Abstract:
A method and apparatus optimize handover optimization in a broadband wireless communication system. Operations of a base station includes determining whether at least one of a plurality of time intervals included in a total time taken by handover, exceeds a threshold time. Whether to advance or delay a handover point is determined based on which at least one of the time interval exceeds the threshold time. And at least one handover parameter is changed to advance or delay the handover point.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application is related to and claims the benefit under 35 U.S.C. §119(a) to a Korean patent application filed in the Korean Intellectual Property Office on Aug. 24, 2009 and assigned Serial No. 10-2009-0077996, the entire disclosure of which is hereby incorporated by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to a broadband wireless communication system. More particularly, the present invention relates to an apparatus and a method for optimizing handover performance in the broadband wireless communication system. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a broadband wireless communication system, a user terminal accesses an Internet Protocol (IP) network for a service by connecting to a base station over a radio channel. The base station has a fixed coverage. This coverage is determined by transmit powers of the base station and the terminal and, accordingly, is physically limited. Hence, the broadband wireless communication system is constructed to deploy a plurality of the base stations at proper intervals by taking into account the coverage of the base stations. 
         [0004]    The terminal accesses the base station with the best channel among the base stations at the current location. The base station with the best channel can change according to the movement of the terminal. When this happens, it is advantageous for the terminal to alter its serving base station. When the change of the serving base station interrupts the service, the service quality is degraded. Thus, the broadband wireless communication system supports inter-cell handover to support the mobility of the terminal. 
         [0005]    Optimization of the handover performance aims at providing the seamless service in the change of the serving base station and minimizing overhead of the signaling for the handover. Specific solutions and issues for optimizing the handover performance should be deliberated from various angles based on the characteristics of the system. To enhance the service quality, it is necessary to suggest a specific method for optimizing the handover performance in accordance with the characteristics of the system. 
       SUMMARY OF THE INVENTION 
       [0006]    To address the above-discussed deficiencies of the prior art, it is a primary aspect of the present invention to provide an apparatus and a method for optimizing handover performance in a broadband wireless communication system. 
         [0007]    Another aspect of the present invention is to provide an apparatus and a method for addressing handover latency in a broadband wireless communication system. 
         [0008]    Another aspect of the present invention is to provide an apparatus and a method for adjusting a point of handover in a broadband wireless communication system. 
         [0009]    Another aspect of the present invention is to provide an apparatus and a method for advancing or delaying the point of handover by modifying handover parameters in a broadband wireless communication system. 
         [0010]    Yet another aspect of the present invention is to provide an apparatus and a method for detecting increment of handover latency in a broadband wireless communication system. 
         [0011]    According to one aspect of the present invention, an operating method of a base station in a broadband wireless communication system includes determining whether at least one of a plurality of time intervals included in a total time taken by handover, exceeds a threshold time. Whether to advance or delay a handover point is determined according to which of the at least one of the time intervals exceeds the threshold time. And at least one handover parameter is changed to advance or delay the handover point. 
         [0012]    According to another aspect of the present invention, an apparatus of a base station in a broadband wireless communication system includes a determiner for determining whether at least one of a plurality of time intervals included in a total time taken by handover exceeds a threshold time. The apparatus also includes a manager for determining whether to advance or delay a handover point according to which of the at least one of the time intervals exceeds the threshold time, and changing at least one handover parameter to advance or delay the handover point. 
         [0013]    Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
         [0014]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0016]      FIG. 1  illustrates a handover process in a broadband wireless communication system according to an embodiment of the present invention; 
           [0017]      FIG. 2  illustrates locations for handover in the broadband wireless communication system according to an embodiment of the present invention; 
           [0018]      FIG. 3  illustrates operations of a base station in the broadband wireless communication system according to an embodiment of the present invention; and 
           [0019]      FIG. 4  illustrates the base station in the broadband wireless communication system according to an embodiment of the present invention. 
       
