Abstract:
Discloses is a gateway, base station, communication network and synchronization method thereof. The method comprises: at time T 1 , sending from the gateway to the base stations a synchronization request signal; at time T 2 , receiving by the base stations the synchronization request signal; at time T 3 , sending from the base stations to the gateway the synchronization response comprising the times T 2  and T 3 ; at time T 4 , receiving by the gateway the synchronization response signals; and calculating for the base stations a mapping relationship between the time system of the gateway and the time system of the base station is calculated based on the times T 1 , T 2 , T 3  and T 4 . With the configuration and method proposed in present invention, it can be avoided the problem that accurate synchronization between the gateway and the base stations can not be reached for MBMS data packets in LTE because of path delay and jitter error, so that every base station can specify the same transmitting time for MBMS data packets to guarantee that UE could implement correct RF combining.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a synchronization technique in mobile communications, especially to a gateway, a base station and a communication network supporting RF combining and the synchronization method thereof. 
       BACKGROUND OF INVENTION 
       [0002]    In the 3GPP LTE, a two-layer flat network architecture is adopted in the core network, i.e., the four network units of NodeB, RNC, SGSN and GGSN in the WCDMA/HSDPA stage are evolved into such two as the eNodeB, viz., the evolved Node B (eNB) (‘Base Station’ for short hereinafter), and the access gateway (aGW). And the fully IP distributed structure is adopted in the core network to support IMS, VoIP, SIP and MobileIP, etc. 
         [0003]      FIG. 1  illustrates the network structure in an LTE system. The aGW may establish connections to multiple eNBs (e.g., eNB 1 , eNB 2  and eNB 3 ) through interface S1. And the eNBs may establish connections with each other in mesh (the dashed line in  FIG. 1 ) through interface X2. The cells of eNB 1 ˜ 3  have some illustrative user equipments as UE 11 ˜E 12 , UE 21 ˜ 23  and UE 31 ˜ 33  respectively. 
         [0004]    In LTE system, OFDM is adopted as the physical layer downlink transmission scheme for radio interface, and SC-FDMA is adopted as the uplink transmission scheme. With the application of OFDM, the same radio signal in different cells can be naturally combined in the air to improve the signal strength without any extra processing overhead, as is called the radio frequency combining (RF combining). 
         [0005]    Therefore, the requirement to improve the gains on cell boundaries by supporting in-the-air RF combining under single-frequency network (SFN) multiple-cell transmission mode is defined as a baseline for the EMBMS in LTE, for it is necessary for EMBMS to transmit the same service data to different UEs. 
         [0006]    The physical layer frame timing synchronization has been achieved for an eNB in the SFN with the precision satisfying the RF combining requirement for EMBMS. However, to guarantee the effectiveness of RF combining, the radio signals to be combined are required to be MBMS service content synchronous and consistent. That is to say, layer  2  (L 2 ) transmission synchronization should be guaranteed for MBMS service&#39;s multi-cell transmission. 
         [0007]    In addition, in LTE network architecture design, IP multicast transmission has been extended to eNB level in LTE architecture. The MBMS packet will be sent only once to a group of eNBs using IP multicast transmission. And current IP multicast routing protocol can guarantee that the route between each eNB and aGW mainly depends on network topology deployment and will not change unless the involved routers collapse. This instance will rarely happen. Besides, the router processing capability and transport network loading will be optimized during the network planning. So the only fact of the different transmission time delay is the different transmission route from aGW to eNBs. That is to say in spite of physical layer time synchronization in SFN area, different eNBs may receive the same MBMS packet at different time by different route. 
         [0008]      FIG. 2  illustrates the transmission delays that the same data packet is transmitted from the aGW to different eNBs. As shown in  FIG. 2(   a ), the routing through which the data packet is transmitted from aGW to eNB 1  is: aGW==&gt;router R 1 ==&gt;eNB 1 . And the routing through which the data packet is transmitted from aGW to eNB 2  is: aGW==&gt;router R 2 ==&gt;router R 3 ==&gt;eNB 2 . Different delays are resulted from that the same data packet is transmitted through different paths. 
         [0009]    As shown in  FIG. 2(   b ), at time T 0 , the data packet is transmitted from aGW to eNB 1  and eNB 2  respectively. It reaches eNB 1  at time T 1 , and reaches eNB 2  at time T 2 . Therefore, delay TD=T 2 −T 1  causes to the transmission of the same data packet to different eNBs. 
         [0010]    In this way, if the data packet is transmitted out just after it is received respectively by eNB 1  and eNB 2  from aGW, clearly it is asynchronously transmitted by different eNBs to UE. This results in that these data packets can not be combined correctly, or even causes extra interference. Moreover, after the same data packet arrives at the eNBs, it is necessary for each eNB to perform such operations as segmentation, coding and modulation and so on for frame construction. Inconsistent framing time will also affect these data packets&#39; RF combining. 
       SUMMARY OF INVENTION 
       [0011]    For the problem mentioned above, this invention is implemented. It is an object of the present invention to propose a gateway, a base station, a communication network and the synchronization method thereof to satisfy the requirement on MBMS packet&#39;s RF combining in LTE. 
