Patent Publication Number: US-2007109962-A1

Title: Method and apparatus for implementing relay

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
      This application is based on the Chinese Patent Application No. 200510110323.7 filed on Nov. 11, 2005, the disclosure of which is hereby incorporated by reference.  
     FIELD OF THE INVENTION  
      The present invention relates to the field of communication, and particularly to a method and apparatus for implementing relay.  
     BACKGROUND OF THE INVENTION  
      To expand the coverage area of a wireless communication network, an effective method is to adopt wireless network repeaters. A repeater, which is typically deployed at the edge of the base station it belongs to, is used to expand the coverage area of this base station and has the basic function of a base station, whereas its coverage area is relatively small. A base station may be provided with one or more repeaters, and also a repeater may be provided with one or more repeaters so as to form repeater cascade. With repeater cascade, the coverage area of this wireless communication network can be expanded further.  
      With respect to a Worldwide Interoperability for Microwave Access (WiMAX) wireless communication network, a scheme that supports time-division duplex (TDD) same-frequency multi-hop relay has been proposed. According to this scheme, all base stations and repeaters work with same frequency. Futhermore, wireless backhaul of each repeater also adopts the frequency. Specifically, a base station reserves uplink/downlink transmission slot for its stage  1  repeater in its uplink/downlink sub-frame, respectively. This stage  1  repeater sends downlink service to its user station in its downlink slot and reserves a downlink slot for its stage  2  repeater; in its uplink slot, this stage  1  repeater receives uplink service from its user station and reserves an uplink slot for its stage  2  repeater. Reasoning by analogy, the above mechanism can be extended to repeaters at lower stages, such as the stage  3  repeater, the stage  4  repeater, . . . , the stage N repeater. For a repeater, it only communicates with its user station and its lower-stage repeater in the uplink/downlink slot, however, it will occupy the access slot of its higher-stage device (base station or repeater) when the repeater backhauls service to its base station or its higher-stage repeater.  
      A disadvantage of this scheme, however, is that since all base stations and repeaters work with the same frequency, and a base station and/or a repeater reserves uplink/downlink slot for their repeaters, the system&#39;s capacity will decrease drastically when there are many hops. It means the scheme is not suitable for network applications with high density and heavy traffic.  
      Therefore, there is a need to provide a method and a apparatus for implementing relay, which can be adapted to network applications with high density and heavy traffic.  
     SUMMARY OF THE INVENTION  
      According to the first aspect of the present invention, a method is provided for implementing relay in a wireless communication network, said method comprises the steps of: determining that a present-stage backhaul window for use in the backhaul of a present-stage service has started; and switching from a first frequency to a second frequency to complete the backhaul of the present-stage service.  
      According to the second aspect of the present invention, a repeater is provided for implementing relay in a wireless communication network, said repeater comprises: means for determining that a present-stage backhaul window for use in the backhaul of a present-stage service has started; and means for switching from a first frequency to a second frequency to complete the backhaul of the present-stage service.  
      According to the third aspect of the present invention, a method is provided for implementing relay in a wireless communication network, said method comprises the steps of: determining that a lower-stage backhaul window for use in the backhaul of a lower-stage service has started; and sending information needed for correct backhaul of the lower-stage service.  
      And according to the fourth aspect of the present invention, a base station is provided for implementing relay in a wireless communication network, said base station comprises: means for determining that a lower-stage backhaul window for use in the backhaul of a lower-stage service has started; and means for sending information needed for correct backhaul of the lower-stage service.  
      In the present invention, each repeater has its own independent frame whose length is the same as the length of that of the base station. Therefore, the present invention is suitable for network applications with high density and heavy traffic.  
      Additionally, in the present invention, wireless backhaul links are used between a repeater and a base station and between a repeater and another repeater. Therefore, deployment cost is low and operational cost is also low, and rapid deployment of a network can be achieved accordingly. 
    
    
     BRIEF DESCRIPTION ON THE DRAWINGS  
      Other objects and effects of the present invention will become more apparent by following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  shows a WiMAX TDD physical layer frame structure;  
       FIG. 2  shows a WiMAX TDD physical layer frame structure having a backhaul window;  
       FIG. 3  shows a network structure under single-hop single-repeater;  
       FIG. 4  shows an exemplary backhaul operation process under single-hop single-repeater;  
       FIG. 5  shows a working flowchart of a repeater;  
       FIG. 6  shows an exemplary network structure under single-hop multi-repeater;  
       FIG. 7  shows an exemplary backhaul operation process under single-hop multi-repeater;  
       FIG. 8  shows an exemplary network structure under multi-hop single-repeater;  
       FIG. 9  shows an exemplary backhaul operation process under multi-hop single-repeater;  
       FIG. 10  shows an exemplary block diagram of a repeater; and  
       FIG. 11  shows an exemplary block diagram of a base station. 
    
    
      Like reference numerals designate the same, similar or corresponding features or functions throughout the figures above.  
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Here, the wording “exemplary” is used to mean “serving as an example, embodiment or illustration”. Any embodiment described as “exemplary” here can not be necessarily interpreted as being preferable or advantageous over other embodiments.  
      The basic idea of the present invention is that a base station and a repeater do not work with the same frequency at all time. In detail, the base station always works with the same frequency and has only one mode, i.e., master mode. The repeater has both master mode and slave mode. In master mode, the repeater works with another frequency different from the frequency of the base station, and the user station belonging to it accomplishes access by using this another frequency; however, in slave mode, the working frequency of this repeater switches to the frequency of the base station, and at this point, it becomes a slave apparatus of this base station to perform backhaul operation. Moreover, the repeater can be cascaded.  
