Abstract:
Disclosed is a method for allocating a safety channel in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels, each of which is a set of a predetermined number of subbands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels include a safety subchannel allocated to particular subscriber stations (SSs), which is unused in a neighbor base station. The method comprises the steps of: transmitting, by the SS, its channel quality information to the base station (BS) when a channel quality of a subchannel allocated to the SS is less than to predetermined reference channel quality; and allocating a safety subchannel to the SS, by the BS upon receiving the channel quality information indicating that the channel quality of the subchannel is less than to the reference channel quality.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “System and Method for Managing Safety Channel in an Orthogonal Frequency Division Multiple Access Communication System” filed in the Korean Intellectual Property Office on Mar. 12, 2004 and assigned Ser. No. 2004-17060, 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 an Orthogonal Frequency Division Multiple Access (OFDMA) communication system, and in particular, to a system and method for managing a safety channel in a communication system using a Time Division Duplex (TDD)-based OFDMA scheme (hereinafter referred to as a “TDD-OFDMA communication system”).  
         [0004]     2. Description of the Related Art  
         [0005]     Extensive research is being conducted in the 4 th  generation (4G) communication system to provide users with services having various levels of Qualities of Service (QoS) supporting a data rate of about 100 Mbps. Compared with the 4G communication system, the 3 rd  generation (3G) communication system generally supports a data rate of about 384 Kbps in an outdoor channel environment having poorer channel conditions, and supports a data rate of a maximum of 2 Mbps in an indoor channel environment having better channel conditions.  
         [0006]     A wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system generally support a data rate of 20 to 50 Mbps. Extensive research is being conducted in the 4G communication system to develop a new communication system guaranteeing mobility and a QoS in the wireless LAN system and the wireless MAN system guaranteeing a higher data rate. In addition, an extensive study of the communication system is made to support a high-speed service provided in the 4G communication system. To this end, extensive research is also being conducted on an Orthogonal Frequency Division Multiplexing (OFDM) scheme as a scheme useful for attaining the high-speed data transmission through wired/wireless channels in the 4G communication system. The OFDM scheme refers to a scheme of transmitting data using multiple carriers, and is a type of a Multi-Carrier Modulation (MCM) scheme which parallel-converts a serial input symbol stream into parallel symbols and modulates the parallel symbols with a plurality of orthogonal subcarriers, i.e. a plurality of subcarrier channels, before transmission.  
         [0007]     A multiple access scheme based on the OFDM scheme is the OFDMA scheme. In the OFDMA scheme, subcarriers in one OFDMA symbol are divided between a plurality of users, i.e., subscriber stations (SSs). Communication systems using the OFDMA scheme include systems based on an Institute of Electrical and Electronics Engineers (IEEE) 802.16a standard, an IEEE 802.16d standard, and an IEEE 802.16e standard. The IEEE 802.16d communication system is a system to which the OFDMA scheme is applied to support a broadband transmission network to a physical channel for the wireless MAN system. The IEEE 802.16d communication system is a Broadband Wireless Access (BWA) communication system using a TDD-OFDMA scheme. Therefore, the IEEE 802.16d communication system, in which the OFDMA scheme is applied to the wireless MAN system, transmits a physical channel signal using a plurality of subcarriers, thereby enabling high-speed, high-quality data transmission.  
         [0008]     With reference to  FIG. 1 , a description will now be made of a frame structure used in a conventional TDD-OFDMA communication system.  
         [0009]      FIG. 1  is a diagram illustrating a frame structure used in a conventional TDD-OFDMA communication system. Referring to  FIG. 1 , a frame used in the TDD-OFDMA scheme is divided into a downlink (DL)  149  interval and an uplink (UL)  153  interval. A Transmit/receive Transition Gap (TTG)  151  is formed in an interval where the transition occurs from the DL  149  to the UL  153 , as a guard time, and a Receive/transmit Transition Gap (RTG)  155  is formed in an interval where the transition occurs from the UL  153  back to the DL  149 , as a guard time. The TDD-OFDMA frame has a vertical axis comprised of a plurality of subchannels  147  and a horizontal axis comprised of a plurality of OFDMA symbols  145 .  
