Abstract:
A method for supporting P2P communication between two user equipments in TDD CDMA systems, performed by user equipment, comprising: receiving signals transferred via the downlink control channel from network system; acquiring the timeslot allocation information and the spreading code allocation information of other active user equipments allocated in the specific downlink timeslot associated with the direct link used by said user equipments, according to the received signals; and synchronizing the P2P communication signals received by the user equipment and signals from network system, according to the acquired timeslot allocation information and spreading code allocation information, so as to reduce the interference caused by the downlink signals transmitted from network system to other user equipments during the P2P communication process.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and apparatus for supporting P2P communication in TDD CDMA (Time-Division-Duplex Code-Division-Multiple-Access) communication systems, and more particularly, to a method and apparatus for reducing interference caused by conventional communication signals to a UE (user equipment) during P2P communication process in TDD CDMA communication systems.  
       BACKGROUND ART OF THE INVENTION  
       [0002]     In conventional cellular mobile communication systems, a UE (user equipment) has to communicate with another UE only through the relaying of base stations even if the two UEs are very close to each other.  FIG. 1  illustrates the conventional communication mode. However, in some cases when the distance between two UEs who camp on the same cell is very close, it can be a more reasonable way for them to communicate directly, rather than through the relaying of base stations. This method is the so-called peer-to-peer communication, abbr. as P2P.  
         [0003]      FIG. 2  illustrates the P2P communication between two UEs. Referring to  FIG. 2 , assume that the two UEs are camping in the same cell and the distance between them satisfies the requirement for establishing P2P connection, the dashed line represents signaling link between the UTRAN and the UE during P2P communication, the solid line for data link between the two UEs, and the arrowhead for direction of information flow. It can be obviously seen from the figure that only signaling link exists between the UTRAN and the UE, while only data link exists between the two communicating UEs. If additional signal overhead for management is ignored, P2P communication can save about 50% radio resource during the process of direct link. Furthermore, control channels are reserved between the UTRAN and the UEs, so wireless network operators still holds control through the base station over how the UEs utilize radio resources.  
         [0004]     It is commonly accepted that a Time Division Duplex (TDD) air interface is a communication standard that offers a more flexible adaptation to different uplink and downlink traffic requirements. Among existing 3G systems based on TDD communication mode, TD-SCDMA (Time Division—Synchronization Code Division Multiple Access) system is the most suitable system for the combination of P2P communication with conventional communication mode, because the same carrier frequency is applied in both uplink and downlink communications, which can simplify the RF (Radio Frequency) module of the mobile terminal.  
         [0005]     In a TD-SCDMA system that is capable of employing P2P communication mode, the DIRECT mode is introduced to describe the direct communication between two UEs, besides two other working modes—IDLE mode and CONNECT mode defined in conventional TD-SCDMA system. The communication link in direct mode can be defined as FORWARD link (e.g.: the link from UE 1  to UE 2 ) and BACKWARD link (e.g.: the link from UE 2  to UE 1 ) identified according to the information flow direction for one UE to send signals to the other UE or receive signals from the other UE. Because P2P communication mode is constructed in combination with existing TD-SCDMA systems, the UTRAN, the P2P UEs and other conventional UEs allocated in the same timeslot can overhear the information transferred on the FORWARD link or BACKWARD link, i.e.: from the view of the UTRAN, even though the UEs have no connection with the UTRAN, the FORWARD link and BACKWARD link are associated with a certain uplink timeslot or downlink timeslot (the FORWARD link and BACKWARD link can correspond to different uplink timeslot or downlink timeslot depending on different resource allocation schemes). Hence P2P communication will cause signal interference to conventional communication. Similarly, two P2P UEs can also overhear the information transferred in the uplink timeslot or downlink timeslot associated with its FORWARD link or BACKWARD link during P2P communication. Therefore, when conventional links share the same timeslots with the P2P links, conventional uplink or downlink communication will interfere with the communication of the P2P FORWARD link or BACKWARD link, which seriously deteriorates the performance of P2P-enabled TDD CDMA communication systems.  
