Abstract:
A method and system for allocating resources in a communication system including a first system having a use right for a plurality of frequency bands, and a second system having no use right for the plurality of frequency bands. The method includes acquiring first, second and third sensing informations indicating frequency bands available by a base station, a relay station and a mobile station belonging to the second system, among the plurality of frequency bands; and selecting a frequency band to be allocated for communications, based on the acquired sensing informations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application claims the benefit under 35 U.S.C. §119(a) of a Korean Patent Application filed in the Korean Intellectual Property Office on Mar. 14, 2007 and assigned Serial No. 2007-24923, the disclosure of which is incorporated herein by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to a communication system, and in particular, to a communication system for selecting a frequency band and allocating resources in a cognitive radio (CR) communication system, and a method for supporting the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the next generation communication systems, intensive research is being conducted to provide users with services based on various Qualities of Service (QoS). 
         [0004]    The next generation communication system should efficiently use limited resources, since a plurality of cells constituting the communication system use the limited resources (e.g., frequency resource, code resource, time slot resource, etc.) on a shared basis. Particularly, with the rapid progress of the radio communication systems and the advent of various services, there is an increasing need for radio resources. However, since almost all commercially available frequency bands have now been allocated, there is a significant lack of frequency resources for new radio platforms. 
         [0005]    To solve such frequency lacking problems, a cognitive radio (CR) communication system based on a CR scheme has been proposed. The CR communication system senses frequency bands which have been allocated but are not in actual use, and efficiently shares the sensed frequency bands. A typical example of the CR communication system includes an IEEE 802.22 Wireless Regional Area Networks (WRAN) system, and the IEEE 802.22 WRAN system introduces the CR technology in the TV frequency band to use an unused TV band(s) for data transmission/reception. 
         [0006]    However, in the CR communication system, if a primary system, while a secondary system secures and uses frequency resources, intends to use the frequency band secured and used by the secondary system, the secondary system should immediately stop the use of the frequency band. The ‘primary system’ as used herein means a communication system having a legal right to use the frequency band. 
         [0007]    Meanwhile, in order to sense a frequency band which has been allocated to the primary system but is not used actually, the secondary system measures strength of a received signal in the frequency band. Thereafter, the secondary system uses the frequency band, sensing the nonuse of the frequency band by the primary system depending on the measure strength of the received signal. If the strength of the received signal is greater than or equal to a predetermined threshold, the secondary system determines that the desired frequency band is used by another system. 
         [0008]    However, the signal received at the secondary system may include not only the transmission and reception signals of the primary system, but also the interference signal (e.g., a transmission or reception signal of another secondary system). It is difficult for the secondary system to determine whether the received signal is a signal of the primary system or an interference signal. In order to detect the signal of the primary system in the frequency band, the secondary system performs correlation calculation using a periodic characteristic of the signal of the primary system, causing an increase in the system complexity. In addition, when the secondary system uses a cyclo-stationary characteristic to detect the primary system signal or the interference signal in the frequency band, it may suffer from latency, causing an increase in the time required for sensing the frequency band. 
         [0009]    Further, in the CR communication system, when the secondary system uses a multi-hop relay scheme based on a relay station (RS), it is not possible to apply the multi-hop relay scheme to the CR communication system since there is no way to allocate frequency band resources. 
       SUMMARY OF THE INVENTION 
       [0010]    To address the above-discussed deficiencies of the prior art, it is a primary aspect of the present invention to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention provides a resource allocation system for improving data transmission and reception efficiency in a cognitive radio (CR) communication system, and a method for supporting the same. 
         [0011]    Another aspect of the present invention provides a system for allocating resources in a CR communication system to which a multi-hop relay scheme is applied, and a method for supporting the same. 