    
    
       [0020]    Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIGS. 1 through 4 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged broadband wireless communications systems. 
         [0022]    Embodiments of the present invention provide an apparatus and a method for optimizing handover performance in a broadband wireless communication system. Hereinafter, an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system is exemplified. However, the present invention is applicable to other wireless communication systems. 
         [0023]    Issues to address and solutions for optimizing the handover performance can be deliberated from various viewpoints according to characteristics of a system. For example, in a 3 rd  Generation Partnership Project (3GPP2) Long Term Evolution (LTE) system, the point of the handover can be controlled by adjusting parameters involving the handover. A handover process of the LTE system is described. 
         [0024]      FIG. 1  depicts the handover process in the broadband wireless communication system according to an embodiment of the present invention. 
         [0025]    In  FIG. 1 , a terminal  110 , a serving Base Station (BS)  120 , a target BS  130 , a Mobility Management Entity (MME)  140 , and a Serving-Gateway (S-GW)  150  are involved in the handover of the terminal  110 . The terminal  110  is the user equipment, the BSs  120  and  130  manage radio access of the terminal  110 , the MME  140 , which is an upper node of the BSs  120  and  130 , is responsible for the signaling, and the S-GW  150 , which is an upper node of the BSs  120  and  130 , transmits and receives traffic. 
         [0026]    Prior to the handover, the serving BS  120  transmits the handover parameters  101  to the terminal  110 . The parameters include information used to determine when events related to the handover defined by the system occur. The delivery of the parameters can be called measurement control. For example, the events are defined as shown in Table 1. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 event 
                 description 
                 Occurrence condition 
                 Release condition 
               
               
                   
               
             
             
               
                 A1 
                 Channel quality of serving 
                 Ms − Hys &gt; Thresh 
                 Ms + Hys &lt; Thresh 
               
               
                   
                 BS is better than a 
               
               
                   
                 threshold. 
               
               
                 A2 
                 Channel quality of serving 
                 Ms + Hys &lt; Thresh 
                 Ms − Hys &gt; Thresh 
               
               
                   
                 BS is worse than a 
               
               
                   
                 threshold. 
               
               
                 A3 
                 Channel quality of neighbor 
                 Mn + Ofn + Ocn − Hys &gt; 
                 Mn + Ofn + Ocn + Hys &lt; 
               
               
                   
                 BS is better than channel 
                 Ms + Ofs + Ocs + Off 
                 Ms + Ofs + Ocs + Off 
               
               
                   
                 quality of serving BS 
               
               
                 A4 
                 Channel quality of neighbor 
                 Mn + Ofn + Ocn − Hys &gt; 
                 Mn + Ofn + Ocn + Hys &lt; 
               
               
                   
                 BS is worse than channel 
                 Thresh 
                 Thresh 
               
               
                   
                 quality of serving BS 
               
               
                 A5 
                 Channel quality of serving 
                 Ms + Hys &lt; Thresh1 &amp; 
                 Ms − Hys &gt; Thresh1 &amp; 
               
               
                   
                 BS is worse than a first 
                 Mn + Ofn + Ocn − Hys &gt; 
                 Mn + Ofn + Ocn + Hys &lt; 
               
               
                   
                 threshold and channel 
                 Thresh2 
                 Thresh2 
               
               
                   
                 quality of neighbor BS is 
               
               
                   
                 better than a second 
               
               
                   
                 threshold and 
               
               
                   
               
             
          
         
       
     
