Abstract:
An apparatus and method for configuring a subframe to support a relay service in a multi-hop relay BWA communication system are provided. The apparatus includes at least one of a BS-MS link subframe, a primary RS-MS link subframe, and a BS-secondary RS link subframe are configured in a first period of the subframe, and at least one of a BS-primary RS link subframe, an RS-RS link subframe, and a secondary RS-MS link subframe is configured in a second period of the subframe.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. §119(a) to Korean Patent Applications filed in the Korean Intellectual Property Office on Apr. 19, 2006 and assigned Ser. No. 2006-35239; filed in the Korean Intellectual Property Office on Nov. 13, 2006 and assigned Ser. No. 2006-111903; and filed in the Korean Intellectual Property Office on Nov. 14, 2006 and assigned Ser. No. 2006-112350, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a multi-hop relay Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for providing synchronization channels to Mobile Stations (MSs) and Relay Stations (RSs) and eliminating near-far interference between a direct service and a relay service in a multi-hop relay BWA communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     One of the most critical requirements for deployment of a 4 th  Generation (4G) communication system is to build a self-configurable wireless network. The self-configurable wireless network refers to a wireless network configured in an autonomous or distributed manner without control of a central system to provide mobile communication services. For the 4G communication system, cells of very small radiuses are defined for the purpose of enabling high-speed communications and accommodating a larger number of calls. Hence, a conventional centralized wireless network design is not viable. Rather, the wireless network should be built to be under distributed control and to actively cope with an environmental change like addition of new Base Stations (BSs). As a result, the 4G communication system requires the self-configurable wireless network.  
         [0006]     For real deployment of the self-configurable wireless network, techniques used for an ad hoc network should be introduced to a wireless access communication system. Such a major example is a multi-hop relay BWA communication system configured by applying a multi-hop relay scheme used for the ad hoc network to a BWA network with fixed BSs.  
         [0007]     In general, since a BS and an MS communicate with each other via a direct link, a highly reliable radio link can be established easily between them in the BWA communication system. However, due to the BSs being fixed, the configuration of a wireless network is not flexible, making it difficult to provide an efficient service in a radio environment experiencing a fluctuating traffic distribution and a great change in the number of required calls.  
         [0008]     The above drawback can be overcome by a relay service that delivers data over multiple hops via a plurality of neighbor MSs or neighbor RSs. The use of the multi-hop relay scheme facilitates fast network reconfiguration adaptive to an environmental change and renders the overall wireless network operation efficient. Also, a radio channel in a better channel status can be provided to an MS by installing an RS between the BS and the MS and thus establishing a multi-hop relay path via the RS. In this way, high-speed data channels can be provided to MSs in a shadowing area or an area where communications with the BS are unavailable. Cell coverage can also be expanded.  
         [0009]      FIG. 1  illustrates service provisioning in a typical multi-hop relay BWA communication system.  
         [0010]     In  FIG. 1 , in the multi-hop relay BWA communication system, MSs  140  to  170  (MS 1  to MS 4 ) can receive the BWA services through a BS  100 , a primary RS (RS 1 )  110 , and secondary RSs (RS 2 )  120  and  130 .  
         [0011]     MS 1  and MS 2  within the service area  101  of the BS  100  communicate with the BS  100  via direct links L 1 . MS 2 , which is located at the cell boundary of the BS  100  and thus placed in a poor channel state, can receive a higher-speed data channel via an RS-MS link L 2  between MS 2  and RS 2   130  than via the direct link L 1 .  
         [0012]     MS 3  and MS 4  outside the service area  101  of the BS  100  communicate with the BS  100  via RS-MS links L 3  provided by RSI  110 . The communication links between the BS  100  and MS 3  and MS 4  via RS 1   110  expand the cell coverage. MS 4 , which is located at the cell boundary of RS 1   110  and thus placed in a poor channel state, can increase its transmission capacity using an RS-MS link L 4  between MS 4  and RS 2   120 .  
         [0013]     As described above, when an MS is in a poor channel state at a cell boundary of a BS or in a shadowing area suffering from a severe shielding effect due to, for example, buildings, the BWA communication system enables the MS to communicate with the BS by providing a better-quality radio channel to the MS via an RS. In other words, the BS can provide high-speed data channels to the cell boundary and the shadowing area and expand its coverage area by the multi-hop relay scheme. The RSs  110 ,  120  and  130  are classified into RS 1  (RS  110 ) that expands cell coverage and RS 2  (the RSs  120  and  130 ) that increases capacity according to their relay capabilities.  
         [0014]     Typically, transmission/reception is carried out between a BS and an MS in frames having the configuration illustrated in  FIG. 2  in the BWA communication system.  FIG. 2  illustrates a Time Division Duplex (TDD) frame structure compliant with Institute of Electrical and Electronics Engineers (IEEE) 802.16, for data transmission/reception between the BS and the MS.  
         [0015]     In  FIG. 2 , a TDD frame  200  is divided into a DownLink (DL) subframe  210  and an UpLink (UL) subframe  220  with a guard region called Transmit/receive Transition Gap (TTG) in between. A guard region called Receive/transmit Transition Gap (RTG) is interposed between TDD frames.  
         [0016]     The DL subframe  210  includes a preamble and a common control channel in mandatory slots. The MSs within the service area of the BS acquire synchronization and control information from the preamble and the common control channel.  
         [0017]     As described above, the BWA communication system provides services to the MSs or RSs outside the cell coverage of the BS or in a shadowing area by use of the RSs. In order to ensure backward compatibility for the MSs, communications are conducted in frames configured as illustrated in  FIG. 2 . That is, an RS operates in the same manner as an MS during initial access and negotiates a relay operation with the BS so that BS can provide a relay service to MSs in frames having the configuration of  FIG. 2 . Because the RS provides the relay service using the same frame configuration as the BS, it has difficulty in concurrently communicating with the BS and the MSs over one frequency band in one frame. To avert a Radio Frequency (RF) isolation problem caused by the frame configuration illustrated in  FIG. 2 , the frames are configured as illustrated in  FIG. 3  so that transmission to and reception from the RS occur in parallel in time.  
         [0018]      FIG. 3  illustrates a TDD frame structure in a conventional multi-hop relay BWA communication system.  
         [0019]     In  FIG. 3 , a DL subframe  300  is divided into a first area  301  and a second area  303 , and a UL subframe  310  is divided into a first area  311  and a second area  313 . For the RS operation transitions, the first areas  301  and  311  are distinguished from the second areas  311  and  313  in time division. The lengths of the first areas  301  and  311  and the lengths of the second areas  311  and  313  are fixed or adaptively adjusted according to a cell environment.  
         [0020]     The BWA communication system provides a direct link service in the first areas  301  and  311  and a relay link service in the second areas  303  and  313 . Hence, the BS provides a synchronization channel, a control channel, and a traffic channel to an MS connected to it by a direct link in the first areas  301  and  311  and a synchronization channel, a control channel, and a traffic channel to an RS in the second areas  303  and  313 .  
         [0021]     Since the RS may move as illustrated in  FIG. 4 , the BWA communication system should consider the mobility of the RS.  
         [0022]      FIG. 4  illustrates movement of the RS in the conventional multi-hop relay BWA communication system.  
         [0023]     In  FIG. 4 , being located in a vehicle such as a bus or a train, RS 1   420  has mobility. Hence, the BWA communication system should provide a synchronization channel to RS 1   420  for synchronization and cell search, taking into account its mobility.  
