Abstract:
Disclosed are a system and a method for assigning ranging codes in a communication system. The communication system classifies rangings between a transmission unit and a reception unit of the broadband wireless access communication system into an initial ranging, a periodic ranging, a bandwidth request ranging, and a drop ranging. The communication system creates a first number of ranging codes used for the rangings and assigning a second number of ranging codes selected from the first number of ranging codes as drop ranging codes used for the drop ranging.

Description:
PRIORITY 
   This application claims priority to an application entitled “System And Method For Selecting Serving Base Station According To Drop Of Mobile Subscriber Station In Broadband Wireless Access Communication System” filed with the Korean Intellectual Property Office on Sep. 4, 2003 and assigned Serial No. 2003-61941, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a broadband wireless access communication system, and more particularly to an apparatus and a method for selecting a serving base station according to a drop of a mobile subscriber station during a communication. 
   2. Description of the Related Art 
   Recently, extensive studies and research have been being carried out for the 4 th  generation (“4G”) communication systems in order to provide subscribers with services having a superior quality of service (“QoS”) at higher transmission speeds. In particular, studies are being actively carried out in relation to the 4G communication systems in order to provide high speed services having a superior QoS through broadband wireless access communication systems, such as wireless local area network (“LAN”) communication systems and wireless metropolitan area network (“MAN”) communication systems, while ensuring the mobility of the broadband wireless access communication systems. 
   The wireless MAN communication system has a wide service coverage area and provides data at a higher transmission speed than a LAN system, and as such the wireless MAN communication system is adaptable for a high-speed communication service. However, the wireless MAN communication system does not take into consideration the mobility of a user, that is, subscriber station (“SS”), so a handover, which is required when the SS moves at a high speed, is not taken into consideration in the wireless MAN communication system. The wireless MAN communication system is one type of broadband wireless access communication system and has a wider service coverage area and higher transmission speed as compared with those of a wireless LAN communication system. 
   In order to provide a broadband transport network for a physical channel of the wireless MAN communication system, an IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system utilizing an orthogonal frequency division multiplexing (“OFDM”) scheme and an orthogonal frequency division multiple access (“OFDMA”) scheme has been suggested. 
   As the IEEE 802.16a communication system applies the OFDM/OFDMA schemes to the wireless MAN system, the physical channel signals can be transmitted through a plurality of sub-carriers so that a high-speed data transmission is possible. In short, the IEEE 802.16a communication system is a broadband wireless access communication system using the OFDM/OFDMA schemes. 
   Hereinafter, a structure of a conventional IEEE 802.16a communication system will be described with reference to  FIG. 1 . 
     FIG. 1  is a structure diagram schematically illustrating the conventional IEEE 802.16a communication system. 
   Referring to  FIG. 1 , the IEEE 802.16a communication system has a single cell structure and includes a base station (BS)  100  and a plurality of SSs  110 ,  120  and  130  managed by the base station  100 . The base station  100  conducts communications with the SSs  110 ,  120  and  130  using the OFDM/OFDMA schemes. 
   Hereinafter, a structure of a downlink frame of the IEEE 802.16a communication system will be described with reference to  FIG. 2 . 
     FIG. 2  is a structure diagram schematically illustrating the structure of the downlink frame of the IEEE 802.16a communication system. 
   Referring to  FIG. 2 , the downlink frame includes a preamble field  200 , a broadcast control field  210 , and a plurality of time division multiplex (“TDM”) fields  220  and  230 . A synchronous signal, that is, a preamble sequence for synchronizing the SSs with the base station, is transmitted through the preamble field  200 . The broadcast control field  210  includes a DL (downlink)_MAP field  211  and a UL (uplink)_MAP field  213 . The DL_MAP field  211  is a field for transmitting a DL_MAP message. Information elements (“IEs”) included in the DL_MAP message are represented in Table 1. 
   
     
       
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               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 
             
             
                Number of DL_MAP Element n 
               16 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 Information Element( ) 
               Variable 
               See corresponding PHY specification 
             
             
                 if!(byte boundary) { 
                4 bits 
               Padding to reach byte boundary 
             
             
                  Padding Nibble 
             
             
                      } 
             
             
                     } 
             
             
                   } 
             
             
                 } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 1, the DL_MAP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, PHY (physical) Synchronization Field corresponding to modulation/demodulation schemes applied to a physical channel for achieving synchronization, DCD Count representing a count according to the variation of a configuration of a downlink channel descript (“DCD”) message including a downlink burst profile, Base Station ID, and Number of DL_MAP Elements n representing the number of elements remaining after the Base Station ID. Although it is not shown in Table 1, the DL_MAP message also includes information related to the ranging codes assigned to each ranging, which will be described later. 
   In addition, the UL_MAP field  213  is a field for transmitting a UL_MAP message. IEs included in the UL_MAP message are represented in Table 2. 
   
     
       
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
                UL_MAP_Message_Format( ) { 
                 
                 
             
             
                Management Message Type=3 
                8 bits 
             
             
                  Uplink Channel ID 
                8 bits 
             
             
                    UCD Count 
                8 bits 
             
             
                Number of UL_MAP Element n 
               16 bits 
             
             
                 Allocation Start Time 
               32 bits 
             
             
               Begin PHY Specific section { 
                 
               See Applicable PHY section 
             
             
                  for (i=1; i&lt;=n; i++) 
                 
               For each UL_MAP element 1 to n 
             
             
               UL_MAP_Information_Element( ) 
               Variable 
               See corresponding PHY specification 
             
             
                     } 
             
             
                   } 
             
             
                  } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 2, the UL_MAP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Uplink Channel ID representing an available uplink channel ID, UCD Count representing a count according to the variation of a configuration of an uplink channel descript (“UCD”) message including an uplink burst profile, and Number of UL_MAP Elements n representing the number of elements remaining after the UCD count. The Uplink Channel ID is allocated only to a medium access control (“MAC”) sub-layer. 
   The TDM field s  220  and  230  are field s corresponding to time slots which are allocated according to the TDM/TDMA (time division multiple access) schemes corresponding to the SSs. The base station transmits broadcast information to the SSs, which are managed by the base station, through the DL_MAP field  211  of the downlink frame by using a predetermined center carrier. As the SSs are powered on, the SSs monitor all frequency bands, which are preset in the SSs, in order to detect a reference channel signal, such as a pilot channel signal having the highest carrier to interference and noise ratio (“CINR”). 
   An SS selects a base station, which has transmitted to the SS the pilot signal having the highest CINR, as a base station for the SS. The SS can then recognize information controlling the uplink and the downlink of the SS and information representing a real data transmission/reception position by checking the DL_MAP field  211  and the UL_MAP field  213  of the downlink frame transmitted from the base station. 
   A configuration of the UCD message is represented in Table 3. 
   
     
       
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
               UCD-Message_Format( ) { 
                 
                 
             
             
                Management Message Type=0 
               8 bits 
             
             
                Uplink channel ID 
               8 bits 
             
             
                Configuration Change Count 
               8 bits 
             
             
                Mini-slot size 
               8 bits 
             
             
                Ranging Backoff Start 
               8 bits 
             
             
                Ranging Backoff End 
               8 bits 
             
             
                Request Backoff Start 
               8 bits 
             
             
                Request Backoff End 
               8 bits 
             
             
                TLV Encoded Information for the overall channel 
               Variable 
             
             
                Begin PHY Specific Section { 
             
             
                 for(i=1; i&lt;n; i+n) 
             
             
                  Uplink_Burst_Descriptor 
               Variable 
             
             
                 } 
             
             
                } 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 3, the UCD message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Uplink channel ID representing an available uplink channel ID, Configuration Change Count counted in the base station, mini-time slot size representing a size of a mini-time slot of an uplink physical channel, Ranging Backoff start representing a start point of backoff using an initial ranging, that is, representing a size of an initial backoff window using the initial ranging, Ranging Backoff End representing an end point of backoff using an initial ranging, that is, representing a size of a final backoff window, Request Backoff start representing a start point of backoff for contention data and requests, that is, representing a size of an initial backoff window, and Request Backoff End representing an end point of backoff for contention data and requests, that is, representing a size of a final backoff window. A backoff value is a waiting time required for the next ranging if the present ranging fails. If the SS fails to perform the ranging, the base station must transmit the backoff value, i.e. the waiting time for the next ranging, to the SS. For instance, if the backoff value is determined as “10” based on the ranging backoff start and the ranging backoff end, the SS must perform the next ranging after by passing 2 10  ranging chances (1024 ranging chances) according to a truncated binary exponential backoff algorithm. 
   A structure of an uplink frame of the IEEE 802.16a communication system will be described with reference to  FIG. 3 . 
     FIG. 3  is a structure diagram schematically illustrating the structure of the uplink frame of the IEEE 802.16a communication system. 
   Prior to explaining  FIG. 3 , a description will be made in relation to rangings, such as an initial ranging, a maintenance ranging, that is, a periodic ranging, and a bandwidth request ranging, used for the IEEE 802.16a communication system. 
   First, the initial ranging will be described. The initial ranging is carried out in order to synchronize the base station with the SS, in which a time offset and a transmit power between the SS and the base station are precisely adjusted. That is, after the SS has been powered on, the SS receives the DL_MAP message and the UL_MAP/UCD message in order to synchronize with the base station. Then, the initial ranging is carried out with respect to the SS in order to adjust the time offset and the transmit power of the SS in relation to the base station. Herein, since the IEEE 802.16a communication system uses the OFDM/OFDMA schemes, ranging sub-channels and ranging codes are required for the initial ranging. Thus, the base station assigns available ranging codes to the SS according to the object or the type of rangings. 
   In detail, the ranging codes are created by segmenting a pseudo-random noise (“PN”) sequence having a predetermined bit length into predetermined ranging code units. In general, two ranging sub-channels, having a 53-bit length, forms one ranging channel and a PN code is segmented through a ranging channel having a 106-bit length, thereby forming the ranging codes. Such ranging codes are assigned to the SS, for instance, a maximum of 48 ranging codes (RC # 1  to RC # 48 ) can be assigned to the SS. At least two ranging codes are used for the initial ranging, the periodic ranging and the bandwidth request ranging as default values with respect to each SS. That is, the ranging codes are differently assigned according to the initial ranging, the periodic ranging and the bandwidth request ranging. For instance, N ranging codes are assigned for the initial ranging, M ranging codes are assigned for the periodic ranging, and L ranging codes are assigned for the bandwidth request ranging. As mentioned above, the assigned ranging codes are transmitted to the SS through the UCD message and the SS performs the initial ranging by using the ranging codes included in the UCD message in match with objects of the ranging codes. 
   Second, the periodic ranging will be described. The periodic ranging is periodically carried out by means of the SS having the time offset and the transmit power adjusted through the initial ranging, in such a manner that the SS can adjust the channel status with respect to the base station. The SS performs the periodic ranging by using ranging codes assigned thereto for the periodic ranging. 
   Third, the bandwidth request ranging will be described. The bandwidth request ranging is carried out by means of the SS having the time offset and the transmit power adjusted through the initial ranging, wherein the SS requests a bandwidth assignment in order to communicate with the base station. 
   Referring back to  FIG. 3 , the uplink frame consists of an initial maintenance opportunities field  300  using the initial ranging and the maintenance ranging, that is, the periodic ranging, a request contention opportunities field  310  using the bandwidth request ranging, and SS scheduled data fields  320  including uplink data of the SSs. The initial maintenance opportunities field  300  includes a plurality of access burst intervals including the real initial ranging and the periodic ranging and a collision interval created because of the collision between the access burst intervals. The request contention opportunities field  310  includes a plurality of bandwidth request intervals including the real bandwidth request ranging and a collision interval created because of the collision between the bandwidth request intervals. In addition, the SS scheduled data fields  320  consist of a plurality of SS scheduled data fields (first SS scheduled data field to SS N th  scheduled data field) and SS transition gaps formed between the SS scheduled data fields (first SS scheduled data field to SS N th  scheduled data field). 
   An uplink interval usage code (“UIUC”) field is provided for recoding information representing the usage of the offset recorded in the offset field. The UIUC field is shown in Table 4. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
                 
