Abstract:
Disclosed are a system and a method for optimizing a handover process in a mobile broadband wireless access system. The method performs handover in a mobile communication system including a mobile subscriber station, a serving base station currently providing service to the mobile subscriber station, and at least one neighbor base stations adjacent to the serving base station. The method includes the steps of receiving a handover request from the mobile subscriber station by the serving base station; instructing the mobile subscriber station to perform handover by the serving base station in response to the handover request from the mobile subscriber station and performing by the mobile subscriber station a network reentry process for fast handover in accordance with information included in a handover instruction from the serving base station.

Description:
PRIORITY 
     This application is a Continuation Application of U.S. application Ser. No. 11/146,683, which was filed on Jun. 7, 2005, and claims priority to an application entitled “System And Method For Optimizing Handover In Mobile Communication System” filed in the Korean Intellectual Property Office on Jun. 7, 2004 and assigned Serial No. 2004-41540, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system and a method for handover in a mobile communication system, and more particularly to a system and a method for optimizing a handover process, which can reduce handover time delay in a broadband mobile communication system. 
     2. Description of the Related Art 
     In a 4 th  generation (hereinafter, referred to as a 4G) communication system, which is the next generation communication system, research has been actively pursued to provide users with services having various quality of services (hereinafter, referred to as a QoSs) and supporting a transmission speed of about 100 Mbps. Especially, in a current 4G communication system, research has been actively pursued to develop a new type of communication system ensuring mobility and QoS in the wireless Local Area Network (LAN) system and the wireless Metropolitan Area Network (MAN) system supporting relatively high transmission speeds, and support a high speed service to be provided by the 4G communication system. As representatives of 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. 
     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 network for a physical channel of the wireless MAN system. 
     The IEEE 802.16a communication system is a system for only a single structure without reflecting the mobility of a Subscriber Station (SS) at all, in which the SS is in a stationary state. In contrast, the IEEE 802.16e communication system is a system reflecting the mobility of SS, and the SS having the mobility will be referred to as a Mobile Subscriber Station (MSS). 
     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 illustrating a structure of a conventional mobile communication system (e.g. the IEEE 802.16e communication system). 
     The IEEE 802.16e communication system has a multi-cell structure including a cell  100  and a cell  150 . Further, the IEEE 802.16e communication system includes a Base Station (BS)  110  controlling the cell  100 , a BS  140  controlling the cell  150 , and a plurality of MSSs  111 ,  113 ,  130 ,  151  and  153 . The transmission/reception of signals between the BS  110  and  140  and the MSSs  111 ,  113 ,  130 ,  151  and  153  is accomplished using an OFDM/OFDMA method. Herein, the MSS  130  is located in an overlapping area (i.e., a handover area) between the cell  100  and the cell  150 . Accordingly, when the MSS  130  moves further into the cell  150  controlled by the BS  140  while transmitting/receiving a signal with the BS  110 , a serving BS for the MSS  130  changes from the BS  110  to the BS  140 . 
     Hereinafter, a typical process of handover will be discussed with reference to  FIG. 2  which is a flow diagram illustrating a typical handover process. 
     First, an MSS  210  linked to a serving BS (BS # 1 )  220  tries to connect to a new BS when a status of the link to the serving BS  220  degrades below a predetermined threshold. In order to determine suitable base stations which can serve as a new serving base station, the MSS  210  scans linkable neighbor BSs, for example, BS # 2   230 , BS # 3   240  and BS # 4   250  (Step  201 ). Thereafter, based on the determination, the MSS  210  selects BSs (e.g., BS # 2   230 , BS # 3   240 ) to which the MSS  210  will request handover. Then, the MSS  210  transmits to the serving BS  220  an MSS handover request (MOB_MSSHO_REQ) message including the recommended neighbor BSs  230  and  240  and the scanned result (Step  203 ). Upon receiving the MOB_MSSHO_REQ message, the serving BS  220  transmits a handover pre-notification (HO_pre_notification) message reporting the handover request by the MSS  210  to the recommended neighbor BSs (i.e., BS # 2   230  and BS # 3   240 ) contained in the MOB_MSSHO_REQ message (Steps  205  and  207 , respectively). The HO_pre_notification message includes a required bandwidth and required service quality information of the MSS  210 . 
     Upon receiving the HO_pre_notification message, the neighbor BSs  230  and  240  transmit to the serving BS  220  a response (HO_pre_notification_response) message including approval or disapproval of the handover request of the MSS  210  which is based on their own resource statuses (Steps  209  and  211 , respectively). Thereafter, the serving BS  220  determines BSs to which the MSS  210  can be handovered and transmits a handover response (MOB_BSHO_RSP) message including the determined BSs to the MSS  210  (Step  215 ). Here, the MOB_BSHO_RSP message includes one or more BSs. Thereafter, the serving BS  220  transmits a handover confirmation (HO_confirm) message to the target BS one of the recommended target BSs (e.g. BS # 3   240 ) included in the MOB_BSHO_RSP message so that the BS can prepare the handover of the MSS  210  (Step  213 ). 
