Patent Publication Number: US-2007104177-A1

Title: System and method for allocating bandwidth in a wireless communication system

Description:
PRIORITY  
      This application claims the benefit under 35 U.S.C. § 119(a) of an application entitled “System and Method for Allocating Bandwidth in a Wireless Communication System” filed in the Korean Intellectual Property Office on Nov. 4, 2005 and assigned Serial No. 2005-105277, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION 1. Field of the Invention  
      The present invention relates generally to bandwidth allocation in a communication system, and in particular, to a system and method for allocating Uplink (UL) bandwidth in a wireless communication system.  
      2. Description of the Related Art  
      The wireless communication system was originally developed to provide voice service and transmit voice data. However, with the rapid growth of technology and ever-increasing user demands, the wireless communication system has evolved to transmit high-speed data. To this end, continuous research and development is actively being performed on the wireless communication system.  
       FIG. 1  is a diagram illustrating a configuration of a conventional wireless communication system.  
      Referring to  FIG. 1 , a wireless communication system generally has a multi-cell configuration including a cell  100  and a cell  150 . Further, the wireless communication system includes a first Base Station (BS)  110  for managing the cell  100 , a second BS  160  for managing the cell  150 , and a plurality of Mobile Stations (MSs)  101 ,  103 ,  130 ,  151  and  153 . The BSs  110  and  160  perform data communication with the MSs  101 ,  103 ,  130 ,  151  and  153  through a Uplink (UL) and a Downlink (DL). The MS  130  is located in a boundary area, i.e. handover region, between the cell  100  and the cell  150 . That is, if the MS  130  moves toward the cell  150  managed by the second BS  160  while exchanging signals with the first BS  110 , a serving BS in communication with the MS  130  is changed from the first BS  110  to the second BS  160 .  
      In the wireless communication system, the link used by the BSs and MSs for communication is generally divided into an uplink and a downlink. The downlink is used for transmitting information from a BS to an MS, and the uplink is used for transmitting information from an MS to a BS.  
      In order to transmit information to and receive information from a BS, an MS performs a link access procedure with the BS. The MS is allocated a bandwidth from the BS, and then performs communication with the BS.  
       FIG. 2  is a signaling diagram illustrating uplink bandwidth allocation in a conventional wireless communication system.  
      Referring to  FIG. 2 , a BS  200  broadcasts an Uplink Channel Descriptor (UCD) message to MSs in its cell in step  201 . The UCD message includes ranging code and ranging channel information.  
      Upon receipt of the UCD message, an MS  250  selects one of ranging codes for uplink bandwidth allocation, included in the UCD message. The MS  250  sends the selected ranging code over a ranging channel to the BS  200  in step  203 .  
      Upon receipt of the ranging code, the BS  200  sends an Uplink-MAP (UL-MAP) message including a Code Division Multiple Access Allocation Information Element (CDMA Allocation IE) to the MS  250  in step  205 . The UL-MAP message has, for example, a 6-byte size. The CDMA Allocation IE transmitted from the BS  200  to the MS  250  is shown in Table 1 below.  
                       TABLE 1                       Syntax   Size   Notes                  CDMA Allocation IE( )               {       Duration   6 bits   Indicates the duration, in units of               OFDMA slots, of the allocation       Repetition Coding   2 bits   0b00-No Repetition       Indication       0b01-Repetition of 2 used               0b10-Repetition of 4 used               0b11-Repetition of 6 used       Ranging Code   8 bits   Indicating the CDMA code used by the               MS       Ranging Symbol   8 bits   Indicating the OFDMA symbol code               used by the MS       Ranging Subchannel   7 bits   Indicating the OFDMA subchannel               used by the MS       BW Request Mandatory   1 bit   Indicates whether the SS shall include               Bandwidth(BW) Request in the               allocation       }                  
 
      The CDMA Allocation IE includes ‘Duration’ indicating the unit of Orthogonal Frequency Division Multiple Access (OFDMA) slots, ‘Repetition Coding Indication’ indicating the number of repetitions (or iterations), ‘Ranging Code’ indicating a CDMA code used by an MS, ‘Ranging Symbol’ indicating an OFDMA symbol used by the MS, ‘Ranging Subchannel’ indicating an OFDMA subchannel used by the MS, and ‘Bandwidth (BW) Request Mandatory’ indicating whether the MS will request a bandwidth.  
