Abstract:
A system and method for performing handover in a wireless mobile communication system are provided. In a Mobile Station (MS)-initiated scanning method of an (MS) in a wireless mobile communication system having at least two frequency bands with different central frequencies, the MS receives from a serving Base Station (BS) a mobile neighbor advertisement message including information about available radio resources of the neighbor BSs, transmits a new scanning request message to the serving BS, if the MS determines to measure Carrier-to-Interference and Noise Ratios (CINRs) of pilot signals from the serving BS and the neighbor BS, receives from the serving BS a response message for the scanning request message, including information about a BS which has reserved radio resources for a predetermined one of the at least two frequency bands, and measures the CINRs of the pilot signals in the predetermined frequency band.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “System And Method For Performing Handover In A Wireless Mobile Communication System” filed in the Korean Intellectual Property Office on Aug. 25, 2005 and assigned Ser. No. 2005-78354,the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a wireless mobile communication system, and in particular, to a system and method for performing a handover for a Mobile Station (MS).  
         [0004]     2. Description of the Related Art  
         [0005]     Provisioning of services with a variety of Quality of Service (QoS) levels at about 100 Mbps to users will be necessary in a future-generation communication system called a 4 th  Generation (4G) mobile communication system. The 4G communication system is envisioned as a new communication system that supports user mobility and QoS to Wireless Local Area Network (WLAN) providing relatively high data rates and a Wireless Metropolitan Area Network (WMAN). A major 4G communication system is known as the Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system.  
         [0006]     With reference to  FIG. 1 , the configuration of a multi-cell IEEE 802.16e communication system will be described as one of the IEEE 802.16 communication systems.  
         [0007]      FIG. 1  illustrates the configuration of the typical IEEE 802.16e communication system.  
         [0008]     Referring to  FIG. 1 , the IEEE 802.16e communication system has a plurality of cells  100  and  150 , Base Stations (BSs)  110  and  140  having the cells  100  and  150  under their control, respectively, and a plurality of MSs  111 ,  113 ,  130 ,  151 , and  153 .  
         [0009]     The MS  130  is located at the boundary between the cells  100  and  150 , i.e., in a handover region. Thus, support of handover for the MS  130  is equivalent to support of mobility for the MS  130 .  
         [0010]     Since the IEEE 802.16e communication system supports mobility to an MS, it can move from a current BS (i.e. a serving BS) to a neighbor BS.  
         [0011]     The process from selection of the serving BS by the MS until before handover will be described below.  
         [0012]     Upon power-on, the MS monitors a total frequency band and detects a pilot channel signal having the highest pilot Carrier-to-Interference and Noise Ratio (CINR). The MS determines that the BS which sent the pilot channel signal is its serving BS. Then the MS receives pilot signals from the serving BS and neighbor BSs and measures the CINRs of the pilot signals. If the CINR of a neighbor BS is higher than that of the serving BS, the MS operates for a handover to the neighbor BS. This step of measuring the channel status of the serving BS and the neighbor BSs is called scanning. With reference to  FIG. 2 , an MS-initiated scanning in the typical IEEE 802.16e communication system will be described.  
         [0013]      FIG. 2  is a diagram illustrating a signal flow for the MS-initiated scanning in the typical IEEE 802.16e communication system.  
         [0014]     A scanning request can be initiated by a BS or an MS. In the former case, the BS may request the MS to scan neighbor BSs in order to distribute its load, while in the latter case, the MS may request scanning to the BS when the CINR of a current channel is lower than a predetermined threshold.  
         [0015]     Referring to  FIG. 2 , a first BS  220  (BS # 1 ) is a serving BS which periodically sends a Mobile_Neighbor-Advertisement (MOB_NBR-ADV) message to an MS  200  in step  202 . BS # 1  broadcasts information about a second BS  240  (BS # 2 ) and a third BS  260  (BS # 3 ) by the MOB_NBR-ADV message. The MOB_NBR-ADV message is formatted as defined in the IEEE 802.16e/Document  8 , the contents of which are incorporated herein by reference.  
