Abstract:
A method for controlling data transmission comprising following steps of: obtaining transmission delays and time differences between a RNC and respective base stations by the RNC; configuring cells and a common channel for data transmitting by the RNC; controlling a start time of data transmission of each cells by the RNC; informing a UE of a frame offset and a chip offset of the common channel through an interface Uu by the RNC; and receiving configuration parameters of the common channel transmitted from the RNC by the UE. In the present invention, merging of signals transmitted from different cells via the common channel has been achieved so that the quality of signals received by the UE has been improved and the throughput of the system has been enlarged.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to 3 rd  Generation (3G) mobile communication system, especially to a method for providing a Multimedia Broadcast/Multicast Service. 
     2. Description of the Related Art 
     The MBMS is a new service under standardization by 3 rd  Generation Mobile Communication System Partnership Project. The MBMS service is a unidirectional point-to-multipoint (p-t-m) (i.e. multimedia data sent from a single data source are transferred to multiple users through network transmission) service. The most remarkable feature of the service is that it can make use of radio resources and network resources efficiently. MBMS service is mainly used in wireless communication network system, e.g. Wideband Code-Division Multiple Access system (WCDMA), Global System of Mobile Communication (GSM), etc. MBMS service data transfer basically includes following several steps, i.e. data source transmission, middle network transmission, destination cell on-the-air transmission and user receiving.  FIG. 1  is a logical figure for network devices of a radio communication system that can provide the MBMS service, in which the MBMS actually makes use of General Packet Radio Data Service (hereinafter referred to as GPRS) as core transmission network. As shown in  FIG. 1 , a Broadcast and Multicast Service Center  70  (hereinafter referred to as BM-SC) is a data source for MBMS data transmission. A Gateway GPRS Supporting Node  60  (hereinafter referred to as GGSN) is used to connect the GRPS network with an external network, such as INTERNET. In the MBMS service, the Gateway GPRS Supporting Node, connects to the BM-SC and sends the MBMS data to a specific Service GPRS Supporting Node  50  (hereinafter referred to as SGSN). The SGSN is used to perform access control on UE as well as mobility management, and sends the MBMS data from the GGSN to a specific Radio Network Controller  30  (hereinafter referred to as RNC) at the same time. The RNC is used to control a group of base stations like station  23  and  24  and sends multimedia data to specific base stations like station  23  and  24 . The base station  23  establishes an air common channel  11  for the MBMS service of cell  21  under the control of the RNC. The base station  24  establishes an air common channel  12  for the MBMS service of cell  22  under the control of the RNC and a terminal User Equipment  10  (hereinafter referred to as UE) is a terminal equipment for MBMS data reception. 
     When implementing a point-to-point communication between the UE and the network, a dedicated channel will be established by the network for the UE and the radio resource occupied by this channel can not be shared by any other UE but exclusively possessed by the UE in the communication process. In order to make use of the radio resource and the network resource effectively when the MBMS service is provided via the network, a common channel is adopted in the air interface for communicating between several UEs simultaneously. After the UE obtains the resource configuration of the common channel, it receives radio signals via the network so as to gain the MBMS service. However, an obvious feature different from the dedicated channel is that no common channel supports soft handover. Soft handover means that the user can simultaneously receive signals from several cells and then merge them to obtain better quality of signal and lower loss rate of data. For the UE  10  in  FIG. 1 , if the transmission channels are established for it in the cell  21  and cell  22  by the network, it can simultaneously receive signals from the cell  21  and cell  22  on the premise that the cell  21  is adjacent to cell  22 . 
     To support soft handover, a certain time delay must be satisfied to the signals received by a physical layer of the UE. For instance, in current WCDMA system, the time delay is required to be controlled within ±148 chip, i.e., 0.03854 milliseconds. Whether this requirement is met or not is controlled by the service RNC of the UE as shown in  FIG. 2 . Reference numeral  21  denotes cell  1 , and  22  denotes cell  2  in  FIG. 1. 201  shows a timing relationship corresponding to the cell  21 , and  202  shows a timing relationship corresponding to the cell  22 . In the following, definitions of respective clock in the WCDMA system are described: 
     BFN: a frame number of the Node B, each frame is 10 milliseconds long, and the frame number repeats from 0 to 4095. 