         [0027]    In Table 1, ‘Ms’ denotes a channel quality measurement value of the serving BS measured at the terminal, ‘Hys’ denotes a hysteresis parameter per event, ‘Mn’ denotes a channel quality measurement value of a neighbor BS measured at the terminal, ‘Ofn’ denotes a frequency-specific offset of the neighbor BS, ‘Ocn’ denotes a specific offset of the neighbor BS, ‘Ofs’ denotes a frequency-specific offset of the serving BS, ‘Ocs’ denotes a specific offset of the serving BS, and ‘Off’ denotes an offset parameter per event. All or some of the events in Table 1 can be selectively used to determine whether to perform a handover. 
         [0028]    Next, the terminal  110 , recognizing at least one event, sends the event occurrence report  103  to the serving BS  120 . In so doing, the terminal  110  transmits identification information of the event and measurement values of variables related to the event. For example, among the events of Table 1, when event A3 occurs, the terminal  110  reports of the event A3 to the Serving BS  120 . The report of the event occurrence is also called a measurement report. The serving BS  120  receiving the event report determines that the reported event involves the handover, and prepares for the handover of the terminal  110  in block  105 . That is, the serving BS  120  determines to process the handover, and signals with the target BS  130  and the MME  140  for the handover. The serving BS  120 , upon completing the preparation for the handover, sends the handover instruction  107  to the terminal  110 . Hence, the terminal  110  and the serving BS  120  conduct the handover in block  109 . In detail, the terminal  110  synchronizes to connect to the target BS  130 , and disconnects from the serving BS  120 . The serving BS  120  forwards data to transmit to the terminal  110  to the target BS  130 . The terminal  110 , upon completing the synchronization with the target BS  130 , sends the handover completion notification  111  to the target BS  130 . Receiving the notification of the handover completion, the target BS  130  changes a data path of the terminal  110  by signaling to the MME  140  of the terminal  110 , and receives the data to transmit to the terminal  110 , from the serving BS  120  in block  113 . 
         [0029]    In the handover process of  FIG. 1 , the time taken to perform the handover can be divided into, on the interval basis, t 1   191  taken for the terminal  110  to report the event occurrence to the serving BS  120 , t 2   192  taken for the serving BS  120  to process the handover preparation procedure, t 3   193  taken for the serving BS  120  to instruct the handover to the terminal  110 , t 4   194  taken for the terminal  110  to synchronize with the target BS  130 , and t 5   195  taken for the terminal  110  to notify the handover completion to the target BS  130 . 
         [0030]    Of the time intervals  191  through  195 , the t 1   191  and the t 3   193  can vary according to the channel quality between the terminal  110  and the serving BS  120 , and the t 4   194  and the t 5   195  can vary according to the channel quality between the terminal  110  and the target BS  130 . 
         [0031]    Problems when the channel quality between the BS and the terminal is hostile are discussed.  FIG. 2  depicts locations for the handover. 
         [0032]    Referring to  FIG. 2 , the handover in a place A  201  is conducted under the most ideal condition. In this situation, the handover optimized in the handover latency is performed. 
         [0033]    The handover in a place D  204  is under the very poor condition of the channel quality between the terminal and the serving BS. At this time, Radio Link Failure (RLF) with the serving BS occurs and the handover fails. 
         [0034]    The handover in a place B  202  is under the better condition than the place D  204  in spite of the poor channel quality between the terminal and the serving BS. While the RLF with the serving BS does not occur, the increasing number of the retransmissions lengthens the t 1   191  or the t 3   193  and extends the handover latency. 
         [0035]    The handover in a place E  205  is performed under the very bad condition of the channel quality between the terminal and the target BS. The message transmission to the target BS fails and the handover fails. 
         [0036]    The handover in a place C  203  is performed under the better condition than the place E  205  even with the poor channel quality between the terminal and the target BS. The message transmission to the target BS does not fail, whereas the increasing number of the retransmissions lengthens the t 4   194  or the t 5   195  and extends the handover latency. 
         [0037]    The problems that result from the handover start point can be addressed by controlling to conduct the handover in the place A  201 . Because the terminal travels from the serving BS toward the target BS, the start point of the handover can be altered by adjusting the handover point. For instance, the handover points in the place B  202  and the place D  204 , which are late, needs to be advanced, and the handover points in the place C  203  and the place E  205 , which are early, need to be postponed. For example, when the event A3 of Table 1 is used as the event for triggering the handover, the BS can alter the handover point by increasing/decreasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, the ‘Hys’, the ‘Ofn’, and the ‘Ocn’. 
         [0038]    The handover to an inappropriate target BS, the unnecessary handover besides the problems at the handover point, and the handover within a certain time after the connection is established degrade the handover performance, and the corresponding measures are required. 
         [0039]    To address those problematic situations, the broadband wireless communication system according to an embodiment defines the problematic situations as shown in Table 2. The BS processes to advance or delay the handover point. 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 problematic situation 
                 description 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Failure due to too late 
                 MS performs handover too late or RLF 
               
               
                   
                 HO triggering 
                 occurs before performing handover. 
               