         [0016]     When the MS  200  desires to scan the neighbor BSs after receiving the MOB_NBR-ADV message, it sends a Mobile_Scanning Interval Allocation-Request (MOB_SCN-REQ) message to BS # 1  in step  204 . The MOB_SCN-REQ has a configuration as defined in the IEEE 802.16e/Document  8 .  
         [0017]     Upon receipt of the MOB_SCN-REQ message, BS # 1  replies with a Mobile_Scanning Interval Allocation-Response (MOB_SCAN-RSP) message containing scanning information (such as scan iteration information, etc.) to the MS  200  in step  206 . The configuration of the MOB_SCN-RSP is also defined in the IEEE 802.16e/Document  8 .  
         [0018]     The MS  200  scans the pilot CINRs of the neighbor BSs for N frames M frames after receiving the MOB_SCN-RSP message in steps  208  and  210 . The parameters M and N are assumed, for convenience sake. How many times the MS  200  is to scan is determined by scan iteration information included in the scanning information.  
         [0019]     The MS  200  reports pilot CINR measurements acquired during the scanning to BS # 1  by a Mobile_Scanning-Report (MOB_SCN-REP) message in step  211 . In step  212 , the MS receives data traffic from BS # 1  during a frame period indicated by interleaving interval information.  
         [0020]     In step  214  through step  218 , the MS  200  repeats the pilot CINR measurement and the data traffic reception a predetermined number of times, i.e., as many times as indicated by the scan iteration information.  
         [0021]     As described above, to support handover in the IEEE 802.16e system, the MS  200  measures the pilot CINRS of the serving BS  220  and the neighbor BSs. If the pilot CINR of the serving BS  220  is lower than that of a neighbor BS  240  or  260 , the MS  200  request a handover (to one of the target BSs  240  or  260 ) to the serving BS  220 .  
         [0022]     Owing to the development of communication technology, MSs demand large-capacity and diverse services. In this context, BSs must satisfy the demand by operating in both a Narrow frequency Band (NB) and a Wide frequency Band (WB). Also, the BSs may use various frequency modes, for example, a combination of Frequency Division Duplex (FDD) and Time Division Duplex (TDD). When a BS uses at least two different frequency bands and/or at least two different frequency modes (i.e., operates in a dual mode), an MS communicating with the corresponding BS must operate the frequency bands and/frequency modes in a similar fashion.  
         [0023]     Accordingly, MSs operating in a dual mode or other similar environment (e.g., the NB and/or WB environments) scan the NB and the WB individually, and/or scan in the individual frequency modes. This results in the increase of scanning time and hardware/software complexity. In addition, the MS cannot receive data traffic during the scanning period. Accordingly, there is a need for a technique for enabling fast, successful handover through efficient scanning of an MS in a communication system using different frequency bands and/or different frequency modes.  
       SUMMARY OF THE INVENTION  
       [0024]     An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, the present invention provides a system and method for enabling an MS to efficiently scan in a Broadband Wireless Access (BWA) communication system using different frequency bands and/or different frequency modes.  
         [0025]     The present invention provides a system and method for enabling an MS to perform handover successfully in a BWA communication system using different frequency bands and different frequency modes.  
         [0026]     According to one aspect of the present invention, in a method of performing an MS-initiated scanning in an MS in a wireless mobile communication system having at least two frequency bands with different central frequencies, the MS receives from a serving BS a mobile neighbor advertisement message including information about available radio resources of neighbor BS, transmits a new scanning request message to the serving BS, if the MS determines to measure the CINRs of pilot signals from the serving BS and the neighbor BS, receives from the serving BS a response message for the scanning request message, including information about a BS which has reserved radio resources for a predetermined one of the at least two frequency bands, and measures the CINRs of the pilot signals in the predetermined frequency band.  
