Abstract:
In a broadband wireless access communication system which includes a mobile station, a serving base station and neighbor base stations located adjacent to the serving base station, the serving base station collects the information of the neighbor base stations, determines if the collected information of the neighbor base stations matches the information of the serving base station, sets up predetermined groups by classifying the neighbor base stations according to a result of the determination, and broadcasts a predetermined message including information of the setup groups, and the mobile station receives the predetermined message broadcasted, confirms the information of the neighbor base stations included in the received message, and performs location information update in accordance with the confirmed information.

Description:
PRIORITY  
       [0001]     This application claims priority to an application entitled “Method And System For Forming And Transmitting/Receiving Neighbor Base Station Information In A Broadband Wireless Access Communication System” filed in the Korean Industrial Property Office on Jun. 18, 2004 and assigned Serial No. 2004-45759, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a Broadband Wireless Access (BWA) communication system, and more particularly to a method and system for forming and transmitting/receiving information of neighbor base stations periodically broadcast by a serving base station.  
         [0004]     2. Description of the Related Art  
         [0005]     A 4 th  generation (4G) communication system is the next generation communication system. Research has been actively pursued to provide users with 4G communications having various improved quality of services (QoS) at high speed. Research on the current 4G communication systems has been focused on ensuring mobility and QoS in BWA communication system, such as wireless Local Area Network (LAN) systems and wireless Metropolitan Area Network (MAN) systems which are capable of supporting relatively high transmission speeds. Representing such new type communication systems, an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and an IEEE 802.16e communication system have been developed.  
         [0006]     The IEEE 802.16a communication system and an IEEE 802.16e communication system employ an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme in order to support broadband transmission networking for a physical channel of the wireless MAN system. The IEEE 802.16a communication system is a system for only a single structure in which the Subscriber Station (SS) is in a stationary state. The IEEE 802.16a standard does not recognize the mobility of an SS at all. In contrast, the IEEE 802.16e communication system does reflects the mobility of the SS. An SS having mobility will be referred to as a Mobile Station (MS).  
         [0007]     Hereinafter, the structure of the conventional IEEE 802.16e communication system will be described with reference to  FIG. 1  which is a block diagram schematically showing the structure of the IEEE 802.16e communication system.  
         [0008]     The IEEE 802.16e communication system has a multi-cell structure including a cell  1   100 , a cell  2   150 , a cell  3   170  and a cell  4   180 . Further, the IEEE 802.16e communication system includes a Base Station (BS)  1   110  controlling the cell  1   100 , a BS  2   140  controlling the cell  2   150 , a BS  3   172  controlling the cell  3   170 , a BS  4   182  controlling the cell  4   180 , and a plurality of MSs  2   111 ,  1   113 ,  8   130 ,  4   151 ,  3   153 ,  6   174 ,  7   176 , and  5   184 . The transmission/reception of signals between the BSs  1   110 ,  2   140 ,  3   172  and  4   182  and the MSs  111 ,  113 ,  130 ,  151 ,  153 ,  174 ,  176  and  184  is accomplished using an OFDM/OFDMA scheme. Herein, the MS  130   8  from among the MSs  111 ,  113 ,  130 ,  151 ,  153 ,  174 ,  176  and  184  is located in a boundary area (i.e., handover area) between the cell  1   100  and the cell  2   150 . Accordingly, it is possible to support the mobility of the MS  8   130  only when the handover for the MS  8   130  is supported. When handover is necessary as the MS  8   130  moves into the cell  2   150  controlled by the BS  2   140  while transmitting/receiving a signal with the BS  1   110  controlling the cell  1   100 , the connection setup of the physical layer and the Medium Access Control (MAC) layer for the communication with the BS  110   1  is no longer available for communication with the BS  2   140 . Therefore, when the MS  8   130  moves from the cell  1   100  to the cell  2   150 , the MS  8   130  must again perform a re-entry process for the connection setup with the BS  2   140  in the same manner as that for the first connection setup with the BS  1   110 .  
         [0009]     In the IEEE 802.16e communication system as described above, an MS receives pilot signals transmitted from a plurality of BSs. The MS measures Carrier to Interference and Noise Ratio (CINR) of the received pilot signals. Then, the MS selects the BS transmitting the pilot signal, having the largest CINR from among the measured CINRs of the received pilot signals, to become the serving BS (i.e. the BS to which the MS currently belongs). In other words, the MS recognizes the BS transmitting the pilot signal, which can be received in best condition by the MS from among the plurality of BSs, as the serving BS for the MS. After selecting the serving BS, the MS transmits and receives data by receiving downlink frames and uplink frames transmitted from the serving BS.  
         [0010]     Dynamic Host Configuration Protocol (DHCP) server  1   190  and DHCP server  2   192  are servers for allocating Internet Protocol (IP) addresses in response to requests from the MSs  111 ,  113 ,  130 ,  151 ,  153 ,  174 ,  176  and  184  connected through the BSs  110 ,  140 ,  172  and  182 . Hereinafter, DHCP server  1   190  and DHCP server  2   192  will be referred to as simply DHCP  1   190  and DHCP  2   192 . In general, the DHCP servers possess different assignable IP addresses, distributed in advance to them due to the finite number of available IP addresses. Referring to  FIG. 1 , BS  1   110  and BS  3   172  are connected to DHCP  1   190  and BS  2   140  and BS  4   182  are connected to DHCP  2   192 . That is, the BSs may be connected to either the same DHCP server or different DHCP servers according to the location of the BSs. Therefore, when MSs are allocated IP addresses from DHCP  1   190  through BS  1   110  and BS  3   172 , they are allocated IP addresses from the same DHCP server and the IP addresses allocated to them have the same address system and the same prefix value. In the same manner, DHCP  2   192  allocates IP addresses having the same prefix value to MSs through BS  2   140  and BS  4   182 .  