         [0012]    According to one aspect of the present invention, there is provided a system for allocating resources in a communication system including a first system having a use right for a plurality of frequency bands, and a second system having no use right for the plurality of frequency bands. The system includes a station for acquiring first, second and third sensing informations indicating frequency bands available by a base station, a relay station and a mobile station belonging to the second system, among the plurality of frequency bands, and selecting a frequency band to be allocated for communications, based on the acquired sensing informations. 
         [0013]    According to another aspect of the present invention, there is provided a method for allocating resources in a communication system including a first system having a use right for a plurality of frequency bands, and a second system having no use right for the plurality of frequency bands. The method includes acquiring first, second and third sensing informations indicating frequency bands available by a base station, a relay station and a mobile station belonging to the second system, among the plurality of frequency bands; and selecting a frequency band to be allocated for communications, based on the acquired sensing informations. 
         [0014]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “station” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular station may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0016]      FIG. 1  is a diagram illustrating a configuration of a CR communication system according to an embodiment of the present invention; 
           [0017]      FIG. 2  is a diagram illustrating a frame structure of a secondary system used in a CR communication system according to an embodiment of the present invention; and 
           [0018]      FIG. 3  is a diagram illustration an operation of a secondary system in a CR communication system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIGS. 1 through 3 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. 
         [0020]    Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication system. 
         [0021]    An embodiment of the present invention, described below, provides a communication system and method in which when a primary system having a right to use a specific frequency band does not use the frequency band, a secondary system is allowed to use the frequency band. In the following description, the communication system allowing the secondary system to use the frequency band unused by the primary system will be referred to as a cognitive radio (CR) communication system. 
         [0022]    In addition, an embodiment of the present invention, described below, provides a resource allocation system and method for a CR communication system to which a multi-hop relay scheme is applied. 
         [0023]    Although a description of the present invention will be given herein for a resource allocation system and method for a CR communication system as an example of the communication system using the Institute of Electrical and Electronics Engineers (IEEE) 802.22 standard, the resource allocation system and method proposed by the present invention can be applied not only to the CR communication system but also to all other communication systems. 
         [0024]      FIG. 1  is a diagram illustrating a configuration of a CR communication system according to an embodiment of the present invention. 
         [0025]    Before a description of  FIG. 1  is given, it will be assumed that the CR communication system includes a primary system and a secondary system, a cell  110  is managed by the primary system and a cell  150  is managed by the secondary system. 
         [0026]    Referring to  FIG. 1 , the cell  110  includes a BS 1   112  (or primary base station (BS)) and an MS 1   114  (or primary mobile station (MS)) for receiving a communication service from the BS 1   112 , and the cell  150  includes a BS 2   152  (or a secondary BS), an MS 2   154  (or a secondary MS) for receiving a communication service from the BS 2   152 , and a relay station (RS)  156  for providing multi-hop between the BS 2   152  and the MS 2   154 . 
         [0027]    The BS 1   112 , while providing a communication service to the MS 1   114  over a specific frequency band, receives the current location information and Channel State Information (CSI) of the MS 1   114 , being fed back from the MS 1   114 . 
         [0028]    At this point, the BS 2   152  senses spectrums in a frequency band whose right-to-use (hereinafter ‘use right’) belongs to the primary system, thereby to sense (search for) a frequency band unused by the primary system. Further, the BS 2   152  generates sensing information indicating the sensed frequency band. The MS 2   154  measures interference information based on its own communication environment, for example, noise and interference situation, and senses spectrums in a frequency band unoccupied in the position where it is now located (i.e., in the frequency band whose use right belongs to the primary system), thereby to sense a frequency band unused by the BS 1   112  and the MS 1   114  of the primary system. Further, the MS 2   154  generates sensing information indicating the sensed frequency band. In addition, the RS  156  senses spectrums in a frequency band whose use right belongs to both the BS 2   152  and the primary system, thereby to sense a frequency band unused by the primary system. Further, the RS  156  generates sensing information indicating the sensed frequency band. 