         [0006]     To improve the performance of P2P-enabled TDD CDMA communication systems, it&#39;s necessary to effectively reduce the signal interference caused by P2P communication mode to the TD-SCDMA communication systems. First of all, analysis will go to the interference signal brought by introducing P2P communication mode in the following, and then how to reduce interference signal will be described. For simplicity in the following, the timeslot in which one UE transmits signals to the other UE through the above FORWARD link or BACKWARD link is called transmit timeslot (Tx timeslot), while the timeslot in which the UE receives signals from said another UE through the above FORWARD link or BACKWARD link is called receive timeslot (Rx timeslot), wherein the Tx timeslot or Rx timeslot is associated with an uplink timeslot or downlink timeslot in the sub-frame in conventional communication respectively.  
         [0007]     1. Interference Associated with Uplink Timeslot between P2P Link and Conventional Link  
         [0008]      FIG. 3  illustrates the interferences between P2P link and conventional link in P2P-enabled TD-SCDMA systems when the P2P link is associated with uplink timeslot. As shown in  FIG. 3 , it is assumed that UE 1  and UE 2  work in P2P mode and UE 3  works in conventional mode, wherein UE 1 &#39;s Tx timeslot is associated with UE 3 &#39;s uplink timeslot, that is, UE 1  and UE 3  are allocated in the same uplink timeslot to transmit signals respectively to UE 2  and the UTRAN. S 1  is the information from UE 1  to UE 2  through direct link (taken as FORWARD link) and S 2  is uplink information from UE 3  to the UTRAN through uplink, moreover, S 1  and S 2  are associated with the same uplink timeslot but with different spreading codes.  
         [0009]     In TD-SCDMA communication systems, one of the most important features is to maintain uplink synchronization, which means signals from different UEs should arrive at the UTRAN at the same time to guarantee the orthogonality of the spreading codes of signals from the main paths of different UEs.  
         [0010]     For conventional communication systems, the UTRAN monitors and controls the UEs&#39; uplink transmitting timing via a specific traffic burst structure in CONNECT mode so as to maintain uplink synchronization for each UE. But for P2P communication mode, the UTRAN is only involved in P2P link establishment procedure and not involved in the P2P communication procedure after P2P link&#39;s establishment. Therefore, during P2P communication, there is no dedicated channel between the UTRAN and the two P2P UEs, so the UTRAN cannot adjust the uplink synchronization advance for the two P2P UEs transmitting signals by using specific traffic burst to maintain uplink synchronization even if it can overhear and estimate the uplink synchronization shift of the two P2P UEs.  
         [0011]     Referring to  FIG. 3 , when UE 1  and UE 3  transmit signals in the same uplink timeslot, the UTRAN can overhear information S 1  transferred from UE 1  to UE 2  (to the UTRAN, S 1  is considered as interference signal  11 ). But as described above, there is no dedicated channel between the UTRAN and UE 1 , so the UTRAN can&#39;t adjust UE 1 &#39;s transmission timing by using the traffic burst in conventional communication mode even if it can overhear information S 1  and estimate UEI&#39;s synchronization shift information, which means UE 1  working in P2P mode may lose uplink synchronization with the UTRAN (UE 3  working in conventional mode can maintain uplink synchronization with the UTRAN with conventional mode). That is, I 1  and S 2  are likely to reach the UTRAN unsynchronously, which will potentially impair uplink synchronization and thus degrade the system performance.  
         [0012]     Similarly, when UE 1  and UE 3  transmit signals in the same allocated uplink timeslot, UE 2  can also overhear signal S 2  transferred from UE 3  to the UTRAN (to UE 2 , S 2  is considered as interference I 2 ), and interference signal I 2  will also produce impact on UE 2 &#39;s receiving S 1 , which may potentially impair the P2P communication quality.  