      Upon receipt of the UL-MAP message, the MS  250  sends a Bandwidth Request Header, which is a message for requesting a bandwidth, to the BS  200  in step  207 . The Bandwidth Request Header has, for example, a 6-byte size. The Bandwidth Request Header transmitted the MS  250  to the BS  200  is shown in Table 2 below.  
                       TABLE 2                       Syntax   Size   Notes                  BW-REQ Header( )               {       HT    1 bit   Header Type = 1       EC    1 bit   Encryption, always set to zero       Type    3 bits   Indicates the type of Bandwidth Request               Header       BR   19 bits   Bandwidth Request       CID   16 bits   Connection Identifier       HCS    8 bits   Header Check Sequence       }                  
 
      Bandwidth Request Header includes ‘Header Type (HT)’ indicating a header type, ‘Encryption (EC)’ indicating encryption, which is always set to ‘0′, ‘Type’ indicating a type of Bandwidth Request Header, ‘Bandwidth Request (BR)’ indicating bandwidth request, ‘Connection Identifier (CID)’ indicating a connection ID, and ‘Header Check Sequence (HCS)’ indicating a header check sequence. Therefore, the MS  250  sends a notification indicating a size of its desired uplink bandwidth to the BS  200  through the Bandwidth Request Header.  
      Upon receipt of the Bandwidth Request Header, the BS  200  sends a UL-MAP message including bandwidth allocation information to the corresponding MS  250  in step  209 .  
      Upon receipt of the allocated uplink bandwidth through the UL-MAP, the MS  250  transmits data to the BS  200  in step  211 . Generally, in communication between a BS and an MS, the MS is allocated a bandwidth through the uplink bandwidth allocation, and can communicate with the BS using the allocated bandwidth. For the bandwidth allocation, the MS  250  can communicate with the BS  200  by sending a request for a service bandwidth for data transmission to the BS  200 .  
      The bandwidth request for the initial network access will be described. In this case, an MS conducts negotiation on the basic function of a Mobile Station (MS), and the MS continuously sends an SS Basic Capability-Request (SBC-REQ) message and a Registration Request (REG-REQ) for registering the MS in a BS to the BS. In order to send these messages, the MS undergoes the above bandwidth request process. When the MS makes initial network access with the BS, the MS should undergo the bandwidth request process every time, causing an increase in the time required for initial access.  
      The wireless communication system, which takes mobility of MSs into account, broadcasts a Mobile Neighbor Advertisement (MOB-NBR-ADV) message including information on neighbor BSs to each of the MSs so that the MSs located in the cell boundary can acquire synchronization with the neighbor BSs without performing initial ranging. Here, the MS acquires synchronization with the neighbor BSs through the MOB-NBR-ADV message, measures signal strengths of the neighbor BSs, and requests uplink bandwidth allocation for handover request if there is any neighbor BS having a signal strength which is higher than that of its serving BS.  
      In the course of the uplink bandwidth allocation, the MS may not normally receive BS signals. For example, if the BS signal strength is low as the MS is located in the cell boundary, or when the wireless channel condition deteriorates, the MS may fail to normally receive the BS signals.  
       FIG. 3  is a signaling diagram illustrating uplink bandwidth allocation when a BS fails to receive a UL-MAP message in a conventional wireless communication system.  
      Referring to  FIG. 3 , a BS  300  broadcasts a UCD message including ranging code and ranging channel information in step  301 .  