         [0012]    According to one aspect of present invention, it provides a method for implementing synchronization between a gateway and bas stations, comprising the steps of: at time T 1 , sending from the gateway to the base stations a synchronization request signal; at time T 2 , receiving by the base stations the synchronization request signal; at time T 3 , sending from the base stations to the gateway the synchronization response comprising the times T 2  and T 3 ; at time T 4 , receiving by the gateway the synchronization response signals; and calculating for the base stations a mapping relationship between the time system of the gateway and the time system of the base station is calculated based on the times T 1 , T 2 , T 3  and T 4 . 
         [0013]    According to another aspect of present invention, it provides a method for implementing synchronization between a gateway and base stations, comprising the steps of: at time T 1 , sending from the gateway to the base stations a synchronization request signal; at time T 2 , receiving by the base stations the synchronization request signal; at time T 3 , sending from the base stations to the gateway the synchronization response signals comprising the time T 2  and the first offset between the base station&#39;s time system and a baseline time; receiving by the gateway the synchronization response signal and calculating the mapping relationship between the gateway&#39;s time system and the base station&#39;s time system for each base station according to the first offset and the second offset between the gateway&#39;s time system and the baseline time. 
         [0014]    According to another aspect of present invention, it provides a gateway comprising: communication means adapted to transmit a synchronization request signal to at least one base station at time T 1 , and to receive from the base station at time T 4  synchronization response signals including the time T 2  when the base station receive the synchronization request signal and the time T 3  when the base station transmit the synchronization response signal; and calculation means adapted to calculating for the base station the mapping relationship between the gateway&#39;s time system and the base station&#39;s time system according to times T 1 , T 2 , T 3  and T 4 . 
         [0015]    According to another aspect of present invention, it provides a base station comprising: communication means which is adapted to receive a synchronization request signal, send information including the time moment when receiving the synchronization request signal and the information including the time when sending the synchronization response signal to a gateway, receive the mapping relationship between the base station&#39;s time system and the gateway&#39;s time system from the gateway and receive from the gateway the data packets including the expected transmitting time; and translating means which is adapted to translate the expected transmitting time into the real transmitting time under the base station&#39;s time system. 
         [0016]    According to another aspect of present invention, it provides a gateway comprising: communication means which is adapted to transmit a synchronization request signal to at least one base station at time T 1 , and receive from the base station synchronization response signals including the time T 2  when the base station receive the synchronization request signal and the first offset between the base station&#39;s time system the baseline time; and calculation means which is adapted to calculating for the base station the mapping relationship between the gateway&#39;s time system and the base station&#39;s time systems according to the first offset and the second offset between the base station&#39;s time system and the baseline time. 
         [0017]    According to another aspect of present invention, a base station comprising: communication means which is adapted to receive a synchronization request signal, send information including the time moment when receive the synchronization request signal and information including the time moment when sending the synchronization response signal to a gateway, receive from the gateway the mapping relationship between the base station&#39;s time systems and the gateway&#39;s time system from the gateway and receive the data packets including the expected transmitting time; and translation means which is adapted to translate the expected transmitting time into the real transmitting time under the base station&#39;s time system. 
         [0018]    According to another aspect of present invention, a communication network comprising at least one gateway as described above and at least one base station as described above. 
         [0019]    With the configuration and method proposed in present invention, it can be avoided the problem that accurate synchronization between the aGW and eNB can not be reached for MBMS data packets in LTE because of path delay and jitter error, so that every eNB can specify the same transmitting time for MBMS data packets to guarantee that UE could implement correct RF combining. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The characteristics and advantages of present invention can be described to be more obvious and detailed with reference to the drawings in which: 
           [0021]      FIG. 1  illustrates a LTE network structure; 
           [0022]      FIG. 2  is a schematic diagram illustrating the delays that the same data packet is transmitted from an aGW to different eNBs; 
           [0023]      FIG. 3  illustrates a network structure proposed according to an embodiment of the present invention; 
           [0024]      FIG. 4  illustrates a block diagram of an aGW according to the first embodiment of the present invention; 
           [0025]      FIG. 5  illustrates a block diagram of an eNB according to the first embodiment of the present invention; 
           [0026]      FIG. 6  illustrates the flow of operations implemented between the aGW and the eNB according to the first embodiment; 
           [0027]      FIG. 7  illustrates the relationship between the frame number counter of the aGW and that of the eNB; 
           [0028]      FIG. 8  illustrates the synchronization between the aGW and eNB in the case of no common reference clock; 
           [0029]      FIG. 9  illustrates how to specify the unified transmitting time and then the relevant transmission embodiment; 
           [0030]      FIG. 10  illustrates how to re-synchronize; 
           [0031]      FIG. 11  illustrates the relationship between two SFNs&#39; physical layer frame timing; 
           [0032]      FIG. 12  illustrates the “jitter” error; 
           [0033]      FIG. 13  illustrates the frame length set for eliminating the “jitter” error; 
           [0034]      FIG. 14  illustrates a network structure according to the second embodiment of the present invention; 
           [0035]      FIG. 15  illustrates a black diagram of the aGW according to the second embodiment of the present invention; 
           [0036]      FIG. 16  illustrates a black diagram of the eNB according to the second embodiment of the present invention; 
           [0037]      FIG. 17  illustrates the relationship between AFN and BFN-i in the case of external common reference clock; 
           [0038]      FIG. 18  illustrates the synchronization between the aGW and eNB in the case of common reference clock; and 
           [0039]      FIG. 19  illustrates the required three-layer synchronization structure for RF combining in LTE. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    Now, let&#39;s get down to detailed description on the preferred embodiments of the present invention with reference to the drawings. Among the attached figures, the same reference numerals (although in different figures) denote the same or similar components. To be clear and concise, description on well known function and structure will be omitted for not disturbing the presentation on present invention&#39;s main idea. 