     Upon receiving the MOB_BSHO_RSP message, the MSS  210  determines a target BS to which handover will be performed from among the recommended target BSs included in the MSS_BSHO_RSP message and transmits to the serving BS  220  a handover indication (MOB_HO_IND) message including the determined BS (Step  217 ). Upon receiving the MOB_HO_IND message from the MSS  210 , the serving BS  220  releases the wireless connection channel with the MSS  210  (Step  219 ). 
     Then, the MSS  210  performs a network reentry process with the neighbor BS (e.g. BS # 3   240 ) included in the MOB_HO_IND message (Steps  221 - 225 ). Hereinafter, the neighbor BS to which the MSS  210  moves and newly connects will be referred to as a target BS. After performing the network reentry process, the MSS  210  sets a new wireless connection channel with the target BS  240 . The network reentry process with the target BS  240  is similar to the network entry process with the serving BS  220  and includes the steps of: (1) downlink/uplink parameter acquisition; (2) ranging; (3) basic capability re-negotiation; (4) re-authorization; (5) re-register; and (6) re-establishment of IP connectivity. Finally, the MSS  210  having established a new wireless connection channel with the target BS  240  can continue the data transmission (Step  227 ). 
     According to the construction as described above, in the course of releasing the wireless connection channel with the existing serving BS and establishing a new wireless connection channel with the target BS, the existing serving BS deletes the service context of the MSS at the following time point. That is to say, the serving BS deletes the service context at the time point when receiving the MOB_HO_IND message from the MSS or at the time point when receiving a handover completion message from the target BS after establishment of a new wireless connection channel between the MSS and the target BS is completed. Here, the handover completion message has yet to be defined in detail. 
     The existing handover method in which the MSS releases the existing wireless connection channel with the serving BS and establishes a new wireless connection channel with the target BS at the time of handover as described above has the following disadvantages. 
     Specifically, according to the existing handover method, frequent handover is necessary when an MSS frequently moves between different cells controlled by different BSs and a network reentry process causing a delay is necessary whenever the handover is performed. Therefore, according to the existing handover method, interruption of data transmission/reception due to handover may increase and become unacceptable. 
     Further, if the handover process is initiated when the signal from the serving BS has a Carrier to Interference and Noise Ratio (CINR) is smaller than that of a neighbor BS, the MOB_MSSHO_REQ message, the MOB_BSHO_RSP message and MOB_HO_IND message must be transmitted even in a state in which the connection between the MSS and the serving BS is poor. In this case, a transmission error or a delay due to a retransmission may occur which can cause further delays and service interruptions. Moreover, a random access delay may occur when the MSS transmits the MOB_MSSHO_REQ message and the MOB_HO_IND message. Therefore, depending on the length of the time delay, the connection with the serving BS may be interrupted even before the handover process is completed. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide a system and a method for optimizing a handover process in a mobile broadband wireless access system. 
     It is another object of the present invention to provide a backbone communication method and polling method for optimizing a handover process in a mobile broadband wireless access system. 
     It is a further another object of the present invention to provide a system and a method for optimizing a handover process, which can reduce a network reentry process at every handover in a mobile broadband wireless access system. 
     It is a still another object of the present invention to provide a system and a method for optimizing a handover process, which can reduce handover time delay by using a polling mechanism in a mobile broadband wireless access system. 
     In order to accomplish the aforementioned objects, there is provided a method for performing a handover in a mobile communication system including a mobile subscriber station, having a serving base station currently providing service to the mobile subscriber station, and having at least one neighbor base stations adjacent to the serving base station. The method including the steps of receiving by the serving base station a handover request from the mobile subscriber station; instructing by the serving base station the mobile subscriber station to perform a handover in response to the handover request from the mobile subscriber station; and performing by the mobile subscriber station a network reentry process for fast handover in accordance with information included in a handover instruction from the serving base station. 
     In accordance with another aspect of the present invention, there is provided a method for performing a handover by a mobile subscriber station in a mobile communication system including the mobile subscriber station, having a serving base station currently providing service to the mobile subscriber station, and having a plurality of neighbor base stations adjacent to the serving base station. The method including the steps of transmitting from the mobile subscriber station to the serving base station a handover request; receiving from the serving base station by the mobile subscriber station response message in response to the handover request; confirming communication level information for handover included in the response message from the serving base station; and performing by the mobile subscriber station and the target base station a network reentry process for fast handover while omitting at least one step of the network reentry process in accordance with the communication level information. 