         [0024]     In the case of the frame configuration illustrated in  FIG. 3 , the lengths of the first and second areas  301  and  303  of the DL subframe  300  may vary depending on a cell environment. The resulting change in the position of the synchronization channel at the start of the second area  303  imposes overhead because RS 1   420  should locate the synchronization channels of the neighbor BSs. Increased interference between neighbor cells due to the power boost of synchronization channels, transmission of information about the neighbor BSs, and search for the synchronization channel of each neighbor BS add to the RS overhead.  
         [0025]     Without providing the synchronization channels, RS 2 &#39;s  120  and  130 , as illustrated in  FIG. 1 , provide the relay service in conjunction with the BS by multiple communications in the cell. In this case, an MS experiences near-far interference because of the power difference between a signal received from BS  100  or the RS 1   110  and a signal received from the RS 2   120  or  130 , as illustrated in  FIG. 5 .  
         [0026]      FIG. 5  illustrates a signal flow for a relay service from an RS in the conventional multi-hop relay BWA communication system.  
         [0027]     In  FIG. 5 , within the cell area of a BS  500 , a first MS  530  (MS 1 ) in a good channel status receives a service from the BS  500  via a direct link and a second MS  520  (MS 2 ) in a poor channel status receives the service via RS 2   510 .  
         [0028]     Although BS  500  and RS 2   510  perform multiple communications using orthogonal resources in the same time area, a BS link signal is overlaid with an RS link signal in the air. Thus, MS 1  may undergo near-far interference as it receives a stronger interference signal from the nearby RS 2   510  than a signal from BS  510 . The near-far interference may also occur to the uplink as RS 2   510  receives a stronger interference signal from MS 1  than a signal from MS 2 .  
       SUMMARY OF THE INVENTION  
       [0029]     An aspect of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, one aspect of the present invention is to provide an apparatus and method for efficiently supporting cell search and synchronization according to the mobility of an RS in a multi-hop relay BWA communication system.  
         [0030]     Another aspect of the present invention is to provide an apparatus and method for efficiently supporting cell search and synchronization according to the mobility of an RS by providing synchronization channels in a multi-hop relay BWA communication system.  
         [0031]     A further aspect of the present invention is to provide an apparatus and method for reducing near-far interference caused by multiple communications within a cell in a multi-hop relay BWA communication system.  
         [0032]     Still another aspect of the present invention is to provide an apparatus and method for eliminating near-far interference between relay communications and direct communications in a cell by time-multiplexing the relay communications and the direct communications in a multi-hop relay BWA communication system.  
         [0033]     Yet another aspect of the present invention is to provide a method for configuring a frame so as to provide synchronization channels and eliminate near-far interference and an apparatus supporting the same in a multi-hop relay BWA communication system.  
         [0034]     According to an aspect of the present invention, there is provided a method for configuring a subframe in order to support a relay service in a multi-hop relay (BWA) communication system. The method includes configuring at least one of a BS-MS link subframe, a primary RS-MS link subframe, and a BS-secondary RS link subframe is configured in a first period of the subframe, the BS-MS link subframe being a subframe for a link between a BS and an MS, the primary RS-MS link subframe being a subframe for a link between a primary RS that provides a synchronization channel; and configuring an MS, and the BS-secondary RS link subframe being a subframe for a link between the BS and a secondary RS that does not provide a synchronization channel, and at least one of a BS-primary RS link subframe, an RS-RS link subframe, and a secondary RS-MS link subframe is configured in a second period of the subframe, the BS-primary RS link subframe being a subframe for a link between the BS and the primary RS, an RS-RS link subframe being a subframe for a link between an RS and another RS, and a secondary RS-MS link subframe being a subframe for a link between the secondary RS and an MS.  
         [0035]     According to another aspect of the present invention, there is provided a method for configuring a downlink subframe in order to support a relay service in a multi-hop relay (BWA) communication system. The method includes configuring a BS-MS link subframe and an RS-MS link subframe are provided in a first period of the downlink subframe, the BS-MS link subframe being a subframe for a link between a BS and an MS and the RS-MS link subframe being a subframe for a link between an RS and an MS; configuring a 1 st  group RS-2 nd  group next-hop RS link subframe is configured in a second period of the downlink subframe, the 1 st  group RS-2 nd  group next-hop RS link subframe being a subframe for a link between an RS of a first group including odd-hop RSs and a next-hop RS of a second group including even-hop RSs; and configuring a BS-1-hop RS link subframe and a 2 nd  group RS-1 st  group next-hop RS link subframe are configured in a third period of the downlink subframe, the BS-1-hop RS link subframe being a subframe for a link between the BS and a 1-hop RS and the 2 nd  group RS-1 st  group next-hop RS link subframe being a subframe for a link between an RS of the second group and a next-hop RS of the first group.  
         [0036]     According to another aspect of the present invention, there is provided a method for configuring a downlink subframe in order to support a relay service in a multi-hop relay BWA communication system. The method includes configuring a BS-MS link subframe and an RS-MS link subframe are configured in a first period of the downlink subframe, the BS-MS link subframe being a subframe for a link between a BS and an MS and the RS-MS link subframe being a subframe for a link between an RS and an MS; configuring a BS-1-hop RS link subframe and a 2 nd  group RS-1 st  group next-hop RS link subframe are configured in a second period of the downlink subframe, the BS-1-hop RS link subframe being a subframe for a link between the BS and a 1-hop RS and the 2 nd  group RS-1 st  group next-hop RS link subframe being a subframe for a link between an RS of a second group including even-hop RSs and a next-hop RS of a first group including odd-hop RSs and configuring a 1 st  group RS-2 nd  group next-hop RS link subframe is configured in a third period of the downlink subframe, the 1 st  group RS-2 nd  group next-hop RS link subframe being a subframe for a link between an RS of the first group and a next-hop RS of the second group.  
         [0037]     According to still another aspect of the present invention, there is provided a method for configuring an uplink subframe in a multi-hop relay BWA communication system. The method includes configuring an MS-BS link subframe and an MS-RS link subframe are configured in a first period of the uplink subframe, the MS-BS link subframe being a subframe for a link between an MS and a BS and the MS-RS link subframe being a subframe for a link between an MS and an RS; configuring a 2 nd  group RS-1 st  group previous-hop RS link subframe is configured in a second period of the uplink subframe, the 2 nd  group RS-1 st  group previous-hop RS link subframe being a subframe for a link between an RS of a second group including even-hop RSs and a previous-hop RS of a first group including odd-hop RSs; and configuring a 1-hop RS-BS link subframe and a 1 st  group RS-2 nd  group previous-hop RS link subframe is configured in a third period of the uplink subframe, the 1-hop RS-BS link subframe being a subframe for a link between a 1-hop RS and the BS and the 1 st  group RS-2 nd  group previous-hop RS link subframe being a subframe for a link between an RS of the first group and a previous-hop RS of the second group.  
         [0038]     According to yet another aspect of the present invention, there is provided a method for configuring an uplink subframe in a multi-hop relay BWA communication system. The method includes configuring an MS-BS link subframe and an MS-RS link subframe are configured in a first period of the uplink subframe, the MS-BS link subframe being a subframe for a link between an MS and a BS and the MS-RS link subframe being a subframe for a link between an MS and an RS: configuring a 1-hop RS-BS link subframe and a 1 st  group RS-2 nd  group previous-hop RS link subframe is configured in a second period of the uplink subframe, the 1-hop RS-BS link subframe being a subframe for a link between a 1-hop RS and the BS and the 1 st  group RS-2 nd  group previous-hop RS link subframe being a subframe for a link between an RS of a first group including odd-hop RSs and a previous-hop RS of a second group including even-hop RSs; and configuring a 2 nd  group RS-1 st  group previous-hop RS link subframe is configured in a third period of the uplink subframe, the 2 nd  group RS-1 st  group previous-hop RS link subframe being a subframe for a link between an RS of the second group and a previous-hop RS of the first group.  