       First Embodiment 
       [0041]      FIG. 3  illustrates a network structure proposed according to an embodiment of the present invention. For the convenience of description, only one aGW, two eNBs (eNB 1  and eNB 2 ) in an SFN area, and UE 11 , UE 12 , UE 21 , UE 22  and UE 23  are shown in  FIG. 3 . Some other devices like routers, etc. are omitted here. Obviously, the adoption of the network structure aims at illustrating the present invention. No network launched in practice bears the exact structure of this one. Ordinary technician in this field can adopt multiple aGWs, more eNBs and other complementary devices in practice. 
         [0042]      FIG. 4  illustrates a block diagram of an aGW according to the first embodiment of present invention.  FIG. 5  illustrates a block diagram of the eNB according to the first embodiment of present invention. 
         [0043]    As shown in  FIG. 4 , the aGW in the first embodiment includes a gateway buffer  110  which buffers the MBMS data packets transmitted from the multimedia broadcast and multicast center, a gateway controller  120  which controls the entire gateway&#39;s operations, a gateway communication unit  130  which transmits data packets and signals to UEs and receives signals from UEs, a gateway frame number counter  150  which acts as the system timer of the aGW, and a calculation unit  140  which calculate the transmission delays and transmitting time according to the signals received by communication unit from UEs. 
         [0044]    As shown in  FIG. 5 , the eNB 1  in the first embodiment includes a communication unit  210  which is responsible for communicating with aGW, an eNB buffer  220  which buffers MBMS data packets transmitted from the aGW, an eNB controller  230  which controls the entire eNB 1 , a translation unit  250  which translate the MBMS data packet&#39;s transmitting time into the real one in BFN format according to the mapping relationship transmitted from the aGW, an eNB frame number counter  270 , a data processing unit  240  which implements such operations as segmentation, frame construction and modulation to the received MBMS data packets, and a transmission unit  260  which transmits the processed data packets in the data processing unit  240  to UEs according to the transmitting time obtained through the translation unit  250 . The eNB 2  bears the same structure as eNB 1 . No detailed description will be given here. 
         [0045]    The structure of aGW and the structure of eNB have been illustrated above in the mode of individually describing their functional blocks respectively. But it only aims at clearly explaining the functions of the aGW and eNB. Ordinary technician in this field can either integrate the one or more or even all functions into single hardware, or implement some functions in hardware and the others in software, or implement all functions in software absolutely. 
         [0046]      FIG. 6  illustrates the flow of operations implemented between the aGW and the eNB in the first embodiment. 
         [0047]    As shown in  FIG. 6 , in step S 10 , the synchronization process is implemented between eNBi (i=1, 2) and the aGW after the aGW is powered on, or in the case that some event (e.g., the preset time everyday) is triggered, so as to obtain the corresponding or mapping relationship (Δ i ) between the aGW&#39;s system frame number AFN and eNBi&#39;s system frame number BFN-i and the transmission delays TD i  between the aGW and eNBi. 
         [0048]    Since no absolute time system (but the respective system frame number) is applied in either the aGW&#39;s system timing or eNBi&#39;s system timing, it is necessary for us to explain the relationship between the two counters, viz., the gateway frame number counter  150  and the eNB frame number counter  270 . 
         [0049]      FIG. 7  illustrates the relationship between the respective frame number counters of the aGW and the eNB in SFN area. 
         [0050]    In  FIG. 7 , AFN denotes the time counter of the aGW in its own frame number format, BFN- 1  denotes the time counter of the eNB 1  in its own frame number format, BFN- 2  denotes the time counter of the eNB 2  in its own frame number format. And the same precision of ⅛ frame is adopted by AFN and BFN. That is to say, BFN- 1  indicates the value of eNB 1 &#39;s frame number counter  270 . Similar to the BFN defined in WCDMA R 6 , it is the time reference adopted by eNB for network synchronization with the value in the range 0˜4095. AFN indicates the value of aGW&#39;s frame number counter  150 . Similar to the RFN defined in WCDMA R 6 , it is the time reference adopted by aGW for network synchronization with the value also in the range 0˜4095. Having been powered on and initialized, both aGW&#39;s frame number counter  270  and the eNB&#39;s frame number counter  150  implement counting independently. In the SFN area, since synchronization has been guaranteed for physical layer frame timing, the BFN-is of the eNBis in this area are aligned to the same frame on the boundaries. But in the case of no common reference clock, an offset usually exists between the frame number of eNBi and that of the aGW, as shown in  FIG. 7 . 
