Patent Publication Number: US-2006009228-A1

Title: System and method for allocating safety channels in a broadband wireless access communication system

Description:
PRIORITY  
      This application claims priority under 35 U.S.C. § 119 to an application entitled “System and Method for Allocating Safety Channels in a Broadband Wireless Access Communication System” filed in the Korean Intellectual Property Office on Jun. 19, 2004 and assigned Serial No. 2004-45891, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to a system and method for allocating safety channels and performing handover for allocation of the safety channels.  
      2. Description of the Related Art  
      Research into the next generation communication system, also known as the 4 th  (4G) generation communication system, is being actively pursued to provide users with various Qualities-of-Service (QoSs) at a data rate of about 100 Mbps. In general, the current 3 rd  generation (3G) communication system supports a data rate of about 384 Kbps in an outdoor channel environment with relatively poor channel conditions, and supports a data rate of 2 Mbps at most indoor channel environments with relatively good channel conditions.  
      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. At present, the 4G communication system is being actively developed to create a new communication system capable of supporting mobility and QoS in the wireless LAN and MAN systems, both of which guarantee a relatively high data rate.  
      In particular, the Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system is communication system employing an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support a broadband transmission network to physical channels in the wireless MAN.  
      The IEEE 802.16 communication system is a BWA communication system using an OFDMA scheme. The IEEE 802.16 communication system, which is a wireless MAN system employing the OFDMA scheme, transmits a physical channel signal using a plurality of subcarriers, thereby enabling high-speed data transmission.  
       FIG. 1  is a diagram illustrating a configuration of a general BWA communication system. Referring to  FIG. 1 , the BWA communication system has a multicell geometry, i.e., has a cell  100  and a cell  150 , and includes a base station (BS)  110  managing the cell  100 , a BS  140  managing the cell  150 , and a plurality of mobile stations (MSs)  111 ,  113 ,  130 ,  151  and  153 . Signal exchange between the base stations  110  and  140  and the MSs  111 ,  113 ,  130 ,  151  and  153  is achieved using the OFDM/OFDMA scheme. Among the MSs  111 ,  113 ,  130 ,  151  and  153 , the MS  130  is located in a boundary region of the cell  100  and the cell  150 , i.e., a handover region. To support mobility for the MS  130 , it is necessary to support handover for the MS  130 .  
       FIG. 2  is a diagram illustrating a frame structure in a general BWA communication system. Referring to  FIG. 2 , a horizontal axis  245  represents OFDMA symbol numbers, and a vertical axis  247  represents subchannel numbers. As illustrated in  FIG. 2 , one OFDMA frame includes a plurality of, for example, 13 OFDMA symbols. In addition, one OFDMA symbol includes a plurality of subchannels, for example, (L+1) subchannels. The BWA communication system aims at acquiring a frequency diversity gain by dispersing all of the subcarriers used therein, especially, data subcarriers over the full frequency band. The BWA communication system performs a ranging operation to adjust a time offset and a frequency offset for a transmission/reception period and to adjust transmission power.  
      In the BWA communication system, a transition from a downlink to an uplink is made for a Transmit/receive Transition Gap (TTG)  251 , and a transition from an uplink to a downlink is made for a Receive/transmit Transition Gap (RTG)  255 . Following the TTG  251  and the RTG  255 , separate preamble fields  211 ,  231 ,  233  and  235  are allocated for acquisition of synchronization between a transmitter and a receiver.  
      In the preamble structure of the IEEE 802.16d communication system, a downlink (DL) frame  249  includes a preamble field  211 , a frame control header (FCH) field  213 , a DL-MAP field  215 , UL-MAP fields  217  and  219 , and DL burst fields, i.e., a DL burst # 1  field  223 , a DL burst # 2  field  225 , a DL burst # 3  field  221 , a DL burst # 4  field  227 , and a DL burst # 5  field  229 .  
      The preamble field  211  transmits a sync signal, i.e., a preamble sequence, for acquisition of synchronization between a transmitter and a receiver. The FCH field  213 , including two subchannels, transmits basic information on the subchannel, ranging, modulation scheme, etc. The DL-MAP field  215  transmits a DL-MAP message, and UL-MAP fields  217  and  219  transmit UL-MAP messages.  
      In a multicell broadband OFDMA communication system, MSs located in neighbor cells communicate using the same frequency band. Therefore, if the MSs are located in a cell boundary, the same subchannels used in different cells may create considerable interference with each other. Thus, the MSs located in the cell boundary are allocated a frequency band that is not used in the neighbor cells. Safety channels are allocated to increase cell capacity by minimizing interference from the neighbor cells, guarantee QoS of the MSs located in the cell boundary, and minimize interference from the neighbor cells.  
       FIG. 3  is a diagram illustrating a frame structure to which safety channels are applied in a general BWA communication system. In the frame structure of  FIG. 3 , a full subcarrier band is divided into a plurality of bands, and each band includes a plurality of bins or tiles. Each of the bins or tiles includes a plurality of subcarriers. The bin includes successive subcarriers within one OFDM symbol, and there are pilot tones and data tones. The tile includes successive subcarriers, and there are pilot tones and data tones.  
      In the frame, 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 remaining band Adaptive Modulation and Coding (AMC) channels, diversity channels, and safety channels are allocated. Therefore, data including MAP or control information is distributed at the head of each frame, and data including subcarriers and OFDM symbols is distributed at the end of each frame.  
      The band AMC channels at the head of the frame are allocated in units of band comprised of bins, and the diversity channels at the end of the frame are allocated in units of subchannel comprised of three tiles dispersed over the full subcarrier band. Because the band AMC channels are allocated the wider band as compared with the diversity channels, they can be used for transmitting/receiving a large volume of data at high speed by applying a modulation technique with a high coding efficiency for the high reception quality.  
      The safety channels are allocated a part crossing all of OFDM symbols and one bin. The safety channels are allocated all symbols for one bin. MSs are allocated safety channels with a frequency band allocable in a BS among the safety channels unused in neighbor cells, i.e., the remaining unallocated frequency band. A MS using the band AMC channels is allocated resources in units of band, and a MS using the diversity channels is allocated resources in units of subchannel. The MS using the safety channels is allocated all of the symbols for one bin. The allocated safety channels are selected from the safety channels unused by the MS in neighbor cells.  
      In the BWA communication system, a MS that is communicating with a BS in a serving cell may move to a neighbor cell region. If interference from a BS in the neighbor cell increases, the MS is allocated channels corresponding to the safety channels of the neighbor cell, currently unused therein, so that it can continue safety communication with the serving BS. When the safety channels of the neighbor cell are allocated to another MS, the MS located in the vicinity of the neighbor cell still suffers considerable interference from the BS in the neighbor cell.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide a system and method for allocating safety channels in a Broadband Wireless Access (BWA) communuication system.  
      It is another object of the present invention to provide a system and method for allocating safety channels according to channel allocation of a neighbor cell in a BWA communication system.  
      It is further another object of the present invention to provide a system and method for allocating a safety channel zone of a neighbor cell to minimize interference with a mobile station (MS) that has moved to a boundary of the neighbor cell in a BWA communication system.  