         [0010]     Describing the DL  149 , a preamble  111  for synchronization acquisition is located in a k th  OFDMA symbol, and broadcast data information that the SSs will receive in common, such as a frame control header (FCH)  113 , DL-MAP  115  and UL-MAP  117 , is located in a (K+1) th  or (K+2) th  OFDMA symbol. The FCH  113  is comprised of two subchannels, and transmits basic information on subchannel, raging and modulation schemes. DL bursts  121 ,  123 ,  125 ,  127  and  129  are located between the (K+2) th  OFDMA symbol exclusive of a UL-MAP  119  and a (K+8) th  OFDMA symbol. Describing the UL  153 , preambles  131 ,  133  and  135  are located in a (K+9) th  OFDMA symbol, and UL bursts  137 ,  139  and  141  are located between a (K+10) th  OFDMA symbol and a (K+12) th  OFDMA symbol. In addition, a ranging subchannel  143  is located between the (K+9) th  OFDMA symbol and the (K+12) th  OFDMA symbol.  
         [0011]     Information on positions and allocation of the UL bursts  137 ,  139  and  141  and the DL bursts  121 ,  123 ,  125 ,  127  and  129  is provided by a base station (BS) controlling a particular cell to the SSs located in the cell through the DL-MAP  115  and the UL-MAP  117 . Then the SSs are variably allocated subchannels, each of which has a combination of frequencies and symbols, through the information every frame, and perform communication using the allocated subchannels. That is, the SSs can use different subchannels every frame instead of fixed subchannels. In the TDD-OFDMA communication system using a frequency reuse factor of ‘1’, a neighbor cell also uses the same frequency band, i.e. the same subchannel. For example, it is assumed that an SS belonging to a cell A is located in a boundary of the cell A and uses an s th  subchannel, and an SS belonging to a cell B neighboring the cell A is located in a boundary of the cell B and also uses the s th  subchannel. In this case, because the SS of the cell A and the SS of the cell B use the same subchannels, the subchannels function as significant interferences to each other in the cell boundary.  
       SUMMARY OF THE INVENTION  
       [0012]     As described above, because SSs located within the cell boundary communicate with a BS using the same subchannels, the subchannels may cause significant interference to each other. Therefore, it is necessary to propose a new channel for minimizing interference from an SS of a neighbor cell and guaranteeing a QoS within a cell boundary in the OFDMA communication system with the foregoing frame structure.  
         [0013]     It is, therefore, an object of the present invention to provide a system and method for minimizing interference between the SSs located with in a cell boundary in a TDD-OFDMA communication system.  
         [0014]     It is another object of the present invention to provide a system and method for guaranteeing a QoS of the SSs located within a cell boundary in a TDD-OFDMA communication system.  
         [0015]     In accordance with one aspect of the present invention, there is provided a method for allocating a safety channel in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels, each of which is a set of a predetermined number of subbands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels include a safety subchannel allocated to particular subscriber stations (SSs), which is unused in a neighbor base station. The method includes the steps of transmitting, by the SS, its channel quality information to the base station (BS) when a channel quality of a subchannel allocated to the SS is less than a predetermined reference channel quality; and allocating a safety subchannel to the SS, by the BS upon receiving the channel quality information indicating that the channel quality of the subchannel is less than the reference channel quality.  
         [0016]     In accordance with another aspect of the present invention, there is provided a method for allocating a safety channel by a base station (BS) in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels, each of which is a set of a predetermined number of subbands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels include a safety subchannel allocated to particular subscriber stations (SSs). The method includes the steps of: receiving channel quality information from the SS; determining channel quality of a subchannel currently received from the SS based on the channel quality information; and selecting a safety channel in a frequency band unused in a neighbor BS, and allocating the selected safety channel to the SS, according to determining the channel quality of the subchannel is less than a predetermined reference channel quality.  
         [0017]     In accordance with further another aspect of the present invention, there is provided a method for receiving an allocated safety channel by a subscriber station (SS) in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels each of which is a set of a predetermined number of subbands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels include a safety subchannel allocated to particular SSs located, which is unused in a neighbor base station. The method includes the steps of: transmitting its channel quality information to the base station (BS) when a channel quality of a subchannel allocated to the SS is less than a predetermined reference channel quality; and communicating with the BS using the safety channel which is successfully allocated from the BS.  