      In other words, the repeater serves as the base station of its user station in normal working condition, while in backhaul working condition, the repeater serves as a user station of its base station or its higher-stage repeater (referred to as father node together), and it is a child node of its father node.  
      Depending on the number of repeaters a base station has and whether said repeater has repeater(s) belonging to it, there may be divided into three basic situations: 1)single-hop single-repeater, 2) single-hop multi-repeater, and 3) multi-hop single-repeater, as will be described in detail. These situations may be combined to constitute multi-hop multi-repeater, for example.  
      Hereinafter, embodiments of the present invention will be described in detail with respect to a WiMAX wireless communication network. However, it should be understood to those skilled in the art that the basic idea of the present invention also can also apply to other types of wireless communication networks, such as wireless local area network (WLAN) defined by IEEE 802.11.  
      To facilitate understanding, first, an introduction will be given to WiMAX TDD physical layer frame structure, such as the structure of a frame transmitted between the base station and the user station. Of course, it should be noted that the present invention also can apply to WiMAX FDD (frequency-division duplex) mode. Here, explanation is made in terms of TDD mode for the purpose of conciseness. In the WiMAX standard there are defined various physical layer standards (such as SC (single carrier), SCa (single carrier advanced), OFDM (orthogonal frequency division multiplexing), OFDMA (orthogonal frequency division multiple access), etc.). Although specific formats thereof are different from one another, the structure is basically same.  
       FIG. 1  shows a WiMAX TDD physical layer frame structure. In the frame structure as shown in  FIG. 1 , a downlink sub-frame comprises a downlink broadcast domain. The downlink broadcast domain comprises a preamble, a frame control header (FCH), various downlink broadcast control messages and the like. Among them, the preamble is used for physical synchronization and equalization of a user station; FCH contains a downlink frame prefix that prescribes characteristics and lengths of various downlink burst transmissions; and the downlink broadcast control messages are used to transmit to user station DL-MAP (downlink map), UL-MAP (uplink map), DCE (downlink channel description), UCD (uplink channel description) link control messages, said messages define ways of dividing between uplink/downlink resources in a frame and properties of physical channels. Only when user stations belonging to a base station correctly receive the downlink broadcast domain, can they perform correct transmission and reception operations.  
      Since the start portion in a WiMAX frame is a downlink broadcast domain and all user stations must receive the downlink broadcast domain to complete synchronization and transmission operations, each base station and each repeater need to utilize the start portion of a frame to send the downlink broadcast domain to its slave apparatuses.  
      Moreover, the basic working process of a repeater is as follows: first it enters user station mode, at which point, it, like common user station, utilizes the original downlink broadcast domain to complete synchronization and transmission operations with its father node; then, it negotiates with its father node to determine the size and location of a backhaul window; afterwards, it enters repeater operational mode and complete access and backhaul operations by switching of master/salve mode.  
      Then, a problem will arise. That is, when a repeater enters repeater operational mode, it needs to receive downlink broadcast domain information from its father node so as to acquire synchronization and relevant control information for the backhaul operation.  
      According to an embodiment of the present invention, the downlink broadcast domain of each frame is provided to a repeater by using the method for mapping downlink broadcast domain. Specifically, the father node of a repeater copies the downlink broadcast domain of a frame into the downlink backhaul window of this repeater. Thus, when this repeater switches to slave mode to perform the backhaul operation, it can receive the downlink broadcast down information of its father node.  
       FIG. 2  shows a WiMAX TDD physical layer frame structure having a backhaul window according to an embodiment of the present invention. As shown in  FIG. 2 , based on the existing WiMAX frame structure, this embodiment defines a dedicated sub-frame that is embedded into the downlink service domain and the uplink service domain to serve as the downlink backhaul window and the uplink backhaul window of the repeater, respectively. Moreover, a downlink broadcast domain map of the father node thereof is inserted to the start portion of the downlink backhaul window.  
      According to the embodiment, when the downlink broadcast domain map is inserted to a backhaul window, the preamble of this downlink broadcast domain map will be modified to differ from the preamble of the original downlink broadcast domain. Thus, the synchronization operation of common user stations under the same father node is prevented from being affected, otherwise, they cannot judge the actual start location of a frame. At the same time, other control information such as FCH, DL-MAP, UL-MAP, DCD and UCD is simplified, removing information for user stations in the original downlink broadcast domain, reserving only information needed by this repeater, so as to save resources.  
      In this way, the backhaul window of the repeater is transparent to common user stations. Only when the repeater enters salve node, is the downlink broadcast domain map identified, and are synchronization and backhaul operations completed.  
      In this embodiment, the preamble of this downlink broadcast domain map is not specifically defined provided it differs from the preamble of the original downlink broadcast domain.  
      Hereinafter, a repeater wireless backhaul operation based on the frame structure as shown in  FIG. 2  will be described in terms of different applications.  
      Single-Hop Single-Repeater  
       FIG. 3  shows a network structure under single-hop single-repeater. As shown in  FIG. 3 , this exemplary network structure  300  comprises a core network  301 , a base station  303 , a user station  304  of the base station  303 , a repeater  305  of the base station  303 , and a user station  306  of the repeater  305 . The repeater  305  is deployed at the edge of the base station  303  and has coverage  305   a , thereby expanding the coverage  303   a  of the base station  303 . Wired backhaul is used between the base station  303  and the core network  301 , while wireless backhaul is used between the base station  303  and the repeater  305 .  