         [0027]     According to another aspect of the present invention, in a scanning method in a serving BS in a wireless mobile communication system having at least two frequency bands with different central frequencies, the serving BS transmits a mobile neighbor advertisement message including information about available radio resources of the neighbor BSs, and determines whether radio resources are available for a current frequency band of an MS in the serving BS and the neighbor BS, if determining that scanning is required for the MS. In the absence of the available radio resources, the serving BS requests the neighbor BS to reserve radio resources for the current frequency band of the MS, and receives a response message for the resource reservation request from the neighbor BS. If the response message indicates a successful resource reservation, the serving BS transmits a scanning response message including information about the neighbor BS to the MS.  
         [0028]     According to a further aspect of the present invention, in a scanning method in a BS neighboring a serving BS in a wireless mobile communication system having at least two frequency bands with different central frequencies, the neighbor BS receives a resource reservation request for a first MS from the serving BS and determines whether available radio resources exist for a first frequency band in which the first MS operates currently. In the absence of the available radio resources, the neighbor BS searches for a second MS operating in the first frequency band of the first MS and capable of transitioning to a second frequency band among MSs managed by the neighbor BS. If the second MS is detected, the neighbor BS releases radio resources used in the first frequency band for the second BS by transitioning the second MS to the second frequency band, reserves the released radio resources for the first MS, and transmits information indicating the radio resource reservation to the serving BS.  
         [0029]     According to still another aspect of the present invention, in a method of performing an MS-initiated scanning in an MS in a wireless mobile communication system having at least two different frequency modes, the MS receives from a serving BS a mobile neighbor advertisement message including information about available radio resources of the neighbor BSs, transmits a new scanning request message to the serving BS, if the MS determines to measure the CINRs of pilot signals from the serving BS and the neighbor BS, receives from the serving BS a response message for the scanning request message, including information about a BS which has reserved radio resources for a predetermined one of the at least two frequency modes, and measures the CINRs of the pilot signals in the predetermined frequency mode.  
         [0030]     According to still further of the present invention, in a scanning method in a serving BS in a wireless mobile communication system having two different frequency modes, the serving BS transmits a mobile neighbor advertisement message including information about available radio resources of the neighbor BSs, and determines whether radio resources are available for a current frequency mode of an MS in the neighbor BSs, if determining that scanning is required for the MS. And the serving BS requests the neighbor BS to reserve radio resources for the current frequency mode of the MS, and receives a response message for the resource reservation request from the neighbor BS. If the response message indicates successful resource reservation, the serving BS transmits a scanning response message including information about the neighbor BS to the MS.  
         [0031]     According to yet another aspect of the present invention, in a scanning method in a BS neighboring a serving BS in a wireless mobile communication system having at least two different frequency modes, the neighbor BS receives a radio resource reservation request for a first MS from the serving BS and determines whether available radio resources exist for a first frequency mode in which the first MS currently operates. In the absence of the available radio resources, the neighbor BS searches for a second MS operating in the first frequency mode of the first MS and capable of transitioning to a second frequency mode among MSs managed by the neighbor BS. If the second MS is detected, the neighbor BS releases radio resources used in the first frequency mode for the second BS by transitioning the second MS to the second frequency mode, reserves the released radio resources for the first MS, and transmits information indicating the radio resources reserved for the first MS to the serving BS.  
         [0032]     According to yet further aspect of the present invention, in a wireless mobile communication system having at least two frequency bands each having different central frequencies, a scanning system includes an MS, a serving BS, and a neighbor BS. The MS receives from the serving BS a mobile neighbor advertisement including information about available radio resources of the neighbor BSs, determines whether to measure CINRs of pilot signals received from the serving BS and the neighbor BS, transmits a new scanning request message to the serving BS, if the MS determines to measure the CINRs of the pilot signals, receives a scanning response message for the scanning request message from the serving BS, the response message including information about a BS which has reserved radio resources for a predetermined one of the at least two frequency bands, and measures the CINRs of the pilot signals from the serving BS and the neighbor BS in the predetermined frequency band. The serving BS transmits the mobile neighbor advertisement message, determines whether radio resources are available for a first frequency band in which the MS currently operates in the serving BS and the neighbor BS, upon receipt of a scanning request from the MS or if determining that scanning is required for the MS, requests the neighbor BS to reserve radio resources for the first frequency band, in the absence of the available radio resources, receives a response message for the resource reservation request from the neighbor BS, and transmits the scanning response message to the MS, if the response message indicates successful resource reservation. The neighbor BS receives the resource reservation request from the serving BS, determines whether available radio resources exist for the first frequency band, searching for a second MS operating in the first frequency band of the first MS and capable of transitioning to a second frequency band among MSs managed by the neighbor BS, in the absence of the available radio resources, releases radio resources used in the first frequency band for the second BS by transitioning the second MS to the second frequency band, if the second MS is detected; reserves the released radio resources for the MS, and sends the response message indicating successful radio resource reservation to the serving BS.  