                 
               Connection 
                 
             
             
               IE name 
               UIUC 
               ID 
               Description 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               reserved 
               0 
               NA 
               Reserved for future use. 
             
             
               Request 
               1 
               any 
               Starting offset of request region. 
             
             
               Initial 
               2 
               broadcast 
               Starting offset of maintenance region (used in Initial Ranging). 
             
             
               Maintenance 
             
             
               Station 
               3 
               unicast 
               Starting offset of maintenance region (used in periodic Ranging). 
             
             
               Maintenance 
             
             
               Data Grant 
               4 
               unicast 
               Starting offset of Data Grant Burst Type 1 assignment. 
             
             
               Burst Type 1 
             
             
               Data Grant 
               5 
               unicast 
               Starting offset of Data Grant Burst Type 2 assignment. 
             
             
               Burst Type 2 
             
             
               Data Grant 
               6 
               unicast 
               Starting offset of Data Grant Burst Type 3 assignment. 
             
             
               Burst Type 3 
             
             
               Data Grant 
               7 
               unicast 
               Starting offset of Data Grant Burst Type 4 assignment. 
             
             
               Burst Type 4 
             
             
               Data Grant 
               8 
               unicast 
               Starting offset of Data Grant Burst Type 5 assignment. 
             
             
               Burst Type 5 
             
             
               Data Grant 
               9 
               unicast 
               Starting offset of Data Grant Burst Type 6 assignment. 
             
             
               Burst Type 6 
             
             
               Null IE 
               10 
               zero 
               Ending offset of the previous grant. 
             
             
                 
                 
                 
               Used to bound the length of the last actual interval allocation. 
             
             
               Empty 
               11 
               zero 
               Used to schedule gaps in transmission. 
             
             
               reserved 
               12-15 
               N/A 
               Reserved. 
             
             
                 
             
           
        
       
     
   
   As shown in Table 4, if “2” is recorded in the UIUC field, the starting offset used for the initial ranging is recorded in the offset field. If “3” is recorded in the UIUC field, the starting offset used for the bandwidth request ranging or the maintenance ranging is recorded in the offset field. As mentioned above, the offset field is provided to record starting offset values used for the initial ranging, the bandwidth request ranging or the maintenance ranging corresponding to information recorded in the UIUC field. Information related to the characteristics of a physical channel transmitted from the UIUC field is recorded in the UCD. 
   A ranging process between the base station and the SS in the IEEE 802.16a communication system will be described with reference to  FIG. 4 . 
     FIG. 4  is a signal flow diagram illustrating the ranging process between the base station and the SS in the IEEE 802.16a communication system. 
   Referring to  FIG. 4 , as an SS  400  is powered on, the SS  400  monitors all of the frequency bands, which are preset in the SS  400 , in order to detect a pilot channel signal having the highest CINR. In addition, the SS  400  selects a base station  420  which has transmitted the pilot signal having the highest CINR to the SS  400  as a base station for the SS  400 , so the SS  400  receives the preamble of the downlink frame transmitted from the base station  420 , thereby obtaining system synchronization with respect to the base station  420 . 
   As described above, when the system synchronization is attained between the SS  400  and the base station  420 , the base station  420  transmits the DL_MAP message and the UL_MAP message to the SS  400  (steps  411  and  413 ). Herein, as described above with reference to Table 1, the DL_MAP message notifies the SS  400  of the information required for the SS  400  to obtain the system synchronization with respect to the base station  420  in the downlink and information about a structure of the physical channel capable of receiving messages transmitted to the SS  400  from the downlink. In addition, as describe above with reference to Table 2, the UL_MAP message notifies the SS  400  of the information about a scheduling period of the SS  400  in the uplink and the structure of the physical channel. In addition, the DL_MAP message is periodically broadcast to all of the SSs from the base station  420 . If a predetermined SS, that is, if the SS  400  can continuously receive the DL_MAP message, it will be represented that the SS  400  is synchronized with the base station  420 . That is, the SS  400  receiving the DL_MAP message can receive all of the messages transmitted to the downlink. In addition, as described above with reference to Table 3, if the SS  400  fails to access to the base station  420 , the base station  420  transmits the UCD message including the information representing the available backoff value to the SS  400 . 
   The SS  400 , which has been synchronized with the base station  420 , transmits a ranging request (“RNG_REQ”) message to the base station  420  (step  415 ). Upon receiving the RNG_REQ message from the SS  400 , the base station  420  transmits a ranging response (“RNG_RSP”) message including information required for correcting frequency for the ranging, time and transmit power to the SS  400  (step  417 ). 
   A configuration of the RNG_REQ message is represented in Table 5. 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
               Syntax 
               Size 
               Notes 
             
             
                 
                 
             
           
           
             
                 
               RNG-REQ_Message_Format( ) { 
                 
                 
             
             
                 
                Management Message Type = 4 
               8 bits 
             
             
                 
                Downlink Channel ID 
               8 bits 
             
             
                 
                Pending Until Complete 
               8 bits 
             
             
                 
                TLV Encoded Information 
               Variable 
               TLV specific 
             
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   In Table 5, the “Downlink Channel ID” is a downlink channel identifier included in the RNG_REQ message received in the SS through the UCD and the “Pending Until Complete” is a priority of transmitted ranging responses. If the “Pending Until Complete” is “0”, a previously transmitted ranging response has a priority, and if the “Pending Until Complete” is not “0”, a presently transmitted ranging response has a priority. 
   A configuration of the RNG_RSP message is represented in Table 6. 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 6 
             
             
                 
                 
             
             
                 
               Syntax 
               Size 
               Notes 
             
             
                 
                 
             
           
           
             
                 
               RNG-RSP_Message_Format( ) { 
                 
                 
             
             
                 
                Management Message Type = 5 
               8 bits 
             
             
                 
                Uplink Channel ID 
               8 bits 
             
             
                 
                TLV Encoded Information 
               Variable 
               TLV specific 
             
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   In Table 6, the “Uplink Channel ID” is an ID of an uplink channel included in the RNG_REQ message. Since the IEEE 802.16a communication system shown in  FIG. 4  relates to a fixed SS, that is, since the IEEE 802.16a communication system shown in  FIG. 4  does not take into consideration the mobility of the SS, the base station  420  communicating with the SS  400  becomes a serving base station. 
   The IEEE 802.16a communication system has the signal cell structure in which the mobility of the SS is not considered. Meanwhile, an IEEE 802.16e communication system is defined as a communication system in which the mobility of the SS is added to the IEEE 802.16a communication system. Thus, the IEEE 802.16e communication system must consider the mobility of the SS under a multi-cell environment. In order to ensure the mobility of the SS under the multi-cell environment, the operations of the SS and the base station must be changed. To this end, various studies have been carried out relating to a handover of the SS in order to provide for the mobility to the SS under the multi-cell environment. 
   A structure of a conventional IEEE 802.16e communication system will be described with reference to  FIG. 5 . 
     FIG. 5  is a structure diagram schematically illustrating the structure of the conventional IEEE 802.16e communication system. 
   Referring to  FIG. 5 , the IEEE 802.16e communication system has a multi-cell structure consisting of cells  500  and  550  and includes a first base station  510  for managing the cell  500 , a second base station  540  for managing the cell  550 , and a plurality of mobile subscriber stations (“MSSs”)  511 ,  513 ,  530 ,  551 , and  553 . The MSS signify an SS having mobility. The base stations  510  and  540  communicate with the MSSs  511 ,  513 ,  530 ,  551 , and  553  using the OFDM/OFDMA schemes. From among the MSSs  511 ,  513 ,  530 ,  551 , and  553 , the MSS  530  is positioned in a boundary cell formed between the cell  500  and the cell  550 , that is, the MSS  530  is positioned in a handover region. Thus, the MSS  530  must be provided with a handover function in order to realize the mobility of the MSS  530 . 
   In the IEEE 802.16e communication system, a MSS receives pilot channel signals transmitted from a plurality of base stations and measures the CINR of the pilot channel signals. In addition, the MSS selects a base station, which has transmitted a pilot signal having a highest CINR, as a base station of the MSS. That is, the MSS regards the base station transmitting the pilot signal having the highest CINR as a serving base station of the MSS. After selecting the serving base station, the MSS receives the downlink frame and the uplink frame transmitted from the serving base station. Herein, the downlink frame and the uplink frame of the IEEE 802.16e communication system have structures identical to those of the downlink frame and the uplink frame of the IEEE 802.16a communication system described with reference to  FIGS. 2 and 3 . 
   The serving base station transmits a mobile neighbor advertisement (“MOB_NBR_ADV”) message to the MSS. A configuration of the MOB_NBR_ADV message is represented in Table 7. 
   
