on the given subframe using the selected D2D subframe type.
 
     The method for selecting an appropriate D2D subframe type from among the said plurality of different D2D subframe types may not require signalling between paired or grouped D2D-UEs to indicate the D2D subframe type selected for use in a given subframe. 
     In a first embodiment (or a group of embodiments forming a first group) the first D2D-UE and second D2D-UE, at least, may be in cellular communication with the same servicing base station. That is, the first D2D-UE and second D2D-UE, at least, may belong to, and be serviced by, one and the same servicing base station. 
     In the first embodiment (or an embodiment within the first group of embodiments), the plurality of different D2D subframe types may include a first D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is either:
           a D2D subframe of the same connection (i.e. a D2D subframe transmitted between the same D2D-UEs) and with any transmission direction (the transmit or receive direction relative to a particular D2D-UE), or   a cellular TDD special subframe if the servicing base station is a TDD base station, and   
           the immediately following subframe is either:
           a D2D subframe of the same transmission direction, or   a cellular downlink (DL) subframe if the base station is a TDD base station.   
               

     The said first D2D subframe type may not have a guard period at the beginning or at the end of the subframe. In the case of normal cyclic prefix (CP) all 14 orthogonal frequency division multiplexing (OFDM) or single carrier frequency division multiple access (SC-FDMA) symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP all 12 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s). 
     Also in the first embodiment (or at least in an embodiment which is again within the first group of embodiments), the plurality of different D2D subframe types may include a second D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is either:
           a D2D subframe of the same connection, or   a cellular TDD special subframe if the servicing base station is a TDD base station, and   
           the immediately following subframe is either:
           a D2D subframe of the same connection with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular UL subframe if the servicing base station is a TDD base station.   
               

     The second D2D subframe type may not have a guard period at the beginning of the subframe but it may have at least one guard period at the end of the subframe. This may assist in handling subframe collision and RX-TX switching. In the case of normal CP the first 13 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the first 11 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal CP and extended CP the last OFDM or SC-FDMA symbol in the subframe may be discontinuous transmission (DTX). 
     Again in the first embodiment (or at least in an embodiment which is again within the first group of embodiments), the plurality of different D2D subframe types may include a third D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular uplink (UL) subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection with the same transmission direction, or   a cellular DL subframe if the servicing base station is a TDD base station.   
               

     The third D2D subframe type may not have a guard period at the end of the subframe but it may have at least one guard period at the start of the subframe. This may assist in handling RX-TX switching. In the case of normal CP the last 13 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the last 11 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal CP and extended CP the first OFDM or SC-FDMA symbol may be DTX. 
     Yet again in the first embodiment (or at least in an embodiment which is again within the first group of embodiments), the plurality of different D2D subframe types may include a fourth D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular UL subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular UL subframe.   
               

     The fourth D2D subframe type may have at least one guard period at the start of the subframe (this may assist in handling TX-RX switching time) and also at least one guard period at the end of the subframe (this may assist in handling subframe collision and RX-TX switching). In the case of normal CP the 2nd to 13th OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the 2nd to 11th OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal CP and extended CP the first and last OFDM or SC-FDMA symbol in a D2D subframe may be DTX. 
     In the first embodiment, transmitting one or a plurality of the different D2D subframe types may optionally include applying a retard or offset, possibly for half of the network configured TA in comparison with the network configured cellular UL subframe reference timing. This may help to bring D2D-UE transmit and D2D-UE receive into approximate D2D subframe reference timing alignment. 
     In a second embodiment (or a group of embodiments forming a second group), the first D2D-UE and second D2D-UE, at least, may be in cellular communication with different servicing base stations. 
     In the second embodiment (or an embodiment within the second group of embodiments), the plurality of different D2D subframe types may include a first D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a D2D subframe, and   the immediately following subframe is a D2D subframe of the same connection and transmission direction.       

     The said first D2D subframe type may not have a guard period at the beginning or at the end of the subframe. In the case of normal CP all 14 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP all 12 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s). 
     Also in the second embodiment (or at least in an embodiment which is again within the second group of embodiments), the plurality of different D2D subframe types may include a second D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a D2D subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection but with the opposite transmission direction, or   a D2D subframe of another connection, or   a cellular subframe, including a cellular UL subframe or a cellular DL subframe.   
               

     The second D2D subframe type may not have a guard period at the start of the subframe but it may have at least one guard period at the end. This may assist in handling subframe collision and RX-TX switching. In the case of normal CP the first 12 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last two symbols may be DTX, or in the case of extended CP the first 11 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last symbol may be DTX. 
     Again in the second embodiment (or at least in an embodiment which is again within the second group of embodiments), the plurality of different D2D subframe types may include a third D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular subframe, including a cellular FDD subframe or a cellular TDD special subframe, and   the immediately following subframe is a D2D subframe of the same connection with the same transmission direction.       

     The third D2D subframe type may not have a guard period at the end of the subframe but it may have at least one guard period at the start of the subframe (this may assist in handling subframe collision and RX-TX switching). In the case of normal CP the last 13 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the last 11 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal and extended CP the first symbol may be DTX. 
     Yet again in the second embodiment (or at least in an embodiment which is again within the second group of embodiments), the plurality of different D2D subframe types may include a fourth D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular subframe, including a cellular UL or DL subframe.   
               

     The fourth D2D subframe type may have at least one guard period at the start of the subframe (this may assist in handling subframe collision and TX-RX switching time) and at least one guard period at the end of the subframe (this may assist in handling subframe collision and RX-TX switching). In the case of normal CP the first OFDM or SC-FDMA symbol may be DTX, the 2nd to 12th OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last two OFDM or SC-FDMA symbols may be DTX, or in the case of extended CP the first OFDM or SC-FDMA symbol may be DTX, the 2nd to 11th OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last OFDM or SC-FDMA symbol may be DTX. 
     Further in the second embodiment (or at least in an embodiment which is again within the second group of embodiments), the different base stations servicing the first D2D-UE and the second D2D-UE respectively may both be TDD base stations, and the plurality of different D2D subframe types may include a fifth D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a certain cellular special subframe (such as subframe #1), and   the immediately following subframe is either:
           a D2D subframe of the same connection but with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular subframe.   
               