      It is yet another object of the present invention to provide a system and method in which the serving BS allocates its own safety channel zone to the MS when a serving base station (BS) cannot allocate a channel zone corresponding to a safety channel zone of a neighbor cell to an MS.  
      It is still another object of the present invention to provide a system and method in which a serving BS allows an MS to perform handover to a neighbor BS having the highest signal reception intensity to allocate its own safety channel zone to an MS.  
      It is still another object of the present invention to provide a system and method in which the serving BS allocates a safety channel zone of a neighbor BS having the second highest reception signal intensity to the MS when a serving BS cannot allocate a channel zone corresponding to a safety channel zone of a neighbor cell to an MS.  
      According to one aspect of the present invention, there is provided a method for allocating a safety channel in a broadband wireless access (BWA) communication system including a serving base station (BS) for providing a service to a mobile station (MS) and neighbor BSs, The method includes upon receiving a request for allocation of the safety channel from the MS, sending a request for allocation of the safety channel to a target BS having the best channel condition among the neighbor BSs; and after sending the request for allocation of the safety channel, upon receiving information indicating that the safety channel can be allocated, controlling the MS to perform communication through the allocated safety channel.  
      According to another aspect of the present invention, there is provided a method for allocating a safety channel in a broadband wireless access (BWA) communication system including a serving base station (BS) for providing a service to a mobile station (MS) and neighbor BSs, The method includes upon receiving a request for allocation of the safety channel from the MS, sending a request for allocation of the safety channel to a target BS having the best channel condition among the neighbor BSs; and after sending the request for allocation of the safety channel, upon receiving information indicating that the safety channel cannot be allocated, controlling the MS to perform handover from the serving BS to the target BS.  
      According to further another aspect of the present invention, there is provided a system for allocating a safety channel in a broadband wireless access (BWA) communication system including a serving base station (BS) for providing a service to a mobile station (MS) and neighbor BSs. The system includes the serving BS for, upon receiving a request for allocation of the safety channel from the MS, sending a request for allocation of the safety channel to a target BS having the best channel condition among the neighbor BSs, and upon receiving information indicating possibility of allocating the safety channel from the target BS, controlling the MS to perform communication through the allocated safety channel; and the MS for, upon detecting a need for receiving a safety channel, sending, to the serving BS, information indicating that the MS should be allocated the safety channel, and receiving a safety channel allocated according to a control signal from the serving BS.  
      According to further another aspect of the present invention, there is provided a system for allocating a safety channel in a broadband wireless access (BWA) communication system including a serving base station (BS) for providing a service to a mobile subscriber station (MS) and neighbor BSs The system includes the serving BS for, upon receiving a request for allocation of the safety channel from the MS, sending a request for allocation of the safety channel to a target BS having the best channel condition among the neighbor BSs, and upon receiving information indicating impossibility of allocating the safety channel from the target BS, controlling the MSS to perform handover to the target BS; and the MS for, upon detecting a need for receiving the safety channel, sending, to the serving BS, information indicating that the MS should be allocated the safety channel, and performing handover from the serving BS to the target BS according to a control signal from the serving BS to receive an allocated safety channel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      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:  
       FIG. 1  is a diagram illustrating a configuration of a general BWA communication system;  
       FIG. 2  is a diagram illustrating a frame structure in a general BWA communication system;  
       FIG. 3  is a diagram illustrating a frame structure to which safety channels are applied in a general BWA communication system;  
       FIG. 4  is a signaling diagram illustrating an operating process of allocating safety channels in a BWA communication system according to an embodiment of the present invention;  
       FIG. 5  is a signaling diagram illustrating an operating process of allocating safety channels by performing handover in a BWA communication system according to an embodiment of the present invention;  
       FIG. 6  is a flowchart illustrating an operating process of an MS for allocating safety channels in a BWA communication system according to an embodiment of the present invention;  
       FIG. 7  is a flowchart illustrating an operating process of a serving BS for allocating safety channels in a BWA communication system according to an embodiment of the present invention;  
       FIG. 8  is a flowchart illustrating an operating process of a neighbor BS for allocating safety channels in a BWA communication system according to an embodiment of the present invention;  
       FIG. 9  is a signaling diagram illustrating an operating process of allocating safety channels in a BWA communication system according to an alternative embodiment of the present invention; and  
       FIG. 10  is a flowchart illustrating an operating process of an MS for allocating safety channels in a BWA communication system according to an alternative embodiment of the present invention;  
       FIG. 11  is a flowchart illustrating an operating process of a serving BS for allocating safety channels in a BWA communication system according to an alternative embodiment of the present invention;  
       FIG. 12  is a flowchart illustrating an operating process of a neighbor BS for allocating safety channels in a BWA communication system according to an alternative embodiment of the present invention; and  
       FIG. 13  is a signaling diagram illustrating an operating process of allocating safety channels in a BWA communication system according to further an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Several preferred embodiments 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.  
      The present invention proposes a scheme for preventing deterioration of a current serving cell signal quality from an increase interference intensity from a base station (BS) in a neighbor cell when a mobile station (MS) approaches coverage of the neighbor cell in a Broadband Wireless Access (BWA) communication system. That is, the present invention proposes a scheme for allocating the unused neighbor cell channels, i.e., safety channels, to a MS to reduce interference from the neighbor cell.  
      In addition, the present invention proposes a scheme in which, upon failure to allocate safety channels of a neighbor cell, a serving BS allows the MS to perform handover to the neighbor cell and use its own safety channels in the neighbor cell. Further, the present invention proposes a scheme for allocating channels corresponding to a neighbor cell safety channel zone having the second highest reception signal intensity, upon failure to allocate safety channels of a neighbor cell having the highest reception signal intensity.  
       FIG. 4  is a signaling diagram illustrating an operating process of allocating safety channels in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 4 , in the BWA communication system, a MS measures a change in intensity of signals received from a neighbor cell, and sends a request for allocation of safety channels to its serving BS according to the measurement result. Then the serving BS allocates the safety channels as channels between the MS and the neighbor cell having the highest reception signal intensity.  
      In step  412 , a MS  410 , while communicating with a serving BS  450  in a serving cell, performs scanning on its neighbor BSs including the serving BS  450 , neighbor BS# 1   460  and a neighbor BS# 2   470 . In step  414 , if there is any change in intensity of signals received from the serving BS and the neighbor BSs as a result of the scanning, the MS  410  transmits the scanning result to the serving BS  450  using a “MOB-SCAN-REPORT message”. The format of the MOB-SCAN-REPORT message is shown in Table 1.  
                       TABLE 1                       Syntax   Size   Notes                  MOB-SCAN-REPORT_Message —                 Format( ) {        Management Message Type = ??   8 bits        Report Mode   2 bits   00: Event-triggered               01-11: reserved        N_NEIGHBORS   8 bits   N_NEIGHBORS               contains Serving BS        For(i=0; i&lt;N_NEIGHBORS; i++)         Neighbor BS-ID   48 bits          BS CINR means   8 bits        }       }                  
 
      As shown in Table 1, the MOB-SCAN-REPORT message includes a plurality of information elements (IEs), i.e., a “Management Message Type” indicating a type of a transmission message, a “Report Mode” indicating transmission of the MOB-SCAN-REPORT message to report occurrence of a particular event to the serving BS, and an “N_NEIGHBORS” indicating the scanning result for BSs by the MS  410 . The N_NEIGHBORS includes a Neighbor BS-ID indicating IDs of the neighbor BSs and a BS CINR mean indicating intensities of signals received from BSs. The N_NEIGHBORS includes not only the neighbor BSs but also the serving BS.  