     
       
             
             
             
           
         
             
               TABLE 7 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
               MOB_NBR-ADV_Message_Format( ) { 
                 
                 
             
             
                Management Message Type = 48 
                8 bits 
             
             
                Configuration Change Count 
                8 bits 
             
             
                N_NEIGHBORS 
                8 bits 
             
             
                For (j=0 : j&lt;N_NEIGHBORS : j++) { 
             
             
                 Neighbor BS-ID 
               48 bits 
             
             
                 Physical Frequency 
               32 bits 
             
             
                 TLV Encoded Neighbor information 
               Variable 
               TLV specific 
             
             
                } 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 7, the MOB_NBR_ADV message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Configuration Change Count representing the number of configurations to be changed, N_NEIGHBORS representing the number of neighbor base stations, Neighbor BS-ID representing identifiers of neighbor base stations, Physical Frequency representing a physical channel frequency of the neighbor base stations, and TLV (type length variable) Encoded Neighbor Information representing variable information about the neighbor base stations. 
   After receiving the MOB_NBR_ADV message, the MSS transmits a mobile scanning interval allocation request (“MOB_SCN_REQ”) message to the serving base station if it is necessary to scan the CINRs of the pilot channel signals transmitted from the neighbor base stations. A scan request time of the MSS for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations does not directly relate to the CINR scanning operation, so it will not be further described below. A configuration of the MOB_SCN_REQ message is represented in Table 8. 
   
     
       
             
             
             
           
         
             
               TABLE 8 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
               MOB_SCN-REQ_Message_Format( ) { 
                 
                 
             
             
                Management Message Type = ? 
                8 bits 
             
             
                Scan Duration 
               16 bits 
               Units are frames. 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 8, the MOB_SCN_REQ message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, and Scan Duration representing a scan interval for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations. The scan duration is formed in a frame unit. In Table 8, the Management Message Type for the MOB_SCN_REQ message is not yet defined (Management Message Type=undefined). 
   After receiving the MOB_SCN_REQ message, the serving base station transmits a mobile scanning interval allocation response (“MOB_SCN_RSP”) message including scan information, which must be scanned by the MSS, to the MSS. A configuration of the MOB_SCN_RSP message is represented in Table 9. 
   
     
       
             
             
             
           
         
             
               TABLE 9 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
               MOB_SCN-RSP_Message_Format( ) { 
                 
                 
             
             
                Management Message Type = ? 
                8 bits 
             
             
                Length 
                8 bits 
               in bytes 
             
             
                For (i=0 : i&lt;Length/3: i++) { 
             
             
                 CID 
               16 bits 
               basic CID of 
             
             
                 
                 
               the MSS 
             
             
                 Duration 
                8 bits 
               in frames 
             
             
                } 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 9, the MOB_SCN_RSP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Connection ID (“CID”) of the MSS, which has transmitted the MOB_SCN_REQ message and Duration. In Table 9, the Management Message Type for the MOB_SCN_RSP message is not yet defined (Management Message Type=undefined). The Duration represents an area in which the MSS scans the CINR of the pilot channel signal. After receiving the MOB_SCN_RSP message including the scanning information, the MSS scans the CINRs of the pilot signals of the neighbor base stations included in the MOB_SCN_RSP message corresponding to scanning information parameters. 
   In order to provide the handover function in the IEEE 802.16e communication system, the MSS must measure the CINRs of the pilot channel signals transmitted from the neighbor base stations and the base station of the MSS, that is, the serving base station. If the CINR of the pilot channel signals transmitted from the serving base station is less than the CINRs of the pilot channel signals transmitted from the neighbor base stations, the MSS sends a signal requesting the handover to the serving base station. 
   A handover process according to the request of the MSS in the conventional IEEE 802.16e communication system will be described with reference to  FIG. 6 . 
     FIG. 6  is a signal flow diagram illustrating the handover process according to the request of the MSS in the conventional IEEE 802.16e communication system. 
   Referring to  FIG. 6 , a serving base station  610  transmits an MOB_NBR_ADV message to an MSS  600  (step  611 ). Upon receiving the MOB_NBR_ADV message from the serving base station  610 , the MSS  600  obtains information related to the neighbor base stations and transmits an MOB_SCN_REQ message to the serving base station  610  if it is necessary to scan (“scan” and “measure” will be used synonymously with respect to determining CINRs) the CINRs of pilot channels signals transmitted from the neighbor base stations (step  613 ). A scan request time of the MSS  600  for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations does not directly relate to the CINR scanning operation, so it will not be further described below. The serving base station  610  receiving the MOB_SCN_REQ message transmits an MOB_SCN_RSP message including scanning information, which must be scanned by the MSS  600 , to the MSS  600  (step  615 ). Upon receiving the MOB_SCN_RSP message including scanning information from the serving base station  610 , the MSS  600  scans parameters included in the MOB_SCN_RSP message, that is, the MSS  600  scans the CINRs of the pilot channel signals of the neighbor base stations obtained through the MOB_NBR_ADV message (step  617 ). Although a process for measuring the CINR signal of the pilot channel signal transmitted from the serving base station  610  is not separately illustrated in  FIG. 6 , the MSS  600  may continuously measure the CINR of the pilot channel signal transmitted from the serving base station  610 . 
   After scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations, if the MSS  600  decides to change the serving base station thereof (step  619 ), that is, if the MSS  600  decides to replace the serving base station  610  with a new base station having a structure different from the structure of the serving base station  610 , the MSS  600  transmits a mobile MSS handover request (“MOB_MSSHO_REQ”) message to the serving base station  610 . Herein, a base station, which can be selected as the new base station due to the handover of the MSS  600 , is called a “target BS”. A configuration of the MOB_MSSHO_REQ message is represented in Table 10. 
   
     
       
             
             
             
           
         
             
               TABLE 10 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
               MOB_MSSHO-REQ_Message_Format( ) { 
                 
                 
             
             
                Management Message Type = 52 
                8 bits 
             
             
                N_Recommended 
                8 bits 
             
             
                For (j=0 : j&lt;N_NEIGHBORS : j++) { 
             
             
                 Neighbor BS-ID 
               48 bits 
             
             
                 BS S/(N+1) 
                8 bits 
             
             
                 Service level prediction 
                8 bits 
             
             
                } 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 10, the MOB_MSSHO_REQ message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, and a scanning result of the MSS  600 . In Table 10, N_Recommended represents the number of neighbor base stations, which have transmitted pilot channel signals having CINRs greater than a predetermined CINR, detected through the scanning operation of the MSS  600  for the CINRs of the pilot channel signals transmitted from the neighbor base station. That is, the N_Recommended represents the number of neighbor base stations capable of performing the handover for the MSS  600 . The MOB_MSSHO_REQ message also includes identifiers of the neighbor base stations represented by the N_Recommended, the CINRs of the pilot signals transmitted from the neighbor base stations, and a service level expected to be transmitted to the MSS  600 . 
   The serving base station  610  receives the MOB_MSSHO_REQ message transmitted from the MSS  600  and detects a list of target base stations allowing the handover of the MSS  600  based on N_Recommended information of the MOB_MSSHO_REQ message (step  623 ). In the following description, the list of target base stations allowing the handover of the MSS will be referred to as a “handover-support target base station list” for the purpose of convenience. According to  FIG. 6 , a first target base station  620  and a second target base station  630  may exist in the handover-support target base station list. Of course, the handover-support target base station list may include a plurality of target base stations. The serving base station  610  transmits a handover notification (“HO_notification”) message to the target base stations included in the handover-support target base station list, such as the first target base station  620  and the second target base station  630  (steps  625  and  627 ). A configuration of the HO_notification message is represented in Table 11. 
   
     
       
             
             
             
           
         
             
               TABLE 11 
             
             
                 
             
             
               Field 
               Size 
               Notes 
             
             
                 
             
           
           
             
               Global Header 
               152-bit 
                 
             
             
               For (j=0): j&lt;Num Records: j++) { 
             
             
                MSS unique identifier 
                48-bit 
               48-bit unique identifier used by MSS (as provided by the 
             
             
                 
                 
               MSS or by the I-am-host-of message) 
             
             
                Estimated Time to HO 
                16-bit 
               In milliseconds, relative to the time stamp, value 0 of this 
             
             
                 
                 
               parameter indicates that no actual HO is pending 
             
             
                Required BW 
                8-bit 
               Bandwith which is required by MSS (to gurarantee minimum 
             
             
                 
                 
               packet data transmission) 
             
             
                Required OoS 
                8-bit 
               Name of Service Class representing AuthorizedQoSParam- 
             
             
                 
                 
               Set 
             
             
               } 
             
             
               Security field 
               TBD 
               A means to authenticate this message 
             
             
               CRC field 
                32-bit 
               IEEE CRC-32 
             
             
                 
             
           
        
       
     
   
   As shown in Table 11, the HO_notification message includes a plurality of IEs, such as an ID of the MSS  600  to be handed-over to the first target base station  620  or the second target base station  630 , an expected handover start time of the MSS  600 , a bandwidth provided from the target base station, that is, a bandwidth provided from a new serving base station according to a request of the MSS  600 , and a service level provided to the MSS  600 . The bandwidth and the service level requested by the MSS  600  are identical to the expected service level information recorded in the MOB_MSSHO_REQ message described with reference to  FIG. 10 . 
   The first and second target base stations  620  and  630  receive the HO_notification message from the serving base station  610  and transmit an HO_notification response message to the serving base station  610  (steps  629  and  631 ). A configuration of the HO_notification response message is represented in Table 12. 
   