         [0033]     According to yet further another aspect of the present invention, in a wireless mobile communication system having at least two different frequency modes, a scanning system includes an MS, a serving BS, and a neighbor BS. The MS receives from the serving BS a mobile neighbor advertisement including information about available radio resources of the neighbor BSs, determines whether to measure CINRs of pilot signals received from the serving BS and the neighbor BS, transmits a new scanning request message to the serving BS, if the MS determines to measure the CINRs of the pilot signals, receives a scanning response message for the scanning request message from the serving BS, the response message including information about a BS which has reserved radio resources for a predetermined one of the at least two frequency modes, and measures the CINRs of the pilot signals from the serving BS and the neighbor BS in the predetermined frequency mode. The serving BS transmits the mobile neighbor advertisement message, determines whether radio resources are available for a first frequency mode in which the MS currently operates in the serving BS and the neighbor BS, upon receipt of a scanning request from the MS or if determining that scanning is required for the MS, requests the neighbor BS to reserve radio resources for the first frequency mode, in the absence of the available radio resources, receives a response message for the resource reservation request from the neighbor BS, and transmits the scanning response message to the MS, if the response message indicates successful resource reservation. The neighbor BS receives the resource reservation request from the serving BS, determines whether available radio resources exist for the first frequency mode, searches for a second MS operating in the first frequency mode of the first MS and capable of transitioning to a second frequency mode among MSs managed by the neighbor BS, in the absence of the available radio resources, releases radio resources used in the first frequency mode for the second BS by transitioning the second MS to the second frequency mode, if the second MS is detected; reserves the released radio resources for the MS, and transmits the response message indicating successful radio resource reservation to the serving BS. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0035]      FIG. 1  is a block diagram configuration of a typical IEEE 802.1 6e communication system;  
         [0036]      FIG. 2  is a flow diagram illustrating a signal flow for an MS-initiated scanning operation in the typical IEEE 802.1 6e communication system;  
         [0037]      FIG. 3  is a flow diagram illustrating a signal flow for a scanning operation for an MS-initiated handover in a BWA communication system according to the present invention;  
         [0038]      FIG. 4  is a flow chart illustrating a signal flow for a scanning operation for a BS-initiated handover in the BWA communication system according to the present invention;  
         [0039]      FIG. 5  is a flowchart illustrating a scanning operation in an MS in the BWA communication system according to the present invention;  
         [0040]      FIG. 6  is a flowchart illustrating a scanning operation in a serving BS in the BWA communication system according to the present invention; and  
         [0041]      FIG. 7  is a flowchart illustrating a scanning operation in a neighbor BS in the BWA communication system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]     Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0043]     The present invention provides a system and method for enabling an MS to efficiently scan in a BWA communication system using at least two different frequency bands and at least two different frequency modes. The MS performs a successful handover with a minimized handover time delay through the efficient scanning. Furthermore, radio resource utilization is maximized across the entire system.  
         [0044]     In accordance with the present invention, the MS scans in a predetermined frequency band and/or a predetermined frequency mode rather than scanning in all frequency bands and/or all frequency modes. A BS secures radio resources for use in the MS beforehand so as to ensure the success of the handover and minimize the handover time delay for the MS. A WB and an NB are taken as the different frequency bands and FDD mode and TDD mode are taken as the different frequency modes, by way of example. Also, the present invention will be described in the context of a BWA communication system as one of wireless mobile communication systems.  