         [0018]     In accordance with further another aspect of the present invention, there is provided a method for allocating a safety channel by a base station (BS) in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels, each of which is a set of a predetermined number of subbands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels include a safety subchannel allocated to particular subscriber stations (SSs), which is unused in a neighbor base station, in a state where the BS and the SS communicate with each other using the safety channel. The method includes the steps of: transmitting to the SS a request message for a report on the safety channel quality information from the SS; determining channel quality of a subchannel transmitted by the SS based on the received channel quality information upon receiving channel quality information from the SS in response to the request; and releasing a safety channel allocated to the SS if it is determined that the channel quality of the subchannel is greater than a predetermined reference channel quality.  
         [0019]     In accordance with further another aspect of the present invention, there is provided a method for receiving an allocated safety channel by a subscriber station (SS) in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels, each of which is a set of a predetermined number of subbands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels include a safety subchannel allocated to particular SSs located, which is unused in a neighbor base station, in a state where the base station (BS) and the SS communicate with each other using the safety channel. The method includes the steps of: transmitting by an SS a channel quality information to the BS in response to a channel quality information request from the BS; and releasing communication through the safety channel upon receiving information indicating release of a safety channel in use from the BS.  
         [0020]     In accordance with further another aspect of the present invention, there is provided a system for allocating a safety channel in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels, each of which is a set of a predetermined number of bands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels are classified into a diversity subchannel for acquiring a frequency diversity gain, an Adaptive Modulation and Coding (AMC) subchannel for high-speed, high-capacity data transmission, and a safety subchannel allocated to particular subscriber stations (SSs), which is unused in a neighbor base station, in a state where the base station (BS) and the SS do not use the safety channel. The system includes a SS for transmitting its channel quality information to a BS if the channel quality of its current subchannel is less than a predetermined reference channel quality; and a BS for allocating a safety channel to the SS upon receiving the channel quality information indicating that the channel quality of the subchannel is less than the reference channel quality.  
         [0021]     In accordance with further another aspect of the present invention, there is provided a system for allocating a safety channel in a wireless communication system which divides a full frequency band into a plurality of subcarrier bands and includes subchannels each of which is a set of a predetermined number of subbands, each of which is a set of a predetermined number of contiguous subcarriers, where the subchannels are classified into a diversity subchannel for acquiring a frequency diversity gain, an Adaptive Modulation and Coding (AMC) subchannel for high-speed, high-capacity data transmission, and a safety subchannel allocated to particular subscriber stations (SSs), which is unused in a neighbor base station, in a state where the base station (BS) and the SS communicate with each other using the safety channel. The system includes a SS for transmitting its channel quality information to a BS in response to a channel quality information request from the BS; and a BS for releasing a safety channel allocated to the SS upon receiving the channel quality information indicating that channel quality of a subchannel transmitted by the SS is greater than a predetermined reference channel quality. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     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:  
         [0023]      FIG. 1  is a diagram illustrating a frame structure used in a conventional TDD-OFDMA communication system;  
         [0024]      FIG. 2  is a diagram illustrating a frame structure used in a TDD-OFDMA communication system according to an embodiment of the present invention;  
         [0025]      FIG. 3  is a flowchart illustrating an operation performed by a BS in response to a safety channel allocation request from an SS in a TDD-OFDMA communication system according to an embodiment of the present invention;  
         [0026]      FIG. 4  is a flowchart illustrating an operation of requesting allocation of a safety channel by an SS in a TDD-OFDMA communication system according to an embodiment of the present invention;  
         [0027]      FIG. 5  is a flowchart illustrating an operation by a BS for a safety channel in use in a TDD-OFDMA communication system according to an embodiment of the present invention;  
         [0028]      FIG. 6  is a flowchart illustrating an operation by a SS for a safety channel in use in a TDD-OFDMA communication system according to an embodiment of the present invention; and  
         [0029]      FIG. 7  is a signaling diagram illustrating a process of releasing an allocated safety channel in a TDD-OFDMA communication system according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]     A preferred embodiment of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for conciseness.  