         [0039]     According to yet another aspect of the present invention, there is provided a method of a BS in a multi-hop relay BWA communication system. The method includes the BS allocating resources to a first period and a second period of a subframe, the first period being for communicating with at least one of an MS and a secondary RS that does not provide a synchronization channel and the second period being for communicating with a primary RS that provides a synchronization channel; communicating with the at least one of the MS and the secondary RS in the first period of the subframe; and communicating with the primary RS in the second period of the subframe.  
         [0040]     According to still another aspect of the present invention, there is provided a method of an RS that provides a synchronization channel in a multi-hop relay BWA communication system. The method includes the RS setting a first period and a second period according to control information received from an upper node, the first period being for communicating with an MS and the second period being for communicating with at least one of the upper node and a lower RS; communicating with the MS in the first period; and communicating with the at least one of the upper node and the lower RS in the second period.  
         [0041]     According to still another aspect of the present invention, there is provided a method of an RS that does not provide a synchronization channel in a multi-hop relay BWA communication system. The method includes the RS setting a first period and a second period according to control information received from an upper node, the first period being for communicating with the upper node and the second period being for communicating with an MS; communicating with the upper node in the first period; and communicating with the MS in the second period.  
         [0042]     According to still yet another aspect of the present invention, there is provided an apparatus of a BS in a multi-hop relay BWA communication system. The apparatus includes a timing controller for providing a timing signal for transmission and reception according to a subframe configuration in which a first period and a second period are defined, the first period being for communicating with at least one of an MS and a secondary RS that doest not provide a synchronization channel and the second period being for communicating with a primary RS that provides a synchronization channel; a transmitter for generating one of a first period signal and a second period signal according to the timing signal and transmits the generated signal; and a receiver for receiving one of the first period signal and the second period signal according to the timing signal and recovers the received signal.  
         [0043]     According to still yet another aspect of the present invention, there is provided an apparatus of an RS that provides a synchronization channel in a multi-hop relay BWA communication system. The apparatus includes a timing controller for providing a timing signal for transmission and reception according to a subframe configuration in which a first period and a second period are defined, the first period being for communicating with at least one of an MS and a secondary RS that doest not provide a synchronization channel and the second period being for communicating with an upper node; a transmitter for generating one of a first period signal and a second period signal according to the timing signal and transmits the generated signal; and a receiver for receiving one of the first period signal and the second period signal according to the timing signal and recovers the received signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0045]      FIG. 1  illustrates a signal flow for a relay service in a typical multi-hop relay BWA communication system;  
         [0046]      FIG. 2  illustrates a frame structure in a typical IEEE 802.16 system;  
         [0047]      FIG. 3  illustrates a frame structure in a conventional multi-hop relay BWA communication system;  
         [0048]      FIG. 4  illustrates an RS movement in the conventional multi-hop relay BWA communication system;  
         [0049]      FIG. 5  illustrates a signal flow for a relay service from an RS in the conventional multi-hop relay BWA communication system;  
         [0050]      FIG. 6  illustrates a frame structure with synchronization channels in a multi-hop relay BWA communication system according to the present invention;  
         [0051]      FIG. 7  illustrates a frame structure in the multi-hop relay BWA communication system according to the present invention;  
         [0052]      FIG. 8  is a diagram illustrating the transmission and reception timings of signals in accordance with the frame structure illustrated in  FIG. 7 ;  
         [0053]      FIG. 9  illustrates a frame structure in the multi-hop relay BWA communication system according to the present invention;  
         [0054]      FIG. 10  illustrates a configuration of the multi-hop relay BWA communication system according to the present invention;  
         [0055]      FIG. 11  illustrates a DL subframe structure in the multi-hop relay BWA communication system according to the present invention;  
         [0056]      FIG. 12  illustrates the positions of synchronization channels in the DL subframe illustrated in  FIG. 11  in the multi-hop relay BWA communication system according to the present invention;  
         [0057]      FIG. 13  illustrates a DL subframe structure in the multi-hop relay BWA communication system according to the present invention;  
         [0058]      FIG. 14  illustrates the positions of synchronization channels in the DL subframe illustrated in  FIG. 13  in the multi-hop relay BWA communication system according to the present invention;  
         [0059]      FIG. 15  illustrates a UL subframe structure in the multi-hop relay BWA communication system according to the present invention;  
         [0060]      FIG. 16  illustrates a UL subframe structure in the multi-hop relay BWA communication system according to the present invention;  
         [0061]      FIG. 17  is a flow diagram illustrating a process of a BS in the multi-hop relay BWA communication system according to the present invention;  
         [0062]      FIG. 18  is a flow diagram illustrating a process of a RS 1  in the multi-hop relay BWA communication system according to the present invention;  
         [0063]      FIG. 19  is a flow diagram illustrating a process of a RS 2  in the multi-hop relay BWA communication system according to the present invention;  
         [0064]      FIG. 20  is a flow diagram illustrating a process of an MS in the multi-hop relay BWA communication system according to the present invention; and  
         [0065]      FIG. 21  is a block diagram of the BS in the multi-hop relay BWA communication system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0066]     Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0067]     The present invention provides a technique for providing a synchronization channel to support the mobility of an RS and eliminating near-far interference caused by multiple communications within a cell in a multi-hop relay BWA communication system. The following description will be made in the context of a Time Division Duplex (TDD)-Orthogonal Frequency Division Multiple Access (OFDM) wireless communication system, while the present invention is also applicable to a communication system using any other multiple access scheme or any other division duplex scheme.  
         [0068]     The term “primary RS” or “RS 1 ” is defined as an RS that expands cell coverage and the term “secondary RS” or “RS 2 ” is defined as an RS that increases capacity. Therefore, the RS 1  provides a synchronization channel, a control channel, and traffic channels to the MSs or the RSs outside the cell area of a BS, whereas the RS 2  provides unicast control and traffic channels to the MSs in a poor channel status though located in the cell area of the BS.  
         [0069]     While it is described herein that a subframe is configured in compliance with the IEEE 802.16 standard for communications between a BS and an RS, it is obviously to be understood that an advanced technology with novel functionalities and usages is also applicable. The same holds true for communications with an upper RS (or superordinated RS) and a lower RS.  
         [0070]      FIG. 6  illustrates a frame structure that provides synchronization channels to the MSs and the RSs in a multi-hop relay BWA communication system according to the present invention.  
         [0071]     In  FIG. 6 , a frame is composed of a DL subframe  610  and a UL subframe  620 . The DL subframe  610  includes time-multiplexed first and second areas  611  and  613 , and the UL subframe  620  includes time-multiplexed first and second areas  621  and  623 . The lengths of the first areas  611  and  621  and the lengths of the second areas  621  and  623  may be fixed or vary depending on a cell environment.  
         [0072]     A BS communicates with an MS connected to it via a direct link in the first areas  611  and  621 , and communicates with an RS in the second areas  613  and  623 . As the lengths of the first areas  611  and  621  and the second areas  613  and  623  may vary dynamically according to the cell environment as mentioned above, the BS allocates synchronization channels at the start of the first area  611  and at the end of the second area  613  so that the MS and the RS can acquire synchronization. The BS also allocates ranging channels at the start of the first area  621  and the end of the second area  623 , for ranging from MSs. The positions of the ranging channels (or ranging slots) in the UL subframe  620  may be indicated by a control channel rather than they are fixed.  