         [0045]      FIG. 3  is a flow diagram illustrating a signal flow for a scanning operation for an MS-initiated handover in a BWA communication system according to the present invention.  
         [0046]     Referring to  FIG. 3 , BSs  330 ,  350  and  370  (BS # 1 , BS # 2 , and BS # 3  respectively) operate in at least two different frequency bands and/or at least two frequency modes. For example, they may use an NB and a WB, and FDD mode and TDD mode. Accordingly, an MS  300  must be able to use the NB and the WB, and/or operate in the FDD mode and/or the TDD mode.  
         [0047]     BS # 1 , which is a serving BS for the MS  300 , transmits (i.e., sends) a MOB_NBR-ADV message to the MS  300  periodically in step  302 . To configure the MOB_NBR-ADV message, a partial modification is made to the conventional MOB_NBR-ADV message, specifically to Available Radio Resource. Table 1 illustrates a comparison between the Available Radio Resource fields of the MOB_NBR-ADV message of the present invention and the conventional MOB_NBR-ADV message.  
                                                 TABLE 1                                   Code   Conventional (%)   Present invention (%)                                        0b0000   0   TDD (or WB) 0           0b0001   20   TDD (or WB) 20           0b0010   40   TDD (or WB) 40           0b0011   60   TDD (or WB) 60           0b0100   80   TDD (or WB) 80           0b0101   100   TDD (or WB) 100           0b0110   Reserved   FDD (or NB) 0           0b0111   Reserved   FDD (or NB) 20           0b1000   Reserved   FDD (or NB) 40           0b1001   Reserved   FDD (or NB) 60           0b1010   Reserved   FDD (or NB) 80           0b1011   Reserved   FDD (or NB) 100           0b1100-0b1110   Reserved   Reserved           0b1111   No info   No info                      
 
         [0048]     Available Radio Resource is activated when the fourth bit of Skip-Optional-Fields bitmap, associated with QoS related fields is set to 0 in MOB_NBR-ADV. Hence, the serving BS  330  periodically sends the MOB_NBR-ADV message with the fourth bit of Skip-Optional-Fields bitmap set to 0 always to the MS  300 . Thus the MS  300  can determine the average number of subchannels per frame and the average amount of symbol resources per frame for TDD and FDD, and the NB and WB.  
         [0049]     After receiving the MOB_NBR-ADV message, if the signal strength of the serving BS  330  is lower than a predetermined threshold, the MS  300  decides on scanning for a handover in step  304 . Thus, the MS  300  sends a MOB_SCN-REQ message to the serving BS  330  in step  306 .  
         [0050]     Upon receipt of the MOB_SCN-REQ message, the serving BS  330  sends a new RESOURCE-RESERVE-REQ message defined by the present invention to neighbor BSs, i.e., BS # 2  and BS # 3  in steps  308  and  310 . Steps  308  and  310  may be skipped if available radio resources exist for the current operation mode (frequency band) of the MS  300  in the serving BS  330  or a neighbor BS.  
         [0051]     If BS # 2  and BS # 3  have radio resources available for a particular frequency band (frequency mode) requested by the MS  300 , they send RESOURCE-RESERVE-RSP messages to the serving BS  330 , indicating that they have reserved the resources in steps  314  and  316 . In the absence of the resources for the requested frequency band (frequency mode), BS # 2  and BS # 3  transition MSs using the frequency band (frequency mode) under their management to another frequency band (frequency mode), if the MSs can be transitioned. BS # 2  and BS # 3  reserve radio resources saved from these MSs for allocation to the MS  300  in step  312  and notify the serving BS  330  of the radio resource reservation in steps  314  and  316 .  
         [0052]     The serving BS  330  then sends a MOB_SCN-RSP message to the MS  300  in step  318 .  