         [0031]     The present invention proposes a new TDD-OFDMA frame structure in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system using a Time Division Duplex (TDD) scheme (hereinafter, referred to as a “TDD-OFDMA communication system”). In particular, the present invention proposes a safety channel and its associated messages to minimize interference between neighbor cells in a cell boundary in the TDD-OFDMA communication system having a plurality of cells, thereby increasing cell capacity.  
         [0032]     Before a description of the present invention is given, it should be noted that in the TDD-OFDMA communication system having a multicell configuration in which all cells use a full frequency band (frequency reuse factor is 1), subscriber stations (SSs) located in a cell boundary receive interference signals from other SSs, located in a neighbor cell, which uses the same frequency band, i.e. the same subchannel. Therefore, it is necessary to allocate a frequency band available in a base station (BS) among frequency bands unused in the neighbor cell, to the SSs located in the cell boundary, thereby minimizing the interference signals.  
         [0033]     With reference to  FIG. 2 , a description will now be made of a new frame structure used in a TDD-OFDMA communication system according to an embodiment of the present invention.  
         [0034]      FIG. 2  is a diagram illustrating a frame structure used in a TDD-OFDMA communication system according to an embodiment of the present invention. Referring to  FIG. 2 , in the frame structure, a full subcarrier band is divided into a plurality of bands, for example Band# 0  to Band# 23  in  FIG. 2 , and each band is comprised of a plurality of bins or tiles. The bin or tile is comprised of a plurality of subcarriers. Herein, the bin is comprised of 9 consecutive subcarriers in one OFDM symbol, and there is 1 pilot tone and 8 data tones in the bin. The tile is comprised of 3 to 6 consecutive subcarriers, and there are 2 pilot tones and 16 data tones in the tile.  
         [0035]     In the frame, the first three OFDM symbols are used for a ranging channel, a Hybrid Automatic Repeat Request (H-ARQ) channel, and a channel quality information (CQI) channel, respectively. The other OFDM symbols are allocated for band Adaptive Modulation and Coding (AMC) channels, diversity channels, and safety channels. The band AMC channels at the front of the frame are allocated on a per band basis and are comprised of 6 bins, and the diversity channels at the rear of the frame are allocated on a subchannel basis and are comprised of three tiles spread over the full subcarrier band. As the band AMC channels are greater than the diversity channels in terms of the area allocated therefor, when the reception quality is high, the band AMC channels can be used for transmitting/receiving large-volume data at high speed by applying a modulation scheme having high coding efficiency. For the safety channel, a part which is laid over all OFDM symbols and one bin is allocated. The safety channel is allocated all symbols in one bin. An SS is allocated a safety channel among safety channels unused in a neighbor cell with a frequency band available in a BS, i.e. an unallocated frequency band.  
         [0036]     In order to allocate an appropriate channel according to a state of an SS, a BS should always recognize the state of the SS. If quality of data from the SS decreases gradually, the BS should transmit a message for requesting a report on a state of a channel to the SS in order to detect a change in the state of the channel. The present invention proposes a new channel status request (CH_STA_REQ) message transmitted from the BS to the SS, and a new channel status response (CH_STA_RSP) message through which the SS reports its channel state to the BS. It should be noted herein that the SS can report its channel state to the BS at its own discretion without receiving the CH_STA_REQ message to request allocation of another channel. A format of the CH_STA_REQ message is illustrated in Table 1.  
                       TABLE 1                       Syntax   Size   Notes                   CH_STA_REQ_Message_Format( ) {               Management_Message_Type = XX   8 bits       Requested Channel Type   2 bits   00 = diversity               channel               01 = band AMC               channel               10 = safety channel               11 = reserved       TLV Encoded Information   variable   TLV specific       }                  
 
         [0037]     As illustrated in Table 1, the CH_STA_REQ message includes the following information elements (hereinafter, it refers to as ‘IE’s). A Requested Channel Type (hereinafter, it refers to as ‘Requested Channel Type’) field indicates a type of field that the BS desires to receive C/I from the SS, and the channel type field has the following meanings according to values recorded in this field:  
         [0038]     00=diversity Channel  
         [0039]     01=band AMC Channel  
         [0040]     10=safety Channel  
         [0041]     11=reserved  
         [0042]     Accordingly, the SS measures C/I of the diversity channel when a value recorded in the channel type field is 00 and report the same to BS. Also, when a value recorded in the channel type filed is 01, the SS measures C/I of the band AMC channel and report the same to BS. In addition, when a value recorded in the channel type field is 10, the SS measures C/I of the safety channel and report the same to BS.  