      Specifically, the base station  303  has a working frequency f 1 , and it works in master mode all the time and accesses its user stations, such as the user station  304 . The repeater  305  has master mode and salve mode. In master mode, its working frequency is f 2 , and the user station belonging to it, namely the user station  306 , achieves access by using this frequency. In slave mode, the repeater  305  switches to the working frequency f 1 , at which point it is a salve apparatus of the base station  303  to perform the backhaul operation.  
       FIG. 4  shows an exemplary backhaul operation process under single-hop single-repeater. In a frame, first the repeater  305  works in master mode, broadcasts synchronization and control information to its user stations such as the user station  306  in a downlink broadcast domain  403  and sends downlink services. According to the prior agreement with the base station  303 , when its downlink backhaul window is arrived, the repeater  305  switches to salve mode, i.e. switches from the frequency f 2  to the frequency f 1 , and receives in the downlink backhaul window the downlink broadcast domain map  405  from the base station  303  to correctly perform backhaul, where the downlink broadcast domain map  405  is a simplified copy of the downlink broadcast domain  401 . Then, the repeater  305  receives downlink backhaul services in the downlink backhaul window. These services are stored temporarily and will be forwarded to the corresponding user station later, for example, in the next frame thereof The downlink backhaul window ends at its downlink sub-frame. After the repeater  305  enters the uplink sub-frame, its uplink backhaul window is determined to have arrived. In its uplink backhaul window, the repeater  305  forwards uplink backhaul services to the base station  303  which forwards the uplink backhaul services to the core network  301 . When the repeater  305  determines that its uplink backhaul window has ended, it returns to master mode, i.e. switches from the frequency f 1  to the frequency f 2 . At this point it receives all sorts of services from its common user stations, such as the user station  306 . Through switching, the repeater  305  achieves access of the user station  306  belonging to it and implements wireless backhaul of services.  
      Obviously, the base station  303  and the repeater  305  do not interfere with each other. When they work in master mode, they are in different frequencies. That is, the base station  303  is in the frequency f 1 , while the repeater  305  is in the frequency f 2 . When the repeater  305  enters slave mode, it achieves wireless backhaul of services by using the frequency resources of the base station  303 , at which point it becomes a user station of the base station  303 .  
      Moreover, when the base station  303  determines that the downlink backhaul window of the repeater  305  has arrived, it adds the downlink broadcast domain map  405  to the start potion of the downlink backhaul window. The downlink broadcast domain map  405  is a simplified copy of the downlink broadcast domain  401 , with its preamble completely differing from that of the downlink broadcast domain  401  so as to avoid the fact that a common user station (such as the user station  304 ) cannot judge the actual start location of a frame. Additionally, other control information such as FCH, DL-MAP, UL-MAP, DCD and UCD is simplified, removing information for the user station  304  in the original downlink broadcast domain  401 , only reserving information needed by the repeater  305 , so as to save resources.  
      In this way, when the repeater  305  enters slave mode, it can acquire synchronization and relevant control information needed for the performance of backhaul operation.  
      Additionally, the base station  303  sends downlink backhaul services to the repeater  305  in this downlink backhaul window.  
      Furthermore, when the base station  303  determines that the uplink backhaul window of the repeater  305  has arrived, it receives uplink backhaul services from the repeater  305  in this uplink backhaul window.  
      By the way, during implementation and for the purpose of simplification, the size and location of the backhaul window are determined through negotiation between the repeater  305  and its father node such as the base station  303  when the repeater  305  is initiated, and afterwards, they do not need to be dynamically adjusted.  
       FIG. 5  shows a working flowchart of a repeater. This repeater is, for example, the repeater  305  as shown in  FIG. 3 .  
      First, the repeater  305  is initiated as a common user station(step S 501 ). Then, the repeater  305  achieves synchronization with its base station  303  by using information such as the preamble in the original downlink broadcast domain (step S 503 ), so that it joins the base station. Afterwards, the repeater  305  negotiates with the base station  303  to determine the location and size of its uplink and downlink backhaul windows (step S 505 ). In this way, both the repeater  305  and the base station  303  can enter wireless backhaul operational mode.  
      Next, the repeater  305  starts the first frame operation (step S 509 ). A frame operation comprises a downlink sub-frame operation and an uplink sub-frame operation. The repeater  305  first performs a downlink sub-frame operation. In the downlink sub-frame operation, the repeater  305  first enters master mode, works with the frequency f 2  and sends downlink information to the user station  306  belonging to it (step S 511 ).  
      Then, the repeater  305  determines whether or not the downlink backhaul window has started (step S 513 ). If the repeater  305  determines that the downlink backhaul window has not started, it waits for a while (step S 514 ) and returns to the determination step S 513 .  
      If the repeater  305  determines that the downlink backhaul window has started, it enters slave mode, switches from the frequency f 2  to the frequency f 1 , i.e. works with the frequency of the base station  303 , and searches the downlink broadcast domain map  405  to complete synchronization for the backhaul operation (step S 515 ).  
      Subsequently, the repeater  305  receives downlink backhaul services from the base station  303  and temporarily stores the received downlink backhaul services so as to forward them to the user station  306  of the repeater  305  in a downlink slot of master mode (step S 517 ).  
      After that, the repeater  305  determines whether or not the downlink backhaul window has ended (step S 519 ). If the repeater  305  determines that the downlink backhaul window has not ended, it waits for a while (step S 520 ) and returns to the determination step S 519 .  