         [0053]     The MS  300  scans the serving BS  330  and the neighbor BSs  350  and  370  in the frequency band (frequency mode) based on information set in the MOB_SCN-RSP message in step  320 . In step  322 , the MS  300  reports the scanning results to the serving BS  330  by a MOB_SCN-REP message.  
         [0054]      FIG. 4  is a flow diagram illustrating a signal flow for a scanning operation for a BS-initiated handover in the BWA communication system according to the present invention.  
         [0055]     Referring to  FIG. 4 , BSs  430 ,  450  and  470  (BS # 1 , BS # 2 , and BS # 3 , respectively) can use both the NB and WB and operate in both the FDD mode and the TDD mode, as in the illustrated case of  FIG. 3 . Accordingly, an MS  400  must be able to use the NB and/or the WB, and/or to operate in the FDD mode and/or the TDD mode.  
         [0056]     BS # 1 , which is a serving BS for the MS  400 , sends a MOB_NBR-ADV message to the MS  400  periodically in step  402 . The MOB_NBR-ADV message has the fields listed in Table 1.  
         [0057]     When BS # 1  determines that handover is required for the MS  400 , it decides on scanning for the handover in step  404 . Thus, BS # 1  determines whether the neighbor BSs  450  and  470  have resources available for the current frequency mode (frequency band) of the MS  400  based on preliminarily acquired information in step  406 . If the resources are available, BS # 1  sends a MOB_SCN-RSP message to the MS  400  in step  420 . On the contrary, in the absence of the resources in the neighbor BSs  450  and  470 , BS # 1  determines whether the MS  400  must be kept in the current frequency mode (frequency band) in step  408 . If mode transition (frequency band transition) is possible, BS # 1  determines mode transition for the MS  400  in step  418  and sends a MOB_SCN-RSP message requesting mode transition (frequency band transition) to the MS  400  in step  420 .  
         [0058]     On the other hand, if the MS  400  must be kept in the current frequency mode (frequency band), BS # 1  sends a RESOURCE-RESERVE-REQ message to the neighbor BSs, i.e. BS # 2  and BS # 3  in steps  410 .  
         [0059]     If BS # 2  and BS # 3  each have radio resources available for the MS  400 , or if there is any other MSs which operates in the same frequency mode (frequency band) as that of the MS  400  and which can transition to the other frequency mode (frequency band) under the management of BS # 2  and BS # 3 , they transition the other MSs to the other frequency mode (frequency band) and reserve radio resources saved from the other MSs for allocation to the MS  400  in step  412  and notify BS # 1  of the radio resource reservation in steps  414  and  416 .  
         [0060]     BS # 1  then sends a MOB_SCN-RSP message to the MS  400  in step  420 . Upon receipt of the MOB_SCN-RSP message, the MS  400  scans BS # 1 , BS #2, and BS # 3  in the frequency mode (frequency band) in step  422  and reports the scanning results to BS # 1  by a MOB_SCN-REP message in step  424 . In this way, the MS  400  can hand over to one of the neighbor BSs  450  and  470  which has reserved the resources for the MS  400 .  
         [0061]      FIG. 5  is a flowchart illustrating a scanning operation in the MS in the BWA communication system according to the present invention.  
         [0062]     Referring to  FIG. 5 , the MS receives a MOB_NBR-ADV message broadcast periodically from the serving BS in step  502 . The MOB_NBR-ADV message has the fourth bit of Skip-Optional-Fields bitmap set to 0 and Available Radio Resource configured as illustrated in Table 1. In step  504 , the MS decides on scanning for handover, if the signal strength of the serving BS is equal to or less than a predetermined threshold. The MS sends a MOB_SCN-REQ message for the handover to the serving BS in step  506 .  
         [0063]     Upon receipt of a MOB_SCN-RSP message from the serving BS in step  508 , the MS scans the neighbor BSs in the current frequency band (frequency mode) irrespective of whether frequency mode transition (frequency band transition) has been performed for the MS in step  510 . The neighbor BSs to be scanned are indicated by the last received MOB_NBR-ADV message. In step  512 , the MS sends a MOB_SCN-REP message containing the scanning results to the serving BS.  