         [0043]     The SS transmits a CH_STA_RSP message to the BS to report the quality of a received channel to the BS upon receiving the CH_STA_REQ message from the BS or when a specific condition designated by broadcast information transmitted by the BS is satisfied. A type of channel state information transmitted through the CH_STA_RSP message is determined according to a type of a channel that the SS is currently using together with the BS. For example, when the SS is communicating using a band AMC channel, the SS transmits CQI for each band AMC channel, and when the SS is communicating using a safety channel, the SS transmits CQI for each safety channel. A format of the CH_STA_RSP message is illustrated in Table 2.  
                       TABLE 2                       Syntax   Size   Notes                   CH_STA_RSP_Message_Format( ) {               Management_Message_Type = XX   8 bits       Reported Channel Type   2 bits   00 = diversity               channel               01 = band AMC               channel               10 = safety               channelBin               11 = reserved       No. Reported Channels       for(i=0; i&lt;No. Reported Channels; i++){        if(Reported Channel Type = = 00) {         Downlink Channel ID   8 bits   0˜255         C/I   5 bits         }        if(Reported Channel Type = = 01) {         Band Index   4 bits   0˜15         C/I   5 bits         }        if(Reported Channel Type = = 10) {         Bin Index   7 bits   0˜127         C/I   5 bits         }        }       TLV Encoded Information   variable   TLV specific                  
 
         [0044]     As illustrated in Table 2, among IEs of the CH_STA_RSP message, a Reported Channel Type uses the same values as the values defined for the CH_STA_REQ message, and one of the values is used according to a state of a channel in use by the SS. That is, a value recoded in the Reported Channel Type means a type of Channel corresponding to a channel wherein the C/I is measured and is reported by the SS. And, a value recorded in the Reported Channel Type field has the same meaning as that recorded in the Requested Channel Type field of the CH_STA_REQ message, as follows:  
         [0045]     00=diversity Channel  
         [0046]     01=band AMC Channel  
         [0047]     10=safety Channel  
         [0048]     11=reserved  
         [0049]     That is, if the SS reports the C/I of the diversity channel, “00” is recorded in the Requested Channel Type filed. And, if the SS reports the C/I of the band AMC Channel, “01” is recorded in the Requested Channel Type. In addition, if the SS reports the C/I of the safety Channel, “10” is recorded in the Requested Channel Type.  
         [0050]     A Downlink Channel ID is an OFDM symbol number indicating each diversity subchannel, a Band Index is an OFDM symbol number for distinguishing each band used for the band AMC, and a Bin Index is an OFDM symbol number indicating a position of a safety channel. Based on a C/I of each band, each bin or a full band, reported by the SS, the BS allocates a diversity channel, a band AMC channel and a safety channel. That is, if a C/I is low as the SS enters a cell boundary, the BS can allocate a safety channel, and if a C/I is greater than a predetermined reference C/I, the BS can normally allocate a diversity channel. The BS can allocate a band AMC channel in order to guarantee a high QoS or transmit/receive data at a high speed.  
         [0051]     In order to use a safety channel, the SS and the BS need the following 3 processes:  
         [0052]     1. safety channel allocation  
         [0053]     2. channel state report for safety channel in use  
         [0054]     3. safety channel release  
         [0055]     A detailed description will now be made of the foregoing steps.  
         [0056]     1. Safety Channel Allocation  
         [0057]     With reference to  FIGS. 3 and 4 , a description will be made of an operation of a BS for a safety channel allocation and an operation by a SS for a safety channel allocation request.  