      In step  416 , upon receiving the MOB-SCAN-REPORT message, the serving BS  450  selects a neighbor BS having the highest reception signal intensity using the MOB-SCAN-REPORT message. The serving BS  450  transmits a “SafetyCH-Info” message after setting an Info-request to ‘0’ to send a request for allocable safety channel information to a neighbor BS. If it is assumed in step  416  that the neighbor BS# 1   460  has the highest reception signal intensity, the serving BS  450  transmits the SafetyCH-Info message with the Info-request=‘0’ to the neighbor BS# 1   460 . A format of the SafetyCH-Info message is shown in Table 2.  
                       TABLE 2                       Syntax   Size   Notes                  Global Header   152 bits           Info-request    1 bit   0: Request safety channel               allocation information               1: Inform safety channel               allocation information       TLV_Safety_channel_info   Variable   Safety channel zone               information for case where               Info-request value is set to 1.                  
 
      As shown in Table 2, the SafetyCH-Info message includes a plurality of IEs, i.e., an “Info-request” indicating that one BS sends a request for allocable safety channel information to another BS, or indicating whether one BS informs another BS of its own safety channels, and a “TLV_Safety_channel_info” indicating the safety channel information. The “TLV Safety_channel info” is used when a BS informs another BS of its own safety channels for the case where the Info-request value is set to ‘1’. The format for the TLV_Safety_channel_info is shown in Table 3.  
                               TABLE 3                                   TLV_Safety_channel_info( ) {   Size   Notes                                                         OFDM symbol offset   8 bits                Subchannel offset   7 bits            No. OFDMA symbols   7 bits            No. subchannels   7 bits           }                      
 
      As shown in Table 3, the TLV_Safety_channel_info includes an “OFDMA symbol offset” indicating an OFDMA symbol offset for safety channels allocable to the MS  410 , a “Subchannel offset” indicating a subchannel offset for a safety channel zone, a “No. OFDMA symbols” indicating the number of OFDMA symbols, and a “No. subchannels” indicating the number of subchannels.  
      In step  416 , the neighbor BS# 1   460  receives a safety channel information request from the serving BS  450 . In step  418 , the neighbor BS# 1   460  transmits, to the serving BS  450 , a “SafetyCH-Info” message including TLV_Safety_channel_info indicating allocable channels among its own safety channels and an Info-request being set to ‘1’. In step  420 , upon receiving the SafetyCH-Info message with Info-request=‘1’, the serving BS  450  determines whether it can allocate its own safety channels included in the SafetyCH-Info message to the MS  410 . If it is determined that the serving BS  450  can allocate the safety channels provided by the neighbor BS# 1   460 , the serving BS  450  transmits a DL-MAP message including information on a selected channel to the MS  410  in step  422 . The format for the DL-MAP message is shown in Table 4.  
                       TABLE 4                       Syntax   Size   Notes                  DL-MAP_Message Format( ) {                 Management Message Type = 2   8 bits         PHY Synchronization Field   Variable   See appropriate PHY               specification         DCD Count   8 bits         Base Station ID   48 bits          Begin PHY Specific Section {       See applicable PHY               section          for(i=1; i&lt;=n,i++){       For each DL-MAP               element 1 to n.           DL-MAP IE( )   Variable   See  corresponding               PHY Specification          }         }         if!(byte boundary){          Padding Nibble   4 bits   Padding to reach byte               boundary.         }       }                  
 
      As shown in Table 4, the DL-MAP message includes a plurality of lEs, i.e., a “Management Message Type” indicating a type of a transmission message, a Physical (PHY) Synchronization that is set according to a modulation scheme and a demodulation scheme applied to a physical channel for sync acquisition, a “DCD count” indicating a count corresponding to a change in configuration of a downlink channel descript (DCD) message including a downlink burst profile, a “Base Station ID” indicating a base station identifier, and a “DL-MAP_IE” indicating burst information of DL-MAP IEs. The format for the DL-MAP_IE is shown in Table 5.  
                       TABLE 5                       Syntax   Size   Notes                  DL-MAP IE( ){                 DIUC   4 bits         if(DIUC==15){           Extended DIUC dependent IE   Variable   See clauses following 8.4.5.3.1         } else{           if(INC_CID==1){       The   DL-MAP  starts  with               INC_CID = 0. INC_CID is toggled               Between 0 and 1 by the CID-               SWITCH IE( )(8.4.5.3.1)             N_CID   8 bits   Number of CIDs assigned for this IE             for(n=0; n&lt;N_CID;n++){               CID   16 bits              }           }           OFDMA Symbol offset   8 bits           Subchannel offset   6 bits           Boosting   3 bits   000: normal(not boosted); 001:               +6 dB; 010: −6 dB; 011: +9 dB; 100:               +3 dB; 101: −3 dB; 110: −9 dB; 111: −12 dB;           No. OFDMA Symbols   7 bits           No. Subchannels   6 bits           Repetition Coding Indication   2 bits   0b00 - No repetition coding               0b01 - Repetition coding of 2 used               0b10 - Repetition coding of 4 used               0b11 - Repetition coding of 6 used         }       }                  
 
      As shown in Table 5, each DL-MAP_IE includes a Downlink Interval Usage Code (DIUC) indicating information for designating an offset of a region where the DL-MAP IEs are recorded, a Connection Identifier (CID) based on which each DL-MAP IE is allocated, an “OFDMA symbol offset” indicating an offset of symbol resources allocated to a DL burst, a “Subchannel offset” indicating an offset of subchannel resources allocated to a DL burst, a “Boosting” indicating a power value by which power is increased during power transmission, a “No. OFDMA Symbols” indicating the number of allocated OFDMA symbols, a “No. Subchannels” indicating the number of allocated subchannels, and a “Repetition Coding Indication” indicating information on a repetition code used for the burst.  
      Therefore, in step  422 , the serving BS  450  transmits to the MS  410  a “DL-MAP” message including a DL-MAP IE in which selected channel information is stored, and a DIUC=‘13’ indicating allocation of safety channels by the DL-MAP IE. In step  424 , the serving BS  450  transmits a “SafetyCH-Alloc-Info” message with an Alloc flag=‘1’ to the neighbor BS# 1   460  indicating that the to safety channels provided by the neighbor BS# 1   460  were allocated to the MS  410 .  
      In step  424 , the serving BS  450  transmits, to the neighbor BS# 1   460 , information indicating whether it has actually allocated the safety channels provided by the neighbor BS# 1   460  to the MS  410 . This is to determine whether the serving BS  450  has actually allocated the channels provided by the neighbor BS# 1   460  to the MS  410  because the serving BS  450  may select other allocable channels when the serving cell cannot use the channels provided by the neighbor BS# 1   460 . In other words, the safety channels of neighbor BSs, actually unused in the serving cell, are returned to the neighbor BS# 1   460 , so that the neighbor BS# 1   460  can use the safety channels in determining the allocable safety channels for another MS.  