      If the repeater  305  determines that the downlink backhaul window has ended, it determines whether or not the downlink sub-frame has ended (step S 521 ). If the downlink sub-frame has not ended, the repeater  305  waits for a while (step S 522 ) and returns to the determination step S 521 .  
      If the repeater  305  determines that the downlink sub-frame has ended, it performs the uplink sub-frame operation.  
      First, the repeater  305  determines whether or not the uplink backhaul window has started (step S 523 ). If the repeater  305  determines that the uplink backhaul window has not started, it waits for a while (step S 524 ) and returns to the determination step S 523 .  
      If the repeater  305  determines that the uplink backhaul window has started, it sends the temporarily stored uplink backhaul services from the user station  306  belonging to it (step S 525 ).  
      Then, the repeater  305  determines whether or not the uplink backhaul window has ended (step S 527 ). If the repeater  305  determines that the uplink backhaul window has not ended, it waits for a while (step S 528 ) and returns to the determination step S 527 .  
      If the repeater  305  determines that the uplink backhaul window has ended, it enters master mode, switches from the frequency f 1  to the frequency f 2 , i.e. re-works with its own frequency, receives uplink services from its user station  306 , and temporarily stores these services so as to backhaul these services to its base station  303  in a subsequent uplink backhaul operation.  
      Afterwards, the repeater  305  determines whether or not the uplink sub-frame has ended (step S 531 ). If the uplink sub-frame has not ended, the repeater  305  waits for a while (step S 532 ) and returns to the determination step S 53   1 .  
      If the repeater  305  determines that the uplink sub-frame has ended, it returns to the downlink sub-frame operation and starts a subsequent frame operation similar to steps S 511  to S 532 .  
      It should be noted that after the repeater  305  enters the downlink backhaul operation, even if the downlink backhaul window has ended, the repeater  305  does not re-switch to master mode but is in an idle state. Only when the uplink backhaul operation has completed, the repeater  305  re-switches to master mode to receive uplink services from its user station  306 . This simplifies the implementation complexity of the master/slave mode switching operation.  
      Of course, those skilled in the art should understand that the present invention is not limited to this. In other words, when the downlink backhaul window has ended, the repeater  305  may switch to master mode to access its user stations. When the uplink backhaul window has started, the repeater  305  switches from master mode to slave mode again so as to perform the backhaul of uplink services.  
      Additionally, the downlink backhaul window and the uplink backhaul window of the repeater  305  are located in the end portion of the downlink sub-frame of a frame and in the start portion of the uplink sub-frame of the frame, respectively. Of course, those skilled in the art should understand that the present invention is not limited to this. The repeater  305  may negotiate with its father node, namely the base station  303 , to arrange its uplink/downlink backhaul window.  
      Single-Hop Multi-Repeater  
       FIG. 6  shows an exemplary network structure under single-hop multi-repeater. As shown in  FIG. 6 , this exemplary network structure  600  comprises a core network  601 , a base station  603 , a user station  604  of the base station  603 , repeaters  605  and  607  of the base station  603 , a user station  606  of the repeater  605  and a user station  608  of the repeater  607 . The repeaters  605  and  607  are deployed at different locations of the edge of the base station  603  and have coverage  605   a  and coverage  607   a , respectively, thereby expanding the coverage  603   a  of the base station  603 . Wired backhaul is used between the base station  603  and the core network  601 , while wireless backhaul is used between the base station  603  and the repeaters  605 ,  607 .  
      Generally, the repeaters  605  and  607  use different frequencies to overcome interference between them.  
      Specifically, the base station  603  has a working frequency f 1 , and it works in master mode all the time and accesses its user stations, such as the user station  604 . The repeaters  605  and  607  each have master mode and salve mode. In master mode, the working frequency of the repeater  605  is f 2 , and its user station, namely the user station  606 , achieves access by using this frequency. In slave mode, the repeater  605  switches to the working frequency f 1 , at which point it is a salve apparatus of the base station  603 , to perform a backhaul operation. Additionally, in master mode, the working frequency of the repeater  607  is f 3 , and its user station, namely the user station  608 , achieves access by using this frequency. In slave mode, the repeater  607  switches to the working frequency f 1 , at which point it becomes a slave apparatus of the base station  603 , to perform a backhaul operation.  
       FIG. 7  shows an exemplary backhaul operation process under single-hop multi-repeater. In a frame, first the repeaters  605  and  607  work in master mode, broadcast synchronization and control information to their respective user stations such as the user stations  606  and  608  in the downlink broadcast domain  703  and  705 , respectively, and send downlink services.  
      According to the prior agreement with the base station  603 , when its downlink backhaul window is determined to have arrived, the repeater  607  switches to salve mode, i.e. switches from the frequency f 3  to the frequency f 1 , and receives in the downlink backhaul window the downlink broadcast domain map  707  from the base station  603  to correctly perform backhaul, where the downlink broadcast domain map  707  is a simplified copy of the downlink broadcast domain  701 . Then, the repeater  607  receives downlink backhaul services in the downlink backhaul window. These services are stored temporarily and will be forwarded to the corresponding user station later, for example, in the next frame thereof  
      After the downlink backhaul window of the repeater  607  has ended, according to the prior agreement with the base station  603 , the repeater  605  determines that its downlink backhaul window has arrived. Therefore, the repeater  605  switches to slave mode, i.e. switches from the frequency f 2  to the frequency f 1 , and receives in the downlink backhaul window the downlink broadcast domain map  709  from the base station  603  to correctly perform backhaul, where the downlink broadcast domain map  709  is a simplified copy of the downlink broadcast domain  701 . Then, the repeater  605  receives downlink backhaul services in the downlink backhaul window. These services are stored temporarily and will be forwarded to the corresponding user station later, for example, in the next frame thereof The downlink backhaul window ends at the downlink sub-frame.  
