Abstract:
A system and method for transmitting downlink schedule in a WiMax/WiBro relay system is proposed in present invention. In present invention, the task of transmitting the schedule is dispersed from the BS to both of the BS and RS so that several RS may transmit the schedule items simultaneously and therefore the resource of the system is saved.

Description:
TECHNICAL FIELD 
     The present invention relates to a relay cell system, especially to a system and method for transmitting a downlink schedule in a WiMax/WiBro relay system. 
     BACKGROUND 
     In a conventional WiMax/WiBro system, it is necessary for a BS to transmit a schedule in a header of each frame so that users may receive or transmit data at some location. In general, this schedule should be decoded by any user even the user with worse channel quality in a cell. Thus, the schedule is processed with lower level of modulation and encoding scheme and retransmitted for several times. 
     SUMMARY OF THE INVENTION 
     With introduction of relay, a space multiplexing scheme is mostly adopted between relay stations to make full use of the system&#39;s frequency resource, i.e., several relay stations may simultaneously transmit or receive data, as shown in  FIG. 2 . In this way, several data or control messages are transferred in a cell at some time. Therefore, items of the schedule in each frame increases with the increasing of relays. Since a schedule is usually processed with lower level of modulation and encoding scheme and retransmitted for several times, it occupies more system frequency resources than data transmission. It is necessary to adopt a more effective method to transmit the schedule so as to reduce the resource increased for transmitting the schedule with the increasing of relays. 
     A system and method for transmitting a schedule in a WiMax/WiBro relay system is provided in present invention. In this method, a task of transmitting the schedule is dispersed from the BS to both of BS and RSs so that several RSs may transmit the schedule items simultaneously and therefore the resource of the system is saved. 
     To achieve the object mentioned above, a system for transmitting a schedule in a WiMax/WiBro relay system comprising: 
     a relay scheduling function information module in a BS, for receiving relay scheduling function information transmitted from a relay scheduling function transmitting module in a RS with or without a function of scheduling, the information indicating whether a RS bears the function of scheduling or not; 
     a transmission location information transmitting module in the BS, for transmitting the transmission location information on a schedule for the next frame to a transmission location information receiving module of the RS with or without the function of scheduling; 
     a user information transmitting module in the BS, for transmitting the user information to the RS with the function of scheduling; 
     an item calculating module in the BS for calculating schedule items for all links except for the links between the RSs with the function of scheduling and the relay user in the cell; 
     a schedule item calculating module in the RS with the function of scheduling for calculating items needed to be transmitted in the next frame to user; 
     a schedule item transmitting module in the BS for transmitting the schedule items from the RS to a RS without function of scheduling in the next frame to users of the RS without the function of scheduling; 
     wherein the BS, schedule item declaration modules with or without the function of scheduling simultaneously transmit their own schedule items to users, in frequency domain, the BS transmits the schedule items via some sub-channels and a plurality of RSs transmit schedule items via other sub-channels. 
     In present invention, the task of transmitting the schedule is dispersed from the BS to both of the BS and RSs so that several RSs may transmit the schedule items simultaneously and therefore the resource of system is saved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a process that BS, the RSs with or without the function of scheduling cooperatively implements the transmission of schedule effectively; 
         FIG. 2  illustrates space multiplexing for RSs resulted from that several RSs simultaneously transmit data to users; 
         FIG. 3  illustrates in one mode of in transmitting a downlink schedule; 
         FIG. 4  illustrates a high effective mode in transmitting a downlink schedule; 
         FIG. 5  illustrates a message flow in the process that BS, the RSs with or without the function of scheduling cooperatively implement the transmission of schedule effectively. 