         [0011]     Therefore, when MS  1   113  is located within cell  1   100  so that MS  1   113  connects with and is allocated an IP address from DHCP  1   190  through BS  1   110  and when MS  1   113  is located within cell  3   170  so that MS  1   113  connects with and is allocated an IP address from DHCP  1   190  through BS  3   172 , the IP addresses allocated in both cases have the same prefix value because they are allocated by the same DHCP  1   190 . The MS  1   113  is allocated an IP address having the same prefix value from DHCP  1   190  when it is located in either cell  1   100  or cell  3   170 . Even after moving into cell  3   170 , the MS  1   113  can use the IP address allocated to the MS  1   113  when the MS  1   113  is located in cell  1   100 . In contrast, even after moving into cell  1   100 , the MS  1   113  can use the IP address allocated to the MS  1   113  when the MS  1   113  is located in cell  3   170 . The use of the same address system between cells as described above enables an MS to continue using an IP address allocated in one cell even after moving between cells (e.g. between cell  1   100  and cell  3   170 ). A network constructed by the cells (e.g. between cell  1   100  and cell  3   170 ) using the same IP address is called the ‘same IP subnet’.  
         [0012]     However, when MS  1   113  moves from cell  1   100  to cell  2   150  in  FIG. 1 , MS  1   113  is allocated IP addresses from different DHCP servers because BS  1   110  of cell  1   100  is connected to DHCP  1   190  and BS  2   140  of cell  2   150  is connected to DHCP  2   192 . In this case, the IP address allocated by DHCP  1   190  when the MS  113  is located within cell  1   100  and the IP address allocated by DHCP  2   192  when the MS  113  is located within cell  2   150  have different prefix values. Therefore, the IP addresses allocated by DHCP  1   190  and the IP address allocated by DHCP  2   192  when the MS  113  moves from cell  1   100  to cell  2   150  have different prefix values. Therefore, the MS  113  cannot use the IP address allocated by DHCP  1   190  of cell  1   100  in cell  2   150 . The cells using IP addresses having different prefix values constructs a different IP subnet. In other words, the same IP subnet refers to a network in which cells use IP addresses having the same prefix value, and in which an MS can use an IP address without change even when moving between cells. A different IP subnet refers to a network in which cells use IP addresses having different prefix value, and in which an MS must change the IP address when moving between cells.  
         [0013]     Here, in order to perform handover to a neighbor cell, the MS must know information of neighbor cells (i.e. information of neighbor BSs). Therefore, the MS collects information of neighbor BSs by receiving periodic broadcast messages from the serving BS which currently provides service to the MS. In general, the message containing the information of neighbor BSs is referred to as a Mobile Neighbor Advertisement (MOB_NBR-ADV) message.  
         [0014]      FIG. 2  is a table illustrating a structure of a typical MOB_NBR-ADV message.  
         [0015]     The MOB_NBR-ADV message includes a plurality of Information Elements (IEs) such as ‘Management Message Type’ indicating the type of the transmitted message, ‘Configuration Change Count’ indicating the number of times by which the configuration changes, ‘N 13  Neighbors’ indicating the number of neighbor BSs, ‘Neighbor BS-ID’ indicating the number of identifiers of the neighbor BSs, ‘DL Physical Frequency’ indicating the physical channel frequency of the neighbor BS, and ‘TLV Encoded Neighbor Information’ indicating other information in relation to the neighbor BSs including Type, Length and Value (TLV).  
         [0016]     The MOB_NBR-ADV message is periodically transmitted from the serving BS as described above and the MS can acquire scanning information for measuring the signal intensity of the neighbor BSs by receiving the MOB_NBR-ADV message. That is, the MS can identify neighbor BSs by using the field ‘Neighbor BS-ID’ and can recognize physical frequency band scan information necessary for scanning by using the field ‘DL Physical Frequency’. Further, the field ‘TLV Encoded Neighbor Information’ may include 16 bits of paging zone identifier (ID) information of corresponding neighbor BSs.  
         [0017]     Now, the paging zone will be briefly discussed. In a communication system having a multi-cell structure, a plurality of neighbor cells may be constructed into one logical group (paging group) according to locations of the cells for paging of an MS. A Paging and Location Management (PLM) server (not shown) may provide a paging zone ID to each paging group as constructed above, so as to perform paging to multiple MSs located in the same paging zone. Then, the MSs located in the same paging zone confirms the paging zone ID in the field ‘TLV Encoded Neighbor Information’ of the MOB_NBR-ADV message, determines if the paging zone has been changed by comparing the paging zone ID of the currently received MOB_NBR-ADV message with the paging zone ID of the previously received MOB_NBR-ADV message, and then updates the location of the MS based on the determination. If an MS recognizes a change of the paging zone, it must perform a network re-entry process together with the corresponding BS of the changed paging zone. A more detailed description about location information update according to paging zone change of the MS will be given below with reference to  FIG. 4 .  
         [0018]      FIG. 3  is a network re-entry process of an MS in a typical IEEE 802.16e communication system.  
         [0019]     In step  311 , according to handover, the MS acquires a system sync with a new serving BS to which the handover of the MS is performed by receiving a preamble of a downlink frame transmitted from the new serving BS. Then, the MS acquires a downlink sync by receiving BS information contained in various messages broadcasted by the BS, such as a Downlink Channel Descriptor (DCD) message, an Uplink Channel Descriptor (UCD) message, a DL_MAP message, a UL_MAP message and a MOB_NBR-ADV message.  