     
       
             
             
             
           
         
             
               TABLE 12 
             
             
                 
             
             
               Field 
               Size 
               Notes 
             
             
                 
             
           
           
             
               Global Header 
               152-bit 
                 
             
             
               For (j=0: j&lt; 
             
             
               Num Records: 
             
             
               j++) { 
             
             
                MSS unique 
                48-bit 
               48-bit unique identifier used by MSS (as 
             
             
                identifier 
                 
               provided by the MSS or by the 
             
             
                 
                 
               I-am-host-of message) 
             
             
                QoS 
                8-bit 
               Bandwidth which is provided by BS (to 
             
             
                Estimated 
                 
               guarantee minimum packet data transmission) 
             
             
                 
                 
               TBD how to set this field 
             
             
                BW Estimated 
                8-bit 
               Quality of Service level 
             
             
                 
                 
                Unsolicited Grant Service (UGS) 
             
             
                 
                 
                Real-time Polling Service (rtPS) 
             
             
                 
                 
                Non-real-time Polling Service (nrtPS) 
             
             
                 
                 
                Best Effort 
             
             
                ACK/NACK 
                1-bit 
               Acknowledgement or Negative 
             
             
                 
                 
               acknowledgement 
             
             
                 
                 
                1 is Acknowledgement which means that the 
             
             
                 
                 
                 neighbor BS accepts the HO-notification 
             
             
                 
                 
                 message from the serving BS 
             
             
                 
                 
                0 is Negative acknowledgement which means 
             
             
                 
                 
                 that the neighbor BS may not accept the  
             
             
                 
                 
                 HO-notification message from the serving 
             
             
                 
                 
                 BS 
             
             
               } 
             
             
               Security field 
               TBD 
               A means to a authenticate this message 
             
             
               CRC field 
                32-bit 
               IEEE CRC-32 
             
             
                 
             
           
        
       
     
   
   As shown in Table 12, the HO_notification response message includes a plurality of IEs, such as an ID of the MSS  600  to be handed-over to the target base stations, ACK/NACK representing a response of the target base stations with regard to a handover request of the MSS  600 , and information related to the bandwidth and the service level which must be provided from each target base station when the MSS  600  is handed-over to the target base station. 
   The serving base station  610  receives the HO_notification response message from the first and second target base stations  620  and  630  and analyzes the HO_notification response message in order to select a final base station capable of providing an optimal bandwidth and an optimal service level to the MSS  600  when the MSS  600  is handed-over to the base station. For instance, if the service level provided from the first target base station  620  is less than the service level requested by the MSS  600  and the service level provided from the second target base station  630  is identical to the service level requested by the MSS  600 , the serving base station  610  selects the second target base station  630  as the final target base station performing a handover operation in relation to the MSS  600 . Thus, the serving base station  610  transmits an HO_notification conform message to the second target base station  630  in response to the HO_notification response message (step  633 ). A configuration of the HO_notification confirm message is represented in Table 13. 
   
     
       
             
             
             
           
         
             
               TABLE 13 
             
             
                 
             
             
               Field 
               Size 
               Notes 
             
             
                 
             
           
           
             
               Global Header 
               152-bit 
                 
             
             
               For (j=0: j&lt; 
             
             
               Num Records; 
             
             
               j++) { 
             
             
                MSS unique 
                48-bit 
               48-bit universal MAC address of the MSS 
             
             
                identifier 
                 
               (as provided to the BS on the RNG-REQ 
             
             
                 
                 
               message) 
             
             
                QoS Estimated 
                8-bit 
               Bandwidth which is provided by BS 
             
             
                 
                 
               (to guarantee minimum packet data 
             
             
                 
                 
               transmission) TBD how to set this field 
             
             
                BW Estimated 
                8-bit 
               Quality of Service level 
             
             
                 
                 
                Unsolicited Grant Service (UGS) 
             
             
                 
                 
                Real-time Polling Service (rtPS) 
             
             
                 
                 
                Non-real-time Polling Service (nrtPS) 
             
             
                 
                 
                Best Effort Service (BE) 
             
             
               } 
             
             
               Security field 
               TBD 
               A means to authenticate this message 
             
             
               CRC field 
                32-bit 
               IEEE CRC-32 
             
             
                 
             
           
        
       
     
   
   As shown in Table 13, the HO_notification confirm message includes a plurality of IEs, such as an ID of the MSS  600  to be handed-over to the selected target base station, and information about a bandwidth and a service level which must be provided from the selected target base station when the MSS  600  is handed-over to the selected target base station. 
   In addition, the serving base station  610  transmits a mobile handover response (“MOB_HO_RSP”) message to the MSS  600  in response to the MOB_MSSHO_REQ message (step  635 ). The MOB_HO_RSP message includes information about the target base station performing the handover operation in relation to the MSS  600 . A configuration of the MOB_HO_RSP message is represented in Table 14. 
   
     
       
             
             
             
           
         
             
               TABLE 14 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
               MOB_HO-RSP_Message_Format( ) { 
                 
                 
             
             
                Management Message Type = 53 
                8 bits 
             
             
                Estimated HO time 
                8 bits 
             
             
                N_Recommended 
                8 bits 
             
             
                For (j=0 : j&lt;N_NEIGHBORS : j++) { 
             
             
                 Neighbor BS-ID 
               48 bits 
             
             
                service level prediction 
                8 bits 
               This parameter exists 
             
             
                 
                 
               only when the message 
             
             
                 
                 
               is sent by the BS 
             
             
                } 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 14, the MOB_HO_RSP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, an expected handover start time, and target serving stations selected from the serving base stations. In addition, N_Recommended of the MOB_HO_RSP message represents the number of target base stations capable of providing the bandwidth and service level requested by the MSS  600  among target base stations included in the handover-support target base station list. The MOB_HO_RSP message is marked with the identifiers of the target base stations represented by the N_Recommended and a service level expected to be provided to the MSS  600  from the target base station. Although  FIG. 6  illustrates that the information of one target base station, that is, information about the second target base station  630 , is only included in the MOB_HO_RSP message from among the target base stations included in the handover-support target base station list, if there are a plurality of target base stations capable of providing the bandwidth and service level requested by the MSS in the handover-support target base station list, the MOB_HO_RSP message may include information related to the plurality of target base stations. 
   Upon receiving the MOB_HO_RSP message, the MSS  600  analyzes information included in the MOB_HO_RSP message in order to select a target base station for performing the handover operation in relation to the MSS  600 . After selecting the target base station, the MSS  600  transmits a mobile handover indication (“MOB_HO_IND”) message to the serving base station  610  in response to the MOB_HO_RSP message (step  637 ). A configuration of the MOB_HO_IND message is represented in Table 15. 
   
     
       
             
             
             
           
         
             
               TABLE 15 
             
             
                 
             
             
               Syntax 
               Size 
               Notes 
             
             
                 
             
           
           
             
               MOB_HO_IND_Message_Format( ) { 
                 
                 
             