         [0073]     To facilitate synchronization and cell search, the BS provides the MS with a synchronization channel (referred to as BS synchronization channel) in the form of a preamble and the RS with a synchronization channel (referred to as RS synchronization channel) in the form of a postamble. As the synchronization channels reside at the start and end of the DL subframe  610 , the MS and the RS can acquire synchronization information and neighbor BS information from the fixed synchronization channels. The RS synchronization channel can further be used for interference measurement.  
         [0074]     Multiple communications take place among the BS, RS 1 , RS 2  and MSs in the multi-hop relay BWA communication system.  
         [0075]     Since the BS and RS 2  provide a service to the MSs within the service area of the BS, they use orthogonal resources to avoid interference between two links, i.e. a BS-MS link and an RS 2 -MS link. When a plurality of RS 2 &#39;s exist in the cell, the orthogonal resources allocated to the RS 2  can be reused among them by spatial multiplexing.  
         [0076]     Therefore, the BS-MS link signal is distinguished from the RS 2 -MS link signal in the frequency domain, but they overlap each other in the time domain. When the BS and RS 2  communicate with the MSs at the same time, near-far interference occurs.  
         [0077]     In this context, the same time resources are not allocated to the BS-MS link and the RS 2 -MS link as illustrated in  FIG. 7 . Specifically, the BWA communication system allocates predetermined resources of the second areas  613  and  623  to the RS 2 -MS link.  
         [0078]      FIG. 7  illustrates a frame structure in the multi-hop relay BWA communication system according to the present invention. According to the frame structure, a frame is so configured that resources are allocated to the RS 1  and the RS 2  in frequency division.  
         [0079]     In  FIG. 7 , each frame is composed of a DL subframe  720  and a UL subframe  730 . The DL subframe  720  includes a time-multiplexed first and second areas  721  and  723 , and the UL subframe  730  includes time-multiplexed first and second areas  731  and  733 . The lengths of the first areas  721  and  731  and the lengths of the second areas  723  and  733  are fixed or dynamically vary depending on a cell environment.  
         [0080]     In an RS 2  frame  710 , the RS 2  communicates with the BS in the first areas  721  and  731  and communicates with an MS in predetermined parts  711  and  713  of the second areas  723  and  733 . If a plurality of RS 2 &#39;s exist, the second areas  723  and  733  are reused among them by spatial division multiplexing. The RS 1  provides a transparent relay service to an MS in the first areas  721  and  731  and communicates with the BS in the second areas  723  and  733 .  
         [0081]     When a frame is configured by the spatial multiplexing and the time multiplexing as described above, the transmission and reception of the BS, RS 1 , RS 2 , and the MSs are in the relationship illustrated in  FIG. 8 .  
         [0082]      FIG. 8  is a diagram illustrating the transmission and reception timings of signals in accordance with the frame structure illustrated in  FIG. 7 .  
         [0083]     In  FIG. 8 , in the DL subframe  720 , a BS  800  sends a synchronization channel, a control channel, and a traffic burst to an RS 2   820  or an MS  830  connected to the BS  800  via a direct link in the first area  721  and then sends a control channel, a traffic burst, and a synchronization channel to an RS 1   810  in the second area  723 . The BS  800  provides the MS  830  and the RSs  810  and  820  with a BS synchronization channel and an RS synchronization channel at the start of the first area  721  and at the end of the second area  723 , respectively.  
         [0084]     The RS 1   810  sends a synchronization channel, a control channel, and a traffic burst to the RS 2   820  or the MS  830  connected to the RS 1   810  via a relay link in the first area  721 . Then, the RS 1   810  receives the synchronization channel, the control channel, and the traffic burst from the BS  800  in the second area  723 .  
         [0085]     The RS 2   820  receives the control channel and the traffic burst needed for the relay service from the BS  800  in the first area  721 . Then, the RS 2   820  sends the traffic burst to the MS  830  connected to the RS 2  via a relay link in the predetermined part  711  of the second area  723 .  
         [0086]     The MS  830  receives the synchronization channel, the control channel, and the traffic burst from the BS  800  or the RS 1   810  in the first area  721 . Then, the MS  830  receives a signal from the RS 1   810  or the RS 2   820  in the second area  723 . Particularly, the MS  830  receives the traffic burst from RS 2   820  in the part of the second area  723 .  
         [0087]     To avoid near-far interference, the second areas  723  and  733  are allocated to a RS 2 -MS link and a BS-RS 1  link by frequency division. In another embodiment of the present invention, the second areas  723  and  733  are allocated to the RS 2 -MS link and the BS-RS 1  link by time division.  
         [0088]      FIG. 9  illustrates a frame structure in the multi-hop relay BWA communication system according to the present invention. A BS frame  700  and an RS 2  frame  710  are shown.  
         [0089]     In  FIG. 9 , each of the frames  700  and  710  is composed of the DL subframe  720  and the UL subframe  730 . The DL subframe  720  includes the time-multiplexed first and second areas  721  and  723 , and the UL subframe  730  includes the time-multiplexed first and second areas  731  and  733 . The second areas  723  and  733  are divided into the RS 2 -MS areas  900  and  920  and BS-RS 1  areas  910  and  930 . The BS provides the RS 1  with an RS synchronization channel taking the form of a postamble at a fixed position. Therefore, the RS 2 -MS areas  900  and  920  precede the BS-RS 1  areas  910  and  930  in the second areas  723  and  733 .  
         [0090]     The BS communicates with the RS 2  or an MS in the first areas  721  and  731  and communicates with RS 1  in the second areas  723  and  733 . Notably, the BS leaves the RS 2 -MS areas  900  and  920  empty in the second areas  723  and  733  to avoid intra-cell interference. Accordingly, the BS communicates with RS 1  in the BS-RS 1  areas  910  and  930 .  
         [0091]     For RS 1 , the BS provides the RS synchronization channel and a ranging channel at the ends of the BS-RS 1  areas  910  and  930 .  
         [0092]     In the RS frame  710 , RS 2  communicates with the BS in the first areas  721  and  731  and then communicates with an MS in the RS 2 -MS areas  900  and  920  of the second areas  723  and  733 . Notably, the RS 2  does not use the BS-RS 1  areas  910  and  930  to avoid the intra-cell interference.  
         [0093]     The above description has been made in the context of a two-hop multi-hop relay BWA communication system. The BWA communication system may be configured so that an MS communicates with a BS over multiple hops, as illustrated in  FIG. 10 .  
         [0094]      FIG. 10  illustrates a configuration of the multi-hop relay BWA communication system according to the present invention.  
         [0095]     In  FIG. 10 , a BS  1001  communicates with an MS  1019  via relay links established by a plurality of RSs  1011 ,  1013 ,  1015 , and  1017 .  
         [0096]     The RSs  1011 ,  1013 ,  1015 , and  1017  can be grouped into a 1 st  group and a 2 nd  group. For example, if the BS  1001  is set as a 0-hop RS, the 1 st  group is an even-hop group including the BS  1001 , a 2-hop RS  1013 , a 4-hop RS, and other even-hop RSs and the 2 nd  group is an odd-hop group including a 1-hop RS  1011 , a 3-hop RS  1015 , and other odd-hop RSs.  