               
                 2 
                 Increased HO latency 
                 Handover latency increases since MS 
               
               
                   
                 due to too late HO 
                 performs handover too late. 
               
               
                   
                 triggering 
               
               
                 3 
                 Failure due to too early 
                 RLF occurs since MA performs handover 
               
               
                   
                 HO triggering 
                 too early. 
               
               
                 4 
                 Increased HO latency 
                 Handover latency increases since MS 
               
               
                   
                 due to too early HO 
                 performs handover too early. 
               
               
                   
                 triggering 
               
               
                 5 
                 Failure due to HO to a 
                 RLF occurs since MS performs handover 
               
               
                   
                 wrong cell 
                 to a wrong cell. 
               
               
                 6 
                 Unnecessary HO 
                 Resource is wasted since MS performs 
               
               
                   
                   
                 unnecessary handover. 
               
               
                 7 
                 Unwanted HO 
                 MS performs handover within a certain 
               
               
                   
                 subsequent to connection 
                 time after connection setup. 
               
               
                   
                 setup 
               
               
                   
               
             
          
         
       
     
         [0040]    The occurrence of the problematic situations of Table 2 is determined based on whether the RLF occurs, whether the handover is conducted late, and whether the handover is conducted early. 
         [0041]    The occurrence of the RLF is determined as follows. According to the signal quality in the physical layer, in-sync/out-of-sync is determined. When the out-of-sync consecutively occurs over a certain number of times, the terminal observes the occurrence of the in-sync for a certain time. When the in-sync does not take place over the certain number of times within the certain time, the terminal determines the occurrence of the RLF. Conversely, when the in-sync takes place over the certain number of times within the certain time, the terminal operates normally. In the 3GPP2 LTE system, the certain number of times for the out-of-sync is defined as the variable N 310 , the certain number of times for the in-sync is defined as the variable N 311 , and the certain time is defined as the variable T 310 . 
         [0042]    The RLF due to the late handover is determined as follows. A BS-A detects the RLF with the terminal and records the occurrence time of the RLF. According to the occurrence of the RLF, the terminal scans the accessible cell through cell reselection. When selecting a BS-B, the terminal performs re-establishment. The BS-B records source cell Identifier (ID) information contained in the message received in the re-establishment and the message reception time, and provides the recorded information to the BS-B pointed by the source cell ID. Hence, the BS-A recognizes the RLF due to the late handover (i.e. the situation ‘1’ of Table 2) by comparing and analyzing the occurrence time of the RLF and the information received from the BS-B. 
         [0043]    The RLF due to the early handover is determined as follows. After completing the handover between the BS-A and the BS-B, the terminal notifies the handover completion to the BS-B. The BS-B records the source cell ID and the hand-in event occurrence time with respect to the terminal. Next, the BS-B notifies the handover completion to the BS-A, and the BS-A records the handover success of the terminal and the time of the success notification. The RLF of the terminal takes place, and the BS-B records the time of the RLF. The BS-B informs the BS-A indicated by the source cell ID of the RLF. Concurrently, the terminal scans an accessible cell through the cell reselection. The BS-A is selected, and the terminal performs the re-establishment with the BS-A. The BS-A determines, based on the message received in the re-establishment, that the terminal hands over from the BS-A to the BS-B, and records the time of the re-establishment. Hence, the BS-A recognizes the RLF due to the early handover (i.e. the situation ‘3’ of Table 2) by analyzing the record of the terminal history and the information received from the BS-B. 