         [0058]      FIG. 3  is a flowchart illustrating an operation by a SS performed in response to a safety channel allocation request from an SS in a TDD-OFDMA communication system according to an embodiment of the present invention. Referring to  FIG. 3 , in step  301 , the BS does not use a safety channel for an SS. In step  303 , the BS receives a CH_STA_RSP message transmitted by the SS. In step  305 , the BS determines if a C/I value included in the CH_STA_RSP message transmitted by the SS satisfies a safety channel allocation requested condition, i.e. if the C/I value is less than a predetermined reference C/I value (indicative of bad channel quality). If the safety channel allocation requested condition is satisfied, the BS determines if it can allocate a safety channel taking into consideration the channel quality-related information included in the CH_STA_RSP message and the resource situations of a neighbor BS and a serving BS.  
         [0059]     If it is determined that the BS can allocate a safety channel to the SS, the BS proceeds to step  307 , and otherwise, the BS returns to step  303 . In step  307 , the BS allocates to the SS a safety channel in a frequency band that includes an empty safety channel for a neighbor cell of the SS. After allocating an available safety channel to the SS, the BS communicates with the SS using the allocated safety channel in step  309 .  
         [0060]     With reference to  FIG. 3 , a description has been made of an operation by a BS performed in response to a safety channel allocation request from an SS in a TDD-OFDMA communication system according to an embodiment of the present invention. With reference to  FIG. 4 , a description will be made of an operation of requesting allocation of a safety channel by an SS in a TDD-OFDMA communication system according to an embodiment of the present invention.  
         [0061]      FIG. 4  is a flowchart illustrating an operation of requesting allocation of a safety channel by an SS in a TDD-OFDMA communication system according to an embodiment of the present invention. Referring to  FIG. 4 , in step  401 , the SS does not use a safety channel. In step  403 , the SS measures a C/I to determine the channel quality. If it is determined that the channel quality is poor, i.e. less than a threshold, the SS transmits a CH_STA_RSP message in step  405 , recognizing the need for safety channel allocation. The SS transmits a CH_STA_RSP in response to a safety channel allocation request. Alternatively, the SS can report its channel state to the BS at its own discretion through the CH_STA_RSP message. Thus, after transmitting the CH_STA_RSP message to the BS, the SS determines in step  407  if a safety channel has been successfully allocated from the BS. If a safety channel has been successfully allocated from the BS, the SS communicates with the BS using the allocated safety channel in step  409 . However, if it is determined in step  407  that no safety channel has been allocated from the BS, the SS retransmits the CH_STA_RSP message to the BS after a lapse of a predetermined time.  
         [0062]     2. Channel State Report for Safety Channel in Use  
         [0063]     With reference to  FIGS. 5 and 6 , a description will now be made of BS and SS operations for a channel state report for the safety channel in use.  
         [0064]      FIG. 5  is a flowchart illustrating an operation of a BS for a safety channel in use in a TDD-OFDMA communication system according to an embodiment of the present invention. Referring to  FIG. 5 , in step  501 , the BS allocates a safety channel to an SS. In step  502 , the BS determines if it needs to receive channel state information from the SS or it has received a CH_STA_RSP message from the SS. After the state decision, the BS proceeds to step  503  or step  507 . For example, if the BS needs to receive channel state information from the SS, the BS proceeds to step  503 , and if the BS has received a CH_STA_RSP message from the SS, the BS proceeds to step  507 . If the BS determines in step  503  that it needs to receive channel state information from the SS, the BS transmits a CH_STA_REQ message to the SS to request a report on a channel state in step  505 . However, if the BS has received the CH_STA_RSP message from the SS in step  507 , the BS analyzes information included in the received CH_STA_RSP message in step  509  to determine if it will release the safety channel. If the BS determines not release the safety channel, it returns to step  502 . However, if the BS determines to release the safety channel, it releases in step  511  the safety channel allocated to the SS. In step  513 , the BS communicates with the SS using a diversity channel. As a result, the BS in communication with the SS using a safety channel communicates with the SS using a diversity channel due to the release of the safety channel.  
         [0065]     With reference to  FIG. 5 , a description has been made of an operation of a BS for a safety channel in use in a TDD-OFDMA communication system according to an embodiment of the present invention. Next, with reference to  FIG. 6 , a description will be made of an operation of an SS for a safety channel in use in a TDD-OFDMA communication system according to an embodiment of the present invention.  