         [0020]     In step  313 , the MS transmits a ranging request (RNG_REQ) message to the BS and receives a ranging response (RNG_RSP) message from the BS as a response to the RNG_REQ message, thereby acquiring an uplink sync with the BS. In step  315 , the MS adjusts the frequency and power.  
         [0021]     In step  317 , the MS negotiates with the BS for the basic capability of the MS. In step  319 , the MS performs authentication with the BS, thereby acquiring a Traffic Encryption Key (TEK) allocated to the MS. In step  321 , the MS requests registration of the MS itself to the BS and the BS performs the registration. In step  323 , the MS performs an IP connection with the BS. The IP connection consumes a relatively long time interval of several seconds because it requires a handover process of the IP layer in which the DHCP server newly allocates an IP address and newly registers the location information.  
         [0022]     In step  325 , the MS downloads management information through an Internet protocol connected to the BS. In step  327 , the MS performs a service flow connection with the BS. Here, the service flow refers to a flow by which MAC-Service Data Units (MAC-SDUs) are transmitted/received through a connection having a predetermined QoS. In step  329 , the MS performs the service provided by the BS and then ends the process.  
         [0023]     As described above, the IP connection (step  323 ) in the network re-entry process of the MS takes a relatively long time. However, if the BSs are located in the same IP subnet, the existing IP address can be used without change. Therefore, the IP connection as shown in step  323  (i.e. a process of allocating a new IP address) can be omitted. However, no specific scheme for achieving such omission has yet been defined in the current 802.16 specification.  
         [0024]      FIG. 4  is a signal flow diagram for illustrating a process of updating location information of an MS in a conventional BWA communication system.  
         [0025]     Before discussing  FIG. 4 , an idle mode and an awake mode will be described. The MAC layer of the BWA communication system supports two kinds of operation modes including an awake mode and a idle mode. First, the idle mode, or sleep mode has been proposed in order to minimize power consumption of the MS in an idle interval in which packet data transmission is not performed during at least a predetermined time interval. That is to say, when there is no packet data transmission, the MS transitions from the awake mode to the idle mode in order to minimize power consumption of the MS in the idle interval in which packet data are not transmitted. In general, the packet data are burst when generated. Therefore, it is unreasonable to perform the same operation in the interval in which packet data are not transmitted as that in the interval in which packet data are transmitted. For this reason, the idle mode has been proposed. In contrast, when packet data to be transmitted occur while the MS stays in the idle mode, the MS transits into the awake mode, or active mode and then transmits and receives the packet data. However, because the packet data has a property highly reliant on the traffic mode, the operation in the idle mode must be integrated in consideration of the traffic characteristics and transmission scheme characteristics of the packet data.  
         [0026]     First, handover when the MS  410  in the idle mode moves between different paging zones (i.e. paging zone using different paging zone IDs) will be described with reference to  FIG. 4 . First, the serving BS  430  transmits a mobile idle response (MOB_DL_RSP) message to the MS  410  (step  411 ). Although  FIG. 4  does not show any specific reason why the serving BS  430  transmits the MOB_IDL_RSP message to the MS  410 , the serving BS  430  may transmit the MOB_IDL_RSP message either in response to a mobile idle request (MOB_IDL_REQ) message transmitted from the MS  410  to the serving BS  430  or in an unsolicited manner without any request. Upon receiving the MOB_IDL_RSP message from the serving BS  430 , the MS  410  transitions from the awake mode into the idle mode.  
         [0027]     Then, while the MS  410  is in the idle mode, the MS  410  moves from the service area controlled by the serving BS  430  to another service area controlled by another BS (target BS  450 ) different from the serving BS  430  (step  413 ). Here, it is assumed that the serving BS  430  and the target BS  450  are located in different paging zones. After the MS  410  moves as described above, the MS  410  has no connection for communication with the serving BS  430  and cannot receive a paging request (MOB_PAG_REQ) message even when the MS  410  wakes up at the paging time point and performs monitoring. Therefore, when the MS  410  detects the movement as described above, the MS  410  receives BS information through the DL_MAP message, the UL_MAP message, the DCD message and the UCD message broadcasted by the new BS after the movement, that is, the target BS  450  (step  415 ). As described above, the paging zone ID of the target BS  450  may be included in the DL_MAP message.  
         [0028]     By receiving the BS information broadcasted by the target BS  450  as described above, the MS  410  recognizes the paging zone ID of the target BS  450  and thus recognizes that the serving BS  430  and the target BS  450  are located within different paging zones (step  417 ). When the MS  410  determines that the serving BS  430  and the target BS  450  are located within different paging zones, the MS  410  performs the initial ranging (step  419 ). By performing the initial ranging, the MS  410  acquires a basic Connection ID (CID) and a primary management CID. The MS  410  transmits a mobile station location update request (MOB_LU_REQ) message to the target BS  450  by using the primary CID previously acquired through the initial ranging (step  421 ). The MOB_LU_REQ message includes the paging zone ID stored in the MS  410  (the existing PZID).  
         [0029]     Upon receiving the MOB_LU_REQ message from the MS  410 , the target BS  450  transmits a location update request (LOCATION_UPDATE_REQUEST) message to a Paging and Location Management (PLM) server  470  (step  423 ). The LOCATION_UPDATE_REQUEST message includes a MAC address of the MS requesting the location information update and a paging zone ID of the serving BS  430  with which the MS belonged before the handover. Upon receiving the LOCATION_UPDATE_REQUEST message, the PLM server  470  updates the location of the MS  410  by referring to the paging zone ID and MAC address included in the LOCATION_UPDATE_REQUEST message, and transmits a location update response (LOCATION_UPDATE_RESPONSE) message to the target BS  450  as a response to the LOCATION_UPDATE_REQUEST message (step  425 ). Upon receiving the LOCATION_UPDATE_RESPONSE message from the PLM server  470 , the target BS  450  transmits a location update response (MOB_LU_RSP) message to the MS  410  (step  427 ). After receiving the MOB_LU_RSP message from the target BS  450 , the MS  410  performs mode transition into the idle mode in accordance with the selection calling period, etc. included in the MOB_LU_RSP message.  