         [0029]    In this way, the BS 2   152 , MS 2   154  and RS  156  each sense a frequency band unused by the primary system, and generate sensing information indicating the sensed frequency band. Thereafter, they transmit the generated sensing information in a message. Also, each entity receives sensing information indicating a frequency band sensed by other elements of the secondary system except for the entity itself. 
         [0030]    That is, the BS 2   152  broadcasts its generated sensing information to the MS 2   154  and the RS  156  along with a broadcast message (e.g., MAP-message) and the MS 2   154  includes its generated sensing information in feedback information, and then transmits a control message with the feedback information included therein to the RS  156  and the BS 2   152 . Also, the RS  156  broadcasts its generated sensing information to the MS 2   154  along with a MAP-message, includes the generated sensing information in feedback information, and then transmits a control message with the feedback information included therein to the BS 2   152 . 
         [0031]    As a result, the sensing information transmitted by the BS 2   152  indicates information on an available frequency band of the secondary system, which has been sensed by sensing spectrums in a frequency band whose use right belongs to the primary system. The BS 2   152  receives sensing information for an available frequency band of the secondary system from the RS  156  and the MS 2   154 . The MS 2   154  transmits, to the BS 2   152  and the RS  156 , sensing information indicating available frequency band information of the secondary system, sensed by sensing spectrums in a frequency band whose use right belongs to the primary system, and receives sensing information for an available frequency band of the secondary system from the BS 2   152  and the RS  156 . In addition, the RS  156  transmits, to the BS 2   152  and the MS 2   154 , sensing information indicating available frequency band information of the secondary system, sensed by sensing spectrums in a frequency band whose use right belongs to the primary system, and receives sensing information for an available frequency band of the secondary system from the BS 2   152  and the MS 2   154 . 
         [0032]    Therefore, upon receiving available frequency band information of the secondary system, sensed by sensing spectrums in the frequency band whose use right belongs to the primary system from other elements of the secondary system except for the corresponding entity itself, each of the BS 2   152 , MS 2   154  and RS  156  compares an available frequency band of the secondary system, sensed by the corresponding entity itself, with an available frequency band of the secondary system, sensed by other elements of the secondary system, and selects the optimal frequency band available in the secondary system, i.e., the optimal frequency band available by the BS 2   152 , MS 2   154  and RS  156  depending on the comparison result. 
         [0033]      FIG. 2  is a diagram illustrating a frame structure of a secondary system used in a CR communication system according to an embodiment of the present invention. Herein, shown in  FIG. 2  is a frame structure of a secondary system in a CR communication system according to an embodiment of the present invention. 
         [0034]    Referring to  FIG. 2 , the frame  200  includes a downlink (DL) frame  210  and an uplink (UL) frame  250 . The DL frame  210  includes a region  207  where a BS of a secondary system transmits a signal to an RS and an MS of the secondary system, located in its own cell, and a region  217  where the RS of the secondary system transmits a signal to an MS of the secondary system, to which a relay path to the BS of the secondary system is formed via the RS itself. The UL frame  250  includes a region  253  where the MS of the secondary system transmits a signal to the BS of the secondary system, which provides a service to the MS itself, and to the RS of the primary system, to which a relay path to the MS itself is formed, and a region  259  where the RS of the secondary system transmits a signal to the BS of the secondary system. 
         [0035]    More specifically, the DL frame  210  includes a preamble region  201  for transmitting a synchronization signal for synchronization acquisition of a transmission/reception interval of the secondary system (i.e., synchronization acquisition between the BS and the MS of the secondary system or between the BS and the RS of the secondary system); a DL-MAP region  203  and a UL-MAP region  205  for transmitting DL-MAP information and UL-MAP information, respectively; a first data transmission region (T 1 )  207  where the BS of the secondary system transmits data to the RS and the MS of the secondary system, located in its own cell; and a guard region  209  for separation between a BS transmission region and an RS transmission region of the secondary system. 