     
    
    
     DETAILED DESCRIPTION 
     In the structure of present invention, the relay end without the function of scheduling includes following modules: 
     Transmission location information receiving module  101  that receives a transmission location information message from a BS. With this message, the RS obtains where (for example, in which one of the OFDMA time frequency grid) the schedule items are transmitted in the next frame. After the RS receives this message, it records the transmission location for subsequent frames in the transmission of schedule items. Before this message indicating the next transmission location is not received, the transmission location is kept unchanged; 
     Schedule item receiving module  102  that receives the schedule item message sent from the schedule item transmitting module  302  in the BS. This message indicates the schedule needed to transmit in the next frame for this RS; 
     Schedule item declaration module  103  that transmits the schedule item message received by the relay schedule item receiving module  102  in the previous frame via the recorded transmission location; 
     Relay scheduling function transmitting module  104  that transmits the information to the BS, indicating that it has no function of scheduling. 
     The relay end with the function of scheduling includes following modules: 
     Transmission location information receiving module  201  that receives the transmission location information from BS. With this message, the RS obtains where (for example, in which one of the OFDMA time frequency grid) the schedule items are transmitted in the next frame. After the RS receives this message, it records the transmission location for subsequent frames in the transmission of schedule items. Before the message indicating the next transmission location is not received, the transmission location is kept unchanged; 
     Module  202  that can receive and buffer user information, which receives the messages indicating a destination address from the BS for the relay user, and then buffers them; 
     Schedule item calculating module  203  that calculates the schedule items that are necessary to transmit to users according to the buffered messages; 
     Schedule item declaration module  204  that transmits the schedule items obtained by the schedule item calculating module  203  via the recorded transmission location; 
     Relay scheduling function transmitting module  205  that transmits the information to BS, indicating that it bears function of scheduling. 
     BS includes following modules: 
     Schedule item calculating module  301  that calculates the schedule items for all links except for the links between the RSs with the function of scheduling and the relay user in the cell; 
     Schedule item transmitting module  302  that can transmit the schedule items which from the RS without the function of scheduling to the user of the relay in the next frame to the RS in the current frame; 
     Schedule item declaration module  303  that transmits the schedule items in current frame to RSs and the users; 
     Relay scheduling function information receiving module  304  that receives the scheduling function information from RSs so as to know whether the RS has the scheduling function or not; 
     Transmission location information transmitting module  305  that transmits the transmission location message of the relay schedule to the RS. In this message, the location (i.e., in which one of the OFDMA time frequency grid) is specified in the next frame for transmitting the schedule items; 
     User information transmitting module  306  that transmits the user information on the destination address to the RS. These messages are forwarded to the users through the RS. 
     The user end includes following modules: 
     Schedule item receiving module  401  that receives the schedule items via the locations specified in the system. 
     Based on the structure above, high effective transmission of downlink schedule is implemented cooperatively by BS and RS, as shown in  FIG. 1 . 
       FIG. 5  illustrates an example message transmission process. 