         [0030]     Hereinafter, problems of the conventional process for updating the location of an MS, as described above, will be discussed. After an MS moves into another paging zone in an idle mode, the MS periodically awakes and receives the MOB_NBR-ADV message transmitted from a BS. The MS compares a paging zone recognized by a currently received MOB_NBR-ADV message with a paging zone recognized by a previously received MOB_NBR-ADV message and performs location information update according to the result of the comparison. However, in performing the location information update of the MS as described above, if the MS moves into another paging zone directly after the period at which the MS awakes passes only one time, the MS cannot transition into the awake mode before the next period at which the MS awakes.  
         [0031]     Also, in the conventional MOB_NBR-ADV message, information of neighbor BSs is arranged without any standard. In other words, in the conventional MOB_NBR-ADV message, the BS information recorded at the first order is not the information of the target BS having the highest priority and the BS information recorded at the final order is not the information of the target BS having the lowest priority. Therefore, the conventional MOB_NBR-ADV message includes only the information necessary for the scanning of neighbor BSs by the MS and does not include the information necessary for the MS&#39;s determination of the target BS to which the handover will be actually performed. It is also possible to use the TLV field for each neighbor BS in order to include paging zone ID of the neighbor BSs. However, because most of the BSs in the actual list belong to the same paging zone and have the same paging zone ID, the use of the TLV field for each BS may cause repetitive transmission of unnecessary information, thereby wasting resources of radio channels.  
         [0032]     Further, according to the conventional process, even when the MS has moved between cells belonging to the same IP subnet, the MS must be allocated a new IP address. As a result, an unnecessarily long time delay occurs in the conventional network re-entry process of the MS. Therefore, it is necessary to define a new structure for the MOB_NBR-ADV message, which includes IP subnet information, but which does not include the 16 bits of paging zone ID since this information is overhead unnecessarily included in the TLV field of the conventional MOB_NBR-ADV message.  
       SUMMARY OF THE INVENTION  
       [0033]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to propose a new broadcast message of a neighbor BS in a Broadband Wireless Access (BWA) communication system. It is another object of the present invention to provide a method for transmitting/receiving a newly proposed neighbor BS broadcast message in a BWA communication system. It is another object of the present invention to provide a method for fast network re-entry of an MS in a BWA communication system. It is another object of the present invention to provide a method for fast location information update according to location change of an MS in a BWA communication system.  
         [0034]     In order to accomplish this object, a method is provided for transmitting and receiving information of neighbor base stations in a broadband wireless access communication system which includes a mobile station, a serving base station and the neighbor base stations located adjacent to the serving base station, the serving base station broadcasting information of the neighbor base stations to the mobile station. The method includes collecting the information of the neighbor base stations, determining if the collected information of the neighbor base stations is to the same as information of the serving base station, setting up predetermined groups by classifying the neighbor base stations according to a result of the determination, and broadcasting a predetermined message including information of the setup groups, respectively by the serving base station; and receiving the predetermined message broadcasted, confirming the information of the neighbor base stations included in the received message, and performing location information update in accordance with the confirmed information, respectively by the mobile station.  
         [0035]     In accordance with another aspect of the present invention, a method is provided for constructing information of neighbor base stations by a serving base station in a broadband wireless access communication system which includes a mobile station, the serving base station and the neighbor base stations located adjacent to the serving base station, the serving base station broadcasting the information of the neighbor base stations to the mobile station. The method includes collecting the information of the neighbor base stations and determining if the collected information of the neighbor base stations is the same as the information of the serving base station; and setting up predetermined groups by classifying the neighbor base stations according to a result of the determination, and constructing a broadcasted message by combining the groups.  
         [0036]     In accordance with another aspect of the present invention, is a method is provided for performing a location information update by a mobile station in a broadband wireless access communication system which includes the mobile station, a serving base station and neighbor base stations located adjacent to the serving base station, the serving base station broadcasting the information of the neighbor base stations to the mobile station. The method includes receiving information including paging zone identifiers identified by logical areas in order to page the mobile station and network address identifiers of the neighbor base stations from the serving base station; and confirming the information of the neighbor base stations included in the received information and performing location information update in accordance with the confirmed information.  