      Upon receipt of the UCD message, an MS  350  selects one of ranging codes for uplink bandwidth allocation, included in the UCD message, and transmits the selected ranging code over a ranging channel to the BS  300  in step  303 .  
      Upon receipt of the ranging code, the BS  300  sends a UL-MAP message including a CDMA Allocation IE in step  305 . Herein, it is assumed that the MS  350  fails to normally receive the UL-MAP message transmitted by the BS  300 .  
      Upon failure to receive the UL-MAP message from the BS  300 , the MS  350  retransmits the ranging code transmitted in step  303  to the BS  300  after a lapse of a predetermined time in step  307 .  
      Upon receipt of the ranging code, the MS  350  retransmits a UL-MAP message including the CDMA Allocation IE to the BS  300  in step  309 .  
      Upon receipt of the UL-MAP message, the MS  350  sends a Bandwidth Request Header, which is a message for requesting a bandwidth, to the BS  300  in step  311 .  
      Upon receipt of the Bandwidth Request Header, the BS  300  sends a UL-MAP message including bandwidth allocation information to the corresponding MS  350  in step  313 .  
      Upon receipt of the allocated uplink bandwidth through the UL-MAP message, the MS  350  transmits data to the BS  300  in step  315 .  
      In some cases, as described above, the MSs located in the cell boundary or having a poor wireless channel condition may fail to normally receive the information for uplink bandwidth allocation, i.e. the UL-MAP message.  
      When the MS fails to normally receive the UL-MAP message transmitted from the BS, the MS retransmits the ranging code after a lapse of an exponential random backoff, i.e. predetermined time, causing an increase in the time required for uplink bandwidth allocation.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide an uplink bandwidth allocation system and method for transmitting data without time delay in a wireless communication system.  
      It is another object of the present invention to provide a system and method for stably performing uplink bandwidth allocation taking into account an MS that fails to normally receive BS signals in a wireless communication system.  
      According to the present invention, there is provided a method for allocating bandwidth in a wireless communication system, including broadcasting, at least one ranging code which is mapped with a bandwidth size for bandwidth allocation is set, receiving, from a mobile station (MS), a ranging code which is one of the at least one ranging code, allocating a bandwidth corresponding to the received ranging code to the MS and transmitting a message including the bandwidth allocation information to the MS.  
      According to the present invention, there is provided a method for allocating bandwidth in a wireless communication system, including receiving, from a base station (BS), a broadcast message which includes at least one ranging code mapped with a bandwidth size, selecting a ranging code, for which a bandwidth size desired to be allocated is set, from the received ranging codes, transmitting the selected ranging code to the BS and receiving a message including a bandwidth allocation information from the BS.  
      According to the present invention, there is provided a system for allocating bandwidth in a wireless communication system, including a BS for broadcasting at least one ranging code for which a bandwidth size is set for bandwidth allocation, receiving one of the ranging codes, allocating a bandwidth corresponding to the received ranging code to an MS and transmitting a message including the bandwidth allocation information to the MS, and the MS for receiving the at least one ranging code, selecting a ranging code for which a bandwidth size desired to be allocated from the BS, transmitting the selected ranging code to the BS and receiving the message including the bandwidth allocation information from the BS thereby receiving a bandwidth allocated from the BS.  
      According to the present invention, there is provided a system for allocating bandwidth in a wireless communication system, including a BS for broadcasting at least one ranging code for which a bandwidth size is set for bandwidth allocation, receiving a selected one of the at least one ranging codes, allocating a bandwidth corresponding to the received ranging code to an MS and transmitting a message including the bandwidth allocation information to the MS.  
      According to the present invention, there is provided a system for allocating bandwidth in a wireless communication system, including an MS for receiving from a BS at least one ranging code for which a bandwidth size is set, selecting one of the at least one ranging codes for which a bandwidth size desired to be allocated is set, transmitting the selected bandwidth to the BS and receiving a message including bandwidth allocation information from the BS, thereby receiving a bandwidth allocated from the BS. 