     The fifth D2D subframe type may have a guard period at the start of the subframe (this may assist in handling subframe collision) and at least one guard period at the end of the subframe (this may assist in handling subframe collision and RX-TX switching). In the case of normal CP the first 12 OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s), the last two OFDM or SC-FDMA symbols may be DTX and a retardation may be applied to save one OFDM/SC-FDMA symbol, or in the case of extended CP the first OFDM/SC-FDMA symbol may be DTX, the 2nd to 11th OFDM or SC-FDMA symbols may be used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last OFDM/SC-FDMA symbol may be DTX. 
     In the second embodiment, transmitting one or a plurality of the different D2D subframe types may optionally include applying a retard or offset, possibly for half of the network configured TA in comparison with the network configured cellular UL subframe reference timing. This may help to bring D2D-UE transmit and D2D-UE receive into approximate D2D subframe reference timing alignment. 
     The following is a further discussion of the invention (or aspects or embodiments thereof). In some areas, the discussion below may use slightly different wording/nomenclature to that used above, or it may present the invention (or aspects or embodiments thereof) from a slightly different perspective. 
     In one form of the invention at least, a method is proposed for implementation by D2D capable UEs (so called D2D-UEs) in D2D communication which may assist in managing and/or resolving intra-UE collision and/or inter-UE interference due to, for example, timing misalignment. This may help to enable direct communication to achieve higher end-user data rates in comparison to traditional cellular communication (alone). The method may also help to allow D2D communication sharing the same UL resource, or UL subframes dynamically, on a subframe by subframe basis. 
     In many embodiments, the method involves selecting an appropriate D2D subframe type from among a number of specially designed subframe types, and transmitting the selected D2D subframe type carrying D2D channel(s) and/or D2D signal(s) from a first D2D-UE to (a) second D2D-UE(s), wherein the D2D-UEs may belong to the same cellular servicing base station or different cellular servicing base stations. The method for selecting the appropriate D2D subframe type from among the said number of specially designed subframe types for transmitting D2D channel(s) and/or D2D signal(s) is preferably transparent (i.e. inherently understood by all D2D-UEs), thus eliminating the need for any signalling to inform a receiving D2D-UE in a D2D-UE pair/group of the particular (appropriate) specially designed subframe type used in a given instance. 
     In the transmission and reception of D2D subframe(s) between or among D2D-UEs which form a pair or group for D2D communication and which are under the same cell coverage or the same servicing base station, the said servicing base station may be FDD or TDD. 
     In the transmission and reception of D2D subframe(s) between or among D2D-UEs which form a pair or group for D2D communication and which are under different cell coverage or different servicing base stations, the said servicing base stations may be of FDD and FDD respectively, TDD and TDD respectively, or FDD and TDD respectively. Due to imperfect timing synchronisation, there may be a timing offset (which may be up to a maximum of 10 μsecs in practical implementations) between or among the servicing base stations. 
     In a first aspect of the invention, the first D2D-UE and second D2D-UE(s) that together form a pair (or group) for direct communication utilising cellular FDD UL resource or TDD UL subframes, belong to the same servicing base station. That is, the said servicing base station provides cell coverage to both the first D2D-UE and the second D2D-UE(s). Prior to transmitting and receiving D2D subframes in D2D communication, the first D2D-UE and second D2D-UE(s) may be implicitly or explicitly informed that the D2D-UEs in the pair (or group) have the same cell coverage in order to allow determination/selection of the correct/appropriate (specially designed) D2D subframe type for use in transmitting and receiving D2D channel(s) and/or D2D signal(s). The correct/appropriate D2D subframe type for transmitting D2D channel(s) and/or signal(s) on a particular cellular UL subframe may be determined/selected by the transmitting D2D-UE, and the paired (grouped) receiving D2D-UE(s) may implicitly understand which D2D subframe type is selected in order to perform correct reception and decoding of the D2D channel(s) and/or signal(s). 
     As mentioned above, there may be a number of subframe types which are specially designed for use in selected D2D subframes carrying D2D channel(s) and/or D2D signal(s) from one D2D-UE to another. In the first aspect of the invention presently being discussed, there may be four such specially designed subframe types. 
     The first of these specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is either a D2D subframe or a cellular special subframe if cellular TDD UL subframe(s) are being shared for D2D communication, and its immediately coming subframe is either a D2D subframe of the same connection with the same transmission direction or a cellular DL subframe if cellular TDD UL subframe(s) are being shared for D2D communication. For normal cyclic prefix (CP), this first D2D subframe type may comprise 14 OFDM or SC-FDMA symbols allocated for mapping of D2D channel(s) and/or D2D signal(s). For extended CP, this first D2D subframe type may comprise 12 OFDM or SC-FDMA symbols allocated for mapping of D2D channel(s) and/or D2D signal(s). 
     In the first aspect of the invention, the second of the specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is either a D2D subframe or a cellular special subframe if cellular TDD UL subframe(s) are being shared for D2D communication, and its immediately coming subframe is a D2D subframe of the same connection with opposite transmission direction, a D2D subframe of another connection, or cellular UL subframe. For normal CP, this second D2D subframe type may comprise 14 OFDM or SC-FDMA symbols with the first 13 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last symbol being DTX. For extended CP, this second D2D subframe type may comprise 12 OFDM or SC-FDMA symbols with the first 11 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last symbol being DTX. 
     In the first aspect of the invention, the third of the specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is cellular UL subframe, and its immediately coming subframe is either a D2D subframe of same connection with same transmission direction or a cellular TDD DL subframe if cellular TDD UL subframe(s) are being shared for D2D communication. For normal CP, this third D2D subframe type may comprise 14 OFDM or SC-FDMA symbols with the first symbol being DTX and the following 13 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s). For extended CP, this third D2D subframe type may comprise 12 OFDM or SC-FDMA symbols with the first symbol being DTX and the following 11 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s). 
     In the first aspect of the invention, the fourth of the specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is a cellular UL subframe, and its immediately coming subframe is a D2D subframe of the same connection with opposite transmission direction, or a D2D subframe of another connection, or a cellular UL subframe. For normal CP, this fourth D2D subframe type may comprise 14 OFDM or SC-FDMA symbols with the first symbol being DTX, the 2nd to 13th symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last symbol being DTX. For extended CP, this fourth D2D subframe type comprises 12 OFDM or SC-FDMA symbols with the first symbol being DTX, the 2nd to 11th symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last symbol being DTX. 
     In a second aspect of the invention, the first D2D-UE and second D2D-UE(s) which together form a pair (or group) for direct communication utilising cellular FDD UL resource or TDD UL subframes, belong to different servicing base stations. That is, for example, a first servicing base station may provide cell coverage to the first D2D-UE and a second servicing base station may provide cell coverage to a second D2D-UE. Prior to transmitting and receiving D2D subframes in D2D communication, the first D2D-UE and the second D2D-UE(s) may be implicitly or explicitly informed that the pair (or group) have different cell coverage and services in order to allow determination/selection of the correct/appropriate (specially designed) D2D subframe type for use in transmitting and receiving D2D channel(s) and/or D2D signal(s). Just like in the first aspect of the invention above, the correct/appropriate D2D subframe type for transmitting D2D channel(s) and/or signal(s) on a particular cellular UL subframe may be determined/selected by the transmitting D2D-UE, and the paired (or grouped) receiving D2D-UE(s) may implicitly understand which D2D subframe type is selected in order to perform correct reception and decoding of the D2D channel(s) and/or signal(s). 
     Like in the first aspect of the invention, in the second aspect there may again be a number of subframe types which are specially designed for use in selected D2D subframes carrying D2D channel(s) and/or D2D signal(s) from one D2D-UE to another. In the second aspect of the invention, there may be five such specially designed subframe types. 
     The first of these specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is a D2D subframe, and its immediately coming subframe is a D2D subframe of the same connection with same transmission direction. For normal CP, this first D2D subframe type may comprise 14 OFDM or SC-FDMA symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s). For extended CP, this first D2D subframe type may comprise 12 OFDM or SC-FDMA symbols allocated for mapping of D2D channel(s) and/or D2D signal(s). 
     In the second aspect of the invention, the second of the specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is a D2D subframe, and its immediately coming subframe is D2D subframe of the same connection with opposite transmission direction, or a D2D subframe of a different connection, or a cellular UL subframe. For normal CP, this second D2D subframe type may comprise 14 OFDM or SC-FDMA symbols with the first 12 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last two symbols being DTX. For extended CP, this second D2D subframe type may comprise 12 OFDM or SC-FDMA symbols with the first 11 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last symbol being DTX. 
     In the second aspect of the invention, the third of the specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is a cellular subframe, and its immediately coming subframe is a D2D subframe of same connection with same transmission direction. For normal CP, this third D2D subframe type may comprise 14 OFDM or SC-FDMA symbols with the first symbol being DTX and the following 13 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s). For extended CP, this third D2D subframe type may comprise 12 OFDM or SC-FDMA symbols with the first symbol being DTX and the following 11 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s). 
     In the second aspect of the invention, the fourth of the specially designed D2D subframe types may be used by a transmitting D2D-UE when its immediately prior subframe is a cellular subframe, and its immediately coming subframe is a D2D subframe of same connection with opposite transmission direction, or a D2D subframe of another connection, or a cellular subframe. For normal CP, this fourth D2D subframe type may comprise 14 OFDM or SC-FDMA symbols with the first symbol being DTX, the 2nd to 12th symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) mapping and the last two symbols being DTX. For extended CP, this fourth D2D subframe type may comprise 12 OFDM or SC-FDMA symbols with the first symbol being DTX, the 2nd to 11th symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last symbol being DTX. 