         [0013]      2 . Interference Associated with Downlink Timeslot between P2P Link and Conventional Link  
         [0014]      FIG. 4  illustrates the interferences between P2P link and conventional link in a P2P-enabled TD-SCDMA system when the P2P link is associated with downlink timeslot. It is assumed that UE 1  and UE 2  work in P2P mode and UE 3  works in conventional mode, wherein UE 1 &#39;s Rx timeslot is associated with UE 3 &#39;s downlink timeslot, that is: UE 1  and UE 3  are allocated in the same downlink timeslot to respectively receive signals from UE 2  and the UTRAN. S 3  is the P2P link information from UE 2  to UE 1  via direct link (taken as BACKWARD link) and S 4  is downlink information from the UTRAN to UE 3  via downlink, furthermore, S 3  and S 4  are associated with the same uplink timeslot but with different spreading codes.  
         [0015]     In  FIG. 4 , the downlink information S 4  transmitted from the UTRAN to UE 3  may produce interference to other UEs who share the same downlink timeslot with UE 3  but use different spreading codes to receive signals. Such interference is called multi-access interference (MAI).  
         [0016]     Referring to  FIG. 4 , when UE 1  and UE 3  receive signals in the same allocated downlink timeslot, UEI can overhear information S 4  transferred from the UTRAN to UE 3  via downlink (to UE 1 , S 4  is considered as interference signal I 4 ), and generally the transmission power of signals from the UTRAN is relatively strong, so the interference signal I 4  is likely to impair the direct communication quality seriously.  
         [0017]     Similarly, when UE 1  and UE 3  receive signals in the same allocated downlink timeslot, UE 3  can also overhear information S 3  transferred from UE 2  to UE 1  (to UE 3 , S 3  is considered as interference signal I 3 , and meanwhile UE 2  can be taken as the pseudo-UTRAN of transmission information in downlink timeslot), and the interference signal I 3  will impair the communication quality of UE 3  near UE 2  and other UEs in the same timeslot as UE 3  to receive signals.  
         [0018]     3. Interference between P2P Direct Link Pairs  
         [0019]      FIG. 5  illustrates the interferences between two P2P direct link pairs in a P2P-enabled TD-SCDMA system, wherein a UE in one of the two P2P link pairs receive transmit signals to another UE in another P2P link pair. Assume that UE 1  and UE 2  work in one P2P link pair while UE 3  and UE 4  in another P2P link pair.  
         [0020]     Because the P2P link pairs are symmetrical, in the associated timeslot, signal S 5  or S 6  from UE 1  to UE 2  will become interference I 5  or I 6  to UE 4  who is receiving signals from UE 3 . Obviously these interferences may also greatly impair the direct communication quality.  
         [0021]     As noted above, after P2P link is introduced in conventional TD-SCDMA systems, there exist  6  possible interference signals I 1 , I 2 , I 3 , I 4 , I 5  and I 6 . Depending on whether the UTRAN is involved, the above 6 interference signals can be divided into two types. The first type includes interferences between the UEs, such as I 2 , I 3 , I 5 , and I 6 ; and the second type includes the interferences with UTRAN involved, such as I 1  and I 4 .  
         [0022]     To guarantee the communication quality of a P2P-enable TD-SCDMA communication system, effective methods needs to be researched to cancel the above 6 interferences (it&#39;s better to achieve that without changing the physical layer structure of existing communication systems). Among the above 6 interference signals, the first type can be reduced or cancelled by efficiently limiting the radio range supported by P2P and adopting intelligent radio resource control scheme, while interference signal I 1  can be cancelled as described in a patent application entitled “A Method and Apparatus for Uplink Synchronization Maintenance with P2P Communication in Wireless Communication Networks,” filed by KONINKLIJKE PHILIPS ELECTRONICS N.V., on Mar. 7, 2003, Attorney&#39;s Docket No. CN030004, application Ser. No. 0319894.5, the disclosures of which are hereby incorporated by reference. As for interference signal I 4  of the second type, there is no effective solution yet now.  
         [0023]     From the foregoing interference analysis, it can be seen that interference signal I 4  is introduced by the UTRAN&#39;s transmitting signals to UE 3  via the downlink to UE 1  in the same downlink timeslot as UE 3 . Usually, the signal transmission power of the UTRAN is relatively strong enough that all UEs sharing the same downlink timeslot in the same cell can overhear the signal transmitted, and moreover the signal is the mixed one including redundant information of many other UEs, hence, I 4  can&#39;t be ignored. In UE 1 , UE 1  must adopt MUD (multi-user detection) or JD (joint detection) to cancel the interference signal, so as to guarantee the direct communication quality.  