         [0097]     If the BS  1001  is not classified as the 0-hop RS, the 1 st  group is an odd-hop group including the 1-hop RS  1011 , the 3-hop RS  1015 , and other odd-hop RSs and the 2 nd  group is an even-hop group including the 2-hop RS  1013 , the 4-hop RS, and the other even-hop RSs.  
         [0098]     When the multi-hop links are grouped into the first and second groups in this way, communications are carried out in frames having the configurations illustrated in FIGS.  11  to  16  in the BWA communication system. The following description is made on the assumption that the 1 st  group is the odd-hop group and the second area illustrated in  FIG. 6  is further divided into second and third areas.  
         [0099]     A DL subframe has the configuration illustrated in  FIG. 11  or  FIG. 13  in the BWA communication system.  
         [0100]      FIG. 11  illustrates a DL subframe structure in the multi-hop relay BWA communication system according to the present invention.  
         [0101]     Referring to  FIG. 11 , each DL subframe  1100  includes a time-multiplexed first, second, and third areas  1101 ,  1103 , and  1105 , respectively.  
         [0102]     In a BS subframe  1110 , the BS sends the downlink subframes to an MS within its service area in the first and second areas  1101  and  1103 . To avoid interference between the MS and an RS, null data can be filled in the second area  1103 , instead of the downlink subframe. The BS provides the MS with a preamble as a synchronization channel at the start of the first area  1101 . The BS sends a downlink subframe to a 1-hop RS of the 1 st  group in the third area  1105 . The BS provides the 1-hop RS with a postamble as a synchronization channel at the end of the third area  1105 .  
         [0103]     In a 1 st  group RS subframe  1120 , a 1 st  group RS sends a downlink subframe to an MS within its service area in the first area  1101 . The 1 st  group RS provides the MS with a synchronization channel in the form of a preamble at the start of the first area  1101 . The 1 st  group RS sends a downlink subframe to a next-hop RS of the 2 nd  group in the second area  1103 . The 1 st  group RS provides the next-hop RS with a synchronization channel in the form of a postamble at the end of the second area  1103 . The 1 st  group RS receives a downlink subframe from a previous-hop RS of the 2 nd  group in the third area  1105 . If the 1 st  group RS is the 1-hop RS, the 1 st  group RS receives the downlink subframe from the BS in the third area  1105 . A TTG is interposed between the second area  1103  and the third area  1105 , for an operation transition of the 1 st  group RS. Hence, the 1 st  group RS sends the synchronization channel to the 2 nd  group RS using resources of the second area  1103  before the TTG  
         [0104]     In a 2 nd  group RS subframe  1130 , a 2 nd  group RS sends a downlink subframe to an MS within its service area in the first area  1101 . The 2 nd  group RS provides the MS with a synchronization channel in the form of a preamble at the start of the first area  1101 . The 2 nd  group RS receives a downlink subframe from a previous-hop RS of the 1 st  group in the second area  1103 . The 2 nd  group RS sends a downlink subframe to a next-hop RS of the 1 st  group in the third area  1105 . The 2 nd  group RS provides the next-hop RS with a synchronization channel in the form of a postamble at the end of the third area  1105 . For operation transitions of the 2 nd  group RS, a TTG is interposed between the first area  1101  and the second area and an RTG intervenes between the second area  1103  and the third area  1105 .  
         [0105]     While not shown, if the 1 st  group RS is a last-hop RS, the 1 st  group RS sends the downlink subframes to the MSs within its service area in the first and second areas  1101  and  1103 . To avoid interference between the MSs and the RS, the second area  1103  may have null data. Then, the last-hop RS receives a downlink subframe from a previous-hop RS of the 2 nd  group in the third area  1105 .  
         [0106]     If the last-hop RS is a 2 nd  group RS, the 1 st  group RS sends the downlink subframes to the MSs within its service area in the first and third areas  1101  and  1105 . To avoid interference between the MSs and the RS, the third area  1105  may have null data. The last-hop RS receives a downlink subframe from a previous-hop RS of the 1 st  group in the second area  1103 .  
         [0107]     In accordance with the DL frame structure illustrated in  FIG. 11 , the subframes of the first, second, and third areas of the DL subframe can be configured in compliance with IEEE 802.16 standards, as illustrated in  FIG. 12 .  
         [0108]      FIG. 12  illustrates the positions of synchronization channels in the DL subframe illustrated in  FIG. 11  in the multi-hop relay BWA communication system according to the present invention.  
         [0109]     In  FIG. 12 , a DL subframe  1200  includes time-multiplexed first, second, and third areas  1201 ,  1203 , and  1205 , respectively.  
         [0110]     A BS subframe  1210  carries a synchronization channel, a control channel, and a DL burst to an MS within the service area of the BS. The BS positions the synchronization channel for the MS in the form of a preamble at the start of the first area  1201 . If the BS uses the second area  1203 , the BS frame  1210  includes a DL burst in the second area  1203 . The third area  1205  has a control channel, a DL burst, and a synchronization channel for the 1-hop RS. Thus, the BS provides the 1-hop RS with the synchronization channel in the form of a postamble at the end of the third area  1205 .  
         [0111]     In a 1 st  group RS subframe  1220 , the first area  1201  carries a synchronization channel, a control channel, and a DL burst to an MS within the service area of the 1 st  group RS. The 1 st  group RS provides the MS with the synchronization channel in the form of a preamble at the start of the first area  1201 . The second area  1203  has a control channel, a DL burst, and a synchronization channel for the next-hop RS of the 2 nd  group. Thus, the 1 st  group RS provides the next-hop RS with the synchronization channel in the form of a postamble at the end of the second area  1203 . The 1 st  group RS receives a downlink subframe from a previous-hop RS or the BS in the third area  1205 .  
         [0112]     In a 2 nd  group RS frame  1230 , the first area  1201  includes a synchronization channel, a control channel, and a DL burst for an MS within the service area of the 2 nd  group RS. That is, the 2 nd  group RS provides the synchronization channel for the MS in the form of a preamble at the start of the first area  1201 . The third area  1205  carries a synchronization channel, a control channel, and a DL burst for a next-hop RS of the 1 st  group. That is, the 2 nd  group RS provides the next-hop 1 st  group RS with a synchronization channel in the form of a postamble at the end of the third area  1205 . The 2 nd  group RS receive a downlink subframe from a previous-hop RS in the second area  1203 .  
         [0113]      FIG. 13  illustrates a DL subframe structure in the multi-hop relay BWA communication system according to the present invention.  
         [0114]     In  FIG. 13 , each DL subframe  1300  includes a time-multiplexed first, second, and third areas  1301 ,  1303 , and  1305 .  
         [0115]     In a BS subframe  1310 , the BS sends the downlink subframes to the MSs within its service area in the first and third areas  1301  and  1305 . To avoid interference between the MSs and an RS, null data can be filled in the third area  1305 , instead of the downlink subframe. The BS provides a preamble as a synchronization channel at the start of the first area  1301 , for the MSs. The BS sends a downlink subframe to a 1-hop RS of the 1 st  group in the second area  1303 . The BS provides the 1-hop RS with a postamble as a synchronization channel at the end of the second area  1303 . Since an RTG exists between the second area  1303  and the third area  1305  in a 1 st  group RS subframe  1320 , the BS sends the synchronization channel to the 1-hop RS before the RTG in the second area  1303 .  