         [0037]     In accordance with another aspect of the present invention, a system is provided for transmitting and receiving information of neighbor base stations in a broadband wireless access communication system which includes a mobile station, a serving base station and the neighbor base stations located adjacent to the serving base station, the serving base station broadcasting information of the neighbor base stations to the mobile station. The system includes the serving base station for collecting the information of the neighbor base stations, determining if the collected information of the neighbor base stations is to the same as information of the serving base station, setting up predetermined groups by classifying the neighbor base stations according to a result of the determination, and broadcasting a predetermined message including information of the setup groups; and the mobile station for receiving the predetermined message broadcasted, confirming the information of the neighbor base stations included in the received message, and performing location information update in accordance with the confirmed information. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     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:  
         [0039]      FIG. 1  is a block diagram schematically showing the structure of a typical IEEE 802.16e communication system;  
         [0040]      FIG. 2  is a table illustrating a structure of a typical MOB_NBR-ADV message;  
         [0041]      FIG. 3  is a network re-entry process of an MS in a typical IEEE 802.16e communication system;  
         [0042]      FIG. 4  is a signal flow diagram for illustrating a process of updating location information of an MS in a conventional BWA communication system;  
         [0043]      FIG. 5  illustrates a table showing a structure of a MOB_NBR-ADV message proposed in a BWA communication system according to the present invention;  
         [0044]      FIGS. 6A and 6B  illustrate a flow diagram of a process in which a serving BS constructs a MOB_NBR-ADV message in a BWA communication system according to the present invention;  
         [0045]      FIG. 7  is a flow diagram of an operational process of an MS after receiving the MOB_NBR-ADV message in a BWA communication system according to the present invention;  
         [0046]      FIG. 8  is a signal flow diagram for illustrating a location information update process performed by an MS in a BWA communication system according to an embodiment of the present invention; and  
         [0047]      FIG. 9  is a network re-entry process of an MS in a BWA communication system according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0048]     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention unclear.  
         [0049]     The present invention proposes a new format for the Mobile Neighbor Advertisement (MOB_NBR-ADV) message in a Broadband Wireless Access (BWA) communication system and proposes a method for message transmission/reception between a Base Station (BS) and a Mobile Station (MS) by using the proposed format.  
         [0050]      FIG. 5  illustrates a table showing a structure of a MOB_NBR-ADV message proposed in a BWA communication system according to the present invention.  
         [0051]     The MOB_NBR-ADV message includes fields of ‘Neighbor_Type_Code’, ‘N_Type-1_Neighbors’, ‘N_Type-2_Neighbors’, ‘N_Type-3_Neighbors’ and ‘N_Type-4_Neighbors’ in addition to the conventional structure thereof shown in  FIG. 2 .  
         [0052]     Hereinafter, the fields of ‘Neighbor_Type_Code’, ‘N_Type-1_Neighbors’, ‘N_Type-2_Neighbors’, ‘N_Type-3_Neighbors’ and ‘N_Type-4_Neighbors’ will be described in detail.  
         [0053]     First, the field of ‘Neighbor_Type_Code’ newly added to the MOB_NBR-ADV message indicates the meaning (or standard) by which the information of the BSs according to an embodiment of the present invention is classified, arranged and grouped. When the field of ‘Neighbor_Type_Code’ has a value of “0000”, the information of the BSs is classified, arranged and grouped according to whether the Internet Protocol (IP) subnet information and paging zone identifier (paging zone ID) of the neighbor BSs have changed or not. The preferred embodiment of the present invention, as described later, is based on an assumption that the field of ‘Neighbor_Type_Code’ has a value of “0000”.  
         [0054]     Otherwise, when the information of the BSs is classified, arranged and grouped based on other information of the neighbor BSs instead of the IP subnet information and paging zone ID information of the neighbor BSs, the field of ‘Neighbor_Type_Code’ has a value between “0001” and “1111”. When the field of ‘Neighbor_Type_Code’ has a value between “0001” and “1111”, the field of ‘N_Type_Neighbor’ located after the field ‘Neighbor_Type_Code’ may be used in a more expanded manner. The field is used to store information of the neighbor BSs, other than the IP subnet information and paging zone ID information. Hereinafter, the case in which the field ‘Neighbor_Type_Code’ has a value of “0000”, that is, the case in which the IP subnet information and paging zone ID information from among the information of the neighbor BSs is used, will be described.  
         [0055]     The fields of ‘N_Type-1_Neighbors’, ‘N_Type-2_Neighbors’, ‘N_Type-3_Neighbors’ and ‘N_Type-4_Neighbors’ newly added as is the field ‘Neighbor_Type_Code’ indicate information of neighbor BSs which are classified according to whether the IP subnet information and paging zone ID information of the neighbor BSs coincide with those of the current serving BS of the MS, when the field ‘Neighbor_Type_Code’ has a value of “0000”. These fields can be classified as follows.  
         [0056]     ‘N_Type-1_Neighbors’: BSs using the same IP subnet and the same paging zone ID as those of the serving BS.  
         [0057]     ‘N_Type-2_Neighbors’: BSs using the same IP subnet as that of the serving BS and a paging zone ID different from that of the serving BS.  
         [0058]     ‘N_Type-3_Neighbors’: BSs using an IP subnet different from that of the serving BS and the same paging zone ID as that of the serving BS.  
         [0059]     ‘N_Type-4_Neighbors’: BSs using an IP subnet and a paging zone ID different from those of the serving BS.  
         [0060]     By receiving the MOB_NBR-ADV message broadcasted by the serving BS, the MS can understand in advance if the IP subnets and the paging zone ID of the neighbor BSs are the same or different from each other. As a result, when the MS moves, the MS can determine in advance whether to perform handover of the IP layer or update location information according to the change of the paging zone. That is, considering the fact that very long time delay is necessary until the handover of the higher IP layer is completed, the MS may try handover to a BS within the same IP subnet, thereby causing the handover of the higher IP layer in determining the target BS to which the handover will be performed. Further, when the MS in an idle mode moves between cells, the MS need not wait until the calling period or awake period and can determine whether to perform the IP connection reset or the paging zone location information update by comparing the received information with the stored neighbor BS information even in the idle mode.  
         [0061]     According to the preferred embodiment of the present invention, the field ‘Neighbor_Type_Code’ has a value of “0000” and information of the neighbor BSs is classified, arranged and grouped according to coincidence of the IP subnet information and the paging zone ID information. Therefore, if the value of the field ‘Neighbor_Type_Code’ is defined in a way different from that in the preferred embodiment of the present invention, the fields of ‘N_Type_Neighbor’ also may have different meaning from that according to the preferred embodiment of the present invention.  