             
                Management Message Type = 54 
                8 bits 
             
             
                TLV Encoded Information 
               Variable 
               TLV specific 
             
             
                Target_BS_ID 
               48 bits 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   As shown in Table 15, the MOB_HO_IND message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, an identifier of a final target base station selected by the MSS  600 , and TLV Encoded Information representing variable Encoded information. 
   The serving base station  610  receiving the MOB_HO_IND message recognizes that the MSS  600  will be handed-over to the target base station, that is, the second target base station  630  based on the MOB_HO_IND message so that the serving base station  610  releases a link connecting the serving base station  610  to the MSS  600  (step  639 ). If the link connecting the MSS  600  to the serving base station  610  has been released, the MSS  600  is handed-over to the second target base station  630 . 
   The handover process according to the request of the base station in the conventional IEEE 802.16e communication system will be described with reference to  FIG. 7 . 
     FIG. 7  is a signal flow diagram showing the handover process according to the request of the base station in the conventional IEEE 802.16e communication system. 
   It is noted that the handover process according to the request of the base station may occur when an overload is applied to the base station so that it is necessary to distribute the load of the base station to neighbor base stations or when it is necessary to deal with the status variation of the uplink of the MSS. 
   Referring to  FIG. 7 , a serving base station  710  transmits an MOB_NBR_ADV message to an MSS  700  (step  711 ). Upon receiving the MOB_NBR_ADV message from the serving base station  710 , the MSS  700  obtains information relating to the neighbor base stations and transmits an MOB_SCN_REQ message to the serving base station  710  if it is necessary to scan the CINRs of pilot channels signals transmitted from the neighbor base stations (step  713 ). A scan request time of the MSS  700  for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations does not directly relate to the CINR scanning operation, so it will not be further described below. The serving base station  710  receiving the MOB_SCN_REQ message transmits an MOB_SCN_RSP message including the scanning information, which must be scanned by the MSS  700 , to the MSS  700  (step  715 ). Upon receiving the MOB_SCN_RSP message including the scanning information from the serving base station  710 , the MSS  700  scans the parameters included in the MOB_SCN_RSP message, that is, the MSS  700  scans the CINRs of the pilot channel signals of the neighbor base stations obtained through the MOB_NBR_ADV message (step  717 ). Although a process for measuring the CINR signal of the pilot channel signal transmitted from the serving base station  710  is not separately illustrated in  FIG. 7 , the MSS  700  may continuously measure the CINR of the pilot channel signal transmitted from the serving base station  710 . 
   When the serving base station  710  determines that it is necessary to perform the handover of the MSS  700  managed by the serving base station  710  (step  719 ), the serving base station  710  transmits an HO_notification message to the neighbor base stations (steps  721  and  723 ). Herein, the HO_notification message includes information relating to a bandwidth and a service level which must be provided from a target base station, that is, a new serving base station of the MSS  700 . In  FIG. 7 , the neighbor base stations of the serving base station  710  are first and second target base stations  720  and  730 . 
   Upon receiving the HO_notification message, the first and second target base stations  720  and  730  transmit the HO_notification response message to the serving base station  710  in response to the HO_notification message (step  725  and  727 ). As described with reference to Table 12, the HO_notification response message includes ACK/NACK representing a response of the target base stations, that is, a response of the neighbor base stations with regard to the handover requested by the serving base station  710 , and information about a bandwidth and a service level of the target base stations, which must be provided to the MSS  700 . 
   The serving base station  710  receives the HO_notification response message from the first and second target base stations  720  and  730  and selects the target base stations capable of providing an optimal bandwidth and an optimal service level to the MSS  700 . For instance, if the service level provided from the first target base station  720  is less than the service level requested by the MSS  700  and the service level provided from the second target base station  730  is identical to the service level requested by the MSS  700 , the serving base station  710  selects the second target base station  730  as a final target base station performing a handover operation in relation to the MSS  700 . Thus, the serving base station  710  selecting the second target base station  730  as a final target base station transmits an HO_notification conform message to the second target base station  730  in response to the HO_notification response message (step  729 ). 
   The serving base station  710  transmits the MOB_HO_RSP message to the MSS  700  (step  731 ) after transmitting the HO_notification conform message to the second target base station  730 . The MOB_HO_RSP message includes N_Recommended information selected by the serving base station  710 , that is, information related to the bandwidth and the service level which must be provided to the MSS  700  from the selected target base stations (the second target base station  730  in  FIG. 7 ) and target base stations. Upon receiving the MOB_HO_RSP message, the MSS  700  recognizes that the handover is requested by the serving base station  710  so that the MSS  700  selects the final target base station performing the handover operation in relation to the MSS  700  based on N_Recommended information included in the MOB_HO_RSP message. After that, the MSS  700  transmits the MOB_HO_IND message to the serving station  710  in response to the MOB_HO_RSP message (step  733 ). As the MOB_HO_IND message is received in the serving base station  710 , the serving base station  710  recognizes that the MSS  700  will be handed-over to the target base station based on the MOB_HO_IND message so that the serving base station  710  releases a link connecting the serving base station to the MSS  700  (step  735 ). If the link connecting the MSS  700  to the serving base station  710  has been released, the MSS  700  is handed-over to the second target base station  730 . 
   As described above, according to the conventional IEEE 802.16e communication system, the MSS is handed-over to the neighbor base station. The MSS is handed-over to the target base station, which is different from the serving base station, when the CINR of the pilot cannel signal of the serving base station becomes reduced so that it is impossible for the MSS to properly communicate with the serving base station, or when the handover is requested by the MSS or the serving base station. However, if an MSS drop occurs during the handover operation in the conventional IEEE 802.16e communication system, the MSS monitors all of the frequency bands in a similar way as to the operation of the MSS after the MSS is powered on in order to detect a pilot channel signal having the highest CINR and selects the base station, which has transmitted the pilot channel signal halving the highest CINR, as a base station for the MSS. In addition, if an MSS drop occurs while the MSS is communicating with the serving base station in the conventional IEEE 802.16e communication system, the MSS monitors all of the frequency bands in the same manner as the MSS drop so as to detect a pilot channel signal having the highest CINR and selects the base station, which has transmitted the pilot channel signal having the highest CINR, as a base station for the MSS. 
   According to the above two cases, the MSS monitors all of the frequency bands although the MSS is communicating with the serving base station, requiring a relatively long period of time for selecting the serving base station, thereby lowering service quality. Therefore, it is necessary to provide an improved procedure capable of allowing the MSS subject to the drop during a communication to resume communication with a minimum time delay. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made to solve at least the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a system and a method for selecting a serving base station for an MSS when the MSS is subject to a drop during a communication in a broadband wireless communication system. 
   Another object of the present invention is to provide a system and a method for selecting a serving base station for an MSS when the MSS is subject to a drop during a handover operation in a broadband wireless communication system. 
   Still another object of the present invention is to provide a system and a method capable of allowing an MSS to primarily reestablish communication when the MSS is subject to a drop during a communication in a broadband wireless communication system. 
   Still another object of the present invention is to provide a system and a method for reducing the time required for an MSS to reestablish a communication link when the MSS subject to a drop reenters a network in a broadband wireless communication system. 
   In order to accomplish these objects, the present invention provides a system for assigning ranging codes in a broadband wireless access communication system, the system including a transmission unit for classifying rangings defined in the broadband wireless access communication system into an initial ranging, a periodic ranging, a bandwidth request ranging and a drop ranging, creating Q ranging codes used for the rangings, assigning A ranging codes selected from the Q ranging codes as drop ranging codes used for the drop ranging, and transmitting first information representing the drop ranging codes; and a receiving unit for receiving the first information transmitted from the transmission unit, and performing the drop ranging by using the drop ranging codes included in the first information. 
   In order to accomplish these objects, the present invention provides a system for selecting a new serving base station when a drop occurs in a mobile subscriber station in a broadband wireless access communication system including the mobile subscriber station, a serving base station communicating with the mobile subscriber station, and a plurality of neighbor base stations different from the serving base station, the system including the mobile subscriber station receiving information related to the neighbor base stations from the serving base station communicating with the mobile subscriber station, monitoring the frequency bands of the neighbor base stations included in the information about the neighbor base stations if the drop is detected in order to detect target base stations capable of serving as a new serving base station, selecting the new serving base station from the detected target base stations, and notifying the new serving base station of a reestablishment of communication caused by the drop occurring in the mobile subscriber station; and the new serving base station primarily assigning channel resources to the mobile subscriber station when the new serving base station receives a notification about the reestablishment of communication from the mobile subscriber station. 
   In order to accomplish these objects, the present invention provides a method of assigning ranging codes in a broadband wireless access communication system, the method including the steps of classifying rangings between a transmission unit and a receiving unit of the broadband wireless access communication system into an initial ranging, a periodic ranging, a bandwidth request ranging and a drop ranging; and creating Q of ranging codes used for the rangings and assigning A ranging codes selected from the Q ranging codes as drop ranging codes used for the drop ranging. 
   In order to accomplish these objects, according to a first aspect of the present invention, there is provided a method of selecting a new serving base station when a drop occurs in a mobile subscriber station in a broadband wireless access communication system including the mobile subscriber station, a serving base station making communication with the mobile subscriber station, and a plurality of neighbor base stations different from the serving base station, the method including the steps of detecting the drop based on information related to the neighbor base stations transmitted from the serving base station communicating with the mobile subscriber station; monitoring frequency bands of the neighbor base stations included in the information related to the neighbor base stations if the drop is detected; detecting target base stations capable of serving as a new serving base station for communicating with the mobile subscriber station when the drop occurs in the mobile subscriber station according to a monitoring result for the frequency bands of the neighbor base stations; and selecting the new serving base station from the detected target base stations. 
   In order to accomplish these objects, according to a second aspect of the present invention, there is provided a method of selecting a new serving base station when a drop occurs in a mobile subscriber station during an handover operation of the mobile subscriber station from a serving base station to a predetermined neighbor base station in a broadband wireless access communication system including the mobile subscriber station, the serving base station communicating with the mobile subscriber station, and n neighbor base stations different from the serving base station, the method including the steps of receiving information related to the n of neighbor base stations transmitted from the serving base station communicating with the mobile subscriber station and monitoring frequency bands of the n neighbor base stations included in the information about the n neighbor base stations; allowing the serving base station to determine the handover operation and detecting a m neighbor base stations (m≦n) capable of serving as new serving station according to a monitoring result for the frequency bands of the n neighbor base stations; sending a signal requesting a handover to the serving base station based on information related to the m neighbor base stations; monitoring frequency bands of the m of neighbor base stations if the drop occurs after requesting the handover; detecting target base stations capable of serving as a new serving station according to a monitoring result for the frequency bands of the m neighbor base stations; and selecting the new serving station from the detected target base stations. 
   In order to accomplish these objects, according to a third aspect of the present invention, there is provided a method of selecting a new serving base station when a drop occurs in a mobile subscriber station during an handover operation of the mobile subscriber station from a serving base station to a neighbor base station in a broadband wireless access communication system including the mobile subscriber station, the serving base station communicating with the mobile subscriber station, and n neighbor base stations different from the serving base station, the method including the steps of receiving information relating to the neighbor base stations transmitted from the serving base station making communication with the mobile subscriber station and monitoring frequency bands of the neighbor base stations included in the information related to the neighbor base stations; monitoring each frequency band of neighbor base stations if the drop is detected after monitoring frequency bands of the neighbor base stations; detecting target base stations capable of serving as a new serving station according to a monitoring result for each frequency band of neighbor base stations; and selecting the new serving station from the detected target base stations. 
   In order to accomplish these objects, according to a fourth aspect of the present invention, there is provided a method of selecting a new serving base station when a drop occurs in a mobile subscriber station during an handover operation of the mobile subscriber station from a serving base station to a neighbor base station in a broadband wireless access communication system including the mobile subscriber station, the serving base station communicating with the mobile subscriber station, and n neighbor base stations different from the serving base station, the method including the steps of receiving information relating to the n neighbor base stations transmitted from the serving base station communicating with the mobile subscriber station and monitoring frequency bands of the n neighbor base stations included in the information related to the n neighbor base stations; determining by the serving base station the handover operation and detecting a m neighbor base stations (m≦n) capable of serving as a new serving station according to a monitoring result for the frequency bands of the n neighbor base stations; sending a signal requesting a handover to the serving base station based on information related to the m neighbor base stations; receiving a handover response including information related to a k recommended neighbor base stations (k≦m), to which the mobile subscriber station is handed-over from the serving base station, according to a request for the handover; monitoring frequency bands of the k recommended neighbor base stations if the drop occurs after the request for the handover; detecting target base stations capable of serving as a new serving station according to a monitoring result for the frequency bands of the k recommended neighbor base stations; and selecting the new serving station from the detected target base stations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a structure diagram schematically illustrating a conventional IEEE 802.16a communication system; 
       FIG. 2  is a structure diagram schematically illustrating a structure of a downlink frame of a conventional IEEE 802.16a communication system; 
       FIG. 3  is a structure diagram schematically illustrating a structure of an uplink frame of a conventional IEEE 802.16a communication system; 
       FIG. 4  is a signal flow diagram illustrating a ranging process between a base station and an SS in a conventional IEEE 802.16a communication system; 
       FIG. 5  is a structure diagram schematically illustrating a structure of a conventional IEEE 802.16e communication system; 
       FIG. 6  is a signal flow diagram illustrating a handover process according to a request of an MSS in a conventional IEEE 802.16e communication system; 
       FIG. 7  is a signal flow diagram illustrating a handover process according to a request of a base station in a conventional IEEE 802.16e communication system; 