         [0051]    Although offsets exist between the frame number of the aGW and that of eNBs, no negative affection will be caused to data transmission, for data transmission starts from each frame&#39;s boundary and each frame is marked with frame number as its time stamp. Therefore, the transmission layer&#39;s synchronization requirement can be satisfied under condition that the same MBMS data frames to be transmitted from eNBi in SFN area are transmitted starting from the aligned BFN frame boundary. 
         [0052]      FIG. 8  illustrates the synchronization between the aGW and the eNB in the case of no common reference clock. 
         [0053]    As shown in  FIG. 8 , in the first place, the gateway controller  120  sends a downlink synchronization signal (e.g., the downlink synchronization control frame) to the eNBs in SFN area through the gateway communication unit  130 , requesting to synchronize with the eNBi. It records the current frame number in the aGW&#39;s frame number counter  150  as the transmitting time T 1  (the first time moment), which is included in the downlink synchronization signal. Obviously, the transmitting time T 1  (the first time moment) is just the one under the aGW&#39;s time system. 
         [0054]    Then, each eNBi receives the downlink synchronization signal in virtue of the eNB&#39;s communication unit  210  and records the current frame number in the eNB&#39;s frame number counter  270  as the time T 2   i  (the second time moment) for the receiving of downlink synchronization signal, where i=1, 2, the index of the eNBi. Now, the eNBi responds an uplink node synchronization signal like the uplink node synchronization control frame to the aGW, including at least the time T 2 ; (which is in the format of eNB&#39;s frame number) when corresponding eNBi receives the downlink synchronization signal, and the time T 3   i  (the 3 rd  time moment) when corresponding eNBi transmits the uplink node synchronization signal, or including the transmitting time T 1  as shown in  FIG. 8 . Similarly, both T 2   i  (the 2 nd  time moment) and T 3 ; (the 3 rd  time moment) are the ones under the eNB&#39;s time system. 
         [0055]    Now, the gateway controller  120  receives the uplink node synchronization signals from the eNBi in virtue of the gateway communication unit  130  and records the current frame number in the gateway counter  150  as the time T 4   i  (the 4 th  time moment) for the receiving of the uplink node synchronization signals. Please note that the counting precision on all above frame numbers is ⅛ frames, and T 4   i  (the 4 th  time moment) is the one under the aGW&#39;s time system. 
         [0056]    Here, we suppose that the same delay exists in both the downlink transmission and the uplink transmission. For each eNBi, we have (following descriptions are done to all eNBs, so the subscript i is omitted): 
         [0000]      ( T 2 i   ′−T 1)mod 4096=( T 4 i   −T 3 i )mod 4096  (1)
 
         [0000]    where T 2   i ′ and T 3   i ′ respectively indicate the frame numbers (time moments) in AFN format corresponding to T 2   i  and T 3   i  in BFN format, thus: 
         [0000]        T 2 i =( T 2 i ′+Δ i )mod 4096 ,T 3 i =( T 3 i ′+Δ i )mod 4096  (2)
 
         [0000]      So: 
         [0000]      ( T 2 i −Δ i   −T 1)mod 4096=( T 4 i   −T 3 i +Δ i )mod 4096  (3)
 
         [0057]    Now, the mapping relationship between BFN and AFN is obtained as: 
         [0000]      Δ 1 =Round[(( T 2 i   −T 1 +T 3 i   −T 4 i )mod 4096)/2]  (4)
       and the transmission delay between the aGW and the eNB in this routing is obtained as:       
 
         [0000]        TD   i =Ceil[(( T 2 i   −T 1 +T 4 i   T 3 i )mod 4096)/2]  (5)
 
         [0000]    where Round denotes the rounding function and Ceil denotes the function of upper rounding into integer. And according to the received time moments and formulae (4) and (5), the calculation unit  140  calculates the mapping relationship and the transmission delay. 
         [0059]    Now, with the help of concrete data illustrated in  FIG. 8 , let&#39;s take the relationship and transmission delay exist in the aGW and eNB 1  as an example to further describe how to obtain the synchronization time information. 
         [0060]    As shown in  FIG. 8 , the aGW&#39;s communication unit  130  transmits the downlink node synchronization signal to the eNB at the current frame number in the aGW&#39;s frame number counter  150 , viz., at the time T 1 =4094.250, including the transmitting time T 1 . Then, the eNB 1 &#39;s communication unit  210  receives the downlink node synchronization signal at the current frame number in the eNB&#39;s frame number counter  270 , viz., at the time T 2   1 =149.875. 
         [0061]    The eNB 1 &#39;s communication unit  210  transmits the uplink node synchronization signal at the current frame number in the eNB&#39;s frame number counter  270 , viz., at the time T 3   1 =151.125, including the aGW&#39;s transmitting time T 1 , the time T 2   1  when eNB 1  receives the downlink node synchronization signal and the time T 3   1  when eNB 1  transmits the uplink node synchronization signal. However, as mentioned above, since the aGW can record the time T 1 , it is not necessary to include the information on T 1  in the uplink node synchronization signal. 
         [0062]    The aGW&#39;s communication unit  130  receives the uplink node synchronization signal at the current frame number in the aGW&#39;s frame number counter  150 , viz., at the time T 4   1 =2.875. 