      The SafetyCH-Alloc-Info message can be with or after the DL-MAP message. The format of the SafetyCH-Alloc-Info message is illustrated in Table 6.  
                       TABLE 6                       Syntax   Size   Notes                  Global Header   152 bits           Alloc flag    1 bit   Indicate whether the BS allocates safety               channel zone, which provided from other BS.               0: the BS cannot allocate the same channel               zone, which provided from other BS.               1: the BS allocates the same channel zone,               which provided from other BS.       MS unique identifier    48 bits   48 bit unique identifier used by MS       TLV_Safety_channel_info   Variable   Safety channel zone information for case               where Alloc flag value is set to 0. (the same               format with TLV_Safety_channel_info in               SafetyCH_Info message)                  
 
      As illustrated in Table 6, the SafetyCH-Alloc-Info message includes a plurality of IEs, i.e., a “MS unique identifier” indicating ID information of an MS allocated the safety channels, an “Alloc flag” indicating whether the serving BS has actually allocated the safety channels provided from the neighbor BS to the MS, and a “TLV_Safety_channel_info” indicating the safety channels to be allocated to the MS.  
      The MS unique identifier is included to inform a neighbor BS that the MS will perform safety channel handover to the neighbor BS and use safety channels of the serving BS, and to allocate a fast ranging period for the MS when the MS cannot be allocated the safety channels of the neighbor BS. Alloc flag=‘1’ indicates that the serving BS has allocated the safety channels provided from the neighbor BSs to the MS, and Alloc flag=‘0’ indicates that the serving BS has allocated its own safety channels to the MS so that the MS can use the safety channels after performing handover to the neighbor BS because it cannot allocate the safety channels provided from the neighbor BSs to the MS. Where the Alloc flag value is set to ‘0’, the TLV_Safety_channel_info, is included to inform a neighbor BS of safety channels of the serving BS, allocated to the MS. The structure of the TLV_Safety_channel_info is shown in Table 3.  
      In step  426 , upon receiving the DL-MAP message transmitted by the serving BS  450 , the MS  410  communicates with the serving BS  450  using a channel for the DL-MAP IE bursts.  
       FIG. 5  is a signaling diagram illustrating an operating process of allocating safety channels by performing handover in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 5 , in step  512 , an MS  510 , while communicating with a serving BS  550  in a serving cell, scans its neighbor BSs, which include the serving BS  550 , a neighbor BS# 1   560  and a neighbor BS# 2   570 . In step  514 , if there is any change in intensity of signals from the serving BS and the neighbor BSs as a result of the scanning, the MS  510  transmits the scanning result to the serving BS  550  using a “MOB-SCAN-REPORT” message. In step  516 , upon receiving the “MOB-SCAN-REPORT” message, the serving BS  550  selects a neighbor BS having the highest reception signal intensity, i.e., the best channel condition, and transmits a “SafetyCH-Info” message after setting an Info-request to ‘0’ to request allocable safety channel information from the neighbor BS. That is, in step  516 , the serving BS  550  transmits the SafetyCH-Info message to the neighbor BS# 1   560 , which has the highest reception signal intensity.  
      In step  518 , the neighbor BS# 1   560  transmits, to the serving BS  550 , a “SafetyCH-Info” message including the TLV_Safety_channel_info of Table 3 indicating its own safety channel information after setting an Info-request to ‘1’. In step  520 , upon receiving the SafetyCH-Info message with Info-request=‘1’, the serving BS  550  determines whether it can allocate its own safety channels of the neighbor BS# 1   560  to the MS  510  through the safety channel information from the neighbor BS# 1   560 , included in the SafetyCH-Info message. If it is determined in step  520  that another MS is using the safety channels provided by the neighbor BS# 1   560 , the serving BS determines that it is impossible to allocate the channels to the MS  510 . In step  522 , the serving BS  550  transmits a Mobile BS Handover Request (“MOB-BSHO-REQ”) message to the MS  510  thereby instructing the MS  510  to perform handover to the neighbor BS# 1   560 . The format of the MOB-BSHO-REQ message is shown in Table 7.  
                       TABLE 7                          MOB-BSHO-REQ_Message_Format( ){                 Management Message Type = 52    8 bits         Handover Mode    2 bits   00:  Network handover not               supported               01: Network handover supported               10: safety channel handover               11: reserved         For(j=0; j&lt;N_Recommended; j++)       N_Recommended can be derived               from the known length of the               message           Neighbor BS-ID   48 bits           Service level prediction    8 bits         }         Temporary CID   16 bit   Activated in case where Handover               Mode value is set to 10         HMAC Tuple   21 bytes   See 11.4.11       }                  
 
      As shown in Table 7, the MOB-BSHO-REQ message includes a plurality of IEs, i.e., a “Management Message Type” indicating the type of transmission message, a “Handover Mode” indicating a handover requited by the serving BS, a “Neighbor BS-ID” indicating information on target BSs selected by the serving BS, a “Temporary CID” indicating a temporary connection identifier, and a “HMAC Tuple” indicating a Hash-based Message Authentication Code (HMAC) Tuple.  
      The Handover Mode indicates whether a network assisted handover is performed or a safety channel handover is performed. The “N_Recommended” indicates the number of neighbor BSs selected by the serving BS as recommended target BSs. Further, the “N_Recommended” represents IDs for the neighbor BSs and information on a bandwidth and a service level that the neighbor BSs can provide to the MS. In addition, the MOB-BSHO-REQ message includes a Temporary CID that is activated when the Handover Mode indicates the safety channel handover, i.e., Handover Mode=‘10’, and a HMAC Tuple for authentication of the MOB-BSHO-REQ message.  
      Therefore, the Handover Mode of the MOB-BSHO-REQ message transmitted in step  522  is set to the safety channel handover mode, i.e., ‘10’. In addition, the N_Recommended value becomes 1, and neighbor BS information indicated by the N_Recommended includes an ID of the neighbor BS# 1   560 . In step  524 , after receiving the MOB-BSHO-REQ message, the MS  510  transmits a Mobile Handover Indication (“MOB-HO-IND”) message in response to the MOB-BSHO-REQ message if the Handover Mode indicates a safety channel handover, and then performs handover to the neighbor BS# 1   560  indicated by the N_Recommended field in the MOB-BSHO-REQ message. The format of the MOB-HO-IND message is shown in Table 8.  
                       TABLE 8                       Syntax   Size   Notes                  MOB-HO-IND_Message_Format( ){                 Management Message Type = 56    8 bits         Reserved    6 bits   Reserved; shall be set to zero         HO_IND_type    2 bits   00: Serving BS release               01: HO cancel               10: HO reject               11: reserved         Target_BS_ID   48 bits   Applicable only when HO_IND-type               is set to 00.         HMAC Tuple   21 bytes   See 11.4.11       }                  
 
      As shown in Table 8, the MOB-HO-IND message includes a plurality of IEs, i.e., a “Management Message Type” indicating the type of transmission message, a “HO_IND_type” indicating whether a MS has determined, canceled or rejected handover to a selected final target BS, a “Target_BS_ID” indicating an ID of a final target BS selected by the MS where the MS has performed handover, and a “HMAC Tuple” for authentication of the MOB-HO-REQ message.  