    
    
     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 conventional wireless communication system;  
       FIG. 2  is a signaling diagram illustrating uplink bandwidth allocation in a conventional wireless communication system;  
       FIG. 3  is a signaling diagram schematically illustrating uplink bandwidth allocation when a BS fails to receive a UL-MAP message in a conventional wireless communication system;  
       FIG. 4  is a signaling diagram illustrating uplink bandwidth allocation in a wireless communication system according to the present invention;  
       FIG. 5  is a signaling diagram illustrating uplink bandwidth allocation for which dedicated ranging codes for handover are used in a wireless communication system according to the present invention;  
       FIG. 6  is a flowchart illustrating a BS operation of allocating uplink bandwidths in a wireless communication system according to the present invention; and  
       FIG. 7  is a flowchart illustrating an MS operation of receiving an allocated uplink bandwidth in a wireless communication system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      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 the sake of clarity and conciseness.  
      The present invention provides a system and method for bandwidth allocation, and in particular, for allocating Uplink (UL) bandwidth in a wireless communication system. According to the present invention, if a BS allocates a bandwidth for at least one ranging code and broadcasts the allocated bandwidth, an MS transmits a ranging code corresponding to its desired bandwidth to the BS, is allocated an uplink bandwidth corresponding to the ranging code from the BS, and communicates with the BS using the allocated bandwidth.  
       FIG. 4  is a signaling diagram illustrating uplink bandwidth allocation in a wireless communication system according to the present invention.  
      Referring to  FIG. 4 , a BS  400  transmits, i.e. broadcasts, a UCD message to an MS  450  in its cell in step  401 . In the UCD message, a size of the bandwidth to be allocated to an MS is set according to ranging codes for uplink channel allocation, and a different bandwidth allocation size is set for each ranging code. The UCD message including ranging codes with different bandwidth allocation sizes is shown in Table 3 below.  
                           TABLE 3                       Name   Type(1 Byte)   Length   Value                  . . .                    Initial Ranging Codes   150   1   Number of                   initial ranging                   CDMA codes.       Periodic Ranging Codes   151   1       BW-REQ Ranging   152   1       Codes       HO Ranging Codes   194   1       Dedicated Ranging   200   Variable       Codes       Dedicated Ranging   201   1       Codes for Handover       . . .                   
 
      The UCD message includes ‘Initial Ranging Codes’ indicating initial ranging Code Division Multiple Access (CDMA) code numbers, ‘Periodic Ranging Code’, ‘Bandwidth-Request (BW-REQ) Ranging Codes’, and ‘Handover (HO) Ranging Code’. In particular, the UCD message includes ‘Dedicated Ranging Codes’, shown in Table 4 below, and ‘Dedicated Ranging Code for Handover’.  
                               TABLE 4                                   Syntax   Size   Notes                          Dedicated Ranging Codes( )                   {           Number of Dedicated           Ranging Codes (N Dedicated )           for (i = 0; i &lt; N Dedicated ; i++)           {           Number of Codes   1           Allocation Size   1   In units of Bytes           }           Dedicated Ranging Codes   1           for Handover           }                      
 
      As shown in Table 4, in the ‘Dedicated Ranging Codes’, a byte-sized bandwidth is allocated for each ranging code. The BS  400  can efficiently perform message transmission for uplink bandwidth allocation by broadcasting the UCD message in which a bandwidth is independently allocated for each individual ranging code. In addition, with the use of ‘Dedicated Ranging Code for Handover’, the BS  400  can broadcast a UL-MAP message a number of times to the MSs that have failed to receive BS signals. The ranging codes transmitted through the UCD message will now be described with reference to Table 5 below.  