         [0116]     In the 1 st  group RS subframe  1320 , a 1 st  group RS sends a downlink subframe to an MS within its service area in the first area  1301 . The 1 st  group RS provides the MS with a synchronization channel in the form of a preamble at the start of the first area  1301 . The 1 st  group RS receives a downlink subframe from a previous-hop RS of the 2 nd  group in the second area  1303 . If the 1 st  group RS is the 1-hop RS, the 1 st  group RS receives the downlink subframe from the BS in the second area  1303 . The 1 st  group RS sends a downlink subframe to a next-hop RS of the 2 nd  group in the third area  1305 . The 1 st  group RS provides the next-hop RS with a synchronization channel in the form of a postamble at the end of the third area  1305 . For operation transitions of the 1 st  group RS, a TTG is interposed between the first area  1301  and the second area  1303  and an RTG exists between the second area  1303  and the third area  1305  in the 1 st  group RS subframe  1320 .  
         [0117]     In a 2 nd  group RS subframe  1330 , a 2 nd  group RS sends a downlink subframe to an MS within its service area in the first area  1301 . The 2 nd  group RS provides the MS with a synchronization channel in the form of a preamble at the start of the first area  1301 . The 2 nd  group RS sends a downlink subframe to a next-hop RS of the 1 st  group in the second area  1303 . The 2 nd  group RS provides the next-hop RS with a synchronization channel in the form of a postamble at the end of the second area  1303 . The 2 nd  group RS receives a downlink subframe from a previous-hop RS of the 1 st  group in the third area  1305 .  
         [0118]     While not shown, if a last-hop RS belongs to the 1 st  group, the last-hop RS sends the downlink subframes to the MSs within its service area in the first and third areas  1301  and  1305 . To avoid interference between the MSs and the RS, the third area  1305  may have null data. The last-hop RS receives a downlink subframe from a previous-hop RS of the 2 nd  group in the second area  1303 .  
         [0119]     If the last-hop RS is a 2 nd  group RS, the last-hop RS sends the downlink subframes to the MSs within its service area in the first and second areas  1301  and  1303 . To avoid interference between the MSs and the RS, the second area  1303  may have null data. The last-hop RS receives a downlink subframe from a previous-hop RS of the 1 st  group in the third area  1305 .  
         [0120]     In accordance with the DL frame structure illustrated in  FIG. 13 , the subframes of the first, second and third areas in the DL subframe can configured in compliance with IEEE 802.16 standards, as illustrated in  FIG. 14 .  
         [0121]      FIG. 14  illustrates the positions of synchronization channels in the DL subframe illustrated in  FIG. 13  in the multi-hop relay BWA communication system according to the present invention  
         [0122]     In  FIG. 14 , a DL subframe  1400  includes a time-multiplexed first, second, and third areas  1401 ,  1403 , and  1405 .  
         [0123]     A BS subframe  1410  carries a synchronization channel, a control channel, and a DL burst to an MS within the service area of the BS. That is, the BS positions the synchronization channel for the MS in the form of a preamble at the start of the first area  1401 . If the BS uses the third area  1405 , the BS subframe  1410  includes a downlink burst in the third area  1405 . The second area  1403  of the BS subframe  1410  has a control channel, a DL burst, and a synchronization channel for the 1-hop RS. Thus, the BS provides the 1-hop RS with the synchronization channel in the form of a postamble at the end of the second area  1403 .  
         [0124]     In a 1 st  group RS subframe  1420 , the first area  1401  carries a synchronization channel, a control channel, and a DL burst to an MS within the service area of the 1 st  group RS. The 1 st  group RS provides the MS with the synchronization channel in the form of a preamble at the start of the first area  1401 . The third area  1405  has a control channel, a DL burst, and a synchronization channel for the next-hop RS of the 2 nd  group. Thus, the 1 st  group RS provides the next-hop RS with the synchronization channel in the form of a postamble at the end of the third area  1405 . The 1 st  group RS receives a downlink subframe from a previous-hop RS or the BS in the second area  1403 .  
         [0125]     In a 2 nd  group RS subframe  1430 , the first area  1401  includes a synchronization channel, a control channel, and a DL burst for an MS within the service area of the 2 nd  group RS. That is, the 2 nd  group RS provides the synchronization channel to the MS in the form of a preamble at the start of the first area  1401 . The second area  1403  carries a synchronization channel, a control channel, and a DL burst for a next-hop RS of the 1 st  group. The 2 nd  group RS provides the synchronization channel to the next-hop 1 st  group RS in the form of a postamble at the end of the second area  1403 . The 2 nd  group RS receive a downlink subframe from a previous-hop RS in the third area  1405 .  
         [0126]     The BWA communication system configures a UL subframe as illustrated in  FIG. 15  or  FIG. 16 .  
         [0127]      FIG. 15  illustrates a UL subframe structure in the multi-hop relay BWA communication system according to the present invention.  
         [0128]     In  FIG. 15 , each UL subframe  1500  includes a time-multiplexed first, second, and third areas  1501 ,  1503  and  1505 .  
         [0129]     In a BS subframe  1510 , the BS receives the uplink subframes from the MSs within its service area in the first and second areas  1501  and  1503 . To avoid interference between the MSs and an RS, null data can be filled in the second area  1503 , instead of the uplink subframe. The BS receives an uplink subframe from a 1-hop RS of the 1 st  group in the third area  1505 .  
         [0130]     In 1 st  group RS subframes  1520  and  1540 , a 1 st  group RS receives an uplink subframe from an MS within its service area in the first area  1501 . The 1 st  group RS receives an uplink subframe from a next-hop RS of the 2 nd  group in the second area  1503 . For example, a 1-hop RS of the first group receives an uplink subframe from a 2-hop RS of the second group. A 3-hop RS of the first group receives an uplink subframe from a 4-hop RS of the second group. The 1 st  group RS sends an uplink subframe to a previous-hop RS of the second group in the third area  1505 . If the 1 st  group RS is the 1-hop RS, the 1 st  group RS sends an uplink subframe to the BS in the third area  1505 . For an operation transition of the 1 st  group RS, an RTG is interposed between the second area  1503  and the third area  1505 .  
         [0131]     In a 2 nd  group RS frame  1530 , a 2 nd  group RS receives an uplink subframe from an MS within its service area in the first area  1501 . The 2 nd  group RS sends an uplink subframe to a previous-hop RS of the 1 st  group in the second area  1503 . The 2 nd  group RS receives an uplink subframe from a next-hop RS of the 1 st  group in the third area  1505 . For operation transitions of the 2 nd  group RS, an RTG exists between the first area  1501  and the second area  1503  and a TTG is interposed between the second area  1503  and the third area  1505 .  
         [0132]     While not shown, if a last-hop RS belongs to the first group, the last-hop RS receives the uplink subframes from the MSs within its service area in the first and second areas  1501  and  1503 . To avoid interference between the MSs and the RS, the second area  1503  may have null data. The last-hop RS sends an uplink subframe to a previous-hop RS of the 2 nd  group in the third area  1505 .  
         [0133]     If the last-hop RS is a 2 nd  group RS, it receives uplink subframes from the MSs within its service area in the first and third areas  1501  and  1505 . To avoid interference between the MSs and the RS, the third area  1505  may have null data. The last-hop RS sends an uplink subframe to a previous-hop RS of the 1 st  group in the second area  1503 .  
         [0134]      FIG. 16  illustrates a UL subframe structure in the multi-hop relay BWA communication system according to the present invention.  
         [0135]     In  FIG. 16 , each UL subframe  1600  includes a time-multiplexed first, second, and third areas  1601 ,  1603 , and  1605 , respectively.  