     In the second aspect of the invention, the fifth of the specially designed D2D subframe types may be used by a transmitting D2D-UE only in situations where both the first base station and the second base station are TDD and when its immediately prior subframe is a certain (i.e. it is known to be a) cellular TDD special subframe, and its immediately coming subframe is a D2D subframe of the same connection with opposite transmission direction, or a D2D subframe of another connection, or a cellular subframe. For normal CP, this fifth D2D subframe type may comprise 14 OFDM or SC-FDMA symbols with the first 12 symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last two symbols being DTX, and at the transmitting D2D-UE the fifth D2D subframe type is retarded (preferably for a LTE sample timing unit, which may be 1644 Ts) relative to the start of the corresponding cellular UL subframe. For extended CP, this fifth D2D subframe type may comprises 12 OFDM or SC-FDMA symbols with the first symbol being DTX, the 2nd to 11th symbols being allocated for mapping of D2D channel(s) and/or D2D signal(s) and the last symbol being DTX. 
     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. 
     Advantageous Effects of Invention 
     The present invention may help to resolve or manage intra-UE collision and inter-UE interference due to, for instance, timing misalignment, and may hence help to allow cellular communication and D2D communication sharing the same resource on a subframe basis. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       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. The Detailed Description will make reference to a number of drawings as follows: 
         FIG. 1  schematically illustrates an advanced wireless communication system in which UEs that are transmitting and receiving D2D subframe(s) from one another are serviced by a single cellular base station. 
         FIG. 2  schematically illustrates an advanced wireless communication system in which UEs that are transmitting and receiving D2D subframe(s) from one another are serviced by different cellular base stations. 
         FIG. 3A  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by a single cellular FDD base station. 
         FIG. 3B  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by a single cellular FDD base station. 
         FIG. 4A  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by a single cellular TDD base station. 
         FIG. 4B  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by a single cellular TDD base station. 
         FIG. 5A  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by two cellular FDD base stations. 
         FIG. 5B  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by two cellular FDD base stations. 
         FIG. 5C  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by two cellular FDD base stations. 
         FIG. 6A  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by two cellular TDD base stations. 
         FIG. 6B  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by two cellular TDD base stations. 
         FIG. 6C  contains a schematic illustration of a wireless communication system, and a related timing analysis diagram, for the situation of D2D communication between UEs serviced by two cellular TDD base stations. 
         FIG. 7A  contains schematic illustrations of wireless communication systems, and respective related timing analysis diagrams, for situations of D2D communication between UEs serviced by two cellular base stations; in one case the first base station is FDD and the second base station is TDD, and in the other case the first base station is TDD and the second base station is FDD. 
         FIG. 7B  contains schematic illustrations of wireless communication systems, and respective related timing analysis diagrams, for situations of D2D communication between UEs serviced by two cellular base stations; in one case the first base station is FDD and the second base station is TDD, and in the other case the first base station is TDD and the second base station is FDD. 
         FIG. 7C  contains schematic illustrations of wireless communication systems, and respective related timing analysis diagrams, for situations of D2D communication between UEs serviced by two cellular base stations; in one case the first base station is FDD and the second base station is TDD, and in the other case the first base station is TDD and the second base station is FDD. 
         FIG. 7D  contains schematic illustrations of wireless communication systems, and respective related timing analysis diagrams, for situations of D2D communication between UEs serviced by two cellular base stations; in one case the first base station is FDD and the second base station is TDD, and in the other case the first base station is TDD and the second base station is FDD. 
         FIG. 7E  contains schematic illustrations of wireless communication systems, and respective related timing analysis diagrams, for situations of D2D communication between UEs serviced by two cellular base stations; in one case the first base station is FDD and the second base station is TDD, and in the other case the first base station is TDD and the second base station is FDD. 
         FIG. 8  graphically represents a proposed type 1 subframe structure for D2D communication. 
         FIG. 9  illustrates the application of the type 1 subframe structure. 
         FIG. 10  graphically represents a proposed type 2 subframe structure for D2D communication. 
         FIG. 11  illustrates the application of the type 2 subframe structure. 
         FIG. 12  graphically represents a proposed type 2″ subframe structure for D2D communication. 
         FIG. 13  illustrates the application of the type 2″ subframe structure. 
         FIG. 14  graphically represents a proposed type 3 subframe structure for D2D communication. 
         FIG. 15  illustrates the application of the type 3 subframe structure. 
         FIG. 16  graphically represents a proposed type 4 subframe structure for D2D communication. 
         FIG. 17  illustrates the application of the type 4 subframe structure. 
         FIG. 18  graphically represents a proposed type 4″ subframe structure for D2D communication. 
         FIG. 19  illustrates the application of the type 4″ subframe structure. 
         FIG. 20  graphically represents a proposed type 5 subframe structure for D2D communication. 
         FIG. 21  illustrates the application of the type 5 subframe structure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  helps to illustrate a wireless communication system and a method for transmitting and receiving D2D subframes.  FIG. 1  also depicts certain associated apparatus with reference to which certain embodiments of the invention will be analysed and discussed. The wireless communication system  00  in  FIG. 1  is a single-cell cellular network comprising an access node  01  representing a cellular base station which provides coverage  02  and services a plurality of user equipments (UEs)  03 ,  04 , and  06  which are operating within coverage  02 . The base station may operate using FDD or TDD. Hence, the wireless communication system  00  may be a FDD or a TDD system. 
     Among the plurality of UEs, there may be multiple UEs that are be capable of performing both cellular communication and also direct (or so called D2D) communication, such as UEs  04  and  06 . Such UEs may be referred to as D2D-UEs. D2D-UEs  04  and  06  may happen to be within close proximity of each other (i.e. separated by a distance d 3  ( 08 )), they may be able to discover each other, and they may be configured or at least allowed by their servicing base station  01  to use cellular UL resources to directly communicate with each other. While directly communicating with each other, paired D2D-UEs  04  and  06  are required to use their own cellular UL timing as reference timing for transmitting D2D subframe(s). A UE UL timing used in cellular communication is provided by the servicing base station in the form of a timing advance (TA). According to previously proposed 3GPP LTE, TA is configured based on the distance between a servicing base station and a communicating UE; such as for example distance d 1  ( 05 ) for UE  04 , and distance d 2  ( 07 ) for UE  06 , in  FIG. 1 . 
     D2D communication is also possible for D2D capable UEs (D2D-UEs) belonging to different servicing base stations.  FIG. 2  helps to illustrate this. In  FIG. 2 , the wireless communication system  10  is an extension of cellular network  00  above in that wireless communication system  10  comprises a plurality access nodes, namely access node  11  representing a cellular base station having coverage  12  and providing services to a plurality of UEs  13 ,  14  operating within coverage  12 , and also access node  16  which represents another cellular base station having coverage  17  and providing services to a plurality of UEs  18 ,  19  operating within coverage  17 . The said servicing base stations  11  and  16  may both be FDD, or both TDD, or one may be FDD and the other TDD. They may also operate on the same carrier frequency or different carrier frequencies. 
     Among the plurality of UEs, there may be multiple UEs, such as for example D2D-UE  14  belonging to servicing base station  11  and D2D-UE  19  belonging to servicing base station  16 , that are be capable of performing cellular communication and also D2D communication. These D2D-UEs may happen to be within close proximity of each other (i.e. separated by a distance d 3  ( 21 )), they may be able to discover each other, and they may be configured or at least allowed (each by its own servicing base station  11  or  16 ) to use a selected cellular UL resource to directly communicate with each other. While directly communicating with each other, each of paired D2D-UEs  14  and  19  is required to use its own cellular UL timing as reference timing for transmitting D2D subframe(s). As noted above, a UE&#39;s UL timing used in cellular communication is configured by its servicing base station as a TA, and according to previously proposed 3GPP LTE, the TA is configured based on the distance between the servicing base station and the UE, such as for example distance d 1  ( 15 ) for D2D-UE  14 , and distance d 2  ( 20 ) for D2D-UE  19 , in  FIG. 2 . 
     Furthermore, a base station such as base station  11  may have reference timing  23 , and another base station such as base station  16  may have another reference timing  22 . Due to imperfect system timing synchronisation, reference timing  22  and reference timing  23  may have a timing offset  24  (this may be e.g. 3-10 μsec) to either advance or retard and such a timing offset may introduce additional and potentially unpredictable timing misalignment, as discussed below. 
     A first aspect of the present invention relates to a method and a D2D subframe structure for sending and receiving a D2D channel(s) and/or D2D signal(s) in such a way as to avoid or at least help to manage intra-UE collision and/or inter-UE interference, and which allows paired D2D-UEs to negotiate dynamic selection of any UL subframe in a radio frame for D2D communication while maintaining their cellular UL transmission, in a system where paired D2D communicating UEs belong to the same servicing base station. The said servicing base station may provide services for FDD communication or TDD communication. 
       FIGS. 3A and 3B  illustrates, among other things, a system  100 . a  which represents a worst case scenario wherein maximum D2D subframe collision and/or interference occurs in a FDD system. In  FIGS. 3A and 3B , D2D-UEs  104  and  105  which form a pair for D2D communication are separated by a maximum allowed distance  108 . (In practical implementations, the maximum allowed distance may be 1.5 miles.) D2D-UE  105  is also located at a position such that the distance  107  between the FDD servicing base station  101  and FDD D2D-UE  105  is equal to the distance  106  between the FDD servicing base station  101  and FDD D2D-UE  104  plus distance  108  between FDD D2D-UE  104  and FDD D2D-UE  105 . A timing diagram  110 . a  representing transmitting and receiving cellular subframes and transmitting and receiving D2D subframes in this scenario is also illustrated in  FIGS. 3A and 3B  and discussed below. 
     In timing diagram  110 . a  (which relates to the scenario depicted as  100 . a  in  FIGS. 3A and 3B  discussed immediately above), the FDD base station (eNB#1) TX-RX timing  111  servicing FDD D2D-UE  104  is the same as the FDD base station (eNB#2) TX-RX timing  112  servicing FDD D2D-UE  105 , because D2D-UEs  104  and  105  belong to a single (i.e. one and the same) servicing base station  101 . In other words, eNB#1 and eNB#2 are one and the same in this situation. (Recall that, in the first aspect of the invention presently being discussed, paired D2D communicating UEs belong to the same servicing base station.) 