         [0044]    The values of the t 1  through the t 5  are determined as follows. In the process of the handover of  FIG. 1 , the terminal records the time transmitting the event occurrence report. The serving BS records the time at which the event occurrence report is received and the time at which the handover command is transmitted to the terminal. The terminal records the time at which the handover command is received and the time at which the handover completion notification is transmitted. Upon transmitting the notification of the handover completion, the terminal transmits the time at which the event occurrence report is transmitted, the time at which the handover command is received, and the time at which the handover completion notification is transmitted to the target BS. The target BS records the time at which the handover completion notification is received. Next, the target BS transmits the time record information received from the terminal and the time at which the handover completion notification received to the serving BS. Thus, the serving BS calculates the values of the t 1  through the t 5  using the time record information received from the target BS. 
         [0045]    To detect the problematic situations of Table 2, the BS determines the occurrence of the RLF as stated above and collects the handover history information per neighbor cell. Items in the handover history information include a handover success rate, a handover failure rate, a frequency of the RLF due to the late handover, a frequency of the RLF due to the early handover, a frequency of the RLF due to the handover to a wrong cell, a frequency of the unnecessary handover, a frequency of the handover within a certain time after the connection setup, and average values of the t 1  through the t 5 . Based on the handover history information collected per neighbor cell, the BS determines whether the problematic situations take place, and takes the corresponding action. That is, the BS adjusts the handover point in response to the problematic situation. 
         [0046]    For instance, when detecting the situation ‘1’ or the situation ‘2’, the BS can advance the handover point by decreasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by increasing at least one of the ‘Ofn’ and the ‘Ocn’. When detecting the situation ‘3’ or the situation ‘4’, the BS can postpone the handover point by increasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by decreasing at least one of the ‘Ofn’ and the ‘Ocn’. Consequently, the location of the terminal at the point of the handover gets close to the place A  201  of  FIG. 2 . 
         [0047]    Hereafter, a structure and the operations of the BS for optimizing the handover performance as above are elucidated. 
         [0048]      FIG. 3  illustrates the operations of the BS in the broadband wireless communication system according to an embodiment of the present invention. 
         [0049]    In block  301 , the BS collects the handover history information. More specifically, every time the accessed terminal performs a handover, the BS records the times for transmitting and receiving the messages related to the handover and the times of the events, and determines the handover success rate, the handover failure rate, the frequency of the RLF due to the late handover, the frequency of the RLF due to the early handover, the frequency of the RLF due to the handover to a wrong cell, the frequency of the unnecessary handover, the frequency of the handover within a certain time after the connection setup, and the average values of the t 1  through the t 5  by analyzing the recorded information. The handover history information is managed per neighbor BS. Block  301  is continuously performed throughout this process. 
         [0050]    In block  303 , the BS determines whether the problematic situation results from the inappropriate handover point (i.e. due to the early or late handover). Herein, the problematic situation due to the inappropriate handover point indicates the situations ‘1’ through ‘4’ of Table 2. The occurrence of the problematic situation is determined periodically at the regular time intervals, or every time the handover history information is updated. 
         [0051]    When the problematic situation results from the inappropriate handover point, the BS checks whether the problematic situation includes the RLF in block  305 . That is, the BS determines whether the problematic situation taking place is one of the situation ‘1’ and the situation ‘3’ of Table 2. 
         [0052]    When the problematic situation taking place includes the RLF, that is, when the problematic situation taking place is one of the situation ‘1’ and the situation ‘3’ of Table 2, the BS determines new handover parameters according to the action taken for the problematic situation in block  307 . Specifically as for the situation ‘1’, the BS determines new handover parameters to advance the handover point. As for the situation ‘3’, the BS determines new handover parameters to delay the handover point. For example, when the event A3 of Table 1 is the condition triggering the handover, the BS can advance the handover point by decreasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by increasing at least one of the ‘Ofn’ and the ‘Ocn’. Also, the BS can postpone the handover point by increasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by decreasing at least one of the ‘Ofn’ and the ‘Ocn’. In so doing, the change of the handover point depends on the change of the parameters. 