         [0136]     In a BS subframe  1610 , the BS receives the uplink subframes from the MSs within its service area in the first and third areas  1601  and  1605 . To avoid interference between an MS and an RS, null data can be filled in the third area  1605 , instead of the UL subframe. The BS receives an uplink subframe from a 1-hop RS of the 1 st  group in the second area  1603 .  
         [0137]     In 1 st  group RS subframes  1620  and  1640 , a 1 st  group RS receives an uplink subframe from an MS within its service area in the first area  1601 . The 1 st  group RS sends an uplink subframe to a previous-hop RS of the second group in the second area  1603 . If the 1 st  group RS is a 1-hop RS, the 1 st  group RS sends an uplink subframe to the BS in the second area  1603 . In the third area  1605 , the 1 st  group RS receives an uplink subframe from a next-hop RS of the 2 nd  group. For operation transitions of the 1 st  group RS, an RTG is interposed between the first area  1601  and the second area  1603  and a TTG exists between the second area  1603  and the third area  1605 .  
         [0138]     In a 2 nd  group RS frame  1630 , a 2 nd  group RS receives an uplink subframe from an MS within its service area in the first area  1601 . The 2 nd  group RS receives an uplink subframe from a next-hop RS of the 1 st  group in the second area  1603  and sends an uplink subframe to a previous-hop RS of the 1 st  group in the third area  1605 .  
         [0139]     While not shown, if a last-hop RS belongs to the first group, the last-hop RS receives the uplink subframes from the MSs within its service area in the first and third areas  1601  and  1605 . To avoid interference between the MSs and the RS, the third area  1605  may have null data. The last-hop RS sends an uplink subframe to a previous-hop RS of the 2 nd  group in the second area  1603 .  
         [0140]     If the last-hop RS is a 2 nd  group RS, the last-hop RS receives the uplink subframes from the MSs within its service area in the first and second areas  1601  and  1603 . To avoid interference between the MSs and the RS, the second area  1603  may have null data. The last-hop RS sends an uplink subframe to a previous-hop RS of the 1 st  group in the third area  1605 .  
         [0141]     The BWA communication system may configure a frame by combining the DL subframe illustrated in  FIG. 11  and the UL subframe illustrated in  FIG. 15  or  FIG. 16 . On the other hand, the BWA communication system may configure a frame by combining the DL subframe illustrated in  FIG. 13  and the UL subframe illustrated in  FIG. 15  or  FIG. 16 .  
         [0142]     As described above, the BS sends a synchronization channel to the MSs and the 1-hop RSs according to a frame configuration in the BWA communication system. The 1 st  group RSs send synchronization channels to the MSs and the 2 nd  group RSs according to the frame configuration. The 2 nd  group RSs provide synchronization channels to the MSs and the next-hop RSs of the first group according to the frame configuration.  
         [0143]     The BS, the 1 st  group RSs, and the 2 nd  group RSs send the synchronization channels in every frame or in every predetermined number of frames. Alternatively, they may include the synchronization channels in frames indicated by a control signal. The control signal contains a Frame Control Header (FCH), a MAP, and a Downlink Channel Descriptor (DCD).  
         [0144]     Now a description will be made of operations of the BS, RS 1 , RS 2 , and the MS to communicate using the frame configurations described above in the BWA communication system.  
         [0145]      FIG. 17  is a flow diagram illustrating a process of the BS in the multi-hop relay BWA communication system according to the present invention.  
         [0146]     In  FIG. 17 , the BS defines a direct-link area and a relay-link area in each of the DL and UL subframes in step  1701 . For example, if the BWA communication system spans two hops, a first area for the direct link and a second area for the relay link are defined in each of the DL and UL subframes. If the BWA communication system spans three hops, a first area for the direct link and second and third areas for the relay link are defined in each of the DL and UL subframes.  
         [0147]     In step  1703 , the BS communicates with an MS and RS 2  within its service area in the direct-link area. The BS provides a BS synchronization channel to the MS at the start of the direct-link area. For example, if a frame has the configuration illustrated in  FIG. 6 ,  FIG. 7 , or  FIG. 9 , the BS communicates with the MS in the first areas.  
         [0148]     If the DL subframe and the UL subframe have the configurations illustrated in  FIG. 11  and  FIG. 15 , respectively, the BS communicates with the MS in the first areas or the second areas. If the DL subframe and the UL subframe have the configurations illustrated in  FIG. 13  and  FIG. 16 , respectively, the BS communicates with the MS in the first areas or the third areas.  
         [0149]     The BS communicates with a 1-hop RS in the relay-link area in step  1705 . For the 1-hop RS, the BS provides an RS synchronization channel at the end of the relay-link area.  
         [0150]     For example, if the frame has the configuration illustrated in  FIG. 6 ,  FIG. 7  or  FIG. 9 , the BS communicates with the 1-hop RS in the second areas. Thus, the BS provides the synchronization channel to the 1-hop RS at the end of the second area of the DL subframe.  
         [0151]     If the DL subframe and the UL subframe have the configurations illustrated in  FIG. 11  and  FIG. 15 , respectively, the BS communicates with the 1-hop RS in the third areas. Thus, the BS provides the synchronization channel to the 1-hop RS at the end of the third area of the DL subframe.  
         [0152]     If the DL subframe and the UL subframe have the configurations illustrated in  FIG. 13  and  FIG. 16 , respectively, the BS communicates with the 1-hop RS in the second areas. Thus, the BS provides the synchronization channel to the 1-hop RS at the end of the second area of the DL subframe.  
         [0153]     Then, the BS ends the process.  
         [0154]      FIG. 18  is a flow diagram illustrating a process of the RS 1  in the multi-hop relay BWA communication system according to the present invention.  
         [0155]     In  FIG. 18 , the RS 1  checks the subframe configuration information, i.e. configuration information about direct-link areas and relay-link areas in the DL and UL subframes received from the BS or an upper RS in step  1801 . If a frame has the configuration illustrated in  FIG. 6 ,  FIG. 7 , or  FIG. 9 , the RS 1  checks the configuration information concerning the first and second areas.  
         [0156]     If a DL subframe has the configuration illustrated in  FIG. 11  or  FIG. 13  and a UL subframe has the configuration illustrated in  FIG. 15  or  FIG. 16 , the RS 1  checks configuration information about the first, second, and third areas.  
         [0157]     In step  1803 , the RS 1  communicates with an MS or a RS 2  within its service area in the first areas for the direct link. For the MS, the RS 1  provides a synchronization channel at the start of the first area in the DL subframe.  
         [0158]     The RS 1  communicates with the BS or multi-hop RSs in the relay-link areas in step  1805 . For the lower RSs, RS 1  provide a synchronization channel at the end of an area for communicating with the lower RSs in the DL subframe. If the frame has the configuration illustrated in  FIG. 6 ,  FIG. 7 , or  FIG. 9 , RS 1  communicates with the BS in the second areas.  
         [0159]     If the DL subframe and the UL subframe of the frame have the configurations illustrated in  FIG. 11  and  FIG. 15 , respectively, the RS 1  communicates with the lower RSs in the second areas and communicates with the BS or the upper RSs in the third areas. The RS 1  provides the synchronization channel to the lower RSs at the end of the second area in the DL subframe.  
         [0160]     If the DL subframe and the UL subframe of the frame have the configurations illustrated in  FIG. 13  and  FIG. 16 , respectively, the RS 1  communicates with the lower RSs in the third areas and communicates with the BS or the upper RSs in the second areas. The RS 1  provides the synchronization channel for the lower RSs at the end of the third area in the DL subframe.  
         [0161]     Then the RS 1  ends the process.  