     The distance  106  results in a propagation delay  116  for a cellular DL subframe transmitted from the serving base station  101  to D2D-UE  104 . For a SC-FDMA symbol sent by all UE(s) and arriving at the serving base station  101  within a CP length, the servicing base station  101  configures D2D-UE  104  to have TA  120  that results in UL subframe timing  114 . (According to subframe timing  114 , at least in simplistic terms, the D2D-UE  104  (UE#1) basically transmits a cellular uplink subframe slightly earlier than its servicing base station (eNB#1) reference timing to allow for the time taken for the transmission to travel to and reach the base station (eNB#1), and the D2D-UE  104  (UE#1) is configured to receive a cellular downlink subframe from the base station (eNB#1) slightly later in due to the time taken for that transmission to travel the distance from the base station.) 
     Similarly, the distance  107  results in a propagation delay  117  for a cellular DL subframe transmitted from the serving base station  101  to D2D-UE  105 . For a SC-FDMA symbol sent by all UE(s) and arriving at the serving base station  101  within a CP length, the servicing base station  101  configures D2D-UE  105  to have TA  121  which results in UL subframe timing  115 . (Again, at least in simplistic terms, subframe timing  115  means that the D2D-UE  105  (UE#2) transmits a cellular uplink subframe earlier than its servicing base station (eNB#2) reference timing to allow for the time taken for the transmission to travel to and reach the base station, and the D2D-UE  105  (UE#2) is configured to receive a cellular downlink subframe from the base station later due to the time taken for the transmission to travel the distance from the base station. Note that a transmission from D2D-UE  105  to the base station under subframe timing  115  begins earlier than a transmissions from D2D-UE  104  to the base station under subframe timing  114 , and reception of a transmission from the base station by D2D-UE  105  under subframe timing  115  begins later than reception of a transmission from the base station by D2D-UE  104  under subframe timing  114 . This is because D2D-UE  105  (UE#2) is further away from the base station than D2D-UE  104  (UE#1) meaning that transmissions to and from D2D-UE  105  (UE#2) take longer.) 
     The arrangement described above results in D2D-UEs  104  and  105  having a TX timing difference of  118 . The difference is 8.05 μsec in this particular example, and this happens to be the same as the propagation delay  119  between D2D-UE  104  and D2D-UE  105 . According to a current working assumption endorsed in the above 3GPP D2D SI, cellular UL subframe timing is used as reference timing for sending D2D subframes (see  122  and  123 ), and this makes the timing of D2D RX (see  124  and  125 ) at the receiving D2D-UE ( 104  and  105 ) depend on the timing of D2D TX from the transmitting D2D-UE ( 105  and  104 ). 
     The system and associated timing, as described above, provides a basis for discussing the design of a new D2D subframe structure for transmitting and receiving D2D channel(s) and/or D2D signal(s) under an overlaid FDD cellular network. 
     Still referring to  FIGS. 3A and 3B , and randomly selecting cellular UL subframes #2 and #4 for D2D communication, on UL subframe #2 D2D-UE  105  performs transmission of a D2D subframe  130  and D2D-UE  104  then performs reception  131  of that D2D subframe. On cellular UL subframe #4, D2D-UE  104  performs transmission of a D2D subframe  134  and D2D-UE  105  then performs reception  135  of that D2D subframe. 
     At a D2D subframe transmission site (i.e. the location from which a D2D subframe is transmitted), the start of a transmitted D2D subframe is perfectly aligned in time with the start of the corresponding cellular UL subframe. At the D2D subframe reception site for that D2D subframe (i.e. the location where the said transmitted D2D subframe is received), the received D2D subframe has a subframe start that is later than the start of the corresponding cellular UL subframe start. This is due to the propagation time, and is indicated by for example  136  in  FIGS. 3A and 3B . In the worst case scenario though, a received D2D subframe has a subframe start which is aligned in time with the start of the corresponding cellular UL subframe, as indicated by  132 . 
     Transmitting a D2D subframe immediately after transmitting a cellular UL subframe (e.g.  130  or  134  or  138 ) does not require a D2D-UE to adjust its transmitter. However, receiving a D2D subframe immediately after transmitting an UL cellular subframe (e.g.  131  or  135  or  139 ) does require a D2D-UE to switch from its transmitter to its receiver. Thus, it is proposed to provide a guard period (GP s-1) at the beginning of a transmitted D2D subframe which immediately follows a cellular UL subframe to account for (or allow for) TX-RX switching time at the D2D receiving site. 
     Furthermore, at a D2D subframe transmission site, the end of a transmitted D2D subframe is perfectly aligned with the end of the corresponding cellular UL subframe. However, at the D2D subframe reception site for the said subframe, the end of the received D2D subframe may be aligned with the start of a cellular UL subframe transmitted immediately after the said D2D subframe, as indicated by  133 , or it may collide or overlap with the start of the cellular UL subframe transmitted immediately after the said D2D subframe, as indicated by  137 . 
     Transmitting a cellular UL subframe immediately after transmitting a D2D subframe (e.g.  130  or  134 ) does not require a D2D-UE to adjust its transmitter. However, transmitting an UL cellular subframe immediately after receiving a D2D subframe (e.g.  131  or  136 ) does require a D2D-UE to switch from its receiver to its transmitter. Thus, it is proposed to provide two guard periods (GP e-3 and GP e-1) at the end of a transmitted D2D subframe, where the said transmitted D2D subframe has an immediately following cellular UL subframe, in order to account for (or allow for or prevent) collision and RX-TX switching time at the corresponding D2D receiving site. 
     Multiple consecutive cellular UL subframes may be allocated for D2D communication, such as for example subframes #7, 8 and 9 as illustrated in timing diagram  110 . a  in  FIGS. 3A and 3B . Any two consecutive subframes for D2D communication which have the same direction (e.g. D2D-TX  141  or D2D-RX  142 ) will not require a guard period to account for switching and/or collision handling. However, any two consecutive subframes for D2D communication which are in different directions (e.g. D2D-TX to D2D-RX  143 , or D2D-RX to D2D-TX  140 ) will require a guard period to account for TX-RX or RX-TX switching and collision handling. In the worst case scenario in practical implementations, a guard period for collision handling may take up to 16.1 μsec. Thus, it is proposed to provide two guard periods (GP e-3 and GP e-1) at the end of a transmitted D2D subframe, where the said transmitted D2D subframe has an immediately following D2D subframe in the opposite direction, in order to account for (or allow for or prevent) collision and RX-TX/TX-RX switching time at the corresponding D2D receiving/transmitting site. 
     The proposed guard periods (GP s-1, and GP e-3 and GP e-1) mentioned above for a D2D subframe structure for a FDD system are graphically represented in diagram  160 . a  in  FIGS. 3A and 3B . 
     Turning now to  FIGS. 4A and 4B , this Figure illustrates, among other things, a system which represents a worst case scenario wherein maximum D2D subframe collision and/or interference occurs in a TDD system (this is different to  FIGS. 3A and 3B  above which relates to a FDD system). In  FIGS. 4A and 4B , TDD D2D-UEs  204  and  205  which form a pair for D2D communication under the overlaid TDD cellular network are separated by a maximum allowed distance  208 . (Again, the maximum allowed distance may be 1.5 miles in practical implementations). Also, TDD D2D-UE  205  is located at a position such that the distance  207  between the TDD servicing base station  201  and TDD D2D-UE  205  is equal to the distance  206  between the TDD servicing base station  201  and TDD D2D-UE  404  plus the distance  208  separating TDD D2D-UE  204  and TDD D2D-UE  205 . The timing diagram  210 . a  representing transmitting and receiving cellular subframes and transmitting and receiving D2D subframes in this scenario is also illustrated in  FIGS. 4A and 4B . 
     In the timing diagram  210 . a , a similar approach as was applied in the case of the FDD system above is used as a basis for discussing the design of a new D2D subframe structure for transmitting and receiving D2D channel(s) and/or D2D signal(s) under an overlaid TDD cellular network. 
     Referring to  FIGS. 4A and 4B , and selecting cellular UL subframe #2 and/or #7 (both of which follow immediately after a TDD special subframe) for D2D communication, on cellular UL subframe #2 TDD D2D-UE  205  performs transmission of a D2D subframe  230  and TDD D2D-UE  204  then performs reception  231  of that D2D subframe. On cellular UL subframe #7, TDD D2D-UE  204  performs transmission of a D2D subframe  238  and TDD D2D-UE  205  then performs reception  239  of that D2D subframe. At the D2D subframe transmission site, the start of a transmitted D2D subframe is perfectly aligned with the start of the corresponding cellular UL subframe. At the D2D subframe reception site for that subframe, the start of a received D2D subframe is generally later than the start of the corresponding cellular UL subframe, or in the worst case scenario a received D2D subframe has a subframe start which is aligned or overlaps with the corresponding cellular UL subframe, as indicated by  232 . 
     In the case of TDD, transmitting a D2D subframe immediately after receiving a cellular TDD special subframe (e.g.  230  or  238 ) does not require a TDD D2D-UE to adjust its transmitter as a TDD special subframe is designed to accommodate RX-TX switching. However, receiving a D2D subframe immediately after receiving a cellular TDD special subframe ( 231  or  239 ) results in a collision or overlap (for a maximum of 16.1 μsec in practical implementations) between the end of the cellular TDD special subframe and the start of the D2D RX subframe. Technically, the uplink pilot time slot (UpPTS) region designed for use in cellular TDD subframes is sufficient to deal with this overlapping region. Thus, it is proposed that NO guard period is required at the beginning of a transmitted D2D subframe which immediately follows a cellular special subframe to account for TX-RX switching time at the D2D transmitting site or to account for subframe collision at the D2D receiving site. 
     Similar to the situation in the FDD system discussed above, transmitting a UL cellular subframe immediately after receiving a D2D subframe (e.g.  239 - 240 ) results in subframe collision and also requires a D2D-UE to switch from its receiver to its transmitter. Thus, it is proposed to provide two guard periods (GP e-3 and GP e-1) at the end of a transmitted D2D subframe, where the said transmitted D2D subframe has an immediately following cellular UL subframe, to account for collision and RX-TX switching time at the corresponding D2D receiving site. 
     Also similar to the situation in the FDD system discussed above, multiple consecutive TDD UL cellular subframes may be allocated for D2D communication such as subframes #2, 3 and 4, as illustrated in timing diagram  210 . a . Any two consecutive subframes for D2D communication which have the same direction will not require a guard period to account for switching and/or collision handling. However, any two consecutive subframes for D2D communication which have different directions, such as D2D-TX to D2D-RX ( 234 ) or D2D-RX to D2D-TX ( 235 ), will require guard period to account for RX-TX switching and collision handling. In the worst case scenario in practical implementations, guard period for collision handling may take up to 16.1 μsec ( 236 ). Thus, it is proposed to provide two guard periods (GP e-3 and GP e-1) at the end of a transmitted D2D subframe, where the said transmitted D2D subframe has an immediately following D2D subframe with opposite direction, to account for collision and RX-TX switching time at the corresponding D2D receiving site. 