      In other words, after the downlink backhaul window of the repeater  607  has ended, the downlink backhaul window of the repeater  605  starts.  
      The downlink backhaul window of the repeater  605  ends at the downlink sub-frame. After the repeater  605  enters the uplink sub-frame, its uplink backhaul window is determined to have arrived. In its uplink backhaul window, the repeater  605  forwards uplink backhaul services to the base station  603  which forwards the uplink backhaul services to the core network  601 . When the repeater  605  determines that its uplink backhaul window has ended, it returns to master mode, i.e. switches from the frequency f 1  to the frequency f 2 . At this point it receives all sorts of services from its common user stations, such as the user station  606 .  
      Additionally, after the uplink backhaul window of the repeater  605  has ended, the repeater  607  determines that its uplink backhaul window has arrived. In other words, after the uplink backhaul window of the repeater  605  has ended, the uplink backhaul window of the repeater  607  starts. In its uplink backhaul window, the repeater  607  forwards uplink backhaul services to the base station  603  which forwards the uplink backhaul services to the core network  601 . After the uplink backhaul window of the repeater  607  has ended, the repeater  607  returns to master mode again, i.e. switches from the frequency f 1  to the frequency f 3 . At this point it receives all sorts of services from its common user stations, such as the user station  608 .  
      Through switching, the repeaters  605  and  607  achieve access of their respective user stations  606 ,  608  and implement wireless backhaul of services.  
      With respect to the base station  603 , when it determines that the downlink backhaul window of the repeater  607  has arrived, it adds a downlink broadcast domain map  707  to the start portion of the downlink backhaul window. The downlink broadcast domain map  707  is a simplified copy of the downlink broadcast domain  701 , with its preamble completely differing from that of the downlink broadcast domain  701  so as to avoid the fact that a common user station such as the user station  604  cannot judge the actual start location of a frame. Likewise, other control information such as FCH, DL-MAP, UL-MAP, DCD and UCD is simplified, removing information for the user station  604  in the original downlink broadcast domain  701 , only reserving information needed by the repeater  607 , so as to save resources.  
      In this way, when the repeater  607  enters slave mode, it can acquire synchronization and relevant control information needed for the performance of the backhaul operation.  
      Additionally, the base station  603  sends downlink backhaul services to the repeater  607  in this downlink backhaul window.  
      When the base station  603  determines that the backhaul window of the repeater  605  has arrived, it adds a downlink broadcast domain map  709  to the start portion of the downlink backhaul window. The downlink broadcast domain map  709  is a simplified copy of the downlink broadcast domain  701 , with its preamble completely differing from that of the downlink broadcast domain  701  so as to avoid the fact that a common user station such as the user station  604  cannot judge the actual start location of a frame. Likewise, other control information such as FCH, DL-MAP, UL-MAP, DCD and UCD is simplified, removing information for the user station  604  in the original downlink broadcast domain  701 , only reserving information needed by the repeater  605 , so as to save resources.  
      In this way, when the repeater  605  enters slave mode, it can acquire synchronization and relevant control information needed for the performance of the backhaul operation.  
      Additionally, the base station  603  sends downlink backhaul services to the repeater  605  in this downlink backhaul window.  
      When the base station  603  determines that the uplink backhaul window of the repeater  605  has arrived, it receives uplink backhaul services from the repeater  605  in this uplink backhaul window.  
      When the base station  603  determines that the uplink backhaul window of the repeater  607  has arrived, it receives uplink backhaul services from the repeater  607  in this uplink backhaul window.  
      It can be seen from the foregoing description that the downlink backhaul window of the repeater  605  is arranged in the end portion of the downlink sub-frame and the uplink backhaul window thereof is arranged in the start portion of the uplink sub-frame. The uplink/downlink backhaul windows of the repeater  607  are arranged on the two sides of the backhaul window of the repeater  605 . In the backhaul window of the repeater  605 , the repeater  607  can be used for local access or be in idle state. Being in idle state helps to simplify the implementation complexity of the switching operation. The downlink backhaul windows of the repeater  605  and the repeater  607  have their own downlink broadcast domain maps  709  and  707 . The preambles thereof may be the same, while the frame control headers differ from each other, correspond to their own control information, respectively. The same preambles will not create operation confusion between the repeater  605  and the repeater  607 . This is because that before each repeater joins the base station  603 , it will negotiate with the base station  603  to determine the location and size of the backhaul window, and only after the respective backhaul windows have arrived, the repeaters  605  and  607  switch to slave mode, search for the preambles and complete synchronization processes.  
      Furthermore, it should be noted that although the backhaul window in  FIG. 7  extends from the middle of the frame to both sides thereof, practical applications are not limited to this. A repeater may negotiate with its father node and flexibly arranges uplink/downlink backhaul windows provided each repeater can work normally.  