         [0066]      FIG. 6  is a flowchart illustrating an operation of an SS for a safety channel in use in a TDD-OFDMA communication system according to an embodiment of the present invention. Referring to  FIG. 6 , in step  601 , the SS uses a safety channel. In step  602 , the SS determines if it has received a CH_STA_REQ message from a BS, if it has received a safety channel release request from the BS, or it needs to transmit a CH_STA_RSP message. After the state decision, the SS proceeds to step  603 ,  607  or  611  If the SS has received the CH_STA_REQ message from the BS in step  603 , the SS transmits a CH_STA_RSP message to the BS in response to the CH_STA_REQ message in step  605 . The CH_STA_RSP message includes a Reported Channel Type value determined by the SS and C/I information measured according to the determined Reported Channel Type value.  
         [0067]     If the SS has received in step  607  a CH_STA_REQ message including parameter information indicating a request for releasing the safety channel from the BS due to an increase in its channel state, the SS performs data communication with the BS using a diversity subchannel in step  609  after the safety channel to the BS is released.  
         [0068]     In step  611 , if the SS desires to trigger, or transmit, a CH_STA_RSP message while waiting to transmit the CH_STA_RSP message periodically or depending on a channel state, the SS proceeds to step  613  where it repeatedly transmits the CH_SAT_RSP message to report its channel state to the BS.  
         [0069]     3. Safety Channel Release  
         [0070]     The BS can release a safety channel in use when there is no more data to exchange with the SS. In addition, the BS can stop the use of the safety channel based on the channel state information included in a CH_STA_RSP message received from the SS and a resource state of the BS. If the safety channel in use is released, the SS transitions back to a state where it uses a diversity subchannel.  
         [0071]     With reference to  FIG. 7 , a description will now be made of a signaling procedure for releasing an allocated safety channel in a TDD-OFDMA communication system according to an embodiment of the present invention.  
         [0072]      FIG. 7  is a signaling diagram illustrating a process of releasing an allocated safety channel in a TDD-OFDMA communication system according to an embodiment of the present invention. Referring to  FIG. 7 , an SS  701  and a BS  703  exchange data with each other, using a diversity channel (Step  705 ). Upon detecting a deterioration of its channel state, the SS  701  transmits a CH_STA_RSP message to the BS  703  to request allocation of a safety channel (Step  707 ). If there is a bin unallocated for a safety channel of a neighbor cell, the BS  703  allocates a safety channel to the SS  701  (Step  709 ). Thereafter, the SS  701  and the BS  703  exchange data with each other through the allocated safety channel (Step  711 ). During the data exchange, if the BS  703  requires information related to a channel state of the SS  701 , the BS transmits a CH_STA_REQ message to the SS  701  (Step  713 ). Upon receiving the CH_STA_REQ message, the SS  701  transmits a CH_STA_RSP message to the BS  703  (Step  715 ). Alternatively, the SS  701  transmits the CH_STA_RSP message to the BS  703  at its own discretion (Step  715 ). Even while using the allocated safety channel, the SS  701  periodically measures a C/I of a physical channel and transmits the measured C/I to the BS  703  (Step  717 ). The SS  701  exchanges data with the BS  703  through the safety channel allocated from the BS  703  (Step  719 ). The BS  703  receives a CH_STA_RSP message for a channel of the SS  701 , a C/I of which is greater than a predetermined reference C/I (Step  721 ). In this case, the BS  703  sends a safety channel release request to the SS  701  (Step  723 ). In this manner, if the CQI is greater than the reference C/I, the BS  703  releases the channel. In response to the safety channel release request, the SS  701  releases the safety channel and exchanges data with the BS  703  through a diversity channel (Step  725 ).  
         [0073]     As described above, the present invention proposes a new frame structure for a safety channel and its management scheme to enable an SS located in a cell boundary to minimize interference from an SS of a neighbor cell in a TDD-OFDMA communication system, thereby guaranteeing a QoS of and minimizing the interference to the SS located in the cell boundary, and contributing to an increase in cell capacity.  
         [0074]     While the invention has been shown and described with reference to a certain preferred embodiment 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.