         [0063]    Then, in step S 20 , the calculation unit  140  calculates following information with T 1 , T 2   1 , T 3   1  and T 4   1  and the formulae (4) and (5):
       The mapping relationship between the aGW frame number AFN and the eNB frame number BFN- 1 :       
 
         [0000]      Δ 1 =Round[(( T 2 1   −T 1 +T 3 1   −T 4 1 )mod 4096)/2]=150
 
         [0065]    then BFN- 1 =(AFN+150) mod 4096 
         [0066]    and the transmission delay in the transmission path from the aGW to the eNB 1 : 
         [0000]        TD   1 =Ceil[(( T 2 1   −T 1 +T 4 1   −T 3 1 )mod 4096)/2]=2. 
         [0067]    In this way, we obtain the mapping relationship Δ i  and the transmission delay TD i  between the aGW and the eNBs. 
         [0068]    In addition, since clock drifts exist in both the time systems of aGW and eNBi, the mapping relationship Δ i  between AFN and BFN-i also varies with the clock drifts. According to the minimum requirement on eNB&#39;s frequency deviation 0.1 ppm regulated in 3GPP TS 25.104, we can obtain the clock drift in the eNBi and aGW after a day time as: 
         [0000]      3600*24*0.1*10−6=8.64 ms
 
         [0069]    Therefore, the drift of about one frame at most exists in eNBi and the aGW everyday, so the maximum clock drift between the aGW and eNBi is 2*8.64 ms per day. 
         [0070]    To guarantee the precision on the mapping relationship Δ i  between AFN and BFN-i, the synchronization process should be implemented twice a day between the aGW and the eNBi. 
         [0071]    After the mapping relationship Δ i  and the transmission delay TD i  between AFN and BFN-i are obtained, then in step S 30  the aGW&#39;s controller  120  transmits the node synchronization signal to eNBi in virtue of the aGW&#39;s communication unit  130 , i.e., to send the calculated mapping relationship Δ i  to corresponding eNBi. 
         [0072]    After eNBi receives the node synchronization signal, the controller  230  of every eNBi sends the node synchronization ACK signal to the aGW in virtue of the eNB&#39;s communication unit  210  to confirm that corresponding eNBi has received the mapping relationship Δ i . 
         [0073]    In step S 40 , if there is an MBMS data packet necessary to be transmitted from the aGW to eNBi, the aGW specifies the MBMS data packet&#39;s unified expected transmitting time AFN expect  (which is in the format of AFN) for eNBi. 
         [0074]    When specifying the unified expected transmitting time, it is necessary for the aGW to take such factors into account as the maximum transmission delay MaxTD between eNBi and the aGW, all eNBs&#39; maximum processing time T proc , and an extra guard interval T margin . The sum of the maximum transmission delay MaxTD, the maximum processing time T proc  and the guard interval T margin  is called the waiting time WT. 
         [0075]    On the basis of the process of obtaining the node synchronization time information between the aGW and the eNBi in step S 10 , we can obtain the transmission delays TD i  between the aGW and all relevant eNBs. And of these transmission delays, we can pick out the maximum one, i.e., the MaxTD. 
         [0076]    In addition, since extra processing overhead (such as segmentation, coding, modulation, and so on implemented in the data processing unit  240 ) exists after each eNBi receives an MBMS data packet, it is necessary to preset a maximum processing time T proc , and to embed this factor into the waiting time when specifying the unified expected transmitting time. In general, the maximum processing time T proc  is preset in advance, i.e., to obtain it in advance statistically or according to the eNB&#39;s processing ability. 
         [0077]    In addition, it is necessary to take the fact into account that an MBMS data packet would be segmented into data frames for transmitting after it is transmitted to the eNB. To guarantee consistent segmentation to the MBMS data packet in each eNBi, it is necessary to configure the same TFC parameter in relevant eNBs for the MBMS service data so as to ensure consistent segmentation and coding modulation implemented to the MBMS data packet. Therefore, no cascaded operations will be done by the eNB to the MBMS data packet. Data frames in the same MBMS data packet are continuously transmitted by each eNB. 
         [0078]    During the transmitting of data frames, since the data frames of a data packet should be transmitted right after the transmission of the ones of the previous data packet, it is necessary to consider the factor how many data frames the MBMS data packet can be segmented when we configure the guard interval. 
         [0079]    To implement consistent segmentation to MBMS data packets, a simple process is to pre-define and pre-configure the resource block parameters like IP parameter in both the aGW and the eNBs for each MBMS service data. MBMS data frame&#39;s transmission length is known to the aGW, and such segmentation is fixed to the MBMS service. 
         [0080]    For each MBMS data packet, the unified expected transmitting time is specified by the aGW&#39;s calculation unit  140  as: 
         [0000]        AFN   expect =( AFN   start   +WT )mod 4096=( AFN   start +Max TD+T   proc   +T   margin )mod 4096  (6)
 
         [0081]      FIG. 9  illustrates how to specify the unified expected transmitting time and then gives a corresponding implementation example of the transmission of the unified expected transmitting time. As shown in  FIG. 9 , AFN start =4094, MaxTD=3, T proc =1, T margin =3, i.e., WT=7, then AFN expect =(4094+7)mod 4096=5. 