       SUMMARY OF THE INVENTION  
       [0024]     The object of the present invention is to provide a method and apparatus for supporting P2P communication in TDD CDMA communication systems, so as to reduce interferences to the UE caused by downlink signal from conventional communication transmitted in the same downlink timeslot as the UE during direct communication.  
         [0025]     To achieve the above object, a method for supporting a UE to perform P2P communication in TDD CDMA communication systems in accordance with the present invention, comprising: receiving signals transferred by network system via downlink control channel; acquiring the timeslot allocation information according to the received signals; acquiring the spreading code allocation information of other active UEs allocated in the specific downlink timeslot associated with the direct link used by the UE, according to the signals transferred via downlink control channel; and reducing the interference to the UE caused by downlink signals in conventional communication during P2P communication, according to the acquired timeslot allocation information and spreading code allocation information.  
         [0026]     A method for supporting P2P communication between two UEs , performed by network system, in TDD CDMA communication systems in accordance with the present invention, comprising: sending the timeslot allocation information to the two P2P UEs via downlink control channel; generating spreading code allocation information corresponding to each downlink timeslot in said timeslot allocation information; and sending the said spreading code allocation information to these two UEs respectively so as to synchronize the P2P communication signal of each of the two UEs with the signals from network system. 
     
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS  
       [0027]      FIG. 1  is a schematic diagram illustrating two UEs communicate through the relaying of base stations in conventional communication mode;  
         [0028]      FIG. 2  is a schematic diagram illustrating the P2P communication mode between two UEs;  
         [0029]      FIG. 3  is a schematic diagram illustrating the generation of interference signals between direct link and conventional link employing uplink timeslot to communicate, in a P2P-enabled TD-SCDMA system;  
         [0030]      FIG. 4  is a schematic diagram illustrating the generation of interference signals between direct link and conventional link employing downlink timeslot to communicate, in a P2P-enabled TD-SCDMA system;  
         [0031]      FIG. 5  is a schematic diagram illustrating the generation of interference signals between two direct link pairs in a P2P-enabled TD-SCDMA system;  
         [0032]      FIG. 6  is a schematic diagram illustrating the timeslot allocation information in accordance with the present invention;  
         [0033]      FIG. 7  is a schematic diagram illustrating the spreading code allocation information in accordance with the present invention;  
         [0034]      FIG. 8  is a schematic diagram illustrating the time relationship between receiving downlink signals from the UTRAN and that from UE 2  respectively at UE 2  and UE 1  with the UTRAN as the time benchmark when downlink timeslot is employed to communicate;  
         [0035]      FIG. 9  is a schematic diagram illustrating the method to be performed in the present invention for downlink synchronization for an Rx UE in direct mode. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     According to the analysis to interference signals in a P2P-enabled TD-SCDMA communication system as above, the present invention primarily focuses on addressing the problem of interference signals from conventional downlink for a Rx UE in P2P communication.  
         [0037]     In fact, an advanced receiver has been widely used in the UTRAN of TD-SCDMA communication systems, i.e. using MUD or JD method to cancel MAI interference, as disclosed in detail in the article entitled “Multi-user Detection for DS-CDMA Communication,” written by S. Moshavi, in  IEEE Commun. Mag. ,October 1996, page 124-136, and the article entitled “Zero forcing and minimum mean-square-error equalization for multiuser detection in code division multiple access channels,” written by A. Kelein, G. K. Kaleh, in  IEEE Trans. Vehicular Tech. ,Vol. 45, No. 2, May 1996, pp 276-287, the disclosures of which are hereby incorporated by reference. But to apply interference canceling method like MUD or JD in the receiver of a UE, two conditions needs to be satisfied in advance: 
        1. acquire the spreading code information of all active UEs allocated in the same timeslot as the UE;     2. when receiving signals from direct link, the UE in direct mode should maintain synchronization with associated downlink signals from the UTRAN (the complexity of implementing the receiver can be effectively simplified only with signal synchronization).        