                       TABLE 5                       Name   Code Number   Allocation Size (Byte)                                            Initial Ranging Codes   0˜3           Periodic Ranging Codes   4˜7       Bandwidth Ranging Codes    8˜11       Handover Ranging Codes   12˜15       Dedicated Ranging Codes   16, 17   50       (N Ded 1)       Dedicated Ranging Codes   18, 19   100       (N Ded 2)       Dedicated Ranging Code for   20, 21   50       Handover                  
 
      Table 5 shows code numbers allocated individually to ranging codes. For example, code numbers 0 to 3 are allocated to initial ranging codes, code numbers 4 to 7 are allocated to periodic ranging codes, code numbers 8 to 11 are allocated to bandwidth ranging codes, code numbers 12 to 15 are allocated to handover ranging codes, code numbers 16 and 17 are allocated to first dedicated ranging codes N ded 1, code numbers 18 and 19 are allocated to second dedicated ranging codes N ded 2, and code numbers 20 and 21 are allocated to dedicated ranging code for handover.  
      For example, 50 bytes are allocated to both the first dedicated ranging codes and the dedicated ranging codes for handover, and 100 bytes are allocated to the second dedicated ranging codes. The dedicated ranging codes and the dedicated ranging codes for handover can be additionally extended, and bandwidths can be allocated according thereto.  
      Therefore, the BS  400  can transmit dedicated ranging codes or dedicated ranging codes for handover to the MSs through the UCD message.  
      Upon receipt of the UCD message, the MS  450  selects a ranging code corresponding to its required bandwidth, and transmits the dedicated ranging code to the BS  400  in step  403 .  
      Upon receipt of the dedicated ranging code, the BS  400  allocates an uplink bandwidth corresponding to the ranging code to the MS  450 , and transmits the uplink bandwidth allocation information to the MS  450  through a UL-MAP message in step  405 . The UL-MAP message includes a CDMA Allocation Information Element (CDMA Allocation IE), and also includes bandwidth allocation information in response to an uplink bandwidth request of the MS  450 .  
      Upon receipt of the allocated uplink bandwidth through the UL-MAP message, the MS  450  transmits uplink data to the BS  400  using the allocated uplink bandwidth in step  407 . With reference to Table 5 above, a description will now be made of an exemplary operation in which the MS  450  selects the dedicated ranging code, transmits the selected ranging code to the BS  400  and is allocated a bandwidth for the ranging code.  
      When the MS  450  receives the UCD message, an uplink bandwidth required by the MS  450  for uplink data transmission is assumed to be 100 bytes. Therefore, the MS  450  transmits a ranging code, i.e. dedicated ranging code # 18  for which a 100-byte bandwidth is allocated, to the BS  400 . Then the BS  400  allocates a 100-byte bandwidth corresponding to the dedicated ranging code to the MS that transmitted the ranging code, and transmits the uplink bandwidth allocation information to the MS  450  through a UL-MAP message, thereby allocating an uplink bandwidth.  
       FIG. 5  is a signaling diagram illustrating uplink bandwidth allocation for which dedicated ranging codes for handover are used in a wireless communication system according to the present invention.  
      Referring to  FIG. 5 , a BS  500  transmits, i.e. broadcasts, a UCD message to an MS  550  in its cell in step  501 . In the UCD message, a size of the bandwidth allocated to every ranging code for uplink channel allocation is set, and a different bandwidth size is set for each ranging code. The UCD message includes not only the bandwidth size, but also dedicated ranging codes for which the number of repetitions is also set, i.e. dedicated ranging codes for handover.  
      If it is determined that the MS  550  receiving the UCD message is located in the cell boundary, the MS  550  selects a dedicated ranging code for handover in order to transmit a MOB-MSHO-REQ message or a MOB-HO-IND message to the BS  500  or a serving BS. The MS  550  transmits the dedicated ranging code for handover, such as code # 20  shown in Table 5, to the BS  500  in step  503 .  
      Upon receipt of the dedicated ranging code for handover, the BS  500  transmits a UL-MAP message used for allocating a 50-byte bandwidth allocated to the dedicated ranging code for handover to the MS  550  in step  505 .  