     In accordance with another aspect of the present invention, there is provided a method for supporting handover by a serving base station in a mobile communication system including a mobile subscriber station, the serving base station currently providing service to the mobile subscriber station, and a target base station adjacent to the serving base station. The method including the steps of: transmitting a function profile of the mobile subscriber station upon receiving a handover request from the mobile subscriber station; and transmitting a response message to the mobile subscriber station, the response message having information omitted for performing a fast reentry process with the target base station. 
     In accordance with another aspect of the present invention, there is provided a method for performing a handover by a target base station in a mobile communication system including a mobile subscriber station, having a serving base station currently providing service to the mobile subscriber station, and having the target base station adjacent to the serving base station. The method including the steps of: transmitting to the serving base station response information and communication level information when it is determined during a process of negotiation through a network backbone message with the serving base station that the target base station can provide service to the mobile subscriber station, the communication level information including a communication level at which the target base station and the mobile subscriber station will perform communication; receiving a function profile of the mobile subscriber station from the serving base station; and performing a fast handover together with the mobile subscriber station through a network reentry process omitting at least one predetermined step. 
     In accordance with another aspect of the present invention, there is provided a method for optimizing a handover process in a mobile communication system including a serving base station, a mobile subscriber station, and a target base station. The method including the steps of: dividing the handover process into multiple levels of backbone communication in order to minimize a time delay due to a handover and selecting a corresponding backbone communication level by the target base station in a process of negotiation through a network backbone message between the serving base station and the target base station; transmitting a from the serving base station to the mobile subscriber station handover response message, the handover response message informing the mobile subscriber station of the corresponding backbone communication level selected by the target base station; transmitting to the target base station information of the mobile subscriber station corresponding to the backbone communication level; and performing by the mobile subscriber station a network reentry process omitting a predetermined message transmission/reception process corresponding to the backbone communication level from the serving base station. 
     In accordance with another aspect of the present invention, there is provided a method for performing a handover in a mobile communication system including a mobile subscriber station, a serving base station currently providing service to the mobile subscriber station, and a plurality of neighbor base stations adjacent to the serving base station. The method including the steps of: transmitting from the serving base station to the mobile subscriber station a handover request message, the handover request message having information omitted for performing fast reentry process together with the target base station; and performing fast handover by the mobile subscriber station through a network reentry process omitting at least one predetermined step based on information included in the handover request message. 
     In accordance with another aspect of the present invention, there is provided a handover support system for supporting handover in a mobile communication system including a mobile subscriber station, a serving base station currently providing service to the mobile subscriber station, and a plurality of neighbor base stations adjacent to the serving base station. The handover support comprises a serving base station for receiving a message including a selected backbone communication level from an available target base station during a process of network negotiation, transmitting a function profile of the mobile subscriber station to the target base station in response to the message, and transmitting a handover response message including the appointed backbone communication level to the mobile subscriber station, thereby instructing the mobile subscriber station to perform fast handover omitting a predetermined step corresponding to the backbone communication level; a target base station for receiving information of the mobile subscriber station to be handed over from the serving base station and selecting and transmitting a corresponding backbone communication level to the serving base station in response to the received information; and a mobile subscriber station for receiving the handover response message from the serving base station and performing a network reentry process for fast handover together with the target base station in accordance with the backbone communication level included in the handover response message. 
    