 
         [0040]     Discussions will respectively be given below to the above two conditions:  
         [0041]     1. How to Obtain Spreading Code Allocation Information  
         [0042]     In a TD-SCDMA communication system taken as an example, the spreading codes used by all active UEs are controlled by the base station subsystem UTRAN. During communication, RF signals convey information in form of frame. Every RF frame is divided into two sub-frames, and every sub-frame has 7 timeslots. Each UE allocated in the same timeslot transmits or receives signals using different spreading code allocated by the base station subsystem. Downlink control channel such as BCCH (Broadcast Control CHannel), has fixed position in each radio frame or sub-frame, and a UE can receive information from downlink control channel no matter working in conventional mode or P2P mode, so the UE can obtain the spreading code allocation information of all UEs sharing the same timeslot, according to the control information transferred from the UTRAN via downlink control channel.  
         [0043]     Wherein spreading code allocation information includes two parts: (i) the timeslot allocation information in each frame or sub-frame about whether each timeslot is to be used in uplink or downlink; (ii) the spreading code allocation information associated with each allocated downlink timeslot.  
         [0044]     In the embodiments of the present invention, the above timeslot allocation information and spreading code allocation information respectively store the involved information into the corresponding timeslot allocation map and spreading code allocation map through mapping, especially:  
         [0045]     (1) About the Timeslot Allocation Information  
         [0046]     In a TD-SCDMA communication system taken as an example, each sub-frame has 7 timeslots, denoted as TS 0 -TS 6 , so an octet with 8 bits can be used to map the 7 timeslots in a sub-frame in timeslot allocation map.  
         [0047]     Wherein a timeslot corresponds to a bit in the octet. As  FIG. 6  shows, TS 0 -TS 6  respectively corresponds to Bit 6 -Bit 0  in the octet, while Bit 7  is reserved. For each bit in Bit 6 -Bit 0 , assume that its corresponding timeslot is to be used as downlink timeslot when it is 1, and as uplink timeslot when it is 0. In TD-SCDMA systems, TS 0  is always allocated as downlink and TS 1  as uplink, so let Bit 6 =1 and Bit 5 =0.  
         [0048]     (2) About the Spreading Code Allocation Information Associated with each Allocated Downlink Timeslot  
         [0049]     After the timeslots in an RF frame or sub-frame are allocated, the UTRAN can accordingly generates the spreading code allocation information associated with each associated downlink timeslot according to the downlink timeslots included in the sub-frame. A TD-SCDMA system is also taken as an example in the following.  
         [0050]     In this communication system, a downlink timeslot may have totally up to 16 spreading codes for different UEs or different codes in one UE to use. So 16 bits are needed to represent how each spreading code in a downlink timeslot is used, that is, two octets are needed to represent the spreading code allocation information. It can be leant from the foregoing description that TS 1  is always used as uplink in the 7 timeslots included in each sub-frame, therefore at most 6 timeslots in each sub-frame can be used as downlink, thus totally 12 octets are needed to represent how the spreading codes of the 6 timeslot are used.  
         [0051]     The following embodiment will describe the spreading code allocation information included in a downlink timeslot. Assume that Bit 4 =0 and Bit 1 =0 in the timeslot allocation map shown in  FIG. 6 , that is, TS 2  and TS 5  are used as downlink timeslots, 4 octets are required to be included in the spreading code allocation map that corresponds to  FIG. 7 . Wherein the first two octets correspond to the allocation information of each spreading code in TS 2  and the second two octets correspond to that in TS 5 . The part with lighter background color indicates the first two octets, wherein Bit 15 ˜Bit 0  respectively correspond to the information about spreading codes Code 15 ˜Code 0  in TS 2  to be used by 16 UEs or codes at most; while the part with deeper background color indicates the second tow octets, wherein Bit 15 ˜Bit 0  respectively correspond to the information about spreading codes Code 15 ˜Code 0  in TS 5  to be used by 16 UEs or codes at most. To each bit of the four octets in the above two groups corresponding to each spreading code, it can be defined as: when the bit corresponding to the spreading code is 1, it represents that the spreading code is used by a UE in the corresponding timeslot; when the bit corresponding to the spreading code is 0, it represents that the spreading code is not allocated to any UE in the corresponding timeslot yet. For instance, when Bit 8  and Bit 0  of the two octets in the first group are 1 while other bits are all 0, it represents only the spreading codes corresponding to Bit 8  and Bit 0  are occupied in TS 2  corresponding to the first group, while other spreading codes are not allocated to any UE yet.  