     SFN: a system frame number of the cell under the control of the Node B, each frame is 10 milliseconds long, and the frame number repeats from 0 to 4095. The time difference between the SFN and BFN is one Tcell, which values from 0 to 9 with the granularity of 256 chip (about 1/15 milliseconds), i.e., the Tcell values 0, 2/15, 3/15, 4/15, 5/15, 6/15, 7/15, 8/15, 9/15 milliseconds. 
     CFN: CEN involves a type of channel through which the UE receives signals, if the channel is a common one, it equals SFN modulo 256, and if the channel is a dedicated one, it equals SFN subtracted by a frame offset modulo 256. A CFN 1  and CFN 2  in  FIG. 2  are corresponding to the common channel, while a CFN 1 ′ and CFN 2 ′ are corresponding to the dedicated channel. The only difference consists in that the frame offset of the dedicated channel in  201  is zero while the frame offset of dedicated channel in  202  is 6. In an actual system, whether the dedicated channel or the common channel is in service, a chip-level time difference within 0˜149 256-chips may exist between a start point of the channel&#39;s frame and that of the SFN&#39;s frames. To be simplified, as shown in  FIG. 2 , it is assumed that Dcoff is the chip offset of two cells  21  and  22  corresponding to the dedicated channel, and Soff is the chip offset of the two cells  21  and  22  corresponding to the common channel. 
     Since there is no device such as a GPS is applied in the WCDMA system for all network elements in clock adjusting, it is possible that the clock of every network element be different. Moreover, a deviation may happen to every clock&#39;s frequency. The RNC has its own clock for time control, and the Node B also has its own clock for time control. However, since all resource configurations of the Node B are under the control of the RNC, and the time for data sending is also managed by the RNC, it is necessary to make the RNC have some knowledge to the clock&#39;s characteristics of the Node B.  FIG. 3  illustrates a method of node synchronization between the RNC and the Node B. The clock of the RNC is denoted by RFN, which is the frame number of the RNC side, each frame is 10 milliseconds long and the frame number repeats from 0 to 4095. The time within a cycle can be denoted as from 0 to 40959.875 with the granularity of 0.125 milliseconds, which equals to the length of 480 chip. The RNC sends a downlink node synchronization message to the Node B, which contains time t 1  when the message is sent. After the Node B receives that message, it records time t 2  when the message is received, then sends an uplink node synchronization message to the RNC, which contains the time t 1 , t 2  and t 3 . Here, t 3  is time when the Node B sends the uplink node synchronization message. After the RNC receives the message, it records time t 4  when the message is received. Till now, the node synchronization process between the RNC and the Node B completes. According to the four parameters t 1 , t 2 , t 3  and t 4 , a data transmission delay between the RNC and the Node B can be calculated by the RNC, and a time corresponding relationship between the RNC and the Node B can also be set forth. It is assumed that a RTD denotes the time delay between the RNC and the Node B, RTD=(t 4 −t 1 −t 3 +t 2 )/2. The time corresponding relationship between the RNC and the Node B is that: time of Node B=time of RNC+t 2 −t 1 −RTD. However, in the actual transmission process, the RTD and variation of the RTD should be considered, for the transmission delay in an interface Iub may not involve the actual situation of the transmission network. 
     With the equations above, if it is supposed that the transmission delay between the RNC and the Node B is almost invariable, the RNC can know the time of the Node B clearly with the granularity of the time be 0.125 milliseconds, i.e., 480 chip. 
     It is not enough to know the time of Node B for the RNC, it also clearly controls the transmission time of the common channel. As shown in  FIG. 2 , the transmission time of the common channel should be: BFN+Tcell+Soff. The following requirement should be satisfied to keep the time delays of different cells in transmitting data less than the required: an absolute value of ((BFN 1 +Tcell 1 −Soff 1 )−(BFN 2 +Tcell 2 +Soff 2 ))&lt;the required time delay. 