         [0082]    Now in step S 50 , the aGW indicates the expected transmitting time AFN expect  in the MBMS data frame to be transmitted. In each MBMS data packet, the unified expected transmitting time AFN expect  is transmitted to each eNBi as in-band information. As shown in  FIG. 10 , the unified expected transmitting time AFN expect =5 is included in the data packets transmitted from the aGW to each eNBi. 
         [0083]    In step S 60 , after the eNBi receives the MBMS data packets, it translate the unified expected transmitting time AFN expect  included in the MBMS data packets into the one with its own BFN-i format according to the mapping relationship Δ i  between the aGW&#39;s AFN and its own BFN-i, i.e., 
         [0000]        BFN   transmiti =( AFN   expect +Δ i )mod 4096  (7)
 
         [0084]    As shown in  FIG. 9 , for eNB 1 , the mapping relationship Δ i =150. Therefore, eNB 1 &#39;s transmitting time BFN transmit1 =(5+150)mod 4096=155. And for eNB 2 , the mapping relationship Δ i =404. Therefore, eNB 2 &#39;s transmitting time BFN transmit2 =(5+404)=409. 
         [0085]    After every eNB receives the MBMS data packets and calculates the corresponding BFN transmiti , it compares the calculated frame number with the eNB&#39;s current frame number. If the calculated BFN transmiti  is prior to the current frame number, i.e., the calculated BFN transmiti  is not suitable for this transmission and then the eNB sends a timing adjustment signal to the aGW, requesting to implement the synchronization process once more. 
         [0086]      FIG. 10  illustrates the re-synchronizing process. As shown in  FIG. 10 , if eNB 2  finds out that BFN transmit2  is not suitable for the transmission, it sends a timing adjustment signal to the aGW. After the aGW receives the timing adjustment signal, it again sends the downlink node synchronization signal to the eNBs in the SFN area to implement the synchronization process once more. The subsequent steps are just the same as those mentioned above, details are omitted here. 
         [0087]    The description above is on the synchronization operations of the eNBs in one SFN area for RF combining. In the case that a SFN area is divided into several sub-SFN areas, then in each sub-SFN area, although the physical layer frame timing synchronization has been achieved to every eNB, no exact alignment is reached to the frame timing boundaries in different sub-SFN areas. And less than one frame (10 ms) at most exists on the frame timing boundaries in different sub-SFN areas. This very satisfies the requirement of soft combining, for the maximum transmission delay allowed for soft combining is 40 ms. 
         [0088]      FIG. 11  illustrates the relationship between two sub-SFN areas&#39; physical layer frame timing. As shown in  FIG. 11 , among the four eNBs (eNB 1 - 4 ) of the aGW, eNB 1 - 2  belong to SFN area  1 , and eNB 3 - 4  belong to SFN area  2 . Within these two areas, the respective eNBs&#39; physical layer frame timing is aligned just the same. And between the two areas, some deviation less than 10 ms exists in the frame timing. 
         [0089]    So, with the method proposed in present invention, not only the RF combining requirement in a single SFN area but also the soft combining requirement in different sub-SFN areas can be satisfied. 
         [0090]    In addition, with the application of IP transmission technique, “jitter” error causes to both the data transmitted from the aGW to eNBs and the node synchronization measurement between the aGW and the eNBs. The reason is that the performance of IP transmission technique is related to the network load, i.e., the measured transmission delay in the case of heavy network load differs from that in the case of light load. 
         [0091]      FIG. 12  illustrates the “jitter” error. In  FIG. 12 , AFN denotes the time counter in the format of the aGW&#39;s own frame, BFN- 1  denotes the time counter in the format of eNB 1 &#39;s own frame, and applied precision of AFN and BEN is ⅛ frames. As shown in  FIG. 12 , the signal with AFN&#39;s frame number  4095  should arrive at the time T 2   1 =150.875 (which is in the format of eNB 1 &#39;s frame number). But since the network load varies, the measured actual arrival time is T 2   1 ′=150.500 or T 2   1 ″=151.125. When the measured results are in different frames, the jitter will affect the measurement precision so as to further affect the accuracy on specifying the eNB&#39;s data packet transmitting time BEN transmit . 
         [0092]    To solve the “jitter” problem, several synchronization processes like 5˜10 times can be implemented between the aGW and the eNBs to gain the resulting transmission delay by averaging the ones obtained all synchronization processes. In this way, ‘jitter’ error can be scattered into the measurements. And more accurate result will be obtained so that the affection from jitter error will be reduced. 
         [0093]    The cause of “jitter” lies in that the frame from aGW possibly spans frames when it arrives at the eNB. If the frame is long enough to exceed the maximum transmission delay between the aGW to the eNB, the jitter error problem can be completely avoided.  FIG. 13  illustrates how to configure the frame length to avoid “jitter” error. As shown in  FIG. 13 , if the maximum transmission delay between the aGW and the eNB is 10 ms, the frame length is configured to be over 10 ms, e.g., 40 ms. In this way, the “jitter” error is always less than one frame. And it no longer causes essential affection to data transmission. The super-frame can be adopted in the configuration of long frames. In this case, the granularity on system synchronization is the super-frame. 
         [0094]    The first embodiment above illustrates in detail how to implement the synchronization process in the case of no external reference clock. However, present invention can also be implemented in the case of common external reference clock. 