    
     
       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 block diagram schematically illustrating a structure of a conventional mobile communication system; 
         FIG. 2  is a flow chart for illustrating a typical handover process; 
         FIG. 3  is a flow diagram of a backbone communication process for handover optimization in a mobile broadband wireless access system according to an embodiment of the present invention; 
         FIGS. 4A and 4B  are flow diagrams illustrating a second-level backbone communication for handover optimization in a mobile broadband wireless access system according to a preferred embodiment of the present invention; and 
         FIGS. 5A and 5B  are flow diagrams illustrating a third-level backbone communication for handover optimization in a mobile broadband wireless access system according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, a preferred embodiment according to the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, 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. 
     The present invention proposes a solution for improving a handover process in a Mobile Broadband Wireless Access (MBWA) system. That is, the present invention proposes a solution for optimizing a handover process in order to improve handover performance in a MBWA system. For such optimization, the present invention proposes a stepped backbone communication and polling mechanism and defines a signaling process and message for realizing them. 
     The backbone communication according to the present invention includes three levels, that is, a first-level (Level  1 ) backbone communication, a second-level (Level  2 ) backbone communication and a third-level (Level  3 ) backbone communication (wherein each level is also known as a step). Hereinafter, a method according to an embodiment of the present invention, for optimizing a network reentry process through the three-stepped backbone as described above, will be discussed. In the following description, it should be noted that target BS  1  and target BS  2  are the neighbor BSs to which the MSS can move, that is, target BSs included in the target BS list. 
     In the following description about a handover method according to the present invention, the same operation as that in the conventional handover method as shown in  FIG. 2  will not be repeatedly described and only the stepped backbone communication according to the present invention will be mainly discussed in detail. 
       FIG. 3  is a flow diagram illustrating a first-level backbone communication for handover optimization in a mobile broadband wireless access system according to a preferred embodiment of the present invention. 
     The first-level backbone communication process is a process in which a serving BS  320  transmits a capability profile of an MSS  310  to target BSs  330  and  340 , so that the step of basic capability re-negotiation, that is, the steps of transmitting SBC_REQ and SBC_RSP (as shown in Step  350 ) can be omitted from the network reentry process. 
     The flow diagram shown in  FIG. 3  illustrates the first-step (-level) from among the three-step backbone communication proposed according to the present invention in order to minimize delay in handover. As shown in  FIG. 3 , in the first-level, in the course of negotiation through network backbone messages including the HO_pre_notification message (Steps  309  and  311 ) and the HO_pre_notification_response message (Steps  313  and  315 ) between the serving BS  320  and available target BSs  330  and  340 , the available target BSs insert an acknowledgement (ACK) or not acknowledgement (NACK) in the transmitted HO_pre_notification_response message. The flow diagram illustrated in  FIG. 3  is based on an assumption that the target BS 2   340  is a serviceable target BS. Here, the serviceable target BS 2   340  transmits the HO_pre_notification_response message after inserting a corresponding Backbone Communication Level (BCL, e.g. BCL=1) together with the ACK in the HO_pre_notification_response message (Step  315 ). 
     Then, the serving BS  320  inserts the information of BCL (BCL=1) received from the target BS 2   340  in the MOB_BSHO_RSP message and then transmits the message to the MSS  310  (Step  319 ). Here, the MOB_BSHO_RSP message may include the following numbers for indicating the backbone communication level (BCL): 0=Semi Nomadic; 1=backbone communication level  1  (Omit SBC-REQ/RSP during reentry processing); 2=backbone communication level  2  (Omit SBC-REQ/RSP, PKM-REQ/RSP and REG-REQ/RSP during reentry processing); and 3=backbone communication level  3  (Omit SBC-REQ/RSP, PKM-REQ/RSP and REG-REQ/RSP during reentry processing/ARQ transmission). 
     Further, the serving BS  320  sends information corresponding to the BCL to the target BS  340 . In the first BCL (BCL  1 ), the serving BS  320  sends an original SBC (Basic Capability) profile to the target BS  340  (Step  317 ). 
     As a result, it is possible to omit the step of transmitting SBC_REQ/RSP from the network reentry process. 
     It should be noted that the same as the above description about the first-level backbone communication can also be applied to the second and third-level backbone communication which will be described below. 