     In the WCDMA system, the transmission of a new copy of data can not be started from any of the frames and a transmission time interval TTI is defined for each transmission channel. The value of the TTI is integral multiple to the frame length, such as 10 milliseconds, 20 milliseconds, 40 milliseconds or 80 milliseconds and the corresponding number of frames occupied by each TTI is 1, 2, 4 and 8 respectively. The start point of these TTIs can only be the time when the CFN modulo Fn equals 0, where the Fn is the number of the frames corresponding to the TTIs. In addition, the RNC also controls the cells&#39; or the common channel&#39;s configuration parameters so as to make the start point of the data received by the UE be the same within a TTI. Therefore the formula should be modified as: an absolute value of (((BFN 1 +Tcell 1 +Soff 1 )−(BFN 2 +Tcell 2 +Soff 2 )) mod Fn)&lt;the required time delay. 
     Since the maximum lengths of Tcell and Soff are 9/15 milliseconds and 10 milliseconds respectively, it is impossible to make the time delay difference of data received from the two cells by the UE be less than 10 milliseconds in the case that the BFNs of the two cells are absolutely different, no matter how to regulate Tcell or Scoff. In general, the time of 10 milliseconds is greater than the time delay requirement for the UE in the signal merging. In the MBMS, the user data is transmitted through the common channel. To make all users receive the signal at any location of the cell, the required transmitting power of the common channel is higher. Therefore interference is generally caused to the adjacent cells so that the system capacity is reduced. Thus a method for merging the signals via the common channel of different cells has been proposed, which requires that the time delay of the signals transmitted from adjacent cells via the same common channel should not be over a certain value, otherwise, the object of signal merging and reception quality optimizing can not be reached. However, the time delay of the signals transmitted from adjacent cells via the same common channel can not be guaranteed within a relative small range in the current system at all. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide a method for controlling data transmission, which guarantees that a time delay of signals transmitted from adjacent cells via the same common channel be controlled within a relative small range so that it is possible for a UE to merge signals received from several cells. 
     To achieve the object mentioned above, a method for controlling data transmission comprising following steps of: 
     obtaining transmission delays and time differences between a RNC and respective base stations by the RNC; 
     configuring cells and a common channel for data transmitting by the RNC; 
     controlling a start time of data transmission of each cells by the RNC; 
     informing a UE of a frame offset and a chip offset of the common channel through an interface Uu by the RNC; and 
     receiving configuration parameters of the common channel transmitted from the RNC by the UE. 
     In the present invention, merging of signals transmitted from different cells via the common channel has been achieved so that the quality of signals received by the UE has been improved and the throughput of the system has been enlarged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a logic device diagram of MBMS; 
         FIG. 2  shows a timing sequence diagram of a clock; 
         FIG. 3  shows a timing sequence of synchronization between the RNC and the Node B; 
         FIG. 4  illustrates a block diagram of a structure according to the present invention; 
         FIG. 5  illustrates a definition of clocks in the present invention; 
         FIG. 6  illustrates a clock relationship in the side of the UE of the present invention; 
         FIG. 7  illustrates the first embodiment of the present invention; 
         FIG. 8  illustrates the second embodiment of the present invention; 
         FIG. 9  illustrates the third embodiment of the present invention; and 
         FIG. 10  illustrates a timing sequence of the first embodiment of the present invention. 
         FIG. 11  illustrates the behavior of the UE of the present invention. 
         FIG. 12  illustrates the behavior of the RNC of the present invention. 
         FIG. 13  illustrates the behavior of the Node B of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To make the time differences that the same copy of data is transmitted from different cells be relative small, such as 5 milliseconds, the RNC must know the time differences between each of the Node Bs at first ( 401 ). This function can be realized with reference to the flow chart illustrated in  FIG. 3 , which is applied in  FIG. 4  to obtain step  431 . After the process of node synchronization between the RNC and the Node B  23  and  24  completes respectively, i.e., the signaling interaction between  411  and  412  according to the flow illustrated in  FIG. 3  completes, the RNC will know the difference of BFNs between its controlled Node B  23  and  24 : BFN 1 −BFN 2 =(t 2 −1−t 1 −1−RTD 1 )−(t 2 −2−t 1 −2−RTD 2 ). In an existing system, the granularity of the time difference is 0.125 milliseconds and the algorithms differ from one another for the concrete realization approaches. 