       Second Embodiment 
       [0095]    As described above, common external time can be adopted by both the aGW and eNBs as the reference clock in present invention. For instance, eNBs and the aGW have common external reference clock sources like the GPS system or the Galileo system. And the eNBs and the aGW are synchronous to the external reference clock system. 
         [0096]      FIG. 14  illustrates the network structure of the second embodiment. As shown in  FIG. 14 , satellite STLT like GPS satellite or Galileo satellite provides eNB 1  and eNB 2  (which are connecting with relevant satellite receivers) and the aGW with uniform time baseline. 
         [0097]      FIG. 15  illustrates a block diagram of the aGW&#39;s according to the second embodiment.  FIG. 16  illustrates a block diagram o of the eNB according to the second embodiment. 
         [0098]    As shown in  FIG. 15 , the aGW in the second embodiment includes a gateway buffer  310  which buffers the MBMS data packets transmitted from the multimedia broadcast and multicast center, a gateway controller  320  which controls the entire gateway&#39;s operations, a gateway communication unit  330  which transmits data packets and signals to UEs and receives signals from UEs, a gateway frame number counter  350  which acts as the system timer of the aGW, a calculation unit  340  which calculate the transmission delays and transmitting time according to the signals received by communication unit from UEs, and an external reference clock  360 , e.g., the GPS receiver or the Galileo system receiver. 
         [0099]    As shown in  FIG. 16 , the eNB 1  according to the second embodiment includes a communication unit  410  which is responsible for communicating with aGW, a eNB buffer  420  which buffers MBMS data packets transmitted from the aGW, an eNB controller  430  which controls the entire eNB 1 , a translation unit  450  which translates the MBMS data packet&#39;s transmitting time into the real one in BFN format according to the mapping relationship transmitted from the aGW, an eNB frame number counter  470 , a data processing unit  440  which implements such operations as segmentation, frame construction and modulation to the received MBMS data packets, a transmission unit  460  which transmits the processed data packets in the data processing unit  440  to UEs according to the transmitting time obtained through the translation unit  450 , and an external reference clock  480 , e.g., the GPS receiver or the Galileo system receiver. The eNB 2  bears the same structure as eNB 1 . No detailed description will be given here. 
         [0100]      FIG. 17  illustrates the relationship between AFN and BFN-i in the case of external common reference clock. As shown in  FIG. 17 , the aGW and eNBs respectively have their own frame number counter, viz., the aGW&#39;s AFN and the eNBs&#39; BFN- 1  and BFN- 2 . Although these frame number counters operate independently, they share the same common GPS or Galileo system clock reference. And they are exactly aligned the same on their frame timing boundaries in the SFN area. The frame number offsets AFN offset  and BFN offseti  relative to common GPS or Galileo system clock reference are known to the aGW and the eNBs. 
         [0101]    In this case, the mapping relationship between AFN and BFN is fixed and can be exactly derived according to AFN offset  and BFN offseti . Now let&#39;s get down to the synchronization process in the second embodiment with reference to the steps in  FIG. 6  and the illustration in  FIG. 18 . 
         [0102]      FIG. 18  illustrates the synchronization between the aGW and the eNB in the case of common reference clock. 
         [0103]    As shown in  FIG. 18 , in the first place, the gateway controller  320  sends the downlink node synchronization signal (e.g., the downlink node synchronization control frame) to the eNBs in SFN area through the gateway communication unit  330 , requesting to synchronize with the eNBs. It records the current frame number in the aGW&#39;s frame number counter  150  as the transmitting time T 1 , which is included in the downlink node synchronization signal. Then, each eNB receives the downlink node synchronization signal in virtue of the eNB&#39;s communication unit  410  and records the current frame number in the eNB&#39;s frame number counter  470  as the time T 2 ; for the receiving of downlink node synchronization signal. Now, each eNB responds an uplink node synchronization signal like the uplink node synchronization control frame to the aGW, including at least the time T 2   i  and BFN offseti  (which are in the format of eNB&#39;s frame number) when corresponding eNB receives the downlink synchronization signal, and the time T 3   i  when corresponding eNB transmits the uplink node synchronization signal, or including the transmitting time T 1 , as shown in  FIG. 18 . 
         [0104]    In this case, no assumption is made that the uplink transmission delay be the same as the downlink transmission delay. The mapping relationship Δ i  between AFN and BFN can be directly calculated as: 
         [0000]      Δ i =Round(( AFN   offset   −BFN   offseti )mod 4096)  (8)
       and the transmission delay TD i  between the aGW and the eNB can be calculated as:       
 
         [0000]        TD   i =Ceil[((( T 2 i   +BFN   offseti )mod 4096)−(( T 1 +AFN   offset )mod 4096))mod 4096]  (9)
 
         [0000]    where Round denotes the rounding function and Ceil denotes the function of upper rounding into integer. And according to the received time moments and formulae (8) and (9), the calculation unit  140  calculates the mapping relationship Δ i  and the transmission delay TD i . 
         [0106]    Now, with the help of concrete data illustrated in  FIG. 18 , let&#39;s take the relationship and transmission delay exist in the aGW and eNB 1  as an example to further describe how to obtain the synchronization time information. 
         [0107]    The AFN frame number offset AFN offset  that the aGW is relative to the common reference GPS or Galileo system clock is 4092. And the BFN frame number offset BFN offset1  that eNB 1  is relative to the common reference GPS or Galileo system clock is 3942. 