     In one example, there are one BS and two RSs (for example, RS1 and RS2) in a cell, in which RS1 includes the function of schedule and RS2 does not include a scheduling function; and two users (for example, user1 and user2). Detailed steps that high effective transmission of downlink schedule is implemented cooperatively by BS and RS, as shown in  FIG. 5 : 
     Step 1. in the first frame, the relay scheduling function transmitting module  205  in RS1 transmits the scheduling function message to the BS, indicating that RS1 has the function of scheduling; 
     Step 2. in the first frame, the relay scheduling function information receiving module  304  in the BS receives the scheduling function message from RS1 to know that RS1 has the function of, scheduling; 
     Step 3. in the first frame, the relay scheduling function transmitting module  104  in RS2 transmits the scheduling function message to the BS, indicating that RS2 has no function of scheduling; 
     Step 4. in the first frame, the relay scheduling function information receiving module  304  in the BS receives the scheduling function message from RS2 to know that RS2 has no function of scheduling; 
     Step 5. in the first frame, the transmission location message transmitting module  305  in the BS transmits the message indicating the transmission location of schedule in the next frame to RS1 and RS2; 
     Step 6. in the first frame, the transmission location message receiving module  201  in RS1 receives the transmission location message from BS and records it; 
     Step 7. in the first frame, the transmission location message receiving module  101  in RS2 receives the transmission location message from BS and records it; 
     Step 8. in the first frame, the user information transmitting module  306  in the BS transmits the user information to RS1; 
     Step 9. in the first frame, the user information receiving and buffering module  202  in RS1 receives the transmission user information from BS and buffers it; 
     Step 10. in the first frame, the schedule item calculating module  301  in the BS calculates the schedule items (in the next frame, for example, the second frame) for all links except the ones between the RSs which bear function of scheduling and the relay user in the cell; 
     Step 11. in the first frame, the schedule item calculating module  203  in RS1 calculates the schedule items needed to be transmitted to users via the next frame (for example, the second frame); 
     Step 12. in the first frame, the schedule item transmitting module  302  in the BS transmits the schedule items (from RS2 to the users of RS2) to RS2 via the next frame (for example, the second frame); 
     Step 13. in the first frame, the schedule item receiving module  102  in RS2 receives the schedule items (from RS2 to the users of RS2) from BS via the next frame (for example, the second frame) and records them; 
     Step 14. in the second frame, the schedule item declaration modules (i.e.,  301 ,  204  and  103 ) in the BS, RS1 and RS2 simultaneously transmits their own schedule items, where the location (for example, the OFDMA time frequency unit grid) of the broadcast schedule item of RS1 or RS2 is the one recorded in the first frame; 
     Step 15. in the second frame, the schedule item receiving module  401  of user1 receives the schedule items from both BS and RS1 so as to obtain the schedule items in current frame; 
     Step 16. in the second frame, the schedule item receiving module  401  of user2 receives the schedule items from both BS and RS2 so as to know about the schedule items in current frame. 
     In an illustrative example a cell has one BS, three RSs (for example, RS1, RS2 and RS3) and three users (for example, user1, user2 and user3). Three messages should be transmitted from BS to user1, user2 and user3 respectively on the premise that the message to user1 should be relayed by RS1 the message to user2 by RS2 and the message to user3 by RS3. In this case, it is necessary for the BS to broadcast six (6) schedule items: (1) BS→RS1; (2) BS→RS2; (3) BS→RS3; (4) RS1→user1; (5) RS2→user2; (6) RS3→user3. 
       FIG. 3  shows a PDU structure of MAC in one mode of transmitting the downlink schedule with a dash part indicating that the transmission is not implemented. In this figure, all six (6) schedule items should be transmitted by the BS. Meanwhile, all users have received the PDU that contains the six (6) schedule items. 
       FIG. 4  shows a process of transmitting the schedule items with the high effective transmission method according to present invention. Here,  FIG. 4A  shows the content that BS needs to transmit. It should be noted that BS transmits only the MAC header and the three schedule items which are from BS to RSs.  FIGS. 4B through 4D  respectively show the transmission content of RS1, RS2 and RS3. Each RS transmits the corresponding schedule items to users. Different sub-channels are adopted by BS and RSs to transmit the schedule items. The three RSs share the same sub-channels to transmit the schedule items. Since the three RS are told apart spatially, the PDUs received by user1, user2 and user3 are illustrated in  FIGS. 4E through 4G  respectively. 
     From the comparison between  FIG. 3  and  FIGS. 4A-4G , it will be seen that each user in  FIG. 3  has received the same PDU of six (6) schedule items, and each user in  FIGS. 4A-4G  has received PDUs (which are partially consistent) of four (4) schedule items. Suppose one OFDMA time frequency grid is used for the MAC header and one schedule item, seven OFDMA time frequency grids are needed to transmit the schedule in the conventional method, while only five (5) OFDMA time frequency grids are needed in the high effective transmission method. In practice, with the increment of schedule items occupied by RSs, more system resource will be saved by the high effective transmission method.