      Multi-Hop Single-Repeater  
       FIG. 8  shows an exemplary network structure under multi-hop single-repeater. As shown in  FIG. 8 , this exemplary network structure  800  comprises a core network  801 , a base station  803 , a user station  804  and a repeater  805  of the base station  803 , a user station  806  and a repeater  807  of the repeater  805 , a user station  808  and a repeater  809  of the repeater  807 , and a user station  810  of the repeater  809 . The repeater  805  is deployed at the edge of the base station  803  and has coverage  805   a , thereby expanding the coverage  803   a  of the base station  803 . The repeater  807  is deployed at the edge of the repeater  805  and has coverage  807   a , thereby expanding the coverage  805   a  of the repeater  805 . The repeater  809  is deployed at the edge of the repeater  807  and has coverage  809   a , thereby expanding the coverage  807   a  of the repeater  807 . Wired backhaul is used between the base station  803  and the core network  801 , while wireless backhaul is used between the base station  803  and the repeater  805 , between the repeater  805  and the repeater  807  and between the repeater  807  and the repeater  809 .  
      Generally, the base station  803 , the repeaters  805 ,  807  and  809  use different frequencies.  
      Specifically, the base station  803  has a working frequency f 1 , and it works in master mode all the time and accesses the user stations belonging to it, such as the user station  804 . The repeaters  805 ,  807  and  809  each have master mode and salve mode. In master mode, the working frequency of the repeater  805  is f 2 , and the user station belonging to it, namely the user station  806 , achieves access by using this frequency. In slave mode, the repeater  805  switches to the working frequency f 1 , at which point it is a salve apparatus of the base station  803  to perform backhaul operation. Additionally, in master mode, the working frequency of the repeater  807  is f 3 , and the user station belonging to it, namely the user station  808 , achieves access by using this frequency. In slave mode, the repeater  807  switches to the working frequency f 2 , at which point it becomes a slave apparatus of the repeater  805  to perform backhaul operation. In master mode, the working frequency of the repeater  809  is f 4 , and the user station belonging to it, namely the user station  810 , achieves access by using this frequency. In slave mode, the repeater  809  switches to the working frequency f 3 , at which point it becomes a salve apparatus of the repeater  807  to perform backhaul operation.  
      In other words, in the network structure as shown in  FIG. 8 , a plurality of repeaters work in a cascaded manner. In addition to access the user stations in its own coverage, a repeater is also responsible for relaying of backhaul services of other repeaters, at which point it plays the role of father node and the relayed node is child node. During the backhaul of services, child node utilizes the frequency resource of its father node, and while serving the user stations, child node has its own frequency resource.  
      By the way, when the distance between two nodes (the base station or the repeater) is far enough, the two nodes can use the same frequency in order to save frequency resources provided there is no interference between them. For instance, if the distance between the repeater  809  and the base station  803  is far enough, the repeater  809  can also use the frequency f 1  to access its user station  810 , so that frequency resources are saved.  
       FIG. 9  shows an exemplary backhaul operation process under multi-hop single-repeater. In a frame, first, the repeaters  805 ,  807  and  809  work in master mode, i.e. respectively work with the frequencies f 2 , f 3  and f 4 , broadcast synchronization and control information to their respective user stations such as the user stations  806 ,  808  and  810  in downlink broadcast domain  903 ,  905  and  907 , respectively, and send downlink services.  
      According to the prior agreement with the base station  803 , when the repeater  805  determines that its present-stage downlink backhaul window has arrived, it first switches to salve mode, i.e. switches from the frequency f 2  to the frequency f 1 , and receives in the present-stage downlink backhaul window the downlink broadcast domain map  909  from the base station  803  to correctly perform backhaul, where the downlink broadcast domain map  909  is a simplified copy of the downlink broadcast domain  901 . Then, the repeater  805  receives present-stage downlink backhaul services in the downlink backhaul window. These services are stored temporarily and will be forwarded to the corresponding user station later, for example, in the next frame thereof  
      After the downlink backhaul window of the repeater  805  has ended, the repeater  805  switches back to master mode, i.e. switches from the frequency f 1  to the frequency f 2 .  
      According to the prior agreement with the repeater  807 , when the repeater  805  determines that the downlink backhaul window of the lower-stage repeater  807  has arrived, a downlink broadcast domain map  911  is added to the start portion of the downlink backhaul window of the repeater  807 . The downlink broadcast domain map  911  is a simplified copy of the downlink broadcast domain  903 , with its preamble completely differing from that of the downlink broadcast domain  903  so as to avoid the fact that a common user station such as the user station  806  cannot judge the actual start location of a frame. Likewise, other control information such as FCH, DL-MAP, UL-MAP, DCD and UCD is simplified, removing information for the user station  806  in the original downlink broadcast domain  903 , only reserving information needed by the repeater  807 , so as to save resources.  
      According to the prior agreement with the repeater  805 , when the repeater  807  determines that its present-stage downlink backhaul window has arrived, it first switches to salve mode, i.e. switches from the frequency f 3  to the frequency f 2 , and receives in its present-stage downlink backhaul window the downlink broadcast domain map  911  from the repeater  805  to correctly perform backhaul, where the downlink broadcast domain map  911  is a simplified copy of the downlink broadcast domain  903 . Then, the repeater  807  receives downlink backhaul services in the present-stage downlink backhaul window. These services are stored temporarily and will be forwarded to the corresponding user station later, for example, in the next frame thereof After the downlink backhaul window of the repeater  807  has ended, the repeater  807  switches back to master mode, i.e. switches from the frequency f 2  to the frequency f 3 .  