     After  401  completes, it is necessary for the RNC to determine the particular time  402  for every cell so as to transmit data. In different embodiments, the RNC configures either parameter Tcell or Soff of the cell or both of Tcell and Soff. The ranges of the two parameters must be expanded over their originals. For the convenience of understanding, the expanded Tcell is split into two parameters, one is Tcell which has the same function and range as that defined in the existing system, other is Coff which indicates the difference between the BFN and SFN of every cell of different Node Bs. The expanded Soff indicates a time offset of the common channel with respect to the SFN within one frame. For the convenience of understanding, the expanded Soff is split into two parameters, one is Soff which has the same function and range as that defined in existing system, other is CHoff which indicates a frame offset of CFNs in different MBMS transmission channels with respect to the SFN of the cell. The definitions of the four parameters are shown in  FIG. 5 . As seen from  FIG. 5 , a start time of SFN=a time indicated by BFN−the time indicated by Coff−a time indicated by Tcell and a start time of the common channel CFN=a time indicated by SFN−a time indicated by CHoff−a time indicated by Soff. With such calculation formula, when the Node B is informed of the four parameters through the Iub signaling ( 432 ), it can calculate the corresponding start time for data transmitting. 
     The RNC informs the UE of the frame offset and chip offset of the common transmission channel through the signaling of interface Uu  441 . By reading system information of the cell, the UE obtains the SFN, and the UE can learn about from which frame and chip the data within a TTI starts by following calculation formula of the CFN: CFN=(SFN−Choff) mod 256. The start frame of TTI is the one satisfying that the CFN modulo Fn equals 0, where Fn indicates how many frames the TTI has. 
     Another method proposed is to only regulate the UE in calculating the start frame of the TTI to meet the requirement of time delay. From the previous description, it will be seen that the start frame of the TTI is the one satisfying that CFN modulo Fn equals 0, where Fn indicates how many frames the TTI has. Since the CFNs of the common channel transmitted from different cells are different, in order to make the UE know the start time of the TTI, a CFNoff value should be transmitted, for each common channel that transmits the MBMS data. With this value, the UE can regulate the start time of the TTI correspondingly, as shown in  FIG. 6 . In the cell  22 , as shown in  FIG. 6 , the start time of TTI of the data received by the UE does not satisfy the formula above, one offset should be considered in this case. In this example, the TTI is 40 milliseconds long, i.e., the TTI should start at the frame satisfying that CFN modulo 4 equals 0. In cell  21 , the offset of the CFN is 0, and in cell  22 , the offset of the CFN is 3. 
       FIG. 7  shows the first embodiment of the present invention. 
     The embodiment is implemented through modifying frame offsets of different cell. In  701  and  702 , the transmission delay and the time difference of interface Iub between the RNC and the Node B are obtained through the process of synchronization between the RNC and the Node B illustrated in  FIG. 3 . After the RNC collects the transmission delays and time differences between itself and the other Node Bs, it begins to calculate the offset values of the cells under the configuration of different Node Bs so as to make all cells&#39; time differences of the frames satisfying that SFN modulo Fn equals 0 within the required time delay range. One method for calculating the time difference is to take one Node B as the reference, and make the other Node Bs relate to it, as shown in  FIG. 10 . The time relationship between the SFN and BFN in  FIG. 10  is that: a time indicated by SFN 1 =a time indicated by BFN 1 −a time indicated by Coff−a time indicated by Tcell, where the time indicated by Coff is 0. A time indicated by SFN 2 =a time indicated by BFN 2 −a time indicated by Coff 2 −a time indicated by Tcell 2 , where Coff 2  is three frames long. From  FIG. 10 , it is seen that the difference of SFNs of the two cells is not greater than one frame in case that Fn equals 4. 
     After the RNC configures Coff and Tcell for each cell according to algorithm above, it transfers these parameters to the Node B in  703  and  704 . The Node B configures the transmitting time for cell  705  according to the two parameters and broadcasts SFN of the cell via the broadcast channel. In  706  and  707 , the method for establishing the common channel is just the same as that of the existing technique. The RNC sends the configurations related to the common channel to the Node B and establishes the common channel, whose offset of the transmitting time is Soff. This Soff has the same reference value as that of the existing technique. 