         [0036]    In the CR communication system according to an embodiment of the present invention, the BS of the secondary system senses spectrums in the frequency band whose use right belongs to the primary system. Thereafter, the BS includes, in DL-MAP information, sensing information indicating information on an available frequency band of the secondary system, sensed by sensing the spectrums, and transmits the DL-MAP information to the RS and the MS of the secondary system over the DL-MAP region  203 . In the UL, the BS includes, in UL-MAP information, sensing information indicating information on an available frequency band of the secondary system, and transmits the UL-MAP information to the RS and the MS of the secondary system over the UL-MAP region  205 . 
         [0037]    Further, the DL frame  210  includes a preamble region  211  for transmitting a synchronization signal for synchronization acquisition of a transmission/reception interval, i.e., for synchronization signal between the RS and the MS of the secondary system using a region over which the RS of the secondary system transmits a signal to an MS, like the preamble region  201 ; a DL-MAP region  213  and a UL-MAP region  215  for transmitting DL-MAP information and UL-MAP information, respectively; a second data transmission region (T 2 )  217  where the RS of the secondary system transmits data to the MS of the secondary system, to which a relay path to the RS itself is formed; and a Transmit Transition Gap (TTG) region  219  as a guard interval for separation between the DL frame  210  and the UL frame  250 . 
         [0038]    In the CR communication system according to an embodiment of the present invention, the RS of the secondary system senses spectrums in the frequency band whose use right belongs to the primary system. Thereafter, the RS includes, in DL-MAP information, sensing information indicating information on an available frequency band of the secondary system, sensed by sensing the spectrums, and transmits the DL-MAP information to the BS and the MS of the secondary system over the DL-MAP region  213 . In the UL, the RS includes, in UL-MAP information, sensing information indicating information on available frequency band of the secondary system, and transmits the UL-MAP information to the BS and the MS of the secondary system over the UL-MAP region  215 . 
         [0039]    The UL frame  250  includes a control region  251  over which the MS of the secondary system transmits various control information as feedback information; a third data transmission region (T 3 )  253  over which the MS of the secondary system transmits data to the BS and the RS of the secondary system, which provides a service to the MS itself; and a guard region  255  for separation between the MS transmission region and the RS transmission region of the secondary system. 
         [0040]    In the CR communication system according to an embodiment of the present invention, the MS of the secondary system senses spectrums in the frequency band whose use right belongs to the primary system. Thereafter, the MS includes, in control information, sensing information indicating available frequency band information of the secondary system, sensed by sensing the spectrums, and transmits the control information to the BS and the RS of the secondary system over the control region  251 . 
         [0041]    Further, the UL frame  250  includes a control region  257  over which the RS of the secondary system transmits various control information as feedback information; a fourth data transmission region (T 4 )  259  over which the RS of the secondary system transmits data to the BS of the secondary system, to which a relay path to the RS itself is formed; and a Receive Transition Gap (RTG) region  261  as a guard interval between the UL frame  250  of the current frame and a DL frame of the next frame of the secondary system. 
         [0042]    In the CR communication system according to an embodiment of the present invention, the RS of the secondary system senses spectrums in the frequency band whose use right belongs to the primary system. Thereafter, the RS transmits sensing information indicating available frequency band information of the secondary system, sensed by sensing the spectral region, to the BS of the secondary system over the control region  257 . 
         [0043]    At this point, each of the BS, RS and MS of the secondary system generates sensing information indicating an available frequency band of the secondary system for a period defined by a signal transmission/reception region of the frame, which is irrelevant to the corresponding entity itself. More specifically, the BS of the secondary system senses a frequency band available by the secondary system by sensing spectrums in the frequency band whose use right belongs to the primary system, for periods defined by signal transmission/reception regions irrelevant to the BS itself in the frame, i.e., for a first Quiet Period (QP)  237  of the BS (hereinafter ‘BS QP1’) in the DL frame  210 , and a second QP  275  of the BS (hereinafter ‘BS QP2’) and a third QP  279  of the BS (hereinafter ‘BS QP3’) in the UL frame  250 . Thereafter, the BS of the secondary system generates sensing information indicating a frequency band available by the secondary system. 
         [0044]    The BS QP 1   237  includes the guard region  209 , preamble region  211 , DL-MAP region  213 , UL-MAP region  215 , T 2   217  and TTG region  219  of the DL frame  210 , the BS QP 2   275  includes the guard region  255  of the UL frame  250 , and the BS QP 3   279  includes the RTG region  261  of the UL frame  250 . 