         [0052]     As described above, through mapping the allocation information of each timeslot in the sub-frame and the allocation information of each spreading code included in each downlink timeslot in the sub-frame into the timeslot allocation map and spreading code allocation map, the UTRAN can send the information included in the timeslot allocation map and spreading code allocation map to each UE via downlink control channel, so that the UE can obtain the timeslot allocation information and the spreading code allocation information associated with downlink timeslot.  
         [0053]     Of course, the UE can also obtain the timeslot allocation information and the spreading code allocation information associated with uplink timeslot in a similar way, if needed.  
         [0054]     2. Maintain Synchronization with Associated Downlink Signals from the UTRAN when a UE in Direct Mode is Receiving Signals from Direct Link  
         [0055]     In conventional TD-SCDMA systems, the downlink mixed signals transmitted by the UTRAN should reach the UE synchronously if the multi-path delay effect is ignored. But in a P2P-enabled TD-SCDMA system, there is great difference between the transmission path of the useful signal S 3  from a P2P UE and that of the interference signal I 4  from the UTRAN (referring to  FIG. 4 ), so their arriving time may be quite different, i.e.: when UE 1  in direct mode is receiving signal S 3  over direct link, it can&#39;t be guaranteed to keep synchronization with downlink signal I 4  from the UTRAN. In this case, S 3  and I 4  can&#39;t arrive at UE 1  at the same time, so the complexity of implementing channel estimation or JD (joint detection) will be increased greatly due to time unsynchronization although conventional JD method can still be employed to cancel Interference signal I 4  in UE 1 , even if UE 1  can obtain the timeslot allocation information and spreading code allocation information transmitted by the UTRAN via downlink control channel.  
         [0056]     To greatly simplify the operation of the receiver in UEI and effectively cancel downlink interference signal I 4 , how to implement synchronization between direct link signal S 3  and interference signal I 4  at Rx UE 1 , arises as an important step to guarantee P2P communication quality.  
         [0057]     As  FIG. 4  shown, in the case that the useful signal S 3  and interference signal I 4  can&#39;t arrive at UEI synchronously, there are two methods for them to arrive at UE 1  synchronously: the first is to adjust the TA for the UTRAN to transmit signal I 4  so that signal I 4  transmitted by the UTRAN and signal S 3  transmitted by UE 2  can arrive at UE 1  synchronously; the second is to adjust the TA for UE 2  to transmit signal S 3  so that signal S 3  transmitted by UE 2  and signal I 4  transmitted by the UTRAN can arrive at UE 1  synchronously. Wherein it&#39;s impossible to realize the first method by adjusting the UTRAN&#39;s transmission TA for I 4  and S 3  to arrive at UE 1  synchronously, because there are usually several P2P link pairs using the same downlink timeslot during communication process, which can&#39;t guarantee downlink interference signal I 4  to be synchronized with signals (e.g. S 3 ) received by the UEs in P2P communication just by adjusting a common transmission TA for the UTRAN to transmit signals. So, the TA adjustment must be done at UE 2  for transmitting useful signal S 3  to guarantee that each UE in direct mode can maintain synchronization with the associated downlink signals from the UTRAN when receiving signals from different direct links.  
         [0058]     Before describing the above synchronization procedure in conjunction with  FIG. 8 , two issues need to be clarified in advance:  
         [0059]     (i) Establish and Maintain Synchronization with the UTRAN in Conventional Communication Mode  
         [0060]     During cell search phase, each UE firstly establishes downlink synchronization with the UTRAN in conventional way, and then maintains downlink synchronization with the UTRAN by tracking the pilot channel in the sub-frame and taking the detected pilot channel as the time reference in its sub-frame.  