      According to the prior agreement with the repeater  809 , when the repeater  807  determines that the downlink backhaul window of the lower-stage repeater  809  has arrived, a downlink broadcast domain map  913  is added to the start portion of the downlink backhaul window of the repeater  809 . The downlink broadcast domain map  913  is a simplified copy of the downlink broadcast domain  905 , with its preamble completely differing from that of the downlink broadcast domain  905  so as to avoid the fact that a common user station such as the user station  808  cannot judge the actual start location of a frame. Likewise, other control information such as FCH, DL-MAP, UL-MAP, DCD and UCD is simplified, removing information for the user station  808  in the original downlink broadcast domain  905 , only reserving information needed by the repeater  809 , so as to save resources.  
      According to the prior agreement with the repeater  807 , when the repeater  809  determines that its downlink backhaul window has arrived, it first switches to salve mode, i.e. switches from the frequency f 4  to the frequency f 3 , and receives in its downlink backhaul window the downlink broadcast domain map  913  from the repeater  807  to correctly perform backhaul, where the downlink broadcast domain map  913  is a simplified copy of the downlink broadcast domain  905 . Then, the repeater  809  receives downlink backhaul services in the downlink backhaul window. These services are stored temporarily and will be forwarded to the corresponding user station later, for example, in the next frame thereof.  
      The downlink backhaul window of the repeater  809  ends at the downlink sub-frame. After the repeater  809  enters the uplink sub-frame, its uplink backhaul window is determined to have arrived. In its uplink backhaul window, the repeater  809  forwards uplink backhaul services to the repeater  807 . When the repeater  809  determines that its uplink backhaul window has ended, it returns to master mode again, i.e. switches from the frequency f 3  to the frequency f 4 . At this point it can receive all sorts of services from its common user station, such as the user station  810 .  
      When the repeater  807  determines that the uplink backhaul window of the lower-stage repeater  809  has arrived, it receives uplink backhaul services from the repeater  809 .  
      Next, the repeater  807  determines that its present-stage uplink backhaul window has arrived. Thus, the repeater  807  switches to slave mode, i.e. switches from the frequency f 3  to the frequency f 2 . And, the repeater  807  forwards its uplink backhaul services to the repeater  805  with the frequency f 2  in its uplink backhaul window. When the repeater  807  determines that its uplink backhaul window has ended, it returns to master mode again, i.e. switches from the frequency f 2  to the frequency f 3 . At this point it can receive all sorts of services from its common user station, such as the user station  808 .  
      When the repeater  805  determines that the uplink backhaul window of the lower-stage repeater  807  has arrived, it receives the uplink backhaul services from the repeater  807 .  
      Next, the repeater  805  determines that its present-stage uplink backhaul window has arrived. Thus, the repeater  805  switches to slave mode, i.e. switches from the frequency f 2  to the frequency f 1 . And, the repeater  805  forwards its uplink backhaul services to the base station  803  with the frequency f 1  in its uplink backhaul window. When the repeater  805  determines that its uplink backhaul window has ended, it returns to master mode again, i.e. switches from the frequency f 1  to the frequency f 2 . At this point it can receive all sorts of services from its common user station, such as the user station  806 .  
      When the base station  803  determines that the uplink backhaul window of the lower-stage repeater  805  has arrived, it receives uplink backhaul services from the repeater  805 . Then, the base station  803  forwards these uplink backhaul services to the core network  801 .  
      With respect to the base station  803 , when it determines that the downlink backhaul window of the repeater  805  has arrived, it adds the downlink broadcast domain map  909  to the start portion of the downlink backhaul window. The downlink broadcast domain map  909  is a simplified copy of the downlink broadcast domain  901 , with its preamble completely differing from that of the downlink broadcast domain  901  so as to avoid the fact that a common user station such as the user station  804  cannot judge the actual start location of a frame. Likewise, other control information such as FCH, DL-MAP, UL-MAP, DCD and UCD is simplified, removing information for the user station  804  in the original downlink broadcast domain  901 , only reserving information needed by the repeater  805 , so as to save resources.  
      In this way, when the repeater  805  enters slave mode, i.e. switches from the frequency f 2  to the frequency f 1 , it can acquire synchronization and relevant control information needed for the performance of backhaul operation.  
      And, the base station  803  sends downlink backhaul services to the repeater  805  in this downlink backhaul window.  
      In the network under multi-hop single-repeater as shown in  FIG. 9 , typically a repeater that is closer to the core network  801  has a larger backhaul window. In this situation, in frames of some nodes (such as the base station  803  and the repeater  805  in  FIG. 9 ), the uplink/downlink backhaul windows of their child node is not adjacent to each other, and instead, there is a relatively large time interval between them. Such a time interval can be used for access of the local user stations so as to improve the utilization ratio of resources.  
      Additionally, it would be best if the base station  603  and all the repeaters  805 ,  807  and  809  could work synchronously so as to reduce radio interference, improve the utilization ratio of frequency resources and enable mobile user station to perform effective handing over.  
      Repeater  
       FIG. 10  shows an exemplary block diagram of a repeater. As shown in  FIG. 10 , the repeater  1000  comprises a transceiver means  1010 , a negotiation means  1020 , a storage means  1030  and a scheduling means  1040 . The transceiver means  1010  comprises a switching means  1012 .  
      The negotiation means  1020  is synchronized with the father node of the repeater  1000  by using a preamble, and negotiates with this father node to determine the location and size of a backhaul window for use in the backhaul of a present-stage service by using a pre-appointed message, and notifies the scheduling means  1040  of the negotiation result.  