      If the MS has decided to perform handover to the final target BS, it sets the HO_IND_type to ‘00’, if the MS has determined to cancel the handover, it sets the HO_IND_type to ‘01’, or if the MS has determined to reject the handover, it sets the HO_IND_type to ‘10’ before transmitting the HOB-HO-IND message. Upon receiving the MOB-HO-IND message with HO_IND_type=‘10’, the serving BS  550  updates a recommended target BS list.  
      After transmitting the MOB-HO-IND message to the serving BS  550  in which the neighbor BS# 1   560  is stored as a target BS, the MS  510  changes its connection to the neighbor BS# 1   560 .  
      In step  523 , the serving BS  550  transmits to the neighbor BS# 1   560  a “SafetyCH-Alloc-Info” message after setting Alloc flag to ‘0’ to indicate that it cannot allocate the safety channels provided by the neighbor BS# 1   560  to the MS  510  and the MS  510  will perform handover to the neighbor BS# 1   560 . The format of the SafetyCH-Alloc-Info message is shown in Table 6, and includes safety channel information of the serving BS itself. Step  523  can be performed before or after steps  522  and  524 , or can be performed between steps  522  and  524 .  
      In step  528 , upon receiving the SafetyCH-Alloc-Info message, the neighbor BS# 1   560  transmits an uplink (UL)-MAP message including a “Fast_UL_Ranging_IE” allocated to support fast UL ranging of the MS  510 , recognizing from the message that the MS  510  will perform handover to the neighbor BS# 1   560 . The UL-MAP message includes parameters related to an uplink of the neighbor BS# 1   560 . The neighbor BS# 1   560  transmits the “Fast_UL_Ranging_IE” to the MS  510  to minimize delay from the handover performed by the MS  510 . The MS  510  can perform initial ranging with the neighbor BS# 1   560  on a contention-free basis according to the Fast_UL_Ranging_IE. The format of the Fast_UL_Ranging_IE included in the UL-MAP message is shown in Table 9.  
                       TABLE 9                       Syntax   Size   Notes                  Fast_UL_Ranging_IE{                 MAC address   48 bits   MS MAC address as provided on the RNG-REQ               Message on initial system entry         UIUC    4 bits   UIUC = 15. A four-bit code used to define the type of               uplink access and the burst type associated with that               access.         OFDM Symbol offset   10 bits   The offset of the OFDM symbol in which the burst               starts, the offset value is defined in units of OFDM               symbols and is relevant to the Allocation Start Time               field given in the UL-MAP message.         Subchannel offset    6 bits   The lowest index OFDMA subchannel used for               carrying the burst, starting from subchannel 0.         No. OFDM Symbols   10 bits   The number of OFDM symbols that are used to carry               the UL Burst         No. Subchannels    6 bits   The number OFDMA subchannels with subsequent               indexes, used to carry the burst.         Reserved    4 bits       }                  
 
      The Fast_UL_Ranging_IE of Table 9 includes a (Medium Access Control) MAC address for an MS that will have ranging opportunities, a “M C” (Uplink Interval Usage Code) providing information on a field for recording a start offset value for the fast uplink ranging, an offset for a contention-free-based ranging opportunity period allocated to the MS  510 , a “No. OFDM” symbols indicating the number of OFDM symbols, and a “No. subchannels” indicating the number of subchannels.  
      The MAC address of the MS  510  has been transmitted to the neighbor BS# 1   560  through the “SafetyCH-Alloc-Info” message in step  523 .  
      In step  530 , after receiving the UL-MAP message, the MS  510  transmits a Ranging Request (“RNG-REQ”) message to the neighbor BS# 1   560  according to the “Fast_UL_Ranging_IE”. In step  532 , after receiving the RNG-REQ message, the neighbor BS# 1   560  transmits, to the MS  510 , a Ranging Response (“RNG-RSP”) message including information for correction of frequency, time and transmission power for the ranging.  
      Therefore, the MS,  510  in communication with the serving BS  550 , transmits the scanning result to the serving BS  550  to inform the BS  550  of a change in reception signal intensity. If the serving BS  550  fails to be allocated the safety channels from a neighbor BS, it allows the MS  510  to perform handover to the neighbor BS. Then the MS  510  communicates with the neighbor BS through the safety channels of the serving BS  550 .  
       FIG. 6  is a flowchart illustrating an operating process of an MS for allocating safety channels in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 6 , in step  602 , the MS is in an idle state # 1 . In step  604 , the MS scans on a serving BS and neighbor BSs. In step  606 , the MS determines whether there is any specific event, as a result of the scanning. For example, a change in signal intensity received from the serving BS and the neighbor BS or the case where the intensity of a signal received from the neighbor BS is higher than or equal to a predetermined threshold SafetyCH_Threshold. If the signal intensity from the neighbor BS is higher than or equal to the threshold SafetyCH_Threshold, the MS should be allocated the safety channels of a neighbor cell from the serving BS so that interference from the neighbor cell can be minimized.  
      Therefore, in step  606 , the MS compares the intensity of a signal received from the neighbor BS with the threshold SafetyCH_Threshold. If the intensity of a signal received from the neighbor BS is higher than or equal to the threshold SafetyCH_Threshold, the MS proceeds to step  608 . However, if the intensity of a signal received from the neighbor BS is lower than the threshold SafetyCH_Threshold, the MS returns to step  602  where it performs a general communication process with the serving BS, staying in the idle state # 1 .  
      In step  608 , the MS transmits a MOB-SCAN-REPORT message to the serving BS. In step  610 , the MS stays in an idle state # 2 , and then proceeds to step  612 . The idle state # 2  is not substantially different from the idle state # 1 , and is provided for a simple description of the present invention. In step  612 , the MS determines whether a MOB-BSHO-REQ message with a Handover Mode=‘10’ indicating a safety channel handover process has been received from the serving BS. Upon failure to receive the MOB-BSHO-REQ message with Handover Mode=‘10’, the MS proceeds to step  614 . However, upon receiving the MOB-BSHO-REQ message with Handover Mode=‘10’, the MS proceeds to step  616 .  
      In step  614 , the MS communicates with the serving BS using channels allocated from DL-MAP and transmitted by the serving BS. The channels allocated from the DL-MAP can be either the safety channels of the neighbor BS or channels that were used for communication with the serving BS before receiving safety channel allocation information from the neighbor BS.  
      In step  616 , the MS transmits a MOB-HO-IND message to the serving BS in response to the MOB-BSHO-REQ message. In step  618 , the MS changes its connection to the target BS indicated by the MOB-HO-IND message, i.e., the neighbor BS with the highest reception signal intensity.  
      In step  620 , the MS receives a UL-MAP message including Fast_UL_Ranging_IE from the neighbor BS. In step  622 , the MS transmits an RNG-REQ message to the neighbor BS using a channel provided by the Fast_UL_Ranging_IE. In step  624 , the MS receives an RNG-RSP message that the neighbor BS has transmitted in response to the RNG-REQ message. In step  626 , the MS receives from the neighbor BS a DL-MAP message including channel information corresponding to safety channels of the serving BS. Thereafter, the MS performs communication with the neighbor BS on the safety channels.  