     The proposed guard periods mentioned above for a D2D subframe structure for a TDD system are graphically represented in diagram  260 . a  in  FIGS. 4A and 4B . 
     It is to be recalled that the above timing analyses and proposals for transmitting and receiving D2D channel(s) and/or D2D signal(s), as explained with reference to  FIGS. 3A, 3B, 4A and 4B , relate to cases where both of the D2D-UEs which form a pair for D2D communication belong to the same servicing base station (which may be a FDD or TDD base station). Four types of D2D subframes are proposed for D2D communication where two D2D UEs which are paired for D2D communication belong to the same cellular servicing base station. These are: 
     1. Type 1 D2D Subframe 
     The type 1 D2D subframe  400  is illustrated in  FIG. 8 . This subframe does not require a guard period either at the beginning or at the end of a D2D subframe  401 . In the case of normal CP, all 14 OFDM or SC-FDMA symbols  402  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). In the case of extended CP, all 12 OFDM or SC-FDMA symbols  403  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). 
     Referring to  FIG. 9 , in the case where both of the D2D-UEs which form a pair for D2D communication belong to the same servicing base station, and where the said base station is a FDD base station or a TDD base station, the type 1 D2D subframe  400  is used when: 
     the immediately prior subframe is either a D2D subframe of the same connection with any transmission direction (see  400 . 1 ) or a cellular TDD special subframe (see  400 . 2 ) if the base station is a TDD base station, and 
     the immediately following subframe is either a D2D subframe of the same transmission direction ( 400 . 3 ) or a cellular DL subframe ( 400 . 4 ) if the base station is a TDD base station. 
     2. Type 2″ D2D Subframe 
     The type 2″ D2D subframe  510  is illustrated in  FIG. 12 . This subframe type  511  does not require a guard period at the subframe start but does require guard periods  512  and  513  at the subframe end for handling subframe collision and RX-TX switching respectively. In the case of normal CP, the first 13 OFDM or SC-FDMA symbols  515  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). In the case of extended CP, the first 11 OFDM or SC-FDMA symbols  517  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). For both normal CP and extended CP, the last OFDM or SC-FDMA symbol in the subframe is discontinuous transmission (DTX). 
     Referring to  FIG. 13 , in the case where both of the D2D-UEs which form a pair for D2D communication belong to the same servicing base station, and where the said base station is a FDD base station or a TDD base station, the type 2″ D2D subframe  510  is used when: 
     the immediately prior subframe is either a D2D subframe of the same connection (see  510 . 1 ) or a cellular TDD special subframe ( 510 . 2 ) if the base station is a TDD base station, and 
     the immediately following subframe is either a D2D subframe of the same connection with opposite transmission direction (see  510 . 3 ), or a D2D subframe of another connection, or a cellular UL subframe ( 510 . 4 ) if the base station is a TDD base station. 
     3. Type 3 D2D Subframe 
     The type 3 D2D subframe  420  is illustrated in  FIG. 14 . This subframe  421  does not require a guard period at the subframe end but does require guard periods at the subframe start  423  for handling RX-TX switching. In the case of normal CP, the last 13 OFDM or SC-FDMA symbols  425  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). In the case of extended CP, the last 11 OFDM or SC-FDMA symbols  427  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). For both the normal CP and extended CP cases, the first OFDM or SC-FDMA symbol is DTX. 
     Referring to  FIG. 15 , in the case where both of the D2D-UEs which form a pair for D2D communication belong to the same servicing base station, and where the said base station is a FDD base station or a TDD base station, the type 3 D2D subframe  420  is used when: 
     the immediately prior subframe is a cellular UL subframe (see  420 . 1 ), and 
     the immediately following subframe is either a D2D subframe of the same connection with the same transmission direction ( 420 . 2 ) or a cellular DL subframe ( 420 . 3 ) if the base station is a TDD base station. 
     4. Type 4″ D2D Subframe 
     The type 4″ D2D subframe  530  is illustrated in  FIG. 18 . This subframe requires a guard period  532  at the subframe start for handling TX-RX switching time and it also requires guard periods  533  and  534  at the subframe end for handling subframe collision and RX-TX switching. In the case of normal CP, the 2nd to 13th OFDM or SC-FDMA symbols  537  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). In the case of extended CP, the 2nd to 11th OFDM or SC-FDMA symbols  540  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). For both the normal CP and extended CP cases, the first and last OFDM or SC-FDMA symbol in a D2D subframe are DTX. 
     Referring to  FIG. 19 , in the case where both of the D2D-UEs which form a pair for D2D communication belong to the same servicing base station, and where the said base station is a FDD base station or a TDD base station, the type 4″ D2D subframe  530  is used when: 
     the immediately prior subframe is a cellular UL subframe (see  530 . 1 ), and 
     the immediately following subframe is either a D2D subframe of the same connection with opposite transmission direction (see  530 . 2 ), or a D2D subframe of another connection, or a cellular UL subframe  530 . 3 . 
     In transmitting one or more of the above D2D subframe types, a D2D-UE may optionally retard a subframe transmission, possibly for half of its network configured TA in comparison with the corresponding network configured cellular UL subframe in order to help bring D2D-UE transmit and D2D-UE receive into approximate D2D subframe reference timing alignment. This may help to enhance D2D subframe boundary detection. 
     By way of further explanation, as can be seen from the timing analysis, TA can be considered as 2× propagation delay (i.e. until the propagation delay), so halving the configured TA for both D2D transmit and D2D receive may lead to D2D transmit and D2D receive having, approximately at least, a common reference timing i.e. aligned with base station timing. 
     A second aspect of the present invention relates to a method and a D2D subframe structure for sending and receiving D2D channel(s) and/or D2D signal(s) in such a way as to avoid or at least help manage intra-UE collision and/or inter-UE interference, and a allow paired D2D-UEs to negotiate dynamic selection of any UL subframe in a radio frame for D2D communication while maintaining their cellular UL transmission, in a system where paired D2D communicating UEs each belong to a different servicing base station. (Note: this is different to the first aspect of the invention above which is concerned with the situation where paired D2D communicating UEs belong to the same servicing base station.) In the context of the second aspect of the invention, two respective base stations may operate with both using FDD (i.e. FDD-FDD), or with both using TDD (i.e. TDD-TDD), or with one using FDD and the other using TDD (i.e. FDD-TDD/TDD-FDD). 
     When one D2D-UE is discovering other D2D-UE(s), or is being discovered by another D2D-UE, for the purposes of forming a pair for D2D communication, one D2D-UE may only know whether or not the other D2D-UE shares the same servicing base station. In other words, all that one D2D-UE will know about the other is whether it shares the same base station or belongs to a different base station. In the case where the other D2D-UE belongs to a different base station, the said UE may not know further information such as, for example, if the other servicing base station is FDD or TDD, the UL-DL configuration if the other base station happens to be TDD, if the other servicing base station uses flexible TDD, etc. 
     A FDD or TDD D2D-UE may be configured by its own servicing base station as to the cellular UL resources on which it is allowed to transmit and receive D2D channel(s) and/or D2D signal(s). Set out below are timing analyses for worst case scenarios for different system combinations to identify requirements for D2D communication subframe types and associated applications, bearing in mind that a D2D-UE may not know the system configuration of the other D2D-UE to which it is (or may be) paired. 
       FIGS. 5A,5B and 5C  illustrates, among other things, a system  100 . b  which represents a worst case scenario wherein maximum D2D subframe collision and/or interference occurs in a FDD-FDD system. In the said system, D2D-UEs  104  and  105 , which are paired for D2D communication, each belong to a different FDD servicing base station. More specifically, D2D-UE (UE#1)  104  belongs to the first base station (eNB#1)  101 , and D2D-UE (UE#2)  105  belongs to the second base station (eNB#2)  102 . D2D-UEs  104  and  105  are separated at a maximum allowed distance  108  (1.5 miles in practical implementations). FDD D2D-UE  104  is located at a position such that distance  106  between servicing first base station  101  and FDD D2D-UE  104  results in a minimum possible timing advance (TA) of 0, and FDD D2D-UE  105  (the other D2D-UE) is located at a position such that the distance  107  between second servicing base station  102  and FDD D2D-UE  105  results in a maximum possible timing advance (TA) of 32. Furthermore, the first base station  101  and the second base station  102  may have different timing references due to imperfect inter-base station timing synchronisation. Imperfect timing synchronisation may result in a timing reference offset, which may be up to 10 μsec in practical implementations. 
     In the timing diagram  110 . b  in  FIGS. 5A,5B and 5C , which corresponds to the scenario described immediately above, the first base station&#39;s TX-RX timing  111  servicing FDD D2D-UE  104  has a timing retard  113  (the timing retard  113  being 10 μsec in this example) compared with the second base station&#39;s TX-RX timing  112  servicing FDD D2D-UE  105 . The distance  106  results in a (nil) propagation delay  116  for transmitting a cellular DL subframe from the first servicing base station  101  to FDD D2D-UE  104 . For a SC-FDMA symbol sent by all UE(s) under the first servicing base station and arriving at the first servicing base station  101  within a CP length, the first servicing base station  101  configures FDD D2D-UE  104  to have a (nil) TA  120  that results in cellular UL subframe timing  114 . On the other hand, the distance  107  results in (a non-zero) propagation delay  117  for transmitting a cellular DL subframe from the second serving base station  102  to FDD D2D-UE  105 . For a SC-FDMA symbol sent by all UE(s) under the second servicing base station and arriving at the second servicing base station  102  within a CP length, the second servicing base station  102  configures FDD D2D-UE  105  to have (a non-zero) TA  121  of 32, which results in cellular UL subframe timing  115 . 
     This arrangement results in FDD D2D-UEs  104  and  105  having a maximum TX timing difference  119  (this timing difference is 26.38 μsec in this example). Furthermore, the maximum separation  108  (which is 1.5 miles in this example) between FDD D2D UE  104  and FDD D2D-UE  105  results in a propagation delay  118  (this propagation delay is 8.05 μsec in this example) for transmitting a D2D subframe from FDD D2D-UE  104  to FDD D2D-UE  105  or via versa. According to a current working assumption endorsed in the above 3GPP D2D SI, cellular UL subframe timing of each servicing base station is used as reference timing for sending D2D subframe (see  122  and  123 ), and this makes the D2D RX ( 124  and  125 ) timing at a receiving D2D-UE ( 104  and  105 ) depend on the timing of D2D TX from the transmitting D2D-UE ( 105  and  104 ). 
     The system and associated timing as described above provides a basis for discussing of the design of a new D2D subframe structure for transmitting and receiving D2D channel(s) and/or D2D signal(s) under an overlaid FDD-FDD cellular network. 