      Further, the negotiation means  1020  is synchronized with a child node of the repeater  1000  by using another preamble, and negotiates with this child node to determine the location and size of a backhaul window for use in the backhaul of a lower-stage service by using a pre-appointed message, and notifies the scheduling means  1040  of the negotiation result.  
      When the scheduling means  1040  determines that the present-stage backhaul window for use in the backhaul of a present-stage service has started, it notifies the switching means  1012  to switch from a first frequency to a second frequency so as to complete the backhaul of the present-stage service.  
      Then, the transceiver means  1010  receives the present-stage backhaul service and temporarily stores it to the storage means  1030 .  
      When the scheduling means  1040  determines that the aforesaid present-stage backhaul window has ended, it notifies the switching means  1012  to switch from the second frequency back to the first frequency so as to access a present-stage service.  
      The aforesaid present-stage backhaul window may be a downlink backhaul window, and the present-stage service is a downlink service.  
      The aforesaid present-stage backhaul window may be an uplink backhaul window, and the present-stage service is an uplink service.  
      Prior to the backhaul of a present-stage service, the transceiver means  1010  may further receive information needed for the correct backhaul of a present-stage service. The information needed for the correct backhaul of a present-stage service comprises at least one of following: a preamble, a frame control header, a downlink map, an uplink map, downlink channel description and uplink channel description.  
      Moreover, the preamble comprised in the information needed for the correct backhaul of a present-stage service is not the same as the preamble of the father node of the repeater  1000 .  
      In particular, when the aforesaid present-stage backhaul window is a downlink backhaul window, the present-stage service is a downlink service, and the scheduling means  1040  determines that the present-stage downlink backhaul window has ended and that a present-stage uplink backhaul window has started, it notifies the transceiver means  1010  to perform the backhaul of a present-stage uplink service. And when the scheduling means  1040  determines that the present-stage uplink backhaul window has ended, it notifies the switching means  1012  to switches from the second frequency back to the first frequency so as to access a present-stage service.  
      And, when the scheduling means  1040  determines that a lower-stage backhaul window for use in the backhaul of a lower-stage service has started, it notifies the transceiver means  1010  to send information needed for the correct backhaul of the lower-stage service with the first frequency.  
      The information needed for the correct backhaul of a lower-stage service comprises at least one of following: a preamble, a frame control header, a downlink map, an uplink map, downlink channel description and uplink channel description.  
      Moreover, the preamble comprised in the information needed for the correct backhaul of a lower-stage service is not the same as the preamble of its own.  
      Then, the transceiver means  1010  sends a lower-stage backhaul service.  
      In particular, when the aforesaid lower-stage backhaul window is a downlink window, the lower-stage service is a downlink service, and the scheduling means  1040  determines that the lower-stage downlink backhaul window has ended and that a lower-stage uplink backhaul window has started, it notifies the transceiver means  1010  to receive a lower-stage uplink backhaul service. When the scheduling means  1040  determines that a present-stage uplink backhaul window has started, it notifies the switching means  1012  to switches from the first frequency back to the second frequency so as to complete the backhaul of the present-stage uplink service. Then, the transceiver means  1010  performs the backhaul of the present-stage uplink service. When the scheduling means  1040  determines that the present-stage uplink backhaul window has ended, it notifies the switching means  1012  to switch from the second frequency back to the first frequency so as to access a present-stage service.  
      Base Station  
       FIG. 11  shows an exemplary block diagram of a base station. As shown in  FIG. 11 , the base station  1100  comprises a transceiver means  1110 , a negotiation means  1120  and a scheduling means  1140 .  
      The negotiation means  1120  is synchronized with the repeater  1000  by using a preamble, and negotiates with the repeater to determine the location and size of a backhaul window for use in the backhaul of a service by using a pre-appointed message, and notifies the scheduling means  1140  of the negotiation result.  
      When the scheduling means  1140  determines that the backhaul window for use in the backhaul of a service of the repeater  1000  has started, it notifies the transceiver means  1110  to send information needed for the correct backhaul of the service.  
      The information needed for the correct backhaul of the service comprises a preamble, a frame control header, a downlink map, an uplink map, downlink channel description and uplink channel description.  
      Moreover, the preamble comprised in the information needed for the correct backhaul of the service is not the same as the above preamble which the negotiation means  1120  uses to negotiate with the repeater  1000  so as to determine the location and size of a backhaul window for use in the backhaul of a service.  
      Then, the transceiver means  1110  sends a backhaul service.  
      In particular, when the backhaul window is a downlink backhaul window, the service is a downlink service, and the scheduling means  1140  determines that said downlink backhaul window has ended and that an uplink backhaul window has started, it notifies the transceiver means  1110  to receive an uplink backhaul service.  
      The exemplary embodiments of the present invention have been described with reference to the accompanying drawings. As seen from the foregoing description, each repeater has its own independent frame whose length is the same as the length of the base station. Therefore, the present invention is suitable for network applications with high density and heavy traffic.  
      Additionally, in the present invention, wireless backhaul links are used between a repeater and a base station and between a repeater and another repeater. Therefore, deployment cost is low and operational cost is also low, and rapid deployment of a network can be achieved accordingly.  
      Moreover, the backhaul window can be flexibly arranged in the present invention. Therefore, a relatively small backhaul delay can be achieved, which helps the backhaul of delay-sensitive services.  
      The present invention further has strong scalability.  
      As many different embodiments of the present invention can be made without departing from the spirit and scope thereof, it should be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.