         [0045]    The RS of the secondary system senses a frequency band available by the secondary system by sensing spectrums in the frequency band whose use right belongs to the primary system, for periods defined by signal transmission/reception regions irrelevant to the RS itself in the frame, i.e., for a first QP  231  of the RS (hereinafter ‘RS QP1’) and a second QP  233  of the RS (hereinafter ‘RS QP2’) in the DL frame  210 ; and a third QP  271  of the RS (hereinafter ‘RS QP3’) and a fourth QP  273  of the RS (hereinafter ‘RS QP4’) in the UL frame  250 . Thereafter, the RS of the secondary system generates sensing information indicating a frequency band available by the secondary system. 
         [0046]    The RS QP 1   231  includes the guard region  209  of the DL frame  210 ; the RS QP 2   233  includes the TTG region  219  of the DL frame  210 ; the RS QP 3   271  includes the guard region  255  of the UL frame  250 ; and the RS QP 4   273  includes the RTG region  261  of the UL frame  250 . 
         [0047]    The MS of the secondary system senses a frequency band available by the secondary system by sensing spectrums in the frequency band whose use right belongs to the primary system, for periods defined by signal transmission/reception regions irrelevant to the MS itself in the frame, i.e., for a first QP  235  of the MS (hereinafter ‘MS QP1’) and a second QP  239  of the MS (hereinafter ‘MS QP2’) in the DL frame  210 , and a third QP  277  of the MS (hereinafter ‘MS QP3’) in the UL frame  250 . Thereafter, the MS of the secondary system generates sensing information indicating a frequency band available by the secondary system. 
         [0048]    The MS QP 1   235  includes the guard region  209  of the DL frame  210 ; the MS QP 2   239  includes the TTG region  219  of the DL frame  210 ; and the MS QP 3   277  includes the guard region  255 , control region  257 , T 4   259  and RTG region  261  of the UL frame  250 . 
         [0049]    In this way, each of the BS, RS and MS of the secondary system senses a frequency band available by the secondary system by sensing spectrums in the frequency band corresponding to the corresponding entity itself in the frame  200 . Thereafter, the RS and the MS each generate its sensed sensing information and transmit the sensing information to the BS over the corresponding region of the frame  200 . Here, the sensing information indicates a frequency band(s) available by the secondary system, sensed by each of the BS, RS and MS. 
         [0050]    Upon receiving the sensing information from the RS and the MS, the BS compares a resource occupation rate in each frequency band based on the received sensing information and its generated sensing information. Based on the comparison result, the BS preferentially allocates a frequency band with a lower resource occupation rate to the MS. 
         [0051]    That is, the BS determines, as the lowest level, a resource occupation rate of a first frequency band unused by any station among all frequency bands. The BS determines a resource occupation rate of a second frequency band previously used by the primary system, as a low level corresponding to a level higher than the lowest level. The BS determines a third frequency band currently used by the primary system as the highest level corresponding to a level higher than the low level. 
         [0052]    Therefore, the BS can allocate the first frequency band, the second frequency band and the third frequency band to the MS in order, and for this, transmits a Downlink Stream (DS)-MAP including information indicating the allocated frequency band, to the MS directly or via the RS. If there are frequency bands having the same resource occupation rate as a result of the comparison, the BS selects the RS, which is its nearest station, and selects one of the frequency bands according to the sensing information of the selected RS. 
         [0053]    For example, reference numerals  232  and  234  shown in  FIG. 2  represent sensing information generated by the RS in the RS QP 1   231  and RS QP 2   233 , respectively, and reference numerals  236  and  240  represent sensing information generated in the MS QP 1   235  and MS QP 2   239 , respectively. A shown frequency band  290  has a resource occupation rate corresponding to the lowest level. Therefore, the BS transmits a DS-MAP message with information on the frequency band  290  to the MS via the RS, thereby allocating to the MS the frequency band  290  which is one of the frequency bands having the lowest-level resource occupation rate. 
         [0054]    With reference to  FIG. 2 , a description has been made an exemplary method in which the BS preferentially allocates a frequency band with the lowest resource occupation rate to the MS based on its generated sensing information and the sensing information received from the RS and the MS. Alternatively, however, the MS can select a frequency band with the lowest resource occupation rate based on its generated sensing information and the sensing information received from the BS and the RS, and send a request for allocation of the selected frequency band to the BS. 
         [0055]    A DS-MAP message format the BS transmits to the MS for allocation of a frequency band can be defined as Table 1. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Syntax 
                 Size 
                 Notes 
               