     Still referring to  FIGS. 5A, 5B and 5C , and randomly selecting cellular UL subframes #2 and #4 for D2D communication, on cellular UL subframe #2 FDD D2D-UE  105  performs transmission of a D2D subframe  130  and FDD D2D-UE  104  should then perform reception  131  of that D2D subframe. On UL subframe #4, FDD D2D-UE  104  performs transmission of a D2D subframe  135  and D2D-UE  105  should then perform reception  136  of that D2D subframe. For the D2D transmission on cellular UL subframe #2, at the D2D subframe transmission site (i.e. at the location of D2D-UE  105 ) the start of the transmitted D2D subframe is perfectly aligned with the corresponding cellular UL subframe, whereas at the D2D subframe reception site for that subframe (i.e. at the location of D2D-UE  104 ) the received D2D subframe has a subframe start earlier than the start of the corresponding cellular UL subframe, as indicated by  132 . This is due to the TX timing difference  119  (the timing difference is 26.38 μsec in this example), and it causes a collision region (the collision region is 18.33 μsec in this example) in which transmission of the last symbol(s) of the previous cellular UL subframe collide with reception of the first symbol(s) of the D2D subframe. 
     Furthermore, transmitting a D2D subframe immediately after transmitting an UL cellular subframe (e.g.  130  or  135  or  139 ) does not require a D2D-UE to adjust its transmitter. However, receiving a D2D subframe immediately after transmitting an UL cellular subframe (e.g.  131  or  136  or  140 ) does require a D2D-UE to switch from its transmitter to its receiver. Thus, it is proposed to provide guard periods (GP s-1 and GP s-2) at the beginning of a transmitted D2D subframe which immediately follows a cellular UL subframe to account for TX-RX switching time and subframe colliding at the D2D receiving site. 
     At a D2D subframe transmission site, the end of the transmitted D2D subframe is perfectly aligned with the corresponding cellular UL subframe, whereas at the D2D subframe reception site for the said subframe, the end of the received D2D subframe collides or overlaps (see region  137 , which is 34.43 μsec in this example) with the cellular UL subframe transmitted immediately thereafter. Transmitting a cellular UL subframe immediately after transmitting a D2D subframe (e.g.  130  or  135 ) does not require a D2D-UE to adjust its transmitter. However, transmitting an UL cellular subframe immediately after receiving a D2D subframe (e.g.  131  or  136 ) does require a D2D-UE to switch from its receiver to its transmitter. Thus, it is proposed to provide two guard periods (GP e-2 and GP e-1) at the end of a transmitted D2D subframe which has an immediately following cellular UL subframe to account for collision and RX-TX switching time at the corresponding D2D receiving site. 
     Multiple consecutive UL cellular subframes may be allocated for D2D communication, such as for example subframes #7, 8 and 9 illustrated in the timing diagram  110 . b . Any two consecutive subframes for D2D communication which have the same direction, such as for example D2D-TX  142  or D2D-RX  141 , will not require a guard period to account for switching and/or collision handling. However, any two consecutive subframes for D2D communication which have different directions, such as for example D2D-TX to D2D-RX  139  or D2D-RX to D2D-TX  140 , will require guard period to account for RX-TX switching and collision handling. In the worst case scenario in the depicted example, guard period for collision handling may take up to 34.43 μsec. Thus, it is proposed to provide two guard periods (GP e-2 and GP e-1) at the end of a transmitted D2D subframe, where the immediately following subframe is a D2D subframe in the opposite direction, to account for collision and RX-TX switching time at the corresponding D2D receiving site. 
     The proposed guard periods mentioned above for a D2D subframe structure for a FDD-FDD system are graphically represented in diagram  160 . b  in  FIGS. 5A,5B and 5C . 
     Turning now to  FIGS. 6A, 6B and 6C , this Figure illustrates, among other things, a system  200 . b  which represents a worst case scenario wherein maximum D2D subframe collision and/or interference occurs in a TDD-TDD system. In the said system, D2D-UEs  204  and  205 , which are paired for D2D communication, each belong to a different TDD servicing base station. More specifically, D2D-UE (UE#1)  204  belongs to the first base station (eNB#1)  201 , and D2D-UE (UE#2)  205  belongs to the second base station (eNB#2)  202 . D2D-UEs  104  and  105  are separated at a maximum allowed distance  208  (1.5 miles in practical implementations). 
     D2D-UE  204  is located at position such that the distance  206  between the first servicing base station  201  and TDD D2D-UE  204  results a minimum possible timing advance (TA) of 0, and TDD D2D-UE  205  is located at position such that the distance  207  between second servicing base station  202  and TDD D2D-UE  205  results in a maximum possible timing advance (TA) of 32. Furthermore, the first servicing base station  201  and the second servicing base station  202  may have different timing references due to imperfect inter-base station timing synchronisation. Imperfect timing synchronisation may result in a timing reference offset of up to 10 μsec. 
     In the timing diagram  210 . b , a similar approach as was applied in the case of the FDD-FDD system above is used as a basis for discussing the design of a new D2D subframe structure for transmitting and receiving D2D channel(s) and/or D2D signal(s) under an overlaid TDD-TDD cellular network. 
     Referring to  FIGS. 6A, 6B and 6C , and selecting cellular UL subframes #2 and/or #7 (both of which are immediately after a cellular TDD special subframe) for D2D communication, on cellular UL subframe #2 TDD D2D-UE  205  performs transmission of a D2D subframe  230  and TDD D2D-UE  204  should then perform reception  231  of that D2D subframe. On cellular UL subframe #7, TDD D2D-UE  204  performs transmission of a D2D subframe  238  and D2D-UE  205  should then perform reception  239  of that D2D subframe. At a D2D subframe transmission site, the start of a transmitted D2D subframe is always perfectly aligned with the start of the corresponding cellular UL subframe, but in the worst case scenario at the D2D subframe reception site for the said subframe, due to the D2D-UE&#39;s TX timing difference  218 , the received D2D subframe has a subframe start earlier than that of the corresponding cellular UL subframe, as indicated by  232 . Transmitting a D2D subframe immediately after receiving a cellular TDD special subframe (see  230  or  238 ) does not require a TDD D2D-UE to adjust its transmitter as TDD special subframes are designed to accommodate RX-TX switching. Receiving a D2D subframe immediately after receiving a cellular TDD special subframe (see  231  or  239 ) results in subframe collision or overlap (of a maximum of 18.33 μsec in practical implementations) between the cellular TDD special subframe end and the D2D RX subframe start. Technically, the UpPTS region designed for use in cellular TDD subframes is sufficient for dealing with this overlapping region. Thus, it is proposed that NO guard period is required at the beginning of a transmitted D2D subframe which immediately follows a certain cellular special subframe (such as subframe #1) to account for TX-RX switching time at the D2D transmitting site or to account for subframe collision at the D2D receiving site. 
     As mentioned previously, a D2D-UE belonging to (say) the first servicing base station may not know detailed information such as the UL-DL configuration of the second servicing base station, or if the second servicing base station is flexible TDD or not. Given that one of several different UL-DL configurations may be configured, the subframe #6 may happen to be a cellular DL subframe on the first serving base station and a special subframe on the other base station. Receiving a D2D subframe immediately after receiving a cellular TDD DL subframe may result in a collision or overlap (of a maximum of 18.33 μsec in practice) between the end of the cellular TDD DL subframe and the start of the D2D RX subframe. Transmitting a D2D subframe immediately after receiving a cellular TDD DL subframe may result in subframe collision or overlap (of a maximum of 32.76 μsec in practice) between the end of the cellular TDD DL subframe and the start of the D2D TX subframe. Thus, it is proposed to provide guard periods (GP s-1 and GP s-2) at the beginning of a transmitted D2D subframe which immediately follows an uncertain cellular special subframe (such as subframe #6) or a cellular UL subframe to account for RX-TX/RX-TX switching time at the D2D transmitting/receiving site and to account for subframe collision at the D2D transmitting and receiving site. 
     Similar to the FDD-FDD system discussed previously, transmitting an UL cellular subframe immediately after receiving a D2D subframe  239  results in a subframe collision and also requires a D2D-UE to switch from its receiver to its transmitter. Thus, it is proposed to provide two guard periods (GP e-2 and GP e-1) at the end of a transmitted D2D subframe which has an immediately following cellular UL subframe to account for collision and RX-TX switching time at the corresponding D2D receiving site. 
     Also similar to the FDD-FDD system above, multiple consecutive TDD UL cellular subframes may be allocated for D2D communication, such as subframes #2, 3 and 4 illustrated in the timing diagram  210 . b . Any two consecutive subframes for D2D communication which have the same direction will not require guard period to account for switching and/or collision handling. However, any two consecutive subframes for D2D communication which have different directions, such as D2D-TX to D2D-RX  234  or D2D-RX to D2D-TX  235  will require guard periods to account for RX-TX switching and collision handling. In the worst case scenario in practice, guard period for collision handling may take up to 16.1 μsec  326 . Thus, it is proposed to provide two guard periods (GP e-2 and GP e-1) at the end of a transmitted D2D subframe which has an immediately following D2D subframe with opposite direction to account for collision and RX-TX switching time at the corresponding D2D receiving site. 
     The proposed guard periods mentioned above for a D2D subframe structure for a TDD-TDD system are graphically represented in diagrams  260 . b  (for same UL-DL configuration) and  261 . b  (for different UL-DL configuration). 
     Turning now to  FIGS. 7A, 7B, 7C, 7D and 7E , this Figure illustrates, among other things, systems  300 . a  and  300 . b  which represent worst case scenarios wherein maximum D2D subframe collision and/or interference occur in FDD-TDD/TDD-FDD systems. In both systems  300 . a  and  300 . b , the D2D-UE (UE#1)  304  belongs to the first base station (eNB#1)  301 , the D2D-UE (UE#2)  305  belongs to the second base station (eNB#2)  302 , and D2D-UEs  304  and  305  are paired for D2D communication. However, in system  300 . a  the first base station (eNB#1)  301  is FDD (meaning that D2D-UE  304  is a FDD D2D-UE) and the second base station (eNB#2)  302  is TDD (meaning that D2D-UE  305  is a TDD D2D-UE), whereas in system  300 . b  the first base station (eNB#1)  301  is TDD (meaning that D2D-UE  304  is a TDD D2D-UE) and the second base station (eNB#2)  302  is FDD (meaning that D2D-UE  305  is a FDD D2D-UE). In both systems  300 . a  and  300 . b , the D2D-UEs  304  and  305  are separated by a maximum allowed distance  308  (1.5 miles in practice). 
     In both systems  300 . a  and  300 . b , D2D-UE  304  is located at a position such that the distance  306  between first servicing base station  301  and D2D-UE  304  results in a minimum possible timing advance (TA) of 0, and D2D-UE  305  is located at a position such that the distance  307  between second servicing base station  302  and D2D-UE  305  results in a maximum possible timing advance (TA) of 32. Furthermore, the first base station  301  and second base station  302  may have different timing references due to imperfect inter-base station timing synchronisation. Imperfect timing synchronisation may result in a timing reference offset (which may be up to 10 μsec in practice). 
     In timing diagrams  310 . a  and  310 . b , a similar approach as was applied in the cases of the FDD-FDD and TDD-TDD systems above is used as a basis for discussing the design of a new D2D subframe structure for transmitting and receiving D2D channel(s) and/or D2D signal(s) under an overlaid FDD-TDD or TDD-FDD cellular network. 