      Therefore, the MS, moving to the neighbor cell region recognizes that there is a change in intensity of the BS signal. Next, the MS is allocated safety channels from the serving BS. Alternatively, if the serving BS cannot allocate the safety channels, the MS performs handover to the neighbor BS and then performs communication with the neighbor BS using safety channels of the serving BS.  
       FIG. 7  is a flowchart illustrating an operating process of a serving BS for allocating safety channels in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 7 , in step  702 , the serving BS receives a MOB-SCAN-REPORT message from an MS. In step  704 , the serving BS selects a neighbor BS with the highest reception signal intensity based on the MOB-SCAN-REPORT message. In step  706 , the serving BS transmits a SafetyCH-Info message, after setting an Info-request to ‘0’, to send a request for safety channel information to the selected neighbor BS. In step  708 , the serving BS stays in an idle state, and then proceeds to step  710 . The idle state is a state in which the MS, the serving BS and the neighbor BSs are mutually communicated according to a general communication process until safety channel information is received from the selected neighbor BS.  
      In step  710 , the serving BS receives a SafetyCH-Info message including safety channel information and an Info-request=‘1’ from the selected neighbor BS. In step  712 , the serving BS determines whether it can actually allocate the safety channels provided by the selected neighbor BS to the MS. If the serving BS can allocate its own safety channels provided by the neighbor BS to the MS, it proceeds to step  714 , and if the serving BS cannot allocate the channels to the MS, it proceeds to step  718 .  
      In step  714 , the serving BS transmits a DL-MAP message to the MS in which serving cell safety channels of the neighbor BS are stored. In step  716 , the serving BS transmits a SafetyCH-Alloc-Info message of Table 6 after setting Alloc flag to ‘1’ to indicate that it has actually allocated channels corresponding to safety channels of the neighbor BS to the MS.  
      In step  718 , the serving BS transmits the SafetyCH-Alloc-Info message with Alloc flag=‘0’ to the neighbor BS. The SafetyCH-Alloc-Info message includes safety channel information of the serving BS. By setting the Alloc flag to ‘0’, the serving BS indicates that it has failed to allocate safety channels of the neighbor BS and will allow the MS to perform handover to the neighbor BS. In addition, the serving BS indicates that it will allow the MS and the neighbor BS to perform communication with each other, using its own safety channels in the neighbor BS. The SafetyCH-Alloc-Info message includes an ID of an MS that will perform handover to the neighbor BS, and TLV_Safety_Channel_info indicating the safety channel information of the serving BS.  
      In step  720 , the serving BS cannot allocate safety channels to the MS. Therefore, the serving BS transmits a MOB-BSHO-REQ message with Handover Mode=‘10’ to perform safety channel handover to the neighbor BS. In step  722 , the serving BS receives a MOB-HO-SD message from the MS in response to MOB-BSHO-REQ message. In this case, the serving BS recognizes that the MS will change its connection to the neighbor BS.  
      Therefore, the serving BS allocates channels corresponding to safety channels of the neighbor cell to the MS such that interference from the neighbor cell can be minimized for the MS approaching the neighbor cell. In addition, if the channels cannot be allocated to the MS, the serving BS allows the MS to perform handover to the neighbor BS.  
       FIG. 8  is a flowchart illustrating an operating process of a neighbor BS for allocating safety channels in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 8 , in step  802 , the neighbor BS, i.e., a neighbor BS having the highest reception signal intensity, receives from a serving BS, a SafetyCH-Info message with Info-request=‘0’ indicating a request for safety channel allocation zone information. In step  804 , the neighbor BS transmits a SafetyCH-Info message with Info-request=‘1’ to the serving BS to indicate its own safety channels. In step  806 , the neighbor BS stays in an idle state, and then proceeds to step  808 . The idle state is where a general communication process in the neighbor BS is performed until channel allocation information is actually received from the serving BS.  
      In step  808 , the neighbor BS receives a SafetyCH-Alloc-Info message from the serving BS. In step  810 , the neighbor BS determines whether an Alloc flag value of the SafetyCH-Alloc-Info message is set to ‘0’. The Alloc flag value of the SafetyCH-Alloc-Info message indicates whether the serving BS has allocated, to an MS, the safety channels provided to the serving BS by the neighbor BS, included in the SafetyCH-Info message transmitted in step  804 . If it is determined in step  810  that the Alloc flag value is set to ‘1’, the neighbor BS proceeds to step  812  where it updates information on allocable safety channels, recognizing that the safety channels included in the SafetyCH-Info message of  804  have been allocated for the MS.  
      However, if it is determined in step  810  that the Alloc flag value is set to ‘0’, the neighbor BS proceeds to step  814 , recognizing that the safety channels included in the SafetyCH-Info message of step  804  have not been allocated for the MS. In step  814 , the neighbor BS waits for safety channel handover with the MSS.  
      In step  816 , the neighbor BS acquires synchronization with an MS corresponding to an MS ID included in the SafetyCH-Alloc-Info message received in step  808 , and then transmits a UL-MAP message including a Fast_UL_Ranging_IE for fast ranging of the MS that has performed handover.  
      In step  818 , the neighbor BS receives an RNG-REQ message from the MS. In step  820 , the neighbor BS transmits an RNG-RSP message in response to the RNG-REQ message. In step  822 , the neighbor BS performs the ranging process, and thereafter transmits a DL-MAP message including channel information for communication with the MS. The channels included in the DL-MAP message correspond to safety channels of the serving BS, included in the SafetyCH-Alloc-Info message transmitted in step  808  by the serving BS.  
       FIG. 9  is a signaling diagram illustrating an operating process of allocating safety channels in a BWA communication system according to an embodiment of the present invention where the serving BS fails to allocate safety channels of the neighbor BS having the highest reception signal intensity to the MS, i.e., the case where the serving BS fails to allocate the safety channels to the MS in step  420  of  FIG. 4  and step  520  of  FIG. 5 .  
      Referring to  FIG. 9 , in step  912 , an MS  910 , while communicating with a serving BS  950  in a serving cell, scans on its neighbor BSs, which include the serving BS  950 , a neighbor BS# 1   960  and a neighbor BS# 2   970 . In step  914 , if there is any change in intensity of signals received from the serving BS and the neighbor BSs as a result of the scanning, the MS  910  transmits the scanning result to the serving BS  950  using a “MOB-SCAN-REPORT” message. In step  916 , after receiving the “MOB-SCAN-REPORT” message, the serving BS  950  selects a neighbor BS having the highest reception signal intensity, and transmits a “SafetyCH-Info” message after setting an Info-request to ‘0’ to send a request for safety channel information to the neighbor BS. In other words, the serving BS  950  transmits the SafetyCH-Info message to the neighbor BS# 1   960 , which has the highest reception signal intensity. In step  918 , the neighbor BS# 1   960  transmits, to the serving BS  950 , a “SafetyCH-Info” message including TLV_Safety_channel_info indicating its own safety channel information after setting an Info-request to ‘1’, in response to the safety channel information request from the serving BS. In step  920 , upon receiving the SafetyCH-Info message, the serving BS  950  determines whether it can allocate its own channels corresponding to safety channels of the neighbor BS# 1   960  to the MS  910 , included in the SafetyCH-Info message. If another MS is using the channels, the serving BS proceeds to step  922 , determining that it cannot allocate the channels to the MS  910 .  