               
                   
                   
               
             
             
               
                   
                 RS DS-MAP_Message_Format( ) { 
                   
                   
               
               
                   
                 Management Message Type =1 
                  8 bits 
               
               
                   
                 Synchronization Field 
                 16 bits 
               
               
                   
                 RS ID 
                 48 bits 
               
               
                   
                 Begin PHY Specific Section { 
               
               
                   
                 for (i=1; i≦n; i++) { 
               
               
                   
                 RS DS-MAP_IE( ) 
                 Variable 
               
               
                   
                 } 
               
               
                   
                 RS+BS sensing information 
                 n bits 
               
               
                   
                 Entity selection request 
                 n bits 
               
               
                   
                   
               
             
          
         
       
     
         [0056]    In Table 1, an ‘RS+BS sensing information’ field includes sensing information of the RS and the BS, and an ‘Entity selection request’ field indicates frequency band information selected by the BS. Herein, the ‘RS+BS sensing information’ field can be provided only when the BS needs it. 
         [0057]    A Bulk Measurement Report (BLM-REP) message format the MS transmits for allocation request of a frequency band can be defined as Table 2. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Syntax 
                 Size 
                 Notes 
               
               
                   
               
             
             
               
                 BLM-REP_Message_Format( ) { 
                   
                   
               
               
                 Management Message Type = 41 
                  8 bits 
               
               
                 Transaction ID 
                 16 bits 
               
               
                 Number of Single Measurement 
                  8 bits 
                 The number of single 
               
               
                 Reports 
                   
                 measurement reports 
               
               
                   
                   
                 contained in this 
               
               
                   
                   
                 message. 
               
               
                 sensing information 
                 Variable 
               
               
                 Entity selection response 
                 n bits 
               
               
                   
               
             
          
         
       
     