         [0061]     (ii) Maintain Synchronization with Associated Downlink Ttimeslots in P2P Communication Mode  
         [0062]     When UE 1  and UE 2  are in P2P direct mode, there is a dedicated channel between them, and the traffic burst structure of the dedicated channel is the same as that in conventional TD-SCDMA communication systems, which means that the power control information and synchronization shift information are also included in the traffic bursts of FORWARD link and BACKWARD link in the P2P dedicated channel. In conventional TD-SCDMA communication systems, the synchronization shift information included in traffic burst can be used to maintain uplink synchronization at the UTRAN, similarly the synchronization shift information included in traffic burst of direct link can be used by the P2P UEs to maintain synchronization with associated conventional downlink when receiving direct link signals in P2P communication.  
         [0063]     From the above clarification it can be seen that each UE respectively establishes downlink synchronization with the UTRAN in conventional way at the beginning of communication establishment (cell search phase), so two UEs can have a common time benchmark when establishing P2P communication, to determine synchronization between direct link signals and downlink signals. Meanwhile, the traffic burst of P2P dedicated channel contains the same synchronization shift information as in conventional mode, so the UE can set and adjust its signal transmission TA by using the synchronization shift information during P2P establishment and communication procedure, to guarantee synchronization between direct link signals and downlink signals.  
         [0064]     Descriptions will be given below to synchronization procedure between direct link signals and downlink signals at the Rx UE, in conjunction with  FIG. 8  and  FIG. 4 , wherein for UE 1  the useful signal S 3  and interference signal I 4  share the same downlink timeslot.  
         [0065]     First, taking the UTRAN as the common benchmark, the time relationship between the receiving time for each UE to receive signals and the common benchmark at the UTRAN is illustrated in  FIG. 8 . T 0  is the downlink transmitting time benchmark in the associated downlink timeslot at the UTRAN, T 1  and T 2  are the related receiving time points at UE 1  and UE 2  respectively. UE 1  and UE 2  can identify T 1  and T 2  by finding and tracking the pilot channel from the UTRAN as described above. The durations T UTRA-UE2  (equal to T 2 -T 0 ) and TUTRAN-UE 1  (equal to T 1 -T 0 ) are signal transmission time from the UTRAN to UE 1  and UE 2  respectively. TUE 2 -UE 1  (equal to T 21 -T 2 ) is signal transmission time from UE 2  to UE 1 , provided that UE 2  sends signal to UE 1  at T 2  and the signal arrives at UE 1  at T 21 , wherein T 21  should overlap with T 1  so as to guarantee the useful signal S 3  and interference signal I 4  can arrive at UE 1  at the same time.  
         [0066]     Then, UEI estimates the TA by estimating the channel impulse response of each midamble transmitted by UE 2 , and includes the estimated TA as synchronization shift information in the traffic burst from UE 1  to UE 2 . The traffic burst contains the midamble to be used for estimating the TA, so UE 1  and UE 2  can estimate and adjust signal transmission TA any time, even if the two UEs are both in mobility.  
         [0067]     The above conventional downlink synchronization procedure addressing direct link can be summarized as follows:  
         [0068]     (1) UE 1  and UE 2  establish and maintain downlink synchronization with the UTRAN respectively  
         [0069]     UE 1  and UE 2  establish downlink synchronization during cell search, and maintain downlink synchronization by tracking the pilot channel (step  1 ).  
         [0070]     (2) UE 2  determines its transmitting time  
         [0071]     UE 2  sets time T 2  as its transmitting time for transmitting signals to UE 1  in associated downlink timeslot during direct link establishment procedure (step  2 ).  
         [0072]     (3) UE 1  measures the time difference between direct link signal and downlink signal  
         [0073]     During direct link establishment procedure, when receiving the traffic burst transmitted by UE 2  at T 2 , UEI measures the time difference of T UTRAN-UE2 +T UE2-UE1 −T UTRAN-UE1  (T 21 -T 1 ) according to the midamble information contained in the traffic burst, and transmits the measurement result as feedback information to UE 2  (step  3 ).  