         [0053]    By contrast, when the problematic situation taking place does not include the RLF; that is, when the problematic situation taking place is one of the situation ‘2’ and the situation ‘4’ of Table 2, the BS determines the cause of the handover latency in block  309 . Herein, the cause of the handover latency is the threshold excess of at least one of the handover processing intervals t 1 , t 3 , t 4  and t 5 . It is advantageous to define the threshold independently for each processing interval. 
         [0054]    When the cause of the handover latency is the threshold excess of the t 1  or the t 3 , the BS determines the handover parameters to advance the handover point in block  311 . For example, when the event A3 of Table 1 is the condition triggering the handover, the BS can advance the handover point by decreasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by increasing at least one of the ‘Ofn’ and the ‘Ocn’. 
         [0055]    By contrast, when the cause of the handover latency is the threshold excess of the t 4  or the t 5 , the BS determines the handover parameters to postpone the handover point in block  313 . For example, when the event A3 of Table 1 is the condition triggering the handover, the BS can delay the handover point by increasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by decreasing at least one of the ‘Ofn’ and the ‘Ocn’. 
         [0056]    In block  315 , the BS alters the existing handover parameters to the handover parameters determined in block  307 , the block  311 , or the block  313 . Accordingly, terminals accessing the BS receive the newly determined handover parameters. 
         [0057]    Next, to apply the newly defined handover parameters to the terminals currently accessing the BS, the BS transmits the newly determined handover parameters in block  317 . That is, the measurement control through delivery of the handover parameters  101  in  FIG. 1  is carried out. 
         [0058]    In  FIG. 3 , the BS merely handles the problematic situation due to the inappropriate handover point. Additionally, the BS can determine and deal with other problematic situations. When the RLF results from the handover to the wrong cell, when the unnecessary handover takes place, or when the handover is performed within a certain time after the connection setup, the BS addresses the problematic situation by changing the handover parameters or by changing the condition of the event triggering the handover. 
         [0059]    The embodiment of  FIG. 3  considers the situations ‘1’ through ‘4’ of the problematic situations of Table 2. Alternatively, the BS considers the situation ‘2’ and the situation ‘4’. In this situation, the block  305  and the block  307  can be omitted. Alternatively, the BS considers the situation ‘1’ and the situation ‘3’, where the block  309  through the block  313  can be omitted. 
         [0060]      FIG. 4  is a block diagram of the BS in the broadband wireless communication system according to an embodiment of the present invention. 
         [0061]    The BS of  FIG. 4  includes a backhaul communication part  410 , a wireless communication part  420 , a signaling processor  430 , a data processor  440 , and a controller  450 . 
         [0062]    The backhaul communication part  410  provides the interface for the wire communication with the neighbor BSs and the upper nodes. That is, the backhaul communication part  410  converts a physical signal received from the neighbor BS or the upper node to information bit string, and converts information bit string to transmit to the neighbor BS or the upper node to a physical signal. 
         [0063]    The wireless communication part  420  provides the interface for the wireless communication with the terminals. For example, the wireless communication part  420  provides the radio interface according to the OFDM scheme. To transmit the signal, the wireless communication part  420  converts the bit string fed from the signaling processor  430  or the data processor  440  to complex symbols, maps the complex symbols to subcarriers, constitutes OFDM symbols through Inverse Fast Fourier Transform (IFFT) operation and Cyclic Prefix (CP) insertion, up-converts the OFDM symbols to a Radio Frequency (RF) signal, and then transmits the RF signal via an antenna. When receiving a signal, the wireless communication part  420  down-converts the RF signal received via the antenna, splits to the OFDM symbols, restores the complex symbols mapped to the subcarriers through the CP elimination and the FFT operation, converts the complex symbols to the bit string, and then provides the bit string to the signaling processor  430  or the data processor  440 . 
         [0064]    The signaling processor  430  generates a control message transmitted to the terminal and analyzes a control message received from the terminal. For instance, the signaling processor  430  generates the measurement control message for carrying the handover parameters and the handover command message instructing to perform the handover, and analyzes the measurement report message including the event occurrence report from the terminal and the handover completion message notifying of the handover completion. When analyzing the received control message, the signaling processor  430  informs the controller  450  of the reception of the control message, and provides the controller  450  with the information acquired from the control message. The data processor  430  temporarily stores data transmitted to the terminal and data received from the terminal, and processes functions required by the layers. 