         [0162]      FIG. 19  is a flow diagram illustrating an operation of the RS 2  in the multi-hop relay BWA communication system according to the present invention.  
         [0163]     In  FIG. 19 , the RS 2  checks the control information and the subframe configuration information formed according to its relay capability, received from the BS in step  1901 . For example, if a frame is configured as illustrated in  FIG. 7  or  FIG. 9 , the RS 2  checks the information tconcerning the first and second areas and the RS 2 -MS link areas of the second areas.  
         [0164]     In step  1903 , the RS 2  communicates with the BS in the first area. The RS 2  then communicates with an MS that receives a relay service via the RS 2  in the second area in step  1905 .  
         [0165]     For example, the RS 2  receives a signal from the BS in the first area and sends a signal to the MS in the second area of a DL subframe. In a UL subframe, the RS 2  sends a signal to the BS in the first area and receives a signal from the MS in the second area.  
         [0166]     Then the RS 2  ends the process.  
         [0167]      FIG. 20  is a flow diagram illustrating an operation of the MS in the multi-hop relay BWA communication system according to the present invention.  
         [0168]     In  FIG. 20 , the MS communicates with the BS or the RS 1  in the first area in step  2001 .  
         [0169]     In step  2003 , the MS communicates with RS 2  in the second area. For example, in a DL subframe, the MS receives a signal from the BS or RS 1  in the first area and receives a signal from the RS 2  in the second area. In a UL subframe, the MS sends a signal to the BS or RS 1  in the first area and a signal to the RS 2  in the second area.  
         [0170]     Then the MS ends the process.  
         [0171]     It has been described above that each single-directional subframe is divided into the first and second areas or the first, second, and third areas which are time-division-multiplexed in a TDD system. In another exemplary embodiment of the present invention, the single-directional subframe is divided into first and second areas or first, second, and third areas which are frequency-division-multiplexed in a Frequency Division Duplex (FDD) system. In the FDD system, the DL subframe and the UL subframe are sent/received simultaneously in different frequency bands.  
         [0172]     A description will now be made of the structures of the BS and an RS for providing a relay service in the BWA communication system. Because the BS and the RS have the same configuration, their structures will be described, taking a BS configuration illustrated in  FIG. 21 . The following description is made with the appreciation that signal transmission and reception are carried out using a single transceiver in the BS and the RS.  
         [0173]      FIG. 21  is a block diagram of the BS in the multi-hop relay BWA communication system according to the present invention.  
         [0174]     In  FIG. 21 , the BS includes a transmitter  2101 , a receiver  2103 , a timing controller  2105 , and an RF switch  2107 .  
         [0175]     The transmitter  2101  has a frame generator  2109 , a resource mapper  2111 , a modulator  2113 , and a Digital-to-Analog Converter (DAC)  2115 .  
         [0176]     In operation, the frame generator  2109  configures a DL subframe to send synchronization channels, control channels, and traffic bursts to an MS and a lower RS within the service area of the BS under the timing controller  2105 . Notably, the frame generator  2109  provides a synchronization channel for the MS at the start of a subframe for the MS and a synchronization channel for the lower RS at the end of a subframe for the lower RS in the DL subframe.  
         [0177]     If the BWA communication system spans two hops, the frame generator  2109  configures a subframe to be sent to the MS or the RS 2  in a first area of the DL subframe. Then the frame generator  2109  configures a subframe to be sent to a 1-hop RS in a second area of the DL subframe. The frame generator  2109  positions the synchronization channels at the start of the subframe in the first area and at the end of the subframe in the second area.  
         [0178]     If the BWA communication system spans three or more hops, the frame generator  2109  configures a subframe to be sent to the MS or RS 2  in a first area or first and second areas of the DL subframe. Then, the frame generator  2109  configures a subframe to be sent to the 1-hop RS in a third area of the DL subframe. The frame generator  2109  positions the synchronization channels at the start of the subframe in the first area and at the end of the subframe in the third area.  
         [0179]     The resource mapper  2111  maps the subframes received from the frame generator  2109  to bursts for links corresponding to the subframes.  
         [0180]     The modulator  2113  modulates the mapped subframes in a predetermined modulation scheme.  
         [0181]     The DAC  2115  converts the modulated digital signal to an analog signal and provides the analog signal to the RF switch  2107 .  
         [0182]     The receiver  2103  includes an Analog-to-Digital Converter (ADC)  2117 , a demodulator  2119 , a resource demapper  2121 , and a frame extractor  2123 .  
         [0183]     The ADC  2117  converts an analog signal received through the RF switch  2107  to a digital signal. The demodulator  2119  demodulates the digital signal in a predetermined demodulation scheme.  
         [0184]     The resource demapper  2121  extracts subframes from link bursts received from the demodulator. The frame extractor  2123  extracts a subframe destined for the BS from the subframes.  
         [0185]     The RF switch  2107  switches signals to be sent to or received from the MS, RS 1 , and RS 2  to the transmitter  2101  and the receiver  2103  under the control of the timing controller  2105 .  
         [0186]     The timing controller  2105  controls transmission and reception timings at which the BS communicates with the MS and the lower RS according to a frame configuration.  
         [0187]     The configuration of the RS 1  will be described with reference to  FIG. 21 .  
         [0188]     In  FIG. 21 , the RS 1  includes the transmitter  2101 , the receiver  2103 , the timing controller  2105 , and the RF switch  2107 .  
         [0189]     The transmitter  2101  has the frame generator  2109 , the resource mapper  2111 , the modulator  2113 , and the DAC  2115 .  
         [0190]     In operation, the frame generator  2109  configures a DL subframe to send synchronization channels, control channels, and traffic bursts to an MS and a lower RS within the service area of the RS 1  under the timing controller  2105 . Notably, the frame generator  2109  provides a synchronization channel for the MS at the start of a subframe for the MS and a synchronization channel for the lower RS at the end of a subframe for the lower RS in the DL subframe.  
         [0191]     The frame generator  2109  also generates a UL subframe in which to communicate with the BS or an upper RS.  
         [0192]     The resource mapper  2111  maps the subframes received from the frame generator  2109  to bursts for links corresponding to the subframes.  
         [0193]     The modulator  2113  modulates the mapped subframes in a predetermined modulation scheme.  
         [0194]     The DAC  2115  converts the modulated digital signal to an analog signal and provides the analog signal to the RF switch  2107 .  
         [0195]     The receiver  2103  includes the ADC  2117 , the demodulator  2119 , the resource demapper  2121 , and the frame extractor  2123 .  
         [0196]     The ADC  2117  converts an analog signal received through the RF switch  2107  to a digital signal. The demodulator  2119  demodulates the digital signal in a predetermined demodulation scheme.  
         [0197]     The resource demapper  2121  extracts subframes from link bursts received from the demodulator. The frame extractor  2123  extracts a subframe destined for the BS from the subframes.  
         [0198]     The RF switch  2107  switches signals to be sent to or received from the BS, the MS, the lower RS, and the upper RS to the transmitter  2101  and the receiver  2103  under the control of the timing controller  2105 .  
         [0199]     The timing controller  2105  controls transmission and reception timings at which the RS 1  communicates with the BS, the MS, the lower RS, and the upper RS according to a frame configuration.  
         [0200]     As described above, the multi-hop relay BWA communication system provides synchronization channels to the MSs and the RSs. Therefore, the RSs facilitate synchronization and cell search. Also, time multiplexing between a relay service and a direct service within a cell eliminates near-far interference between them.  
         [0201]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.