      If the number of repetitions is set for the dedicated ranging code for handover, the BS  500  continuously transmits the UL-MAP message at intervals in steps  507  and  509 . For example, in  FIG. 5 , the number of repetitions is set to 3. Therefore, the BS  500  transmits the same UL-MAP message for bandwidth allocation to the MS  550  three times in steps  505 ,  507  and  509 . As a result, even though the MS  550  fails to receive the UL-MAP message in the cell boundary area, the BS  500  transmits the UL-MAP message a number of times. Therefore, it is possible to prevent the delay in time for uplink bandwidth allocation as the MS  550  fails to receive the signal transmitted by the BS  500 . The UL-MAP message includes therein a CDMA Allocation IE, and for example, includes information for 50-byte bandwidth allocation.  
      Upon receipt of the allocated bandwidth from the BS  500 , the MS  550  sends a MOB-MSHO-REQ message or a MOB-HO-IND message for handover request to the BS  500  in step  511 .  
      The ranging codes for handover can also be used by the MSs scheduled to perform handover. In addition, even the MSs that have failed to normally receive BS signals can be allocated uplink bandwidths from the BS using the ranging codes for handover.  
       FIG. 6  is a flowchart illustrating a BS operation of allocating uplink bandwidths in a wireless communication system according to the present invention.  
      Referring to  FIG. 6 , in step  601 , a BS broadcasts a UCD message to MSs in its cell. The broadcasted UCD message includes therein dedicated ranging codes, and for the ranging codes, bandwidths for uplink bandwidth selection required by the MSs in the uplink are set. In addition, the ranging codes include even the ranging codes for handover, i.e. the ranging codes for which not only the bandwidths but also the number of repetitions are set.  
      In step  603 , the BS receives a ranging code from an MS. Here, the bandwidth ranging code received from the MS is the ranging code selected by the MS among the ranging codes included in the UCD message transmitted to the MS by the BS.  
      In step  605 , the BS determines whether the received ranging code is a dedicated ranging code. In other words, the BS determines whether the bandwidth ranging code is a ranging code for which a bandwidth is allocated or the number of repetitions is set. If it is determined that the received ranging code is not the dedicated ranging code, the BS proceeds to step  607 , considering that the received ranging code is a bandwidth request ranging code. However, if it is determined that the received ranging code is the dedicated ranging code, the BS proceeds to step  611 .  
      In step  607 , the BS transmits a UL-MAP to the MS. Here, the UL-MAP includes a CDMA Allocation IE.  
      In step  609 , the BS receives a Bandwidth Request Header from the MS, and then proceeds to step  621 . Upon receipt of the Bandwidth Request Header, the BS allocates a bandwidth to the MS using a UL-MAP.  
      In step  611 , if the received ranging code is the dedicated ranging code, the BS determines whether the ranging code is a first dedicated ranging code. If it is determined that the ranging code is the first dedicated ranging code, the BS allocates in step  613  a bandwidth set for the first dedicated ranging code, for example, the 50-byte bandwidth shown in Table 5, and transmits a UL-MAP including the allocated bandwidth information to the MS, and then proceeds to step  621 .  
      If it is determined that the ranging code is not the first dedicated ranging code, the BS determines in step  615  whether the ranging code is a second dedicated ranging code. If it is determined that the ranging code is the second dedicated ranging code for which a bandwidth is allocated, the BS allocates in step  617  a bandwidth set for the second dedicated ranging code, for example, the 100-byte bandwidth shown in Table 5, and transmits a UL-MAP including the allocated bandwidth information to the MS, and then proceeds to step  621 .  
      If it is determined that the ranging code is not the second dedicated ranging code, indicating that the ranging code is a ranging code for which the number of repetitions is set, then the BS allocates in step  619  a bandwidth allocated for the ranging code, for example, the 50-byte bandwidth shown in Table 5, and continuously transmits the allocated bandwidth information to the MS a number of times, and then proceeds to step  621 .  