       FIGS. 4A and 4B  are flow diagrams illustrating a second-level backbone communication for handover optimization in a mobile broadband wireless access system according to a preferred embodiment of the present invention. 
     The second-level backbone communication according to the present invention can be classified into communications employing a security key reuse scheme as shown in  FIG. 4A  and communications employing a pre-authentication scheme as shown in  FIG. 4B . According to the security key reuse scheme, all or some of the keys used for security in a previous serving BS are used without change for the new target BS. According to the pre-authentication scheme, the serving BS receives a key to be used by the target BS in advance from the target BS and transmits it to the MSS before starting the handover, that is, before sending the HO-IND, so that the MSS can wholly or partially omit the exchange of the PKM_REQ message and PKM_RSP message (shown in Step  450  of  FIGS. 4A and 4B ) when the MSS performs network reentry. 
     Referring to  FIGS. 4A and 4B , for the sake of clarity steps shown in  FIG. 3  correspond with respectively with steps shown in  FIG. 4 , and as such no further description will be given. In the course of negotiation through network backbone messages between the serving BS  420  and available target BSs  430  and  440 , the available target BSs transmits an HO_pre_notification_response message after inserting a corresponding BCL (e.g. BCL=2) in the HO_pre_notification_response message (Steps  407  and  411 ).  FIGS. 4A and 4B  are based on an assumption that the target BS 2   440  is an available target BS and the serviceable target BS 2   440  transmits the HO_pre_notification_response message after inserting a corresponding BCL (e.g. BCL=2) in the HO_pre_notification_response message. Then, the serving BS  420  inserts the information of BCL (BC 2 ) received from the target BS 2   440  in the MOB_BSHO_RSP message and then transmits the message to the MSS  410  (Step  413 ). Here, the serving BS  420  sends information corresponding to the BCL to the target BS 2   440  (Step  415 ). As a result, it is possible to omit all steps of the network reentry process except for the ranging step in the second level backbone communication. Namely, it is possible to omit SBC_REQ/RSP, PKM_REQ/RSP, REG_REQ/RSP message exchange between the MSS and the target BS. 
     Hereinafter, the corresponding information sent from the serving BS  420  to the target BS 2   440  in the second-level backbone communication will be discussed. 
     As described above, the second-level backbone communication can be classified into a security key reuse type and a pre-authentication type. In the second-level backbone communication, the serving BS  420  sends not only the service profile of the MSS  410  but also all Connection Identifiers (CIDs), Service Flow CIDs (SFIDs), and security keys (in the case of employing the key reuse scheme) to the target BS 2   440 . As a result, in the second-level backbone communication, it is possible to omit the basic capability re-negotiation step (i.e., the step of transmitting/receiving the SBC_REQ message and the SBC_RSP message), the re-authorization step (i.e., the step of transmitting/receiving the PKM_REQ message and the PKM_RSP message), and the re-registration step (i.e., the step of transmitting/receiving the REG_REQ message and the REG_RSP message) from the network reentry process. In a Medium Access Control (MAC) layer, each of different QoS classes is identified by a CID. Here, such identification is achieved by using the SFID. Also, when the identified information is encrypted, a key is attached to the encrypted information. The reason why it is necessary to transmit all CIDs is that each connection in the air link is identified by a CID which is separately controlled by each BS. Therefore, the target BS must notify its currently used CID to the MSS, which can be defined by a TLV of the REG_RSP message. That is, it is possible to omit unnecessary transmission/reception steps by transmitting the information as described above to the target BS in the above-mentioned step. In other words, according to the second-level backbone communication process of the present invention, it is possible to omit step  450  in  FIGS. 4A and 4B . 
     As described above, the second-level backbone communication illustrated in  FIGS. 4A and 4B  is proposed by the present invention in order to minimize the time delay required for handover. Further, not only the second-level backbone communication but also the third-level backbone communication (which will be described below) proposes two schemes according to the authentication type, which include the security key reuse scheme and the pre-authentication scheme.  FIG. 4A  illustrates a signaling process using the security-key-reuse scheme and  FIG. 4B  illustrates a signaling process using the pre-authentication scheme. 
     The second-level backbone communication process as described above may include all the steps of the first level backbone communication process. Moreover, in the second-level backbone communication process, the serving BS passes all CIDs including the basic CID, the primary CID, the secondary CID, the transport CID and the SFID to the target BS. Further, in the second-level backbone communication process, Hashed Message Authentication Code (HMAC), Tuple and REG_RSP TLV encoded information are carried by the transmitted RNG_RSP message (Step  423 ), so that it is possible to omit all the steps (i.e. step  450  in  FIGS. 4A and 4B ) of the network reentry process except for the ranging process. For the sake of clarity, in Step  450 , Steps  425 ,  427 ,  429 ,  431 ,  433  and  435  correspond with Steps  329 ,  331 ,  333 ,  335 ,  337  and  339 , respectively, and therefore no further description is given. 