         [0064]      FIG. 6  is a flowchart illustrating a scanning operation in the serving BS in the BWA communication system according to the present invention.  
         [0065]     Referring to  FIG. 6 , the serving BS decides on scanning for handover of the MS for the reason of load distribution, for example, in step  602  and determines whether radio resources are available for the current frequency mode (frequency band) of the MS in any neighbor BS in step  604 . In the presence of the radio resources in the neighbor BS, the serving BS sends a MOB_SCN-RSP message to the MS in step  614 .  
         [0066]     In the absence of the radio resources in the neighbor BS, the serving BS determines whether the MS must be kept in the current frequency mode (frequency band) in step  606 . If frequency mode (frequency band) transition is possible for the MS, the serving BS decides on the frequency mode transition (frequency band) transition for the MS in step  612  and sends a MOB_SCN-RSP message requesting scanning in a transitioned frequency mode (frequency band) to the MS in step  614 .  
         [0067]     However, if the MS can operate in the current frequency mode (frequency band) only, the serving BS requests the neighbor BS to save resources from an MS that can transition to the other frequency mode (frequency band) under its management and reserve the saved resources for the handover MS by a RESOURCE-RESERVE-REQ message in step  608 . That is, the neighbor BS performs frequency mode transition (frequency band transition) for an MS which can transition from the frequency mode (frequency band) of the handover MS to the other frequency mode (frequency band) among MSs operating in the same frequency mode (frequency band) as that of the handover MS under management of the neighbor BS, and reserves resources saved from the MS for the handover MS.  
         [0068]     In step  610 , the serving BS receives a RESOURCE-RESERVE-RSP message from the neighbor BS. If the RESOURCE-RESERVE-RSP message indicates failed resource reservation due to the absence of any MS that can transition to the other frequency mode (frequency band), the serving BS can send a RESOURCE-RESERVE-REQ message to neighbor BSs other than the neighbor BS. If all neighbor BSs fail to reserve radio resources for the handover MS, the serving BS drops the call for the handover MS or requests a general scanning other than the handover scanning to the handover MS.  
         [0069]      FIG. 7  is a flowchart illustrating a scanning operation in the neighbor BS in the BWA communication system according to the present invention.  
         [0070]     Referring to  FIG. 7 , the neighbor BS receives a RESOURCE-RESERVE-REQUEST message from the serving BS in step  702  and determines the presence (or absence of) radio resources available for the handover MS in step  704 . In the presence of the radio resources, the neighbor BS proceeds to step  710  and in the absence of the radio resources, the neighbor BS proceeds to step  706 .  
         [0071]     In step  706 , the neighbor BS determines whether there is any MS for which frequency mode transition (frequency band transition) is possible. In the presence of an MS that can mode-transition (frequency band transition), the neighbor BS goes to step  708  and in the absence of any MS to be mode-transitioned (frequency band-transitioned), the neighbor BS goes to step  712 . The neighbor BS performs frequency mode transition (frequency band transition) for the MS in step  708 , reserves radio resources saved from the mode (band)-transitions MS for the handover MS in step  710 , and proceeds to step  714 . Meanwhile, the neighbor BS determines that radio resources cannot be reserved for the handover MS due to the absence of any MS for which mode transition (frequency band transition) is possible in step  712 , and goes to step  714 .  
         [0072]     In step  714 , the neighbor BS sends to the serving BS a RESOURCE-RESERVE-RSP message indicating successful resource reservation or failed resource reservation for the handover MS.  
         [0073]     In accordance with the present invention as described above, in a communication system where a BS operates in at least two frequency bands having different central frequencies and/or at least two different frequency modes, an MS scans only in one frequency band and/or frequency mode, thereby preventing a data reception delay and data reception errors involved in the scanning. In addition, since a neighbor BS reserves radio resources beforehand for the frequency band (frequency mode) of the MS, handover is implemented successfully.  
         [0074]     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.