      In step  922 , the serving BS  950  transmits a “SafetyCH-Alloc-Info” message with Alloc flag=‘0’ to the neighbor BS# 1   960  that has provided the channels, thereby indicating that it has failed to allocate the safety channels of the neighbor BS# 1   960  to the MS  910 .  
      In step  924 , the serving BS  950  selects the neighbor BS# 2   970  having the second highest reception signal intensity, and transmits a “SafetyCH-Alloc-Info” message with Alloc flag=‘0’ to request safety channel information of the neighbor BS# 2   970 .  
      In step  926 , after receiving the SafetyCH-Alloc-Info message, the neighbor BS# 2   970  transmits, to the serving BS  950 , the “SafetyCH-Info” message with Info-request=‘1’ including safety channel information of the neighbor BS# 2   970 .  
      In step  928 , the serving BS  950  transmits, to the MS  910 , a “DL-MAP” message including channel information of the safety channels of the neighbor BS# 2   970 . In step  930 , the serving BS  950  transmits, to the neighbor BS# 2   970 , a “SafetyCH-Alloc-Info” message of Table 6 after setting Alloc flag to ‘1’ to indicate that it has actually allocated the safety channels of the neighbor BS# 2   970  to the MS  910 . If it is not possible to allocate channels corresponding to safety channels provided by the neighbor BS# 2   970  to the MS  910 , the serving BS  950  continues to perform communication using the currently allocated channels, suspending the operation of allocating safety channels. Alternatively, the serving BS  950  can instruct handover to the neighbor BS with the highest reception signal intensity as described with reference to  FIG. 5 .  
       FIG. 10  is a flowchart illustrating an operating process of an MS for allocating safety channels in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 10 , in step  1002 , the MS stays in an idle state # 1 . In step  1004 , the MS scans a serving BS and neighbor BSs. In step  1006 , the MS determines if there is any specific event, as a result of the scanning. For example, where there is a change in intensity of signals received from the serving BS and the neighbor BS, i.e., the case where the intensity of a signal received from the neighbor BS is higher than or equal to a predetermined threshold SafetyCH_Threshold.  
      If the intensity of a signal received from the neighbor BS is higher than or equal to the threshold SafetyCH_Threshold, the MS should be allocated safety channels of a neighbor cell from the serving BS so that interference from the neighbor cell can be minimized.  
      If it is determined in step  1006  that there is a specific event, the MS proceeds to step  1008  where it transmits a MOB-SCAN-REPORT message to the serving BS. However, if it is determined in step  1006  that there is no specific event, the MS returns to step  1002  where it performs a general communication process with the serving BS, staying in the idle state.  
      In step  1010 , the MS stays in an idle state # 2 , and then proceeds to step  1012 . In step  1012 , the MS receives from the serving BS a DL-MAP message including channel information corresponding to safety channels of the neighbor BS, and performs communication with the serving BS using the allocated channels.  
       FIG. 11  is a flowchart illustrating an operating process of a serving BS for allocating safety channels in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 11 , in step  1102 , the serving BS receives a MOB-SCAN-REPORT message from an MS. In step  1104 , the serving BS selects a neighbor BS# 1  having the highest reception signal intensity based on the MOB-SCAN-REPORT message.  
      In step  1106 , the serving BS transmits a SafetyCH-Info message after setting an Info-request to ‘0’ to send a request for safety channel information to the selected neighbor BS# 1 .  
      In step  1108 , the serving BS stays in an idle state # 1 , and then proceeds to step  1110 . In step  1110 , the serving BS receives a SafetyCH-Info message with Info-request=‘1’ including safety channel information from the neighbor BS# 1 . In step  1112 , the serving BS determines whether it can actually allocate the safety channels provided by the selected neighbor BS# 1  to the MS. If the serving BS can allocate the safety channels, it performs the operations described with reference to  FIGS. 4 and 5 . Therefore, if the serving BS is using the channels corresponding to the safety channels of the neighbor BS# 1  for another MS, the serving BS proceeds to step  1114 , determining that it cannot allocate its own channels corresponding to the safety channels provided by the selected neighbor BS# 1 . In step  1114 , the serving BS transmits a SafetyCH-Alloc-Info message with Alloc flag=‘0’ to the neighbor BS# 1 , indicating it has failed to allocate the safety channels of the neighbor BS# 1 . In step  1116 , the serving BS selects a neighbor BS# 2  having the second highest reception signal intensity.  
      In step  1118 , the serving BS transmits a SafetyCH-Alloc-Info message with Alloc flag=‘0’ to the selected neighbor BS# 2  to request safety channel information. In step  1120 , the serving BS stays in an idle state # 2 , and then proceeds to step  1122 . The idle state # 2  is essentially equal to the idle state # 1 , and refers to a state where the MS, the serving BS and the neighbor BSs are mutually communicated according to a general communication process until the serving BS receives safety channel zone information form the selected neighbor BS# 2 .  
      In step  1122 , the serving BS receives a SafetyCH-Info message with Info-request=‘1’ including safety channel information from the selected neighbor BS# 2 . In step  1124 , the serving BS transmits, to the MS, a DL-MAP message including channel information corresponding to safety channels of the neighbor BS# 2 .  
      In step  1126 , the serving BS transmits, to the neighbor BS# 2 , a SafetyCH-Alloc-Info message of Table 6 with Alloc flag=‘1’ indicating that it has allocated channels corresponding to safety channels provided by the neighbor BS# 2  to the MS.  
       FIG. 12  is a flowchart illustrating an operating process of a neighbor BS for allocating safety channels in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 12 , in step  1202 , the neighbor BS# 2  having the second highest reception signal intensity, receives a SafetyCH-Alloc-Info message with Alloc flag=‘0’ requesting safety channel allocation zone information from the serving BS. In step  1204 , the neighbor BS# 2  transmits to the serving BS a SafetyCH-Info message with Info-request=‘1’ to provide its own safety channel information. In step  1206 , the neighbor BS# 2  stays in an idle state, and then proceeds to step  1208 . The idle state refers to a state in which a general communication process is performed in the neighbor BS# 2  until channel allocation information is actually received from the serving BS.  
      In step  1208 , the neighbor BS# 2  receives a SafetyCH-Alloc-Info message with Alloc flag=‘1’ from the serving BS, and recognizes that the serving BS has allocated safety channels included in the SafetyCH-Info message to the MS. In step  1210 , the neighbor BS# 2  updates the allocable safety channel information.  
      An operation of the neighbor BS# 1  in the signaling diagram of  FIG. 9  is similar to an operation of a neighbor BS in the flowchart of  FIG. 8 .  