         [0062]      FIGS. 6A and 6B  illustrate a flow diagram of a process in which a serving BS constructs a MOB_NBR-ADV message in a BWA communication system according to the present invention.  
         [0063]     First, in step  602 , the serving BS receives information of neighbor BSs from the neighbor BSs and stores the received information. In step  603 , the serving BS selects and determines standard information from among the stored information of the neighbor BSs so as to use the standard information in classifying, arranging and grouping the neighbor BSs. That is, the serving BS determines the value of the field ‘Neighbor_Type_Code’ in step  603 . When the classification is performed based on the IP subnet information and the paging zone ID information from among the information of the neighbor BSs, the value of the field ‘Neighbor_Type_Code’ is determined as “0000”, and step  604  is then performed. In contrast, when the classification is performed based on information of the neighbor BSs other than the IP subnet information and the paging zone ID information, the field ‘Neighbor_Type_Code’ is determined to have a value between “0001” and “1111”, and step  635  is then performed. In step  635 , reserved processes are performed by using the information of the neighbor BSs other than the IP subnet information and the paging zone ID information. That is, classification and grouping of information is performed by using the information of the neighbor BSs other than the IP subnet information and the paging zone ID information. Step  636  is then performed.  
         [0064]     In step  604 , the serving BS confirms the IP subnet information and the paging zone ID information of the neighbor BSs from the stored information of the neighbor BSs. Thereafter, the serving BS sequentially determines if the IP subnet and paging zone ID of the serving BS are equal to the confirmed IP subnet and paging zone ID of each neighbor BS. In step  606 , the serving BS determines if the IP subnet of the serving BS is equal to the confirmed IP subnet of a predetermined neighbor BS (hereinafter, referred to as “first neighbor BS”). When the IP subnet of the serving BS is equal to the confirmed IP subnet of the first neighbor BS, the serving BS proceeds to step  608 . In step  608 , the serving BS determines if the paging zone ID of the serving BS is equal to the confirmed paging zone ID of the first neighbor BS. When the paging zone ID of the serving BS is equal to the confirmed paging zone ID of the first neighbor BS, the serving BS proceeds to step  612 .  
         [0065]     In step  612 , the serving BS classifies the first neighbor BS as Type-1 Neighbor. Then, in step  614 , the serving BS inserts the first neighbor BS in the group of Type-1 Neighbors. In step  616 , the serving BS increases the number of Type-1 Neighbors by ‘1’, that is, the serving BS updates the number of Type-1 Neighbors by adding one to the number and proceeds to step  636 . In step  636 , the serving BS constructs the MOB_NBR-ADV message by setting the number of neighbor BSs classified as Type-1 Neighbor in the field ‘N_Type-1 Neighbors’ of the MOB_NBR-ADV message.  
         [0066]     When the IP subnet of the serving BS is same to the confirmed IP subnet of the first neighbor BS but the paging zone ID of the serving BS is different from the confirmed paging zone ID of the first neighbor BS by the determination in step  608 , the serving BS proceeds to step  618 . In step  618 , the serving BS classifies the first neighbor BS as Type-2 Neighbor. Then, in step  620 , the serving BS inserts the first neighbor BS in the group of Type-2 Neighbors. In step  622 , the serving BS increases the number of Type-2 Neighbors by ‘1’, that is, the serving BS updates the number of Type-2 Neighbors by adding one to the number and proceeds to step  636 . In step  636 , the serving BS constructs the MOB_NBR-ADV message by setting the number of neighbor BSs classified as Type-2 Neighbor in the field ‘N_Type-2 Neighbors’ of the MOB_NBR-ADV message.  
         [0067]     The above description deals with a process of setting the field values of N_Type-1 Neighbor and N_Type-2 Neighbor between the serving BS and the first neighbor BS, and a process of setting the field values of N_Type-3 Neighbor and N_Type-4 Neighbor between the serving BS and the first neighbor BS will be described below.  
         [0068]     When the IP subnet of the serving BS is different from the confirmed IP subnet of the first neighbor BS by the determination in step  606 , the serving BS proceeds to step  610 . In step  610 , the serving BS determines if the paging zone ID of the serving BS is equal to the confirmed paging zone ID of the first neighbor BS. When the paging zone ID of the serving BS is equal to the confirmed paging zone ID of the first neighbor BS, the serving BS proceeds to step  624 . In step  624 , the serving BS classifies the first neighbor BS as Type-3 Neighbor. Then, in step  626 , the serving BS inserts the first neighbor BS in the group of Type-3 Neighbors. In step  628 , the serving BS increases the number of Type-3 Neighbors by ‘1’, that is, the serving BS updates the number of Type-3 Neighbors by adding one to the number and proceeds to step  636 . In step  636 , the serving BS constructs the MOB_NBR-ADV message by setting the number of neighbor BSs classified as Type-3 Neighbor in the field ‘N_Type-3 Neighbors’ of the MOB_NBR-ADV message.  
         [0069]     When the paging zone ID of the serving BS is different from the confirmed paging zone ID of the first neighbor BS by the determination in step  610 , the serving BS proceeds to step  630 . In step  630 , the serving BS classifies the first neighbor BS as Type-4 Neighbor. Then, in step  632 , the serving BS inserts the first neighbor BS in the group of Type-4 Neighbors. In step  634 , the serving BS increases the number of Type-4 Neighbors by ‘1’; that is, the serving BS updates the number of Type-4 Neighbors by adding one to the number and proceeds to step  636 . In step  636 , the serving BS constructs the MOB_NBR-ADV message by setting the number of neighbor BSs classified as Type-4 Neighbor in the field ‘N_Type-4 Neighbors’ of the MOB_NBR-ADV message.  