     Referring to  FIGS. 7A, 7B, 7C, 7D and 7E , cellular UL subframes #2 and/or #7 (which are immediately after cellular TDD special subframes in the second base station for system  300 . a , and likewise in the first base station for system  300 . b ) are specifically selected for D2D communication. In both systems  300 . a  and  300 . b , on cellular UL subframe #2, D2D-UE  305  performs transmission of a D2D subframe  330  and D2D-UE  304  should then perform reception  331  of that D2D subframe. On cellular UL subframe #7, D2D-UE  304  performs transmission of a D2D subframe  338  and D2D-UE  305  should then perform reception  339  of that D2D subframe. At a D2D subframe transmission site, the start of a transmitted D2D subframe is always perfectly aligned with the corresponding cellular UL subframe, but in the worst case scenario at the D2D subframe reception site for the said subframe, due to the D2D-UE&#39;s TX timing difference  318 , a received D2D subframe has a subframe start which is earlier than that of the corresponding cellular subframe, and this causes a collision with the end of the immediately prior cellular subframe, as indicated by  332 . 
     In timing diagram  300 . a , transmitting a D2D subframe immediately after receiving a cellular TDD special subframe (e.g.  330 ) does not require TDD D2D-UE  305  to adjust its transmitter as TDD special subframes are designed to accommodate RX-TX switching. However, receiving a D2D subframe immediately after transmitting a cellular FDD UL subframe ( 331 ) results in a subframe collision or overlap (of maximum 18.33 μsec in practice) between the end of the cellular FDD UL subframe and the start of the D2D RX subframe. Furthermore, receiving a D2D subframe immediately after transmitting a cellular FDD UL subframe ( 331 ,  343 ) requires a FDD D2D-UE to switch from transmitting to receiving. Thus, it is proposed to provide two guard periods (GP s-1 and GP s-2) at the beginning of a transmitted D2D subframe which immediately follows a cellular subframe such as a TDD special subframe or a cellular UL subframe to account for RX-TX/RX-TX switching time at the D2D transmitting/receiving end and to account for subframe collision at the D2D receiving site. 
     Furthermore, transmitting an UL cellular subframe immediately after receiving a D2D subframe (e.g.  340 ) results in a subframe collision and also requires a D2D-UE to switch its receiver to transmitter. Thus, it is proposed to provide two guard periods (GP e-2 and GP e-1) at the end of a transmitted D2D subframe which has an immediately following cellular subframe such as a TDD DL subframe or a UL subframe to account for subframe collision and RX-TX switching time at the corresponding D2D receiving site. 
     Similar to the FDD-FDD and TDD-TDD systems above, multiple consecutive TDD UL cellular subframes may be allocated for D2D communication. Any two consecutive subframes for D2D communication which have the same direction will not require guard period to account for switching and/or collision handling. However, any two consecutive subframes for D2D communication which have different directions, i.e. D2D-TX to D2D-RX or D2D-RX to D2D-TX, will require guard periods to account for RX-TX switching and collision handling. In the worst case scenario in practice, guard period for collision handling may take up to 34.43 μsec. Thus, it is proposed to provide two guard periods (GP e-2 and GP e-1) at the end of a transmitted D2D subframe which has an immediately following D2D subframe with opposite direction to account for collision and RX-TX switching time at the corresponding D2D receiving site. 
     The proposed guard periods mentioned above for a D2D subframe structure for a FDD-TDD or TDD-FDD system are graphically represented in diagram  360  in  FIGS. 7A,7B,7C,7D and 7E . 
     It is to be recalled that the above timing analyses and proposals for transmitting and receiving D2D channel(s) and/or D2D signal(s), as explained with reference to  FIGS. 5A, 5B, 5C, 6A, 6B, 6C, 7A, 7B, 7C, 7D and 7E , relate to cases where each D2D-UE in a pair formed for D2D communication belongs to a different servicing base station, and each said base station may be a FDD or TDD base station (thus giving FDD-FDD, TDD-TDD and FDD-TDD/TDD-FDD as possible system scenarios). Five types of D2D subframes are proposed for D2D communication where two D2D UEs which are paired for D2D communication belong to different cellular servicing base station. These are: 
     1. Type 1 D2D Subframe 
     The type 1 D2D subframe  400  is illustrated in  FIG. 8 . This subframe does not require a guard period either at the beginning or at the end of a D2D subframe  401 . In the case of normal CP, all 14 OFDM or SC-FDMA symbols  402  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). In the case of extended CP, all 12 OFDM or SC-FDMA symbols  403  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). 
     Referring to  FIG. 9 , in the case where both of the D2D-UEs which form a pair for D2D communication belong to different servicing base stations, the type 1 D2D subframe  400  is used when: 
     the immediately prior subframe is a D2D subframe ( 400 . 5 ), and 
     the immediately following subframe is a D2D subframe of the same transmission direction ( 400 . 6 ). 
     2. Type 2 D2D Subframe 
     The type 2 D2D subframe  410  is illustrated in  FIG. 10 . This subframe does not require guard period at the subframe start  411  but does require guard periods at the D2D subframe end  412  and  413  for handling subframe collision and RX-TX switching respectively. In the case of normal CP, the first 12 OFDM or SC-FDMA symbols  415  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last two symbols are DTX. In the case of extended CP, the first 11 OFDM or SC-FDMA symbols  417  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last symbol is DTX. 
     Referring to  FIG. 11 , in the case where D2D UEs which form a pair for D2D communication belong to different cellular servicing base stations (i.e. FDD-FDD, TDD-TDD or TDD-FDD/FDD-TDD), the type 2 D2D subframe  410  is used when: 
     the immediately prior subframe is a D2D subframe  410 . 1 , and 
     the immediately following subframe is either a D2D subframe of the same connection but with the opposite transmission direction  410 . 2 , or a D2D subframe of another connection, or a cellular subframe  410 . 3  including a cellular UL subframe or a cellular DL subframe. 
     3. Type 3 D2D Subframe 
     The type 3 D2D subframe  420  is illustrated in  FIG. 14 . This subframe does not require guard period at the subframe end  421  but it does require guard periods at the subframe start  422  and  423  for handling subframe collision and RX-TX switching. In the case of normal CP, the last 13 OFDM or SC-FDMA symbols  425  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). In the case of extended CP, the last 11 OFDM or SC-FDMA symbols  427  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). For both normal and extended CP, the first symbol is DTX. 
     Referring to  FIG. 15 , in the case where D2D UEs which form a pair for D2D communication belong to different cellular servicing base stations (i.e. FDD-FDD, TDD-TDD or TDD-FDD/FDD-TDD), the type 3 D2D subframe  420  is used when: 
     the immediately prior subframe is a cellular subframe  420 . 4 , including a cellular FDD subframe or cellular TDD special subframe, and 
     the immediately following subframe is a D2D subframe of the same connection with the same TX/RX direction ( 420 . 5 ). 
     4. Type 4 D2D Subframe 
     The type 4 D2D subframe  430  is illustrated in  FIG. 16 . This subframe requires guard periods at the subframe start  432  and  433  for handling subframe collision and TX-RX switching time, and it also requires guard periods at the subframe end  434  and  435  for handling subframe collision and RX-TX switching. In the case of normal CP, the first OFDM or SC-FDMA symbol is DTX, the 2nd to 12th OFDM or SC-FDMA symbols  438  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and the last two OFDM or SC-FDMA symbols are DTX. In the case of extended CP, the first OFDM or SC-FDMA symbol is DTX, the 2nd to 11th OFDM or SC-FDMA symbols  441  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and the last OFDM or SC-FDMA symbol is DTX. 
     Referring to  FIG. 17 , in the case where D2D UEs which form a pair for D2D communication belong to different cellular servicing base stations (i.e. FDD-FDD, TDD-TDD or TDD-FDD/FDD-TDD), the type 4 D2D subframe  430  is used when: 
     the immediately prior subframe is a cellular subframe  430 . 1 , and 
     the immediately following subframe is a D2D subframe of the same connection with opposite TX/RX direction  430 . 2 , or a D2D subframe of another connection, or a cellular subframe  430 . 3  including a cellular UL or DL subframe. 
     5. Type 5 D2D Subframe 
     The type 5 D2D subframe  450  is illustrated in  FIG. 20 . This subframe requires a guard period at the subframe start  452  for handling subframe collision, and it also requires guard periods at the subframe end  453  and  454  for handling subframe collision and RX-TX switching. In the case of normal CP, the first 12 OFDM or SC-FDMA symbols  459  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last two OFDM or SC-FDMA symbols are DTX. Additionally, transmitting a type 5 subframe with normal CP requires a retardation  457  (preferably of 1024.Ts) to save one OFDM/SC-FDMA symbol. In the case of extended CP, the first OFDM/SC-FDMA symbol is DTX, the 2nd to 11th OFDM or SC-FDMA symbols  462  are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and the last OFDM/SC-FDMA symbol is DTX. 
     Referring to  FIG. 21 , in the case where D2D UEs which form a pair for D2D communication belong to different cellular servicing TDD base stations (i.e. TDD-TDD), the type 5 D2D subframe  450  is used when: 
     the immediately prior subframe is a certain cellular special subframe  450 . 1 , such as subframe #1, and 
     the immediately following subframe is a D2D subframe of the same connection but with opposite transmission direction ( 450 . 2 ), or a D2D subframe of another connection, or a cellular subframe  450 . 3 . 
     In transmitting one or more of the above D2D subframe types, a D2D-UE may optionally retard a subframe transmission, possibly for half of its network configured TA in comparison with the corresponding network configured cellular UL subframe in order to help bring D2D-UE transmit and D2D-UE receive into approximate D2D subframe reference timing alignment. This may help to enhance D2D subframe boundary detection. 
     The present invention may provide a number of advantages. For example: 
     The present invention may help to resolve or manage intra-UE collision and inter-UE interference due to, for instance, timing misalignment, and may hence help to allow cellular communication and D2D communication sharing the same resource on a subframe basis. 
     The various D2D subframe types discussed above are carefully designed, each with specific guard periods (GPs) and length, in order to handle intra-UE collision and/or inter-UE interference, and hence help to minimise redundant GPs when a single D2D subframe type is used. 
     The various D2D subframe types are designed to be dynamically selected (or selectable) by a transmitting D2D-UE for transmitting D2D channel(s) and/or D2D signal(s) simply and in a manner which is implicitly understood by receiving D2D-UE(s) without the need for signalling to inform the receiving D2D-UE(s) of the particular D2D subframe type used. 
     The present invention should have little or no impact on the legacy LTE specification. 
     Also, in comparison with conventional IEEE P2P technology (which might be considered, in general terms, to represent a current competing state of the art technology supporting direct communication), the present invention may provide a number of advantages: 
     It allows cellular communication and D2D communication, sharing the same cellular UL resource, on a subframe basis, thereby assisting dual connectivity (i.e. cellular connectivity plus direct connectivity, if this is required or desirable); 
     IEEE P2P is designed to operate on different resources within unlicensed spectrum, whereas D2D communication as presently proposed is designed to operate on licensed spectrum, thus supporting general direct communication without affecting or jeopardizing public safety communication. 
     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. 
     (Supplementary Note 1) 
     A signalling method for use in an advanced wireless communications system, wherein the system includes: 
     at least one servicing base station; and
 