       FIGS. 5A and 5B  illustrate a third-level backbone communication for handover optimization in a mobile broadband wireless access system according to a preferred embodiment of the present invention. 
     Similar to the second-level backbone communication, the third-level backbone communication can be classified into two types. That is to say, the third level backbone communication can be classified into communication employing the security-key-reuse scheme as shown in  FIG. 5A  and communication employing the pre-authentication scheme as shown in  FIG. 5B . 
     Referring to  FIGS. 5A and 5B , in the course of negotiation through network backbone messages between the serving BS  520  and available target BSs  530  and  540 , the available target BSs transmits an HO_pre_notification_response message after inserting a corresponding BCL (e.g. BCL=3) in the HO_pre_notification_response message (Steps  507  and  511 ).  FIGS. 5A and 5B  are based on an assumption that the target BS 2   540  is an available target BS and the serviceable target BS 2   540  transmits the HO_pre_notification_response message after inserting the corresponding BCL (e.g. BCL=3) in the HO_pre_notification_response message (Step  511 ). Then, the serving BS  520  inserts the information of BCL (BCL=3) received from the target BS 2   540  in the MOB_BSHO_RSP message and then transmits the MOB_BSHO_RSP message to the MSS  510  (Step  513 ). Here, the serving BS  520  sends information corresponding to the BCL to the target BS 2   540  (Step  515 ). As a result, it is possible to omit all steps of the network reentry process except for the ranging process in the third-level backbone communication. For the sake of clarity, steps with the same names in  FIGS. 5A and 5B  correspond to each other. 
     Hereinafter, the corresponding information sent from the serving BS  520  to the target BS 2   540  in the third-level backbone communication will be discussed. As described above, the third-level backbone communication can be classified into two types, that is, the security-key-reuse type as shown in  FIG. 5A  and the pre-authentication type as shown in  FIG. 5B . In the third-level backbone communication, the serving BS  520  passes all Medium Access Control (MAC) state information of the MSS  510  to the target BS 2   540 . As a result, in the third level backbone communication, it is possible to omit all the steps of the network reentry process except for the ranging process (i.e. the steps of transmitting/receiving the RNG_REQ message (Step  521 ) and the RNG_RSP message ( 523 )). In other words, according to the third level backbone communication process proposed by the present invention, it is possible to omit step  550  in  FIGS. 5A and 5B . Further, the serving BS passes the ARQ state information to the target BS, so that the ARQ can be reused without being reset. The ARQ refers to information indicating the level to which the MSS has received information. Therefore, when the information has not been normally transmitted, the packet transmission must be restarted from the failure in receiving the ACK. As a result, unnecessary waste of resource can be avoided by reporting the level to which the MAC ARQ has been transmitted. 
     As described above, the third-level backbone communication illustrated in  FIGS. 5A and 5B  is proposed by the present invention in order to minimize the time delay for handover. Further, the third level backbone communication process includes all the steps and uses all the messages of the second level backbone communication processes. In addition to the messages of the second level backbone communication processes, the automatic repeat request (ARQ) state information is passed from the serving BS to the target BS in the third level backbone communication. Therefore, the target BS can reduce the time delay for handover by using the ARQ state information from the serving BS as it is without resetting the ARQ state information. 
     Hereinafter, the BS handover response (MOB_BSHO_RSP) message (as shown in Step  513 ) newly proposed by the present invention as described above will be described in detail with reference to Table 1. The present invention can be applied to not only the handover requested by the MSS as described above but also handover requested by the serving BS. In the latter case, that is, in the handover requested by the serving BS, the BCL information as described above may be inserted in the MOB_BSHO_REQ message in order to perform the stepped backbone communication. The MOB_BSHO_REQ message will be described later in detail with reference to Table 2. 
     First, the MOB_BSHO_RSP message includes information about the target BS to which the MSS will be handovered and has a structure as shown in Table 1. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Syntax 
                 Size 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 MOB_BSHO-RSP_Message_Format 
                   