         [0108]    The aGW transmits the downlink node synchronization signal to the eNB 1  at the current frame number in the aGW&#39;s frame number counter  350 , viz., at the time T 1 =4094, recording current time T 1  and including it in the downlink node synchronization signal. In fact, in the second embodiment, on condition that the aGW transmits a downlink node synchronization signal to the eNB, it indicates that the synchronization request has been transmitted out. And it is not necessary to include the time moment T 1  when the synchronization request is transmitted out in the synchronization request. 
         [0109]    Then, the eNB 1  receives the downlink node synchronization signal at the current frame number in the eNB&#39;s frame number counter  470 , viz., at the time T 2   1 =149.875. Now, the eNB transmits the uplink node synchronization signal at BFN&#39;s current frame number, viz., at the time T 3   1 =151.125, including at least the time T 2   1  when eNB 1  receives the downlink node synchronization signal and the BFN frame number offset BFN offset1  eNB 1  relative to the common reference GPS or Galileo system clock. Of course, the uplink node synchronization signal can also include the aGW&#39;s transmitting time T 1  and the time T 3   1  when eNB 1  transmits the uplink node synchronization signal. 
         [0110]    Next, the aGW receives the uplink node synchronization signal at the current frame number in the aGW&#39;s frame number counter  350 , viz., at the time T 4   1  to obtain at least T 2   1  and BFN offset1 . 
         [0111]    Then, according to T 1  recorded or included in the uplink node synchronization signal, and the obtained information on T 2   1  and AFN offset  and BFN offset1 , the calculation unit  340  can calculate the mapping relationship between AFN and eNB 1 &#39;s BFN as: 
         [0000]      Δ 1 =Round(( AFN   offset   −BFN   offset1 )mod 4096)=150
 
         [0000]      Thus: 
         [0000]        BFN− 1=( AFN+ 150)mod 4096 
         [0112]    and the transmission delay between the aGW and the eNB 1  in this path is: 
         [0000]        TD   1 =Ceil[((( T 2 1   +BFN   offset1 )mod 4096)−(( T 1 +AFN   offset )mod 4096))mod 4096]=2
 
         [0113]    In this way, we obtain the mapping relationship Δ i  and the transmission delay TD i  between the aGW and the eNBs. 
         [0114]    Steps after having obtained the mapping relationship Δ i  and transmission delay TD i  are just the same as that in the first embodiment. No detail will be given here. 
         [0115]    Thus, the synchronization processing mechanism is discussed respectively in present invention, the China patent application 200610029863.7 (Title: Method and Device for Synchronization Between Network Devices in Radio communication system; Filing Date: Aug. 9 th  2006), and the China patent application 200610028109.1 (Title: Method and Device for Resource Scheduling for Broadcast and Multicast in Radio Access Network; Filing Date: Jun. 23 rd  2006), for RF combining in 3GPP long-term evolve EMBMS in three aspects. With the synchronization processing mechanism, the problems are settled for UE in RF combining for LTE MBMS data packets. 
         [0116]      FIG. 19  illustrates three-layer synchronization structure required for RF combining in LTE. 
         [0117]    As shown in  FIG. 19 , detailed hierarchical synchronization structure supporting MBMS RF combining in a SFN area is as follows: 
         [0118]    Physical Layer Frame Timing Synchronization 
         [0119]    This synchronization requires that the physical layer frame timing of all eNB in the SFN area be aligned the same on the frame boundaries so as to guarantee the synchronization of physical layer frame timing, as shown by the ellipse on the left of  FIG. 19 . This requirement is made to layer  1  physical frame timing synchronization with the precision being the level of microsecond (please refer to the China patent application 200610029863.7). 
         [0120]    L 2  Content Transmission Synchronization 
         [0121]    This synchronization requires that MBMS service data with consistent content be transmitted at the same time in the form of radio frames by each eNB in the SFN area. That is to say, as described in the first and second embodiments, synchronization is first implemented between the aGW and the eNB to specify the transmitting time moment for the MBMS data packets. Then, the MBMS data packets are transmitted in the form of IP packets from the aGW to the eNBs. And they are transmitted to UEs in the form of radio frames at the specified transmitting time moment BFN transmit  (see the ellipse in the middle of  FIG. 19 ). In this way, in time RF combining of data with consistent content can be guaranteed in the UEs. 
         [0122]    L 3  Service Scheduling Synchronization 
         [0123]    The in-the-air interface&#39;s RF combining is considered for the physical resource block. It requires that the same MBMS service data be transmitted through consistent time frequency resource by different eNBs in the SFN area, as shown by the ellipse on the most right in  FIG. 19 . That is to say, in different eNBs, the physical resource block patterns for the transmission of MBMS service in each scheduling cycle period must be consistent. As shown in  FIG. 19 , MBMS services  1 - 4  are respectively transmitted in virtue of corresponding time frequency resource (refer to the China patent application 200610028109.1). 
         [0124]    The description above is only suitable for the embodiments of present invention. Technician in this field should understand that any modification or local replacement within the scope of present invention is confined within the claims of present invention. So, the protection scope confined by the claims is the one of present invention.