at least a first and a second direct (D2D) communication capable user equipment (D2D-UE), wherein each D2D-UE is in cellular communication with a servicing base station, and the first D2D-UE and second D2D-UE, at least, are paired or grouped for D2D communication with one another,
 
the method comprising, on a given subframe,
 
performing D2D communication by transmitting D2D channel(s) and/or D2D signal(s) between one D2D-UE and another using a D2D subframe type which allows RX-TX switching time and/or prevents subframe collision at the receiving D2D-UE.
 
     (Supplementary Note 2) 
     The signalling method as noted in Supplementary note 1, wherein there is a plurality of different D2D subframe types which can allow RX-TX switching time and/or prevent subframe collision at the receiving D2D-UE, depending on the subframe type of a preceding and/or subsequent subframe, the method further comprising: 
     selecting an appropriate D2D subframe type from among the plurality of different D2D subframe types for use on a given subframe; and
 
transmitting D2D channel(s) and/or D2D signal(s) from one D2D-UE to another on the given subframe using the selected D2D subframe type.
 
     (Supplementary Note 3) 
     The signalling method as noted in Supplementary note 2, wherein the method for selecting an appropriate D2D subframe type from among the said plurality of different D2D subframe types does not require signalling between paired or grouped D2D-UEs to indicate the D2D subframe type selected for use in a given subframe. 
     (Supplementary Note 4) 
     The signalling method as noted in Supplementary note 2 or 3, wherein the first D2D-UE and second D2D-UE, at least, are in cellular communication with the same servicing base station. 
     (Supplementary Note 5) 
     The signalling method as noted in Supplementary note 4, wherein the plurality of different D2D subframe types includes a first D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is either:
           a D2D subframe of the same connection and with any transmission direction, or   a cellular TDD special subframe if the servicing base station is a TDD base station, and   
           the immediately following subframe is either:
           a D2D subframe of the same transmission direction, or   a cellular downlink (DL) subframe if the base station is a TDD base station.   
               

     (Supplementary Note 6) 
     The signalling method as noted in Supplementary note 5, wherein the first D2D subframe type does not have a guard period at the beginning or at the end of the subframe. 
     (Supplementary Note 7) 
     The signalling method as noted in Supplementary note 6 wherein, in the case of normal cyclic prefix (CP) all 14 orthogonal frequency division multiplexing (OFDM) or single carrier frequency division multiple access (SC-FDMA) symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP all 12 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). 
     (Supplementary Note 8) 
     The signalling method as noted in any one of notes Supplementary note 4-7, wherein the plurality of different D2D subframe types includes a second D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is either:
           a D2D subframe of the same connection, or   a cellular TDD special subframe if the servicing base station is a TDD base station, and   
           the immediately following subframe is either:
           a D2D subframe of the same connection with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular UL subframe if the servicing base station is a TDD base station.   
               

     (Supplementary Note 9) 
     The signalling method as noted in Supplementary note 8, wherein the second D2D subframe type does not have a guard period at the beginning of the subframe but does have at least one guard period at the end of the subframe. 
     (Supplementary Note 10) 
     The signalling method as noted in Supplementary note 9 wherein, in the case of normal CP the first 13 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the first 11 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal CP and extended CP the last OFDM or SC-FDMA symbol in the subframe is discontinuous transmission (DTX). 
     (Supplementary Note 11) 
     The signalling method as noted in any one of notes Supplementary note 4-10, wherein the plurality of different D2D subframe types includes a third D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular uplink (UL) subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection with the same transmission direction, or   a cellular DL subframe if the servicing base station is a TDD base station.   
               

     (Supplementary Note 12) 
     The signalling method as noted in Supplementary note 11, wherein the third D2D subframe type does not have a guard period at the end of the subframe but does have at least one guard period at the start of the subframe. 
     (Supplementary Note 13) 
     The signalling method as noted in Supplementary note 12 wherein, in the case of normal CP the last 13 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the last 11 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal CP and extended CP the first OFDM or SC-FDMA symbol is DTX. 
     (Supplementary Note 14) 
     The signalling method as noted in any one of notes Supplementary note 4-13, wherein the plurality of different D2D subframe types includes a fourth D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular UL subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular UL subframe.   
               

     (Supplementary Note 15) 
     The signalling method as noted in Supplementary note 14, wherein the fourth D2D subframe type has at least one guard period at the start of the subframe and also at least one guard period at the end of the subframe. 
     (Supplementary Note 16) 
     The signalling method as noted in Supplementary note 15 wherein, in the case of normal CP the 2nd to 13th OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the 2nd to 11th OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal CP and extended CP the first and last OFDM or SC-FDMA symbol in a D2D subframe are DTX. 
     (Supplementary Note 17) 
     The signalling method as noted in Supplementary note 2 or 3, wherein the first D2D-UE and second D2D-UE, at least, are in cellular communication with different servicing base stations. 
     (Supplementary Note 18) 
     The signalling method as noted in Supplementary note 17, wherein the plurality of different D2D subframe types includes a first D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a D2D subframe, and   the immediately following subframe is a D2D subframe of the same connection and transmission direction.       

     (Supplementary Note 19) 
     The signalling method as noted in Supplementary note 18, wherein the first D2D subframe type does not have a guard period at the beginning or at the end of the subframe. 
     (Supplementary Note 20) 
     The signalling method as noted in Supplementary note 19 wherein, in the case of normal CP all 14 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP all 12 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s). 
     (Supplementary Note 21) 
     The signalling method as noted in any one of notes Supplementary note 17-20, wherein the plurality of different D2D subframe types includes a second D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a D2D subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection but with the opposite transmission direction, or   a D2D subframe of another connection, or   a cellular subframe, including a cellular UL subframe or a cellular DL subframe.   
               

     (Supplementary Note 22) 
     The signalling method as noted in Supplementary note 21, wherein the second D2D subframe type does not have a guard period at the start of the subframe but does have at least one guard period at the end. 
     (Supplementary Note 23) 
     The signalling method as noted in Supplementary note 22 wherein, in the case of normal CP the first 12 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last two symbols are DTX, or in the case of extended CP the first 11 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last symbol is DTX. 
     (Supplementary Note 24) 
     The signalling method as noted in any one of notes Supplementary note 17-23, wherein the plurality of different D2D subframe types includes a third D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular subframe, and   the immediately following subframe is a D2D subframe of the same connection with the same transmission direction.       

     (Supplementary Note 25) 
     The signalling method as noted in Supplementary note 24, wherein the third D2D subframe type does not have a guard period at the end of the subframe but it does have at least one guard period at the start of the subframe. 
     (Supplementary Note 26) 
     The signalling method as noted in Supplementary note 25 wherein, in the case of normal CP the last 13 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), or in the case of extended CP the last 11 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), and for both normal and extended CP the first symbol is DTX. 
     (Supplementary Note 27) 
     The signalling method as noted in any one of notes Supplementary note 17-26, wherein the plurality of different D2D subframe types includes a fourth D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a cellular subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular subframe.   
               

     (Supplementary Note 28) 
     The signalling method as noted in Supplementary note 27, wherein the fourth D2D subframe type has at least one guard period at the start of the subframe and at least one guard period at the end of the subframe. 
     (Supplementary Note 29) 
     The signalling method as noted in Supplementary note 28 wherein, in the case of normal CP the first OFDM or SC-FDMA symbol is DTX, the 2nd to 12th OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last two OFDM or SC-FDMA symbols are DTX, or in the case of extended CP the first OFDM or SC-FDMA symbol is DTX, the 2nd to 11th OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last OFDM or SC-FDMA symbol is DTX. 
     (Supplementary Note 30) 
     The signalling method as noted in any one of notes Supplementary note 17-29, wherein the different base stations servicing the first D2D-UE and the second D2D-UE respectively are both TDD base stations, and the plurality of different D2D subframe types includes a fifth D2D subframe type which is selected for use in transmitting D2D channel(s) and/or D2D signal(s) on a given subframe when:
         the immediately preceding subframe is a certain cellular special subframe, and   the immediately following subframe is either:
           a D2D subframe of the same connection but with opposite transmission direction, or   a D2D subframe of another connection, or   a cellular subframe.   
               

     (Supplementary Note 31) 
     The signalling method as noted in Supplementary note 30, wherein the fifth D2D subframe type has a guard period at the start of the subframe and at least one guard period at the end of the subframe. 
     (Supplementary Note 32) 
     The signalling method as noted in Supplementary note 31 wherein, in the case of normal CP the first 12 OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s), the last two OFDM or SC-FDMA symbols are DTX and a retardation is applied to save one OFDM/SC-FDMA symbol, or in the case of extended CP the first OFDM/SC-FDMA symbol is DTX, the 2nd to 11th OFDM or SC-FDMA symbols are used for transmitting/receiving D2D channel(s) and/or D2D signal(s) and the last OFDM/SC-FDMA symbol is DTX. 
     (Supplementary Note 33) 
     The signalling method as noted in any one of Supplementary notes 4-32 wherein in transmitting one or more of the said D2D subframe types, a D2D-UE retards a subframe transmission for half of its network configured timing advance (TA) in comparison with a corresponding network configured cellular UL subframe. 
     Note that the present invention is not limited to the above-mentioned exemplary embodiments, and it is obvious that various modifications can be made by those of ordinary skill in the art based on the recitation of the claims. 
     This application is based upon and claims the benefit of priority from Australia Patent Application No. 2013904205, filed on Oct. 31, 2013, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           01 , 16  access node 
           03 , 04 , 06 , 13 , 14 , 18 , 19 , 104 , 105 , 204 , 205 , 304 , 305  user equipment (UE) 
           11 , 101 , 102 , 201 , 202 , 301 , 302  base station 
           130 , 134 , 230 , 238 , 330 , 338 , 400 , 401 , 420 , 430 , 450 , 510 , 530  D2D subframe