                   
                   
               
               
                 ( ) { 
               
               
                 Management Message Type = 54 
                 8 
                 bits 
               
               
                 Estimated HO Start 
                 8 
                 bits 
               
               
                 For (i =0; i &lt;N_Recommended; i++) 
                   
                   
                 Neighbor base stations shall be 
               
               
                 { 
                   
                   
                 presented in an order such that the first 
               
               
                   
                   
                   
                 presented is the one most recommended 
               
               
                   
                   
                   
                 and the last presented is the least 
               
               
                   
                   
                   
                 recommended. 
               
               
                   
                   
                   
                 N_Recommended can be derived from 
               
               
                   
                   
                   
                 the known length of the message 
               
               
                 Neighbor BS ID 
                 48 
                 bits 
               
               
                 service level prediction 
                 8 
                 bits 
               
               
                 Backbone Communication Level 
                 8 
                 bits 
               
               
                  } 
               
               
                  HMAC Tuple 
                 21 
                 bytes 
               
               
                 } 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, the MOB_HSHO_RSP message includes a plurality of information elements (IEs) such as Management Message Type indicating the type of the transmitted message, Estimated HO Start indicating the time point at which it is expected to start the handover process, and N_Recommended indicating the target BSs selected by the serving BS. Here, the N_Recommended includes an identifier (Neighbor BS ID) of each of the selected target BSs and a service level (service level prediction) at which each of the target BSs is expected to provide service to the MSS. Further, the IEs shown in Table 1 include Backbone Communication Level for handover optimization. Here, the Backbone Communication Level includes the first backbone communication level (BCL=1), the second backbone communication level (BCL=2) and the third backbone communication level (BCL=3). 
     The above description discusses a handover process according to a request by the MSS in a mobile broadband wireless access system. However, the present invention is not limited to the above-discussed handover process and can be applied to handover according to a request by the BS without question. In the case of the handover according to a request by the BS, the BCL information is inserted in and carried by the MOB_BSHO_REQ message transmitted to the MSS. The MOB_BSHO_REQ message has a structure as shown in Table 2 below. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Syntax 
                 Size 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 MOB_BSHO-REQ_Message_Format 
                   
                   
                   
               
               
                 ( ) { 
               
               
                 Management Message Type = 52 
                 8 
                 bits 
               
               
                 Network Associated HO supported 
                 1 
                 bit 
               
               
                 For (i =0; i &lt;N_Recommended; i++) { 
                   
                   
                 N_Recommended can be derived from 
               
               
                   