         [0070]     After completing the classification of the first neighbor BS in the way described above, the serving BS sequentially performs the above-described process again for each of the remaining neighbor BSs, thereby constructing the MOB_NBR-ADV message by setting each field thereof. Then, the serving BS broadcasts the MOB_NBR-ADV message.  
         [0071]      FIG. 7  is a flow diagram of an operation process of an MS after receiving the MOB_NBR-ADV message in a BWA communication system according to the present invention.  
         [0072]     In step  702 , the MS receives the MOB_NBR-ADV message broadcasted by the serving BS. In step  703 , the MS confirms the value of the field “Neighbor_Type_Code’ of the received MOB_NBR-ADV message. When the confirmed value of the field “Neighbor_Type_Code’ is “0000” as in the embodiment shown in  FIGS. 5, 6A  and  6 B, it is noted that the information of Type-N Neighbors is classified, arranged and grouped based on the IP subnet information and paging zone ID information. When the field “Neighbor_Type_Code’ has a value of “0000” as described above, the MS proceeds to step  704 , step  710 , step  716  or step  722 . Meanwhile, when the field “Neighbor_Type_Code’ has a value between “0001” and “1111”, it is noted that the information of Type-N Neighbors is classified, arranged and grouped based on information other than the IP subnet information and paging zone ID information. When the field “Neighbor_Type_Code’ has a value between “0001” and “1111 38  , the MS proceeds to step  728  and performs a reserved process. That is, in step  728 , the MS confirms the information of the neighbor BSs included in the received MOB_NBR-ADV message.  
         [0073]     After the MS proceeds to step  704 , step  710 , step  716  or step  722 , the MS reads the value set in each field of the MOB_NBR-ADV message in order to understand the neighbor BS information. Specifically, in step  704 , the MS confirms the number of neighbor BSs classified as Type-1 Neighbor and set in the filed ‘N_Type-1 Neighbors’ of the MOB_NBR-ADV message. Then, in step  706 , the MS confirms the BS information of each of the neighbor BSs the number of which has been confirmed. In step  708 , the MS recognizes to use the same IP subnet and the same paging zone ID of the confirmed neighbor BSs and stores the informations.  
         [0074]     In step  710 , the MS confirms the number of neighbor BSs classified as Type-2 Neighbor and set in the filed ‘N_Type-2 Neighbors’ of the MOB_NBR-ADV message. Then, in step  712 , the MS confirms the BS information of each of the neighbor BSs the number of which has been confirmed. In step  714 , the MS recognizes to use the same IP subnet and the different paging zone ID of the confirmed neighbor BSs and stores the informations.  
         [0075]     In step  716 , the MS confirms the number of neighbor BSs classified as Type-3 Neighbor and set in the filed ‘N_Type-3 Neighbors’ of the MOB_NBR-ADV message. Then, in step  718 , the MS confirms the BS information of each of the neighbor BSs the number of which has been confirmed. In step  720 , the MS recognizes to use the different IP subnet and the same paging zone ID of the confirmed neighbor BSs and stores the informations.  
         [0076]     In step  722 , the MS confirms the number of neighbor BSs classified as Type-4 Neighbor and set in the filed ‘N_Type-4 Neighbors’ of the MOB_NBR-ADV message. Then, in step  724 , the MS confirms the BS information of each of the neighbor BSs the number of which has been confirmed. In step  726 , the MS recognizes to use the different IP subnet and the different paging zone ID of the confirmed neighbor BSs and stores the informations.  
         [0077]      FIG. 8  is a signal flow diagram for illustrating a location information update process performed by an MS in a BWA communication system according to an embodiment of the present invention.  FIG. 8  also provides comparison of the relative timing for common events in the location information update process under the prior art system and the present invention.  
         [0078]     By comparing  FIG. 8  with  FIG. 4 , it is noted that the MOB_NBR-ADV message newly proposed by the present invention can reduce the time for update of location information according to the MS&#39;s change of the paging zone. Referring to  FIG. 8 , first, when an MS  802  in an awake mode  870  receives a mobile idle response (MOB_IDL_RSP) message from a serving BS  804  (step  810 ), the MS transfers into the idle mode  874  at the time point  872 . Although  FIG. 8  does not show any specific reason why the serving BS  804  transmits the MOB_IDL_RSP message to the MS  802 , the serving BS  804  may transmit the MOB_IDL_RSP message either in response to a mobile idle request (MOB_IDL_REQ) message transmitted from the MS  802  to the serving BS  804  or in an unsolicited manner without any request.  
         [0079]     While the MS  802  is in the idle mode, the MS  802  may move from the service area controlled by the serving BS  804  to another service area controlled by target BS  806  (step  812 ). In this case, according to the conventional method, although the MS  802  has actually changed its location, the MS  802  cannot understand the change of its paging zone because it is in the idle mode. That is to say, the MS  802  in the idle mode  882  transitions into the awake mode  886  only from the time point  884  which is the time point at which it is scheduled to end the idle mode. The MS  802  can recognize the difference between the paging zone ID of the serving BS  804  and the paging zone ID of the target BS  806  only after directly receiving the information of the target BS through the UCD/DCD message, the UL_MAP message, the DL_MAP message, etc. as in step  415  of  FIG. 4 .  