       FIG. 8  is a flowchart illustrating a process for detecting a drop by means of an MSS using a periodic ranging procedure in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 9  is a flowchart illustrating a process for detecting a drop by means of a serving base station using a periodic ranging procedure in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 10  is a flowchart illustrating a process for detecting a drop by means of an MSS using a downlink status in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 11  is a flowchart illustrating a procedure for selecting a serving base station when a drop occurs during a non-handover state in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 12  is a flowchart illustrating a procedure for selecting a serving base station when a drop occurs after an MSS has transmitted an MOB_MSSHO_REQ message while a handover operation is being carried out at a request of an MSS in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 13  is a flowchart illustrating a procedure for selecting a serving base station when a drop occurs before an MSS has received an MOB_HO_RSP message while a handover operation is being carried out at a request of a serving base station in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 14  is a flowchart illustrating a procedure for selecting a serving base station when a drop occurs after an MSS has received an MOB_HO_RSP message during a handover operation in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 15  is a flowchart illustrating a procedure for selecting a serving base station when a drop occurs after an MSS has transmitted an MOB_HO_IND message during a handover operation in an IEEE 802.16e communication system according to one embodiment of the present invention; 
       FIG. 16  is a signal flow diagram illustrating a drop ranging procedure of an MSS, which is subject to a drop, by using a drop ranging code in an IEEE 802.16e communication system according to one embodiment of the present invention; and 
       FIG. 17  is a signal flow diagram illustrating a drop ranging procedure of an MSS, which is subject to a drop, by using a drop ranging time slot in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following detailed description, representative embodiments of the present invention will be described. In addition, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention. 
   The present invention provides a method for selecting a serving base station when an mobile subscriber station (“MSS”) is subject to a drop during a communication in an IEEE (Institute of Electrical and Electronics Engineers) 802.16e communication system, a broadband wireless access communication system. In the following description, the expression “an MSS is subject to a drop” and “a drop occurs in an MSS” are used to describe a call drop. According to the present invention, the MSS may instantly select a serving base station (BS) when the MSS is subject to the drop during a communication, so that the MSS can reestablish the communication with regard to the serving base station within a short period of time. In addition, the present invention provides a method for assigning a ranging code, that is, a drop ranging code in order to minimize the time required for network re-entry of the MSS when the MSS is subject to the drop in the IEEE 802.16e communication system. 
   The IEEE 802.16e communication system is a broadband wireless access communication system utilizing an orthogonal frequency division multiplexing (“OFDM”) scheme and an orthogonal frequency division multiple access (“OFDMA”) scheme. Since the IEEE 802.16e communication system uses the OFDM/OFDMA schemes, the physical channel signals can be transmitted through a plurality of sub-carriers so that a high-speed data transmission is possible. In short, the IEEE 802.16e communication system is a broadband wireless access communication system capable of providing for the mobility of the MSS by using a multi-cell structure. 
   The drop signifies that the MSS is disconnected from the serving base station during a communication. The drop is a release of a link connecting the MSS to the serving base station, that is, a call release. The present invention has been made under the assumption that the drop occurs in the MSS after the MSS has received a mobile neighbor advertisement (“MOB_NBR_ADV”) message. If the drop occurs in the MSS, a periodic ranging cannot be normally carried out between the MSS and the serving base station. As described above in relation to the prior art, the periodic ranging is periodically carried out by means of the MSS having a time offset and its transmit power adjusted through an initial ranging, in such a manner that the MSS can adjust a channel status with respect to the base station. 
   A process for detecting a drop by means of an MSS using a periodic ranging procedure will be described with reference to  FIG. 8 . 
     FIG. 8  is a flowchart illustrating the process for detecting the drop by means of the MSS using the periodic ranging procedure in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   Referring to  FIG. 8 , the MSS that obtains an initial synchronization with respect to the serving base station transmits a ranging request (“RNG_REQ”) message to the serving base station (step  801 ). A configuration of the RNG_REQ message is identical to the configuration of the RNG_REQ message, which has been described with reference to Table 5, so it will not be further described below. In step  803 , the MSS waits for a ranging response (“RNG_RSP”) message, which is a response message to the RNG_REQ message. A configuration of the RNG_RSP message is identical to the configuration of the RNG_RSP message, which has been described with reference to Table 6, so it will not be further described below. In step  805 , the MSS determines whether or not the RNG_RSP message is transmitted thereto from the serving base station. If the RNG_RSP message is transmitted to the MSS from the serving base station, the procedure goes to step  807 . In step  807 , since the ranging process has been completed, the MSS is normally operated. 
   If the RNG_RSP message has not been transmitted to the MSS from the serving base station in step  805 , the procedure goes to step  809 . In step  809 , the MSS determines whether or not the number of transmission times of the RNG_REQ message exceeds the number of times for the RNG_REQ RETRIES (“RNG_REQ RETRIES”). Herein, the number of times for the RNG_REQ RETRIES represents the maximum number of transmission times of the RNG_REQ message by means of the MSS in a state in which the MSS does not receive the RNG_RSP message from the base station. If it is determined that the number of transmission times of the RNG_REQ message does not exceed the number of times for the RNG_REQ RETRIES in step  809 , the procedure returns to step  801 . If it is determined that the number of transmission times of the RNG_REQ message exceeds the number of times for the RNG_REQ RETRIES in step  809 , the procedure goes to step  811 . In step  811 , the MSS detects the drop occurring in the MSS. 
   A process for detecting a drop by means of a serving base station using a periodic ranging procedure will be described with reference to  FIG. 9 . 
     FIG. 9  is a flowchart illustrating the process for detecting the drop by means of the serving base station using the periodic ranging procedure in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   Referring to  FIG. 9 , the serving base station that obtains an initial synchronization with respect to the MSS waits for RNG_REQ message transmitted from the MSS (step  901 ). In step  903 , the serving base station determines whether or not the RNG_REQ message is transmitted thereto from the MSS. If the RNG_REQ message is transmitted to the serving base station from the MSS, the procedure goes to step  905 . In step  905 , the serving base station transmits the RNG_RSP message to the MSS in response to the RNG_REQ message. 
   If the RNG_REQ message has not been transmitted to the serving base station from the MSS in step  903 , the procedure goes to step  907 . In step  907 , the serving base station determines whether or not the number of transmission times of the RNG_REQ message exceeds the number of times for the RNG_REQ RETRIES. Herein, the serving base station may increase the number of transmission times for the RNG_REQ message by 1 if the serving base station does not receive the RNG_REQ message within a predetermined time (RNG_REQ_Timeout). If it is determined that the number of transmission times of the RNG_REQ message does not exceed the number of times for the RNG_REQ RETRIES in step  907 , the procedure returns to step  901 . If it is determined that the number of transmission times of the RNG_REQ message exceeds the number of times for the RNG_REQ RETRIES in step  907 , the procedure goes to step  909 . In step  909 , the serving base station detects the drop occurring in the MSS. Accordingly, in the same manner as the normal handover procedure, the serving base station releases a link, that is, releases a call with regard to the MSS which is subject to the drop. 
   A process for detecting a drop by using a downlink status will be described with reference to  FIG. 10 . 
     FIG. 10  is a flowchart illustrating the process for detecting the drop by means of an MSS using the downlink status in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   Referring to  FIG. 10 , the MSS initializes a value of CONSECUTIVE_BAD_FRAME, which is a parameter for counting the number of frames having inferior quality (“bad frame”), as “0” (CONSECUTIVE_BAD_FRAME=0) (step  1001 ). The bad frame signifies a frame having inferior quality, which cannot be used for data communication even if errors created in the frame have been corrected. In step  1003 , the MSS remains in a waiting state. In step  1005 , the MSS receives the downlink frame. In step  1007 , the MSS performs a cyclic redundancy check (CRC) with respect to the received downlink frame. In step  1009 , the MSS determines whether or not an error occurs in the downlink frame. 
   If it is determined that the error is not generated from the downlink frame in step  1009 , the procedure returns to step  1001 . If it is determined that the error is generated from the downlink frame in step  1009 , the procedure goes to step  1011 . In step  1011 , the MSS determines that the received downlink frame is a bad frame, so the MSS increase the value of the CONSECUTIVE_BAD_FRAME by 1 (CONSECUTIVE_BAD_FRAME=CONSECUTIVE_BAD_FRAME+1). In step  1013 , the MSS determines whether or not the value of the CONSECUTIVE_BAD_FRAME exceeds a predetermined limit number of bad frames (“LIMIT_BAD_FRAME”). If it is determined that the value of the CONSECUTIVE_BAD_FRAME does not exceed the LIMIT_BAD_FRAME in step  1013 , the procedure returns to step  1003 . If it is determined that the value of the CONSECUTIVE_BAD_FRAME exceeds the LIMIT_BAD_FRAME in step  1013 , the procedure goes to step  1015 . In step  1015 , the MSS detects the drop occurring the MSS. 
   As described with reference to  FIGS. 8 to 10 , since the link connecting the MSS to the serving base station may be released if the MSS is subject to the drop, the MSS must search the serving base stations in order to reestablish a communication with regard to a new serving base station. According to the prior art, the MSS detecting the drop must monitor all of the frequency bands in the similar way as to the operation of the MSS after the MSS is powered on in order to detect a reference channel, that is, a pilot channel signal having a highest CINR and selects the base station, which has transmitted the pilot channel signal having the highest CINR, as a target base station for the MSS. The MSS also receives a preamble of the downlink frame transmitted from the target base station and obtains a system synchronization with respect to the target base station, thereby selecting the target base station as a new serving base station. A new serving base station different from the present serving base station of the MSS, that is, a serving station capable of performing the handover operation with regard to the MSS, becomes the target station. According to the present invention, the drop may occur in the MSS during a communication after the MSS has received the MOB_NBR_ADV message, so it is not necessary to carry out the step of monitor all of the frequency bands preset in the MSS in the similar way as the operation of the MSS after the MSS is powered in order to detect the pilot channel signal having a highest CINR for selecting the serving base station based on the pilot channel signal. According to the present invention, the new serving base station is selected from among target base stations capable of serving as a new serving base station when the drop occurs in the MSS, thereby minimizing a communication delay. 
   According to the present invention, the MSS which is subject to the drop during communication after receiving the MOB_NBR_ADV message may select the serving base station in a different way as compared with the MSS which is subject to the drop during the handover operation. For this reason, a procedure of the MSS for selecting the serving base station will be described below by considering the two cases of the MSS, that is, a non-handover state of the MSS and a handover state of the MSS. 
     FIG. 11  is a flowchart illustrating a procedure for selecting a serving base station when a drop occurs during a non-handover state of an MSS in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   Referring to  FIG. 11 , the MSS detects the drop occurring in the MSS in step  1101 . In step  1103 , the MSS detects information related to neighbor base stations included in the MOB_NBR_ADV message, which has been transmitted to the MSS from the serving base station before the drop occurs in the MSS, and sets a parameter i, used for monitoring the frequency bands of the neighbor base stations, to “0” (i=0). A configuration of the MOB_NBR_ADV message is identical to the configuration of the MOB_NBR_ADV message, which has been described with reference to Table 7, and the information relating to the neighbor base stations includes the number of the neighbor base stations, identifiers of the neighbor base stations, and a physical channel frequency. In addition, the parameter i represents the number of neighbor base stations subject to the frequency band monitoring. In step  1105 , the MSS sequentially selects the information about the neighbor base stations one by one (i=i+1) in order to monitor the frequency bands of the neighbor base stations. 
   In step  1107 , the MSS determines whether or not the target base station is detected through the frequency band monitoring for the neighbor base stations. As mentioned above, the target base station signifies a base station capable of serving as a new serving base station of the MSS. For instance, a base station providing a pilot channel signal having a CINR higher than a predetermined CINR can be selected as a target base station. If it is determined that the target base station is not detected through the frequency band monitoring for the neighbor base stations in step  1107 , the procedure goes to step  1111 . In step  1111 , the MSS determines whether or not the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the number of neighbor base stations (N_NEIGHBORS) included in the MOB_NBR_ADV message. If it is determined that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the N_NEIGHBORS in step  1111 , the procedure returns to step  1105 . 
   If it is determined that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is not less than the N_NEIGHBORS in step  1111 , the procedure goes to step  1113 . In step  1113 , since the MSS fails to select the target base station from among the neighbor base stations, the MSS monitors all of the frequency bands preset in the MSS. In step  1115 , the MSS determines whether or not the target base station is detected. If it is determined that the target base station is not detected in step  1115 , the procedure returns to step  1113 . If it is determined that the target base station is detected in step  1115 , the procedure goes to step  1109 . 
   If it is determined that the target base station is detected through the frequency band monitoring for the neighbor base stations in step  1107 , the procedure goes to step  1109 . In step  1109 , the MSS selects one target base station from among the detected target base stations as a new serving base station for the MSS. If a plurality of target base stations are detected through step  1107 , the MSS selects one target base station as a new serving base station for the MSS based on the value of the CINR. 
   A procedure for selecting a serving base station when a drop occurs in an MSS after the MSS has transmitted an MOB_MSSHO_REQ message while a handover operation is being carried out at the request of the MSS in an IEEE 802.16e communication system will be described with reference to  FIG. 12 . 
     FIG. 12  is a flowchart illustrating the procedure for selecting the serving base station when the drop occurs in the MSS after an MSS has transmitted an MOB_MSSHO_REQ message while the handover operation is being carried out at the request of the MSS in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   Referring to  FIG. 12 , the MSS detects the drop occurring in the MSS in step  1201 . In step  1203 , the MSS detects information related to neighbor base stations included in the MOB_MSSHO_REQ message, which has been transmitted to the serving base station before the drop occurs in the MSS. Information related to the neighbor base stations included in the MOB_MSSHO_REQ message is information relating to N_RECOMMENDED, which represents the number of base stations transmitting to the MSS the pilot channel signal having a CINR greater than a predetermined CINR, obtained by scanning the CINRs of the pilot channel signals transmitted from N_NEIGHBORS included in the MOB_NBR_ADV message. That is, the information relating to the neighbor base stations included in the MOB_MSSHO_REQ message is information relating to the neighbor base stations capable of serving as a target base station for the MSS, which are selected from among N_NEIGHBORS included in the MOB_NBR_ADV message. 
   In step  1205 , the MSS sequentially orders the detected neighbor base stations according to the value of the CINR and sets a parameter i, used for monitoring the frequency bands of the neighbor base stations, to “0” (i=0). The parameter i represents the number of neighbor base stations subject to the frequency band monitoring. In step  1207 , the MSS sequentially selects the information related to the neighbor base stations one by one (i=i+1) in the order of the CINR value of the neighbor base stations in order to monitor the frequency bands of the neighbor base stations. 
   In step  1209 , the MSS determines whether or not the target base station is detected through the frequency band monitoring for the neighbor base stations. The target base station is a base station capable of serving as a new serving base station of the MSS. For instance, a base station providing a pilot channel signal having a CINR greater than a predetermined CINR can be selected as a target base station. If it is determined that the target base station is not detected through the frequency band monitoring for the neighbor base stations in step  1209 , the procedure goes to step  1213 . In step  1213 , the MSS determines whether or not the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the number of neighbor base stations (N_RECOMMENDED) included in the MOB_MSSHO_REQ message. If it is determined that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the N_RECOMMENDED included in the MOB_MSSHO_REQ message in step  1213 , the procedure returns to step  1207 . 