      In step  621 , the BS receives uplink data from the MS using the allocated bandwidth.  
      Although the present invention has been described with reference to the case where the dedicated ranging code is divided into first and second dedicated ranging codes, the ranging code can be additionally extended to, for example, the dedicated ranging code for which the number of repetitions is set. As a result, the BS can set various bandwidths for the ranging codes to perform uplink bandwidth allocation, and can also add the number of repetitions, for communication with the MS.  
       FIG. 7  is a flowchart illustrating an MS operation of receiving an allocated uplink bandwidth in a wireless communication system according to the present invention.  
      Referring to  FIG. 7 , in step  701 , an MS receives a UCD message from a BS. The UCD message broadcasted from the BS includes dedicated ranging codes, and for the ranging codes, bandwidths required by the MSs in the uplink are set. In addition, the ranging codes include even the ranging codes for handover, i.e. the ranging codes for which not only the bandwidths but also the number of repetitions are set.  
      In step  703 , the MS determines whether to use the dedicated ranging code included in the UCD message received from the BS. If it is determined not to use the dedicated ranging code, the MS proceeds to step  705 . However, if it is determined to use the dedicated ranging code, the MS proceeds to step  711 .  
      In step  705 , the MS transmits a bandwidth request ranging code to the BS. Here, the MS selects one of the uplink bandwidth request ranging codes, and transmits the selected ranging code over a ranging channel to the BS.  
      In step  707 , the MS receives from the BS a UL-MAP message including a CDMA Allocation IE shown in Table 1.  
      In step  709 , the MS transmits to the BS a Bandwidth Request Header for requesting a bandwidth, and then proceeds to step  717 . When the MS does not use the dedicated ranging code, it is allocated an uplink bandwidth from the BS using the conventional uplink bandwidth allocation method.  
      In step  711 , the MS determines whether to perform handover, i.e. whether to use a dedicated ranging code for handover. If it is determined not to use the dedicated ranging code for handover, the MS selects in step  715  a dedicated ranging code according to each bandwidth for uplink data transmission, and then proceeds to step  717 . However, if it is determined to use the dedicated ranging code for handover, i.e. if the MS is in a situation where it has difficulty in receiving BS signals, the MS selects in step  713  a dedicated ranging code for handover and transmits the selected ranging code to the BS, and then proceeds to step  717 . When the MS selects the dedicated ranging code for handover, it receives from the BS a UL-MAP message for bandwidth allocation a number of times according to the number of repetitions.  
      In step  717 , the MS receives a UL-MAP message including uplink bandwidth allocation information from the BS. In step  719 , the MS transmits data to the BS through the uplink bandwidth allocated from the BS. In the case where the MS selects in step  713  the dedicated ranging code for handover and transmits the selected ranging code to the BS, the MS scheduled to perform handover as well as the MSs having low-strength BS signals can select the dedicated ranging code for handover, and continuously receive a UL-MAP message from the BS a number of times according to the number of repetitions.  
      In sum, the present invention sets uplink bandwidth allocation information for individual ranging codes in the UCD message transmitted by the BS and transmits/receives the ranging codes, making it possible to allocate uplink bandwidths for a short time as compared with the conventional uplink bandwidth allocation method. In addition, the BS continuously transmits the UL-MAP message even to the MSs having the low-strength BS signals a number of times previously set for the ranging code, thereby securing stable uplink bandwidth allocation.  
      As can be understood from the foregoing description, the BS includes in the UCD message the ranging codes for which bandwidth allocation information is previously set, before transmission. Upon receipt of the UCD message, the MS selects a ranging code and transmits the selected ranging code to the BS, thereby contributing to a decrease in the time required for uplink bandwidth allocation. In addition, the BS continuously transmits the message for uplink bandwidth allocation taking into account the MSs having low-strength BS signals, increasing the reception probability of the message for uplink bandwidth allocation, thereby securing stable uplink bandwidth allocation.  
      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.