         [0080]     However, according to the present invention, after the MS  802  moves to the target BS  806  (step  812 ), the MS  802  already understands (through the process shown in  FIG. 7 . from the MOB_NBR-ADV message received by the MS  802  from the serving BS  804 ) whether the paging ID of the target BS  806  is equal to the paging ID of the serving BS  804 . Therefore, in step  814 , the MS  802  understands that the paging ID of the target BS  806  is different from the paging ID of the serving BS  804 . Specifically, the MS  802  measures the signal level (e.g. the CINR of the pilot signal) by using the pilot signal which it received from the BS in the idle mode  874  and recognizes its movement into another cell when there is change in the signal level. Therefore, the MS  802  recognizes a BS corresponding to a signal level having the largest value as a current serving BS to which the MS  802  currently belongs and obtains information of the BS from the information stored in advance. After recognizing the change of the paging zone in step  814 , the MS  802  performs the initial ranging together with the target BS  806  (step  816 ) and then performs a process of location information update.  
         [0081]     That is, by performing the initial ranging in step  816 , the MS  802  acquires a basic Connection ID (CID) and a primary management CID. The MS  802  transmits a mobile station location update request (MOB_LU_REQ) message to the target BS  806  by using the primary CID acquired through the initial ranging (step  818 ). The MOB_LU_REQ message includes the paging zone ID stored in the MS  802  (the existing PZID).  
         [0082]     Upon receiving the MOB_LU_REQ message from the MS  802 , the target BS  806  transmits a location update request (LOCATION_UPDATE_REQUEST) message to a Paging and Location Management (PLM) server  808  (step  820 ). The LOCATION_UPDATE_REQUEST message includes a MAC address of the MS requesting the location information update and a paging zone ID of the serving BS  804  to which the MS  802  belonged before the handover. Upon receiving the LOCATION_UPDATE_REQUEST message, the PLM server  808  updates the location of the MS  802  by referring to the paging zone ID and MAC address included in the LOCATION_UPDATE_REQUEST message, and transmits a location update response (LOCATION_UPDATE_RESPONSE) message to the target BS  806  as a response to the LOCATION_UPDATE_REQUEST message (step  822 ). Upon receiving the LOCATION_UPDATE_RESPONSE message from the PLM server  808 , the target BS  806  transmits a location update response (MOB_LU_RSP) message to the MS  802  (step  824 ). After receiving the MOB_LU_RSP message from the target BS  806 , the MS  802  performs mode transition into the idle mode  880  at the time point  878  in accordance with the selection calling period, etc. included in the MOB_LU_RSP message.  
         [0083]     Now,  FIG. 8 , illustrating a process of the present invention, will be briefly compared with  FIG. 4 , illustrating the conventional process. According to the present invention, the MS  802  performs the location information update process in a state which has already recognized the change of the paging zone in step  818 . However, according to the conventional process, the MS  802  still stays in the idle mode  882 . Therefore, the new MOB_NBR-ADV message proposed by the present invention enables the MS to acquire information of neighbor BSs in advance, so that the location information update can be performed without a long time delay, even when the paging zone changes as shown in  FIG. 8 . Therefore, the present invention achieves faster location information update in comparison with the conventional technology.  
         [0084]      FIG. 9  is a network re-entry process of an MS in a BWA communication system according to an embodiment of the present invention.  
         [0085]     In step  902 , according to handover, the MS acquires a system sync with a new serving BS to which the handover of the MS is performed by receiving a preamble of a downlink frame transmitted from the new serving BS. Then, the MS acquires a downlink sync by receiving BS information contained in various messages broadcasted by the BS, such as a Downlink Channel Descriptor (DCD) message, an Uplink Channel Descriptor (UCD) message, a DL_MAP message, a UL_MAP message and a MOB_NBR-ADV message.  
         [0086]     In step  904 , the MS transmits a ranging request (RNG_REQ) message to the BS and receives a ranging response (RNG_RSP) message from the BS as a response to the RNG_REQ message, thereby acquiring an uplink sync with the BS. In step  906 , the MS adjusts the frequency and power.  
         [0087]     In step  908 , the MS negotiates with the BS for the basic capability of the MS. In step  910 , the MS performs authentication with the BS, thereby acquiring a Traffic Encryption Key (TEK) allocated to the MS. In step  912 , the MS requests its registration with the BS and the BS performs the registration. In step  914 , the MS determines by using the neighbor BS information acquired from the received MOB_NBR-ADV message if the target BS uses the same IP subnet as that of the previous serving BS. As a result of the determination, when the two BSs use the same IP subnet, the MS need not be allocated a new IP address and thus proceeds to step  918  without performing step  916 . However, when the two BSs use different IP subnet, the MS must be allocated a new IP address and thus proceeds to step  916  in order to be allocated the new IP address.  
         [0088]     In step  916 , the MS performs an IP connection with the BS. In step  918 , the MS downloads management information through an Internet protocol connected to the BS. In step  920 , the MS performs a service flow connection with the BS. The service flow refers to a flow by which MAC-Service Data Units (MAC-SDUs) are transmitted and received through a connection having a predetermined QoS. In step  922 , the MS performs the service provided by the BS and then ends the process.  
         [0089]     According to the present invention as described above, the serving BS collects information of neighbor BSs including the IP subnet and paging zone ID information, constructs a MOB_NBR-ADV message based on the IP subnet and paging zone ID information, and periodically broadcasts the constructed MOB_NBR-ADV message. The MS stores the information of the neighbor BSs by receiving the MOB_NBR-ADV message. Therefore, according to the present invention, the process of IP connection can be omitted from the handover process between cells using the same IP subnet in the network re-entry process of the MS, so that fast handover can be achieved. Further, according to the present invention, when the MS moves between paging zones in an idle mode, the MS can achieve faster location information update.  
         [0090]     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.