   If it is determined in step  1213  that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring exceeds the N_RECOMMENDED, the procedure goes to step  1215 . In step  1215 , since the MSS fails to select the target base station from among the neighbor base stations, the MSS monitors all of the frequency bands preset in the MSS. In step  1217 , the MSS determines whether or not the target base station is detected. If it is determined that the target base station is not detected in step  1217 , the procedure returns to step  1215 . If it is determined that the target base station is detected in step  1217 , the procedure goes to step  1211 . 
   If it is determined that the target base station is detected through the frequency band monitoring for the neighbor base stations in step  1209 , the procedure goes to step  1211 . In step  1211 , the MSS selects one target base station from among the detected target base stations as a new serving base station for the MSS. If a plurality of target base stations are detected through step  1209 , the MSS selects one target base station as a new serving base station for the MSS based on the value of the CINR. 
     FIG. 13  is a flowchart illustrating the procedure for selecting the serving base station when the drop occurs in the MSS before the MSS has received the MOB_HO_RSP message while the handover operation is being carried out at the request of the serving base station in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   Prior to explaining  FIG. 13 , it should be noted that if the drop occurs in the MSS before the MSS has received the MOB_HO_RSP message while the handover operation is being carried out at the request of the serving base station, it may be equal to a case in which the drop occurs in the MSS after the MSS, receiving a mobile scanning interval allocation response (“MOB_SCN_RSP”) message during the non-handover of the MSS, has scanned the CINRs of pilot channel signals transmitted from the neighbor base stations. Therefore, although  FIG. 13  shows the procedure for selecting the serving base station when the drop occurs in the MSS before the MSS has received the MOB_HO_RSP message, if the drop occurs in the MSS before the MSS has received the MOB_HO_RSP message while the handover operation is being carried out at the request of the serving base station, it may be equal to a case in which the drop occurs in the MSS after the MSS receiving the MOB_SCN_RSP message during the non-handover of the MSS has scanned the CINRs of pilot channel signals transmitted from the neighbor base stations. 
   Referring to  FIG. 13 , the MSS detects the drop occurring in the MSS in step  1301 . In step  1303 , the MSS detects information related to the neighbor base stations included in the MOB_NBR_ADV message, which has been transmitted to the MSS from the serving base station before the drop occurs in the MSS. In step  1305 , the MSS sequentially orders the detected neighbor base stations in the order of the value of the CINR and sets a parameter i, used for monitoring the frequency bands of the neighbor base stations, to “0” (i=0). The parameter i represents the number of neighbor base stations subject to the frequency band monitoring. In step  1307 , the MSS sequentially selects the information related to the neighbor base stations one by one (i=i+1) in the order of the CINR value of the neighbor base stations in order to monitor the frequency bands of the neighbor base stations. 
   In step  1309 , the MSS determines whether or not the target base station is detected through the frequency band monitoring for the neighbor base stations. If it is determined that the target base station is not detected through the frequency band monitoring for the neighbor base stations in step  1309 , the procedure goes to step  1313 . In step  1313 , the MSS determines whether or not the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the number of neighbor base stations (N_NEIGHBORS) included in the MOB_NBR_ADV message. If it is determined that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the N_NEIGHBORS included in the MOB_NBR_ADV message in step  1313 , the procedure returns to step  1307 . 
   If it is determined in step  1313  that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring exceeds the N_NEIGHBORS included in the MOB_NBR_ADV message, the procedure goes to step  1315 . In step  1315 , since the MSS fails to select the target base station from among the neighbor base stations, the MSS monitors all of the frequency bands preset in the MSS. In step  1317 , the MSS determines whether or not the target base station is detected. If it is determined that the target base station is not detected in step  1317 , the procedure returns to step  1315 . In addition, if it is determined that the target base station is detected in step  1317 , the procedure goes to step  1311 . 
   If it is determined that the target base station is detected through the frequency band monitoring for the neighbor base stations in step  1309 , the procedure goes to step  1311 . In step  1311 , the MSS selects one target base station from among the detected target base stations as a new serving base station for the MSS. If a plurality of target base stations are detected through step  1317 , the MSS selects one target base station as a new serving base station for the MSS based on the value of the CINR. 
     FIG. 14  is a flowchart illustrating the procedure for selecting the serving base station when the drop occurs in the MSS after the MSS has received an MOB_HO_RSP message during the handover operation in an IEEE 802.16e communication system according to one embodiment of the present invention. 
   Prior to explaining  FIG. 14 , it should be noted that the MOB_HO_RSP message is transmitted from the MSS to the serving base station during the handover operation carried out at the request of the MSS or the serving base station and the handover operation performed at the request of the MSS must be differentiated from the handover operation performed at the request of the serving base station in the method for selecting the serving base station shown in  FIG. 14 . 
   Referring to  FIG. 14 , the MSS detects the drop occurring in the MSS in step  1401 . In step  1403 , the MSS detects information related to the neighbor base stations included in the MOB_HO_RSP message, which has been transmitted to the MSS from the serving base station before the drop occurs in the MSS. As described with reference to Table 14, the information related to the neighbor base stations included in the MOB_HO_RSP message represents information related to the N_RECOMMENDED target base stations, which are selected from handover-support target base stations and capable of providing the bandwidth and service level requested by the MSS. 
   In step  1405 , the MSS sequentially orders the detected neighbor base stations in the order of the service level and sets a parameter i, used for monitoring frequency bands of the neighbor base stations, to “0” (i=0). Then, the procedure goes to step  1407 . The parameter i represents the number of neighbor base stations subject to the frequency band monitoring. In step  1407 , the MSS sequentially selects the neighbor base stations one by one (i=i+1) in the order of the service level provided from the neighbor base stations so as to monitor the frequency bands of the neighbor base stations. 
   In step  1409 , the MSS determines whether or not the target base station is detected through the frequency band monitoring for the neighbor base stations. If it is determined that the target base station is not detected through the frequency band monitoring for the neighbor base stations in step  1409 , the procedure goes to step  1413 . In step  1413 , the MSS determines whether or not the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the number of neighbor base stations (N_RECOMMENDED) included in the MOB_HO_RSP message. If it is determined that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring is less than the N_RECOMMENDED included in the MOB_HO_RSP message in step  1413 , the procedure returns to step  1407 . 
   If it is determined in step  1413  that the parameter i representing the number of neighbor base stations subject to the frequency band monitoring exceeds the N_RECOMMENDED included in the MOB_HO_RSP message, the procedure goes to step  1415 . In step  1415 , since the MSS fails to detect the target base station from among the neighbor base stations, the MSS monitors all of the frequency bands preset in the MSS. In step  1417 , the MSS determines whether or not the target base station is detected. If it is determined that the target base station is not detected in step  1417 , the procedure returns to step  1415 . If it is determined that the target base station is detected in step  1417 , the procedure goes to step  1411 . 
   If it is determined that the target base station is detected through the frequency band monitoring for the neighbor base stations in step  1409 , the procedure goes to step  1411 . In step  1411 , the MSS selects one target base station from among the detected target base stations as a new serving base station for the MSS. If a plurality of target base stations are detected through step  1417 , the MSS selects one target base station as a new serving base station for the MSS based on the value of the CINR, etc. 
     FIG. 15  is a flowchart illustrating the procedure for selecting the serving base station when the drop occurs in the MSS after the MSS has transmitted the MOB_HO_IND message during the handover operation in the IEEE 802.16e communication system according to one embodiment of the present invention. 
   Prior to explaining  FIG. 15 , it should be noted that the MOB_HO_IND message is transmitted from the MSS to the serving base station during the handover operation carried out at the request of the MSS or the serving base station and the handover operation performed at the request of the MSS must be differentiated from the handover operation performed at the request of the serving base station in the method for selecting the serving base station shown in  FIG. 14 . 
   Referring to  FIG. 15 , the MSS detects the drop occurring in the MSS in step  1501 . In step  1503 , the MSS detects related to the neighbor base stations included in the MOB_HO_IND message, which has been transmitted to the serving base station before the drop occurs in the MSS. The MOB_HO_IND message includes information related to the final target base station of the MSS. A configuration of the MOB_HO_IND message is identical to the configuration of the MOB_HO_IND message described with reference to Table 15, so it will not be further described below. 
   In step  1505 , the MSS monitors the frequency band of the target base station detected from the MOB_HO_IND message. Then, the procedure goes to step  1507 . In step  1507 , the MSS determines whether or not the target base station detected from the MOB_HO_IND message is detected as a target base station of the MSS. If it is determined that the target base station detected from the MOB_HO_IND message is not detected as a target base station of the MSS in step  1507 , the procedure goes to step  1403  shown in  FIG. 14 . In addition, if it is determined that the target base station detected from the MOB_HO_IND message is not detected as a target base station of the MSS in step  1507 , the procedure goes to step  1509 . In step  1509 , the MSS selects the detected target base station as a new serving base station for the MSS. A ranging procedure of an MSS by using a drop ranging code for allowing the MSS to reestablish communication within a short period of time when the drop occurs in the MSS in an IEEE 802.16e communication system will be described with reference to  FIG. 16 . 
     FIG. 16  is a signal flow diagram illustrating the ranging procedure of the MSS by using a drop ranging code when the drop occurs in the MSS in the IEEE 802.16e communication system according to one embodiment of the present invention. 
   Prior to explaining  FIG. 16 , the rangings used for the IEEE 802.16e communication system are classified into an initial ranging, a maintenance ranging, that is, a periodic ranging, and a bandwidth request ranging in the same manner as the rangings used for the IEEE 802.16a communication system. The initial ranging, the periodic ranging, and the bandwidth request ranging used for the IEEE 802.16e communication system are identical to those of the IEEE 802.16a communication system, so they will not be further described below. 
   As described above with regard to the prior art, the base station must assign the available ranging codes according to the objects of the rangings, that is, according to the type of the rangings. In the IEEE 802.16e communication system, the ranging codes are created by segmenting a pseudo-random noise (“PN”) sequence having a predetermined bit length (for example, 2 15 −1 bits) into predetermined ranging code units. For instance, a maximum of Q ranging codes (RC # 1  to RC #Q) can be created. 
   In the current IEEE 802.16e communication system, the Q ranging codes are differently assigned according to the objects of the rangings, that is, according to the initial ranging, the periodic ranging and the bandwidth request ranging. For instance, N ranging codes are assigned for the initial ranging, M ranging codes are assigned for the periodic ranging, and L ranging codes are assigned for the bandwidth request ranging. The total number (Q) of the ranging codes is equal to the sum of the N ranging codes for the initial ranging, the M ranging codes for the periodic ranging and the L ranging codes for the bandwidth request ranging (Q=N+M+L). 
   However, according to the present invention, the Q ranging codes are differently assigned for the purpose of four rangings, that is, the initial ranging, the periodic ranging, the bandwidth request ranging and the drop ranging. For instance, A ranging codes are assigned for the initial ranging, B ranging codes are assigned for the periodic ranging, C ranging codes are assigned for the bandwidth request ranging, and D ranging codes are assigned for the drop ranging. Herein, the total number (Q) of the ranging codes is equal to the sum of the A ranging codes for the initial ranging, the B ranging codes for the periodic ranging, the C ranging codes for the bandwidth request ranging, and the D ranging codes for the drop ranging (Q=A+B+C+D). 
   In addition, the drop ranging suggested by the present invention signifies a ranging carried out for primarily reestablishing a communication when the drop occurs during communication. An operation of the drop ranging is actually similar to that of the initial ranging. When the MSS having the drop performs the drop ranging by using drop ranging codes, the base station determines that the MSS tries to reestablish the communication with regard to the base station after the drop occurs in the MSS, so the base station primarily reestablishes the communication with respect to the MSS. 
   Referring to  FIG. 16 , when an initial synchronization is achieved between an MSS  1600  and a serving base station  1610  (step  1611 ), the MSS  1600  receives a DL_MAP message, an UL_MAP message, a DCD message, and an UCD message from the serving base station  1610  (step  1613 ). As described above, in one embodiment of the present invention, the UL_MAP message includes an information of drop ranging codes. The serving base station  1610  is a new serving base station selected by the MSS  1600  after the drop occurs in the MSS  1600 . 
   The MSS  1600  transmits the drop ranging code to the serving base station  1610  (step  1615 ) in such a manner that the serving base station  1610  can recognize that the MSS  1600  is attempting to reestablish a communication after the drop occurs in the MSS  1600 . Upon receiving the drop ranging code from the MSS  1600 , the serving base station  1610  may recognize that the MSS  1600  is attempting to reestablish the communication with regard to the serving base station  1610  after the drop, so the serving base station  1610  transmits the DL_MAP message, the UL_MAP message, the DCD message, and the UCD message to the MSS  1600  (step  1617 ). The UL_MAP message transmitted to the MSS  1600  in step  1617  may include information related to the time slot assignment for allowing the MSS  1600  to transmit the RNG_REQ message through a time slot. 
   The MSS  1600  transmits the RNG_REQ message including coded information and information related to a former serving base station, which communicates with the MSS  1600  before MSS  1600  has been subject to the drop, to the serving base station  1610  through a time slot corresponding to the time slot assignment information included in the UL_MAP message in order to reestablish the communication with respect to the serving base station  1610  (step  1619 ). The serving base station  1610  then transmits the RNG_RSP message to the MSS  1600  in response to the RNG_REQ message (step  1621 ). 
   Since the MSS  1600  is an MSS for reestablishing communication with regard to the serving base station  1610  after the drop, the serving base station primarily assigns resources to the MSS  1600  in such a manner that the MSS  1600  can perform the network-entry procedure in a contention-free manner. The network-entry procedure of the MSS  1600  includes registration and authentication between the MSS  1600  and the serving base station. Upon receiving the RNG_RSP message from the serving base station  1610 , the MSS  1600  performs the network-entry procedure in relation to the serving base station  1610  (step  1623 ). 
   A drop ranging procedure of an MSS by using a drop ranging time slot for allowing the MSS to reestablish communication within a short period of time when the drop occurs in the MSS in an IEEE 802.16e communication system will be described with reference to  FIG. 17 . 
     FIG. 17  is a signal flow diagram illustrating the drop ranging procedure of the MSS, which is subject to the drop, by using the drop ranging time slot in the IEEE 802.16e communication system according to one embodiment of the present invention. 
   Referring to  FIG. 17 , when an initial synchronization is achieved between an MSS  1700  and a serving base station  1710  (step  1711 ), the MSS  1700  receives a DL_MAP message, an UL_MAP message, a DCD message, and an UCD message from the serving base station  1710  (step  1713 ). As described above, in one embodiment of the present invention, the UL_MAP message includes an information of drop ranging time slots. The serving base station  1710  is a new serving base station selected by the MSS  1700  after the drop occurs in the MSS  1700 . The UL_MAP message may include information related to a drop ranging offset, that is, information related to a drop ranging time slot. A drop ranging information element (Drop_Ranging IE) of the UL_MAP message according to the present invention is represented in Table 16. 
   