         [0065]    The controller  450  controls the functions of the BS. For example, the controller  450  controls the transmission and the reception of the signals, the management of the radio resource, and the access management of the terminal. In particular, the controller  450  includes a handover parameter manager  452  for managing the handover parameters, and a problematic situation determiner  454  and handover history information storage  456  for taking actions for the problematic situation which degrades the handover performance. 
         [0066]    The handover history information storage  456  aggregates the handover history information. The handover history information storage  456  records the times for transmitting and receiving the messages related to the handover and the times of the event occurrence every time the accessing terminal performs a handover, and determines the handover success rate, the handover failure rate, the frequency of the RLF due to the late handover, the frequency of the RLF due to the early handover, the frequency of the RLF due to the handover to a wrong cell, the frequency of the unnecessary handover, the frequency of the handover within a certain time after the connection setup, and the average values of the t 1  through the t 5  by analyzing the recorded information. The handover history information is managed per neighbor BS. 
         [0067]    The problematic situation determiner  454  determines whether the problematic situation which degrades the handover performance takes place. The occurrence of the problematic situation is determined periodically at the regular time intervals, or every time the handover history information is updated. For instance, when the problematic situation results from an inappropriate handover point, the problematic situation determiner  454  determines which one of the situations ‘1’ through ‘4’ of Table 2 is the occurring problematic situation, notifies the problematic situation, and instructs the handover parameter manager  452  to change the handover parameters. Because the situations ‘1’ and ‘3’ are the problematic situations including the RLF, the problematic situation determiner  454  distinguishes the situations ‘1’ and ‘3’ from the situations ‘2’ and ‘4’, according to the RLF. 
         [0068]    Hence, the handover parameter manager  452  changes the handover parameters to alter the handover point. More specifically, when the situation ‘1’ or the situation ‘2’ takes place, the handover parameter manager  452  determines new handover parameters to advance the handover point. When the situation ‘3’ or the situation ‘4’ takes place, the handover parameter manager  452  determines new handover parameters to delay the handover point. For example, when the event A3 of Table 1 is the condition triggering the handover, the handover parameter manager  452  can advance the handover point by decreasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by increasing at least one of the ‘Ofn’ and the ‘Ocn’. Also, the handover parameter manager  452  can postpone the handover point by increasing at least one of the ‘Ofs’, the ‘Ocs’, the ‘Off’, and the ‘Hys’, or by decreasing at least one of the ‘Ofn’ and the ‘Ocn’. Note that the change of the handover point for the situations ‘1’ and ‘3’ should be greater than the change of the handover point for the situations ‘2’ and ‘4’. The change of the handover point depends on the change of the parameters. The handover parameter manager  452  changes the existing handover parameters to the determined handover parameters. Hence, the terminals accessing the BS receive the newly determined handover parameters. 
         [0069]    To apply the newly defined handover parameters to the terminals currently accessing, the controller  450  controls the signaling processor  430  to transmit a message including the newly determined handover parameters. 
         [0070]    In the embodiment of  FIG. 4 , the controller  450  handles only the problematic situation due to the inappropriate handover point. Additionally, the controller  450  can determine and deal with other problematic situations. When the RLF results from the handover to the wrong cell, when the unnecessary handover takes place, or when the handover is performed within a certain time after the connection setup, the controller  450  solves the problematic situation by changing the handover parameters or by changing the condition of the event triggering the handover. 
         [0071]    In the broadband wireless communication system, not only the RLF in the handover and the wrong handover are determined, but also the increment of the handover latency. In this regard, the performance of the handover can be optimized by adjusting the point of the handover. 
         [0072]    Although the present disclosure has been described with an embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.