         [0058]    In Table 2, a ‘sensing information’ field includes sensing information the RS, the BS and the MS, and an ‘Entity selection response’ field indicates frequency band information selected by the MS. Herein, the ‘sensing information’ field can be provided only when the MS needs it. 
         [0059]      FIG. 3  is a diagram illustration an operation of a secondary system in a CR communication system according to an embodiment of the present invention. Shown in  FIG. 3  is a signal transmission/reception flow between a BS, an RS and an MS of a secondary system in a CR communication system according to an embodiment of the present invention. 
         [0060]    Referring to  FIG. 3 , a BS  301  of the secondary system transmits a MAP-message to an RS  303  and an MS  305  of the secondary system over a DL-MAP region  203  of a frame  200  (Steps  311  and  313 ). The MAP-message is transmitted to the RS  303  and the MS  305  of the secondary system in a broadcast message form. Thereafter, the BS  301  of the secondary system transmits data to the RS  303  and the MS  305  of the secondary system over a T 1   207  of the frame  200  (Steps  315  and  317 ). 
         [0061]    Upon receiving data from the BS  301  of the secondary system in this way, the RS  303  and the MS  305  of the secondary system search the frame  200  for a frequency band available by the secondary system during a guard region  209  of a DL frame  210 . That is, the RS  303  of the secondary system senses (searches for) a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during an RS QP 1   231 , and generates the sensing information (Step  319 ). The MS  305  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during an MS QP 1   235 , and generates the sensing information (Step  321 ). 
         [0062]    Thereafter, the RS  303  of the secondary system transmits a MAP-message to the MS  305  of the secondary system over a DL-MAP region  213  of the frame  200  (Step  323 ), and then transmits data to the MS  305  of the secondary system over a T 2   217  of the frame  200  (Step  325 ). Next, the RS  303  and the MS  305  of the secondary system sense a frequency band(s) available by the secondary system during a TTG region  219  of the DL frame  210  in the frame  200 . That is, the RS  303  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during an RS QP 2   233 , and updates the sensing information (Step  327 ). The MS  305  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during an MS QP 2   239 , and updates the sensing information (Step  329 ). 
         [0063]    The BS  301  of the secondary system senses a frequency band available by the secondary system during a guard region  209 , a preamble region  211 , a DL-MAP region  213 , a UL-MAP region  215 , a T 2   217  and a TTG region  219  of the DL frame  210  in the frame  200 . That is, the BS  301  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during a BS QP 1   237  (Step  331 ). Thereafter, the MS  305  of the secondary system transmits control information and data to the BS  301  and the RS  303  of the secondary system over a control region  251  and a T 3   253  of the frame  200  (Steps  333  and  335 ). 
         [0064]    Thereafter, the BS  301  and the RS  303  of the secondary system sense a frequency band available by the secondary system during a guard region  255  of a UL frame  250  in the frame  200 . That is, the BS  301  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during a BS QP 2   275 , and updates the sensing information (Step  337 ). The RS  303  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during an RS QP 3   271 , and updates the sensing information (Step  339 ). 
         [0065]    Thereafter, the RS  303  of the secondary system transmits control information and data to the BS  301  of the secondary system over a control region  257  and a T 4   259  of the frame  200  (Step  341 ). Then the BS  301  and the RS  303  of the secondary system sense a frequency band available by the secondary system during an RTG region  261  of the UL frame  250  in the frame  200 . That is, the BS  301  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during a BS QP 3   279 , and updates the sensing information (Step  343 ). The RS  303  of the secondary system senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during an RS QP 4   273 , and updates the sensing information (Step  345 ). 
         [0066]    The MS  305  of the secondary system senses a frequency band available by the secondary system during a guard region  255 , a control region  257 , a T 4   259  and an RTG region  261  of the UL frame  250  in the frame  200 . That is, the MS  305  senses a frequency band available by the secondary system by sensing spectrums in a frequency band whose use right belongs to the primary system during an MS QP 3   277 , and updates the sensing information (Step  347 ). 
         [0067]    The BS  301  receives the RS sensing information generated in steps  319 ,  327 ,  339  and  345 , and the MS sensing information generated in steps  321 ,  329  and  347  (Steps  349  and  351 ). Therefore, the BS compares a resource occupation rate in each frequency band based on the received RS sensing information and MS sensing information, and its sensing information generated in steps  331 ,  337  and  343 . Thereafter, the BS  301  selects a frequency band having the lowest resource occupation rate (Step  353 ). The BS  301  transmits a DS-MAP message with the selected frequency band information to the MS  305  (Step  355 ). 
         [0068]    A description has been made of a method in which the BS selects the frequency band with the lowest resource occupation rate based on its generated sensing information and the sensing information received from the RS and the MS, and allocates the selected frequency band to the MS. However, in an alternative embodiment, the MS can select a frequency band with the lowest resource occupation rate based on its generated sensing information and the sensing information received from the BS and the RS, and can be allocated a corresponding frequency band by sending a request for allocation of the selected frequency band. In this case, steps  349  to  355  are deleted, and the following process should be added. 
         [0069]    That is, the MS  305  receives the RS sensing information and the BS sensing information, compares a resource occupation rate in each frequency band based on the received RS sensing information and BS sensing information, and its generated sensing information, selects a frequency band with the lowest resource occupation rate, and transmits a BLM-REP message with the selected frequency band information to the BS  301 . 
         [0070]    As is apparent from the foregoing description, in the CR communication system according to the present invention, the BS, the RS and the MS of the secondary system each sense an available frequency band and share the sensing information, making it possible to allocate the optimal frequency band available by the secondary system. In addition, the present invention transmits/receives data using the optimal frequency band in the CR communication system to which the multi-hop relay scheme is applied, thereby contributing to an increase in the data transmission/reception efficiency. 
         [0071]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.