       FIG. 8  shows the second embodiment of the present invention. The embodiment is implemented by modifying the frame offsets of different cells in the common channel. In  801  and  802 , transmission delay and the time difference of interface Iub between the RNC and the Node B are obtained through the process of synchronization between the RNC and the Node B illustrated in  FIG. 3 . In  803  and  804 , the RNC configures the transmission offset Tcell for the cells of every Node B according to available solution. In  805 , the Node B configures the transmission offset for the cells according to the configuration parameters transmitted from the RNC. To establish the common channel for data transmitting, the RNC send  806  to the Node B, where the frame offset and chip offset of the common channel are determined by CHoff and Soff, and the Node B configures the transmitting time of the common channel according to the two parameters. After the RNC succeeds in establishing the common channel in the side of the network, it sends the common channel&#39;s parameters to UE through the signaling RRC. After the UE obtain the common channel&#39;s configuration parameters, the frame offset and chip offset through  808 , it calculates the start time for data receiving. The common channel&#39;s CFN is calculated with the formula: CFN=(SFN−CHoff) mod 256. TTI&#39;s start point is the frame number satisfy that CFN mod Fn=0. The UE can obtain not only the configurations of the current cell&#39;s common channel, but also the configurations of the adjacent cell&#39;s common channel used to provide MBMS service, especially the common channel&#39;s parameter Choff. When the UE simultaneously receives signals from several cells at the edge of a cell, it receives signals via the common channel of the several cells and merges them according to the configurations of the common channel of the several cells. 
       FIG. 9  shows the third embodiment of the present invention. 
     The embodiment is implemented by configuring the offset of the TTI&#39;s start frame in the side of the UE. From  901  to  907 , the embodiment is the same as that of the existing technique. They also can be realized according to the relevant signaling flow in  FIG. 7 . In  908 , after the RNC succeeds in configuring the cells and the common channel in the side of the network, it informs the UE of the offset of the TTI&#39;s start frame. The UE determines the start time of TTI in the common channel according to this offset. The determination method is that: the frame satisfying that (CFN−CFNoff) mod Fn=0. The UE can obtain not only the configurations of the current cell&#39;s common channel, but also the configurations of the adjacent cell&#39;s common channel used to provide MBMS service, especially the offset of the TTI&#39;s start frame of the common channel. When the UE simultaneously receives signals from several cells at the edge of a cell, it regulates the offset of the TTI&#39;s start frame of common channel for different cells according to the configurations of the common channel of the several cells and receives signals via the common channel of the several cells and merges them. 
       FIG. 11  shows the behavior of the UE of the present invention. 
     Before the reception of MBMS data, the UE monitors the COMMON CHANNEL SETUP message via MCCH  1101  (the RRC message name can be different). This message includes the physical layer and MAC layer configuration parameters and common channel frame offset Choff described in  FIG. 8  or the TTI offset described in  FIG. 9. 1102  the UE calculates the starting time of TTI based on these two parameters. If the parameter is Choff, then CFN equals to (SFN−Choff) mod 256. The starting frame of TTI fulfills the equation CFN mod Fn=0. If the parameter is CFNoff, then the starting frame of TTI fulfills the equation (CFN−CFNoff) mod Fn=0. 
       FIG. 12  shows the behavior of the RNC of the present invention. 
     When the RNC decides to provide MBMS on common channels, it calculates the Choff for the common channels of each cell. RNC sends to the Node B COMMON CHANNEL SETUP REQUEST message (the message name can be different)  1201 . This message  806  includes the physical layer and MAC layer configuration of the common channel and Choff described in  FIG. 8 . When the RNC receives the response from the Node B  1202 , it will check whether the response is successful. If yes, the RNC sends COMMON CHANNEL SETUP  1203  to the UE, which includes the physical layer and MAC layer configuration parameters and Choff described in  FIG. 8  or TTI offset described in  FIG. 9 . Otherwise, the MBMS service cannot be transmitted over common channel. 
       FIG. 13  shows the behavior of the Node B of the present invention. 
     When the Node B receives the COMMON CHANNEL SETUP REQUEST (message name could be different) message  1301 , which includes the physical layer and MAC layer configuration parameters and Choff described in  FIG. 8  or TTI offset described in  FIG. 9. 1302  the Node B calculates the TTI starting time for the common channel and send MBMS notification accordingly. CFN fulfills the following equation (SFN−Choff) mod 256. The starting time of TTI fulfills that CFN mod Fn equals to zero.