      With reference to  FIG. 12 , a description has been made of an operating process of the neighbor BS# 2 , in which a serving BS sends a safety channel information request to the neighbor BS# 2  having the second highest reception signal intensity and allocates channels corresponding to safety channels received from the neighbor BS# 2  to the MS. However, if the serving BS cannot allocate channels corresponding to safety channels of the neighbor BS, it allows the MS to perform handover to the neighbor BS. The serving BS can allow the MS to perform handover not only to a neighbor BS having highest reception signal intensity but also to a neighbor BS having a reception signal intensity, for example, a carrier-to-interference and noise ratio (CINR), being higher than a threshold SafetyCH_Threshold. In this case, the serving BS can provide the neighbor BSs with handover information of the MS and safety channel information of the serving BS.  
       FIG. 13  is a signaling diagram illustrating an operating process of allocating safety channels in a BWA communication system according to an alternative embodiment of the present invention. Referring to  FIG. 13 , in step  1312 , an MS  1310 , while communicating with a serving BS  1350  in a serving cell, scans its neighbor BSs including the serving BS  1350 , a neighbor BS# 1   1360  and a neighbor BS# 2   1370 . In step  1314 , if there is any change in intensity of signals received from the serving BS and the neighbor BSs as a result of the scanning, the MS  1310  transmits the scanning result to the serving BS  1350  using a “MOB-SCAN-REPORT” message.  
      In step  1316 , upon receiving the MOB-SCAN-REPORT message, the serving BS  1350  selects a neighbor BS having the highest reception signal intensity, and transmits a “SafetyCH-Info” message after setting an Info-request to ‘0’ to send a request for allocable safety channel information to the neighbor BS. In other words, the serving BS  1350  transmits the “SafetyCH-Info” message to the neighbor BS# 1   1360 , that has the highest reception signal intensity.  
      In step  1318 , the neighbor BS# 1   1360  transmits, to the serving BS  1350 , a “SafetyCH-Info” message including TLV_Safety_channel_info indicating its own safety channel information after setting an Info-request to ‘1’, in response to the safety channel information request from the serving BS  1350 . In step  1320 , the serving BS  1350  determines whether it can allocate its own safety channels of the neighbor BS# 1   1360  to the MS  1310 , included in the SafetyCH-Info message. If it is determined that another MS is using the safety channels, the serving BS determines that it cannot allocate the channels to the MS  1310 .  
      In step  1322 , the serving BS  1350  transmits a “MOB-BSHO-REQ” message to the MS  1310  thereby instructing the MS  1310  to perform handover, and then proceeds to step  1324  and  1326 . The serving BS  1350  can select a neighbor BS having the highest reception signal intensity and a neighbor BS having a CINR being higher than SafetyCH_Threshold as handover candidate neighbor BSs. Therefore, if the neighbor BS# 1   1360  and the neighbor BS# 2   1370  have CINRs higher than SafetyCH_Threshold, the serving BS  1350  can include the neighbor BS# 1   1360  and the neighbor BS# 2   1370  in the MOB-BSHO-REQ message as handover candidate neighbor BSs.  
      In step  1324 , the serving BS  1350  transmits a “SafetyCH-Alloc-Info” message with Alloc flag=‘0’ to the neighbor BS# 1   1360  to indicate that as the serving cell cannot allocate the safety channels to the MS  1310 , the MS  1310  will perform handover to the neighbor BS# 1   1360  selected as the handover candidate neighbor BS. Also, in step  1326 , the serving BS  1350  transmits a “SafetyCH-Alloc-Info” message with Alloc flag=‘0’ to the neighbor BS# 2   1370  to indicate that as the serving cell cannot allocate the safety channels to the MS  1310 , the MS  1310  will perform handover to the neighbor BS# 2   1370  selected as the handover candidate neighbor BS.  
      In step  1328 , after receiving the MOB-BSHO-REQ message, the MS  1310  transmits a “MOB-HO-IND” message in response to the MOB-BSHO-REQ message if the Handover Mode indicates the safety channel handover, and then proceeds to steps  1330  and  1332 . The MS  1310  can use a reception signal intensity of a neighbor BS or a possible service level provided from the neighbor BS as a criterion for selecting a target BS. The MOB-HO-IND message transmitted in step  1328  does not necessarily have to include information on the finally selected neighbor BS.  
      The “SafetyCH-Alloc-Info” message transmitted from the serving BS  1350  includes safety channel information of the serving BS  1350  itself. The steps  1324  and  1326  can be performed before or after the steps  1322  and  1328 , or can be performed between the steps  1322  and  1328 .  
      In steps  1330  and  1332 , upon receiving SafetyCH-Alloc-Info messages, the neighbor BS# 1   1360  and the neighbor BS# 2   1370  recognize that the MS  1310  included in their received messages will perform handover thereto. Further, the neighbor BS# 1   1360  and the neighbor BS# 2   1370  transmit UL-MAP messages each including Fast_UL_Ranging_IE to the MS  1310  to support fast uplink ranging of the MS  1310 , and then proceeds to step  1334 .  
      After transmitting the MOB-HO-IND message to the serving BS  1350 , the MS  1310  changes its connection, i.e., performs handover, to the finally selected neighbor BS# 1 . For the handover, in step  1334 , the MS  1310  receives a UL-MAP message transmitted by the neighbor BS# 1   1360  and transmits an “RNG-REQ” message to the neighbor BS# 1   1360  according to the Fast_UL_Ranging_IE.  
      In step  1336 , the neighbor BS# 1   1360  transmits an “RNG-RSP” message to the MS  1310  in response to the RNG-REQ message. After the ranging process, in step  1338 , the neighbor BS# 1   1360  transmits a “DL-MAP” message to allocate a channel zone corresponding to a safety channel zone of the serving BS  1350  to the MS  1310 .  
      In step  1340 , the neighbor BS# 1   1360  transmits a “SafetyCH-Alloc-Info” message with Alloc flag=‘1’ to inform the serving BS  1350  that it has allocated channels corresponding to safety channel zone to the MS  1310 .  
      The neighbor BS# 2   1370  that is allocating a Fast_UL_Ranging_IE, while waiting for handover of the MS  1310 , cancels allocation of the Fast_UL_Ranging_IE, if the MS  1310  does not perform handover for a predetermined time, or, information indicating that the MS  1310  performs handover to another neighbor BS is received from the serving BS  1350 .  
      Because the process in which a neighbor BS failed to be selected as a final target BS cancels allocation of the Fast_UL_Ranging_IE for the MS is not directly related to the present invention, a detailed description thereof will be omitted. In the foregoing description of  FIG. 13 , if the serving BS fails to allocate the safety channels of the neighbor BS, the serving BS selects a plurality of handover candidate neighbor BSs based on the scanning result by the MS and allows the MS to perform handover to one of the neighbor BSs. Because the description of  FIG. 13  is similar to the description of FIGS.  5  to  8 , a detailed description of an individual operating process of the serving BS  1350 , the neighbor BS# 1   1360  and the neighbor BS# 2   1370  will be omitted.  
      As can be understood from the foregoing description, the present invention proposes a safety channel allocation scheme capable of minimizing inter-cell interference for an MS located in a boundary of a neighbor cell and a safety channel handover operation based on channel conditions in an OFDMA communication system, thereby guaranteeing communication quality of the MS located in the cell boundary.  
      While the present 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 present invention as defined by the appended claims.