                   
                   
                 the known length of the message 
               
               
                 Neighbor BS ID 
                 48 
                 bits 
               
               
                 service level prediction 
               
               
                 Backbone Communication Level 
                 8 
                 bits 
               
               
                  } 
               
               
                 reserved 
                 7 
                 bits 
               
               
                  HMAC Tuple 
                 21 
                 bytes 
               
               
                 } 
               
               
                   
               
             
          
         
       
     
     As shown in Table 2, the MOB_HSHO_REQ message includes a plurality of Information Elements (IEs) such as Management Message Type indicating the type of the transmitted message and N_Recommended indicating the scanned result by the serving BS. The N_Recommended includes an identifier of each of the target BSs, a Carrier to Interference and Noise Ratio (CINR) of a pilot signal of each of the target BSs, and a service level at which each of the target BSs is expected to provide service to the MSS. Further, the IEs in Table 2 include Backbone Communication Level for handover optimization. As described above, the Backbone Communication Level includes the Semi Nomadic (BCL=0), the first backbone communication level (BCL=1), the second backbone communication level (BCL=2) and the third backbone communication level (BCL=3). 
     As described above, the MOB_BSHO_REQ message is transmitted while carrying the BCL information in the case of handover based on the request of the BS according to another embodiment of the present invention. That is, in a state where the information for the stepped backbone communication is set in advance through the backbone communication between the serving BS and the target BS, the serving BS transmits the MOB_BSHO_REQ message including the BCL information to the MSS. Then, the MSS transmits the MOB_HO_message and then directly proceeds to the next step while omitting unnecessary steps. 
     Hereinafter, a method of handover employing a polling mechanism according to another embodiment of the present invention will be described. The present invention proposes a polling mechanism as well as the stepped backbone communication, in order to optimize the network reentry process. 
     According to the polling mechanism proposed by the present invention, in order for a BS to allocate a bandwidth to an MSS in a mobile broadband wireless access system, four steps of message exchange between the BS and the MSS is necessary. That is, the message exchange process includes a first step for bandwidth request ranging, a second step in which the BS allocates uplink burst for the bandwidth request header to the MSS, a third step in which the MSS transmits the bandwidth request header to the BS, and a fourth step in which the BS allocates burst for packet transmission to the MSS. 
     Here, in order to reduce the time delay during handover, the present invention proposes a mechanism i.e., a “polling mechanism”) in which an uplink burst is allocated to the MSS at every frame after the transmission of the MOB_MSSHO_REQ. 
     In other words, according to the polling mechanism proposed by the present invention, the BS must exchange messages with the MSS in four steps in total as described above in order to allocate a bandwidth to the MSS. For such a process, the present invention proposes a mechanism in which a small burst by which a bandwidth request header can be transmitted is allocated every frame until the serving BS receives the MOB_HO_IND message after transmitting the MOB_BSHO_REQ message or receiving the MOB_MSSHO_REQ message from the MSS. 
     Further, the polling mechanism can be applied not only to the serving BS but also to the target BS. Specifically, when the target BS receives the ranging request (RNG_REQ) message from the MSS, the target BS allocates a small burst capable of carrying a bandwidth request header to the MSS in every frame as described above. 
     As described above, the present invention proposes a new stepped backbone communication in order to reduce the network reentry process for each handover and a polling mechanism in order to time delay for handover, thereby providing a system and method capable of optimizing handover capability in a mobile broadband wireless access system. 
     The present invention provides an optimal handover system and scheme for minimizing time delay of handover in a mobile communication system. Especially, the present invention can improve the existing handover capacity by using the stepped backbone communication scheme and the polling mechanism in a mobile communication system. 
     Further, according to the present invention, the serving BS and the target BS preliminarily exchange information corresponding to each level, including basic capability information, security key, ARQ state information, etc., so that it is possible to omit the network reentry process except for the ranging process, thereby reducing the duration during which data transmission/reception due to handover. 
     Moreover, according to the present invention, the network reentry process at every handover can be reduced by using a stepped backbone communication and the handover time delay can be reduced by using the polling mechanism in a mobile broadband wireless access system, so that the handover performance in a mobile broadband wireless access system can be optimized. 
     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.