     
       
             
             
             
           
         
             
               TABLE 16 
             
             
                 
             
             
                 
               size 
               notes 
             
             
                 
             
           
           
             
               Drop Ranging 1E 
                 
                 
             
             
               { 
             
             
                UIUC 
                4 bits 
             
             
                Drop ranging offset 
               12 bits 
               Indicates the start time of the burst 
             
             
                 
                 
               relative to the Allocation Start Time 
             
             
                 
                 
               given in the UL MAP message. 
             
             
                reserved 
                4 bits 
             
             
               } 
             
             
                 
             
           
        
       
     
   
   In addition, the MSS  1700  transmits the RNG_REQ message to the serving base station  1710  using the drop ranging time slot (step  1715 ) in such a manner that the serving base station  1710  can recognize that the MSS  1700  is attempting to reestablish the communication with regard to the serving base station  1710  after the drop occurs in the MSS  1700 . Upon receiving the RNG_REQ message from the MSS  1700 , the serving base station  1710  may recognize that the MSS  1700  is attempting to reestablish the communication after the drop, so the serving base station  1710  transmits the RNG_RSP message to the MSS  1700  in response to the RNG_REQ message (step  1717 ). Since the MSS  1700  is an MSS for reestablishing communication with regard to the serving base station  1710  after the drop, the serving base station  1710  primarily assigns resources to the MSS  1700  in such a manner that the MSS  1700  can perform the network-entry procedure in a contention-free manner. Upon receiving the RNG_RSP message from the serving base station  1710 , the MSS  1700  performs the network-entry procedure in relation to the serving base station  1710  (step  1719 ). 
   As described above, according to the present invention, the number of target base stations, which must be monitored for allowing the MSS to reestablish a communication with regard to the target base station when the MSS is subject to the drop while making communication with the serving base station, can be reduced so that the MSS can reestablish the communication with regard to the target base station within a short period of time in a broadband wireless access communication system using the OFDM/OFDMA schemes, such as the IEEE 802.16e communication system. In addition, when the MSS reestablishes the communication with regard to the serving base station in the IEEE 802.16e communication system, the MSS notifies the new serving base station of the reestablishment of the communication by using the drop ranging codes. Thus, the new serving base station may primarily assign resources to the MSS such that the MSS can reestablish communication with regard to the new serving base station within a short period of time, thereby improving service quality of the IEEE 802.16e communication system. 
   While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.