         [0074]     (4) UE 2  sets its transmission TA  
         [0075]     UE 2  sets its transmission TA according to the feedback information from UE 1  and adjusts its time to transmit signals according to this TA (step  4 ).  
         [0076]     (5) UEI estimates and sends synchronization shift information to UE 2  UE 1  estimates the synchronization shift information by evaluating the channel impulse response of each midamble from UE 2  and sets the synchronization shift information in the traffic burst to be transmitted from UE 1  to UE 2  (step  5 ).  
         [0077]     (6) UE 2  adjusts its transmission TA  
         [0078]     UE 2  adjusts its transmission TA according to the synchronization shift information included in the traffic burst from UE 1  (step  6 ).  
         [0079]     (7) Iterate step  5  and step  6   
         [0080]     Judge whether P2P communication is ended (step  7 ). If the direct communication is not ended yet, the above step  5  and step  6  will be iterated continuously to keep direct link signal S 3  to be synchronized with the associated downlink signal  14 , so long as the direct link still shares the same timeslot as the downlink timeslot.  
         [0081]     Through the above steps, the useful signal S 3  from UE 2  and interference signal I 4  from the UTRAN can arrive at UE 1  at the same time, to guarantee that UE 1  can use MUD or JD algorithms to cancel downlink interference signal I 4  and then to guarantee the performance of P2P-enabled TDD CDMA systems, with downlink synchronization and the acquired spreading code allocation information.  
         [0082]     Of course, in the case when two UEs in direct communication mode are very close to each other, i.e.: when the receiving time points T 1  and T 2  are nearly equal, the time difference between direct link signal S 3  and downlink signal I 4  arriving at UE 1  can almost be ignored. At this time, the above synchronization procedure can be omitted, and only spreading code allocation information is needed. UE 1  can cancel the negative effect caused by interference signal I 4  through JD.  
         [0083]     Moreover, it should be noted that setting and adjusting the transmission TA by UE 2  can only guarantee that the useful signal S 3  and interference signal I 4  arrive at UE 1  at the same time, but can&#39;t guarantee that the useful signal S 4  and interference signal  13  in  FIG. 4  arrive at UE 3  at the same time, thus UE 3 &#39;s receiver can&#39;t effectively cancel the effect caused by interference signal I 3  in UE 3  (or cancel  13 &#39;s effect based on increasing complexity by using asynchronous JD method) according to the acquired spreading code allocation information. Fortunately, in practical communications, when UE 3  extracts the desired information from S 4  by using conventional synchronous JD method, its most interferences are from the received mixed signal S 4 , and interference signal I 3 &#39;s effect to the useful signal S 4  can be ignored at this time.  
         [0084]     The above method for supporting P2P communication in downlink timeslot in TD-SCDMA systems as proposed in the present invention, can be implemented in computer software, or hardware, or in combination of both software and hardware.  
         [0000]     Beneficial Results of the Invention  
         [0085]     As described above, referring to the method and apparatus for supporting P2P communication in TD-SCDMA systems provided in the present invention, a UE in direct communication mode can acquire spreading code allocation information via downlink, thus the UE can effectively cancel the interference caused by the downlink signals transferred in the same timeslot in conventional communication, by utilizing the spreading code allocation information used by other UEs allocated in the same downlink timeslot, with methods like MUD or JD.  
         [0086]     Furthermore, referring to the method and apparatus for supporting P2P communication in TD-SCDMA systems provided in the present invention, some steps are adopted to keep direct link signals and downlink signals in the associated timeslot synchronized, which greatly simplifies the operation of the UE to cancel downlink signal interference according to the spreading code allocation information, and simplifies the hardware setting of the UE&#39;s receiver.  
         [0087]     Although the method and apparatus for supporting P2P communication in TD-SCDMA systems provided in the invention has been shown and described with respect to exemplary embodiments of TD-SCDMA, it should be understood by those skilled in the art that the communication method and apparatus are not limited hereof, but also suitable to other TDD CDMA systems.  
         [0088]     It is also to be understood by those skilled in the art that the method and apparatus for supporting P2P communication in TD-SCDMA systems disclosed in this invention can be modified considerably without departing from the spirit and scope of the invention as defined by the appended claims.