Patent Publication Number: US-2006009212-A1

Title: Apparatus and method for signal processing in a handover in a BWA communication

Description:
PRIORITY  
      This application claims priority to an application entitled “Apparatus and Method for Signal Processing in Handover in BWA Communication System” filed in the Korean Intellectual Property Office on Jun. 25, 2004 and assigned Serial No. 2004-48447, the contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and more particularly to an apparatus and a method for processing received signals in a handover of a Mobile Station (MS) in a BWA communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.  
      2. Description of the Related Art  
      Generally, a representative system of a wireless communication system includes a mobile communication system using a cellular communication scheme. Such a mobile communication system uses a multiple access scheme in order to simultaneously communicate with a plurality of users. A Time Division Multiple Access (TDMA) scheme and a Code Division Multiple Access (CDMA) scheme are used as the multiple access scheme used in the mobile communication system as described above.  
      With the rapid development of communication technology, a mobile communication system using the CDMA scheme has developed into a system capable of transmitting packet data at high speed from a system providing voice-based communication.  
      However, codes, which are resources in the CDMA scheme, have reached a limit in use thereof, such that it becomes more and more difficult to transmit multimedia data. Accordingly, it is required to provide a multiple access scheme capable of identifying many more users and transmitting more data to the identified users. In order to satisfy such requirements, the idea of a BWA communication system using an OFDMA scheme is gathering support.  
      The OFDMA scheme transmits/receives data at high speed using multiple sub-carriers that maintain orthogonality or a sub-channel including at least one sub-carrier.  
      A BWA communication system using the OFDMA scheme accommodates the mobility of a MS. Accordingly, a handover must be performed to facilitate smooth and continuous communication, regardless of the movement of the MS.  
      A handover denotes that a channel is maintained for smooth communication even when a MS during communication moves between Base Stations (BSs). The handover may be largely classified into hard handover and soft handover. In the hard handover, a channel of a serving BS, which is currently communicating with a MS, is blocked when the MS during communication moves between BSs, and the channel of a target BS to which the MS is to be handed over is quickly connected, so that continuity of communication is guaranteed. In the soft handover, both a channel of a serving BS currently communicating with a MS and a channel of a target BS to which the MS is to be handed over are maintained when the MS during communication moves between BSs, and the channel of the serving BS is then released after the MS completely moves to a region of the target BS. That is, when the MS moves between different regions, in each of which a communication service is being provided to the MS, the soft handover enables the MS to connect to the channel of the target BS without communication interruption.  
      As described above, the handover may be classified into a hard handover and a soft handover. Currently, the hard handover has been generalized in the BWA communication scheme. However, using the hard handover may cause interruption of signals. Although the interruption of signals occurs during a very short time period, it may be disadvantageous in the BWA communication system targeting stable transmission/reception of high-speed data. Accordingly, the BWA communication system must consider the soft handover. However, a detailed scheme for the soft handover has not yet been proposed for the BWA communication system.  
      Hereinafter, a description will be given on assumption that the soft handover is performed in the BWA communication system. When the soft handover is performed, a channel may be classified into an uplink and a downlink channel in data transmission/reception between a MS and BSs. More specifically, in the downlink, the MS receives signals from a plurality of BSs. Therefore, the complexity of the MS receiving and processing the signals from the BSs increases and system management for handover cannot be efficiently performed. Consequently, when the soft handover is performed, data transmission/reception is not efficiently performed.  
      As described above, a detailed scheme for performing the soft handover between BSs is necessary for the BWA communication system. Further, when the soft handover is performed, it is necessary to provide a scheme for efficiently performing data transmission/reception.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention has been designed to solve the above and other problems occurring in the prior art. It is an object of the present invention is to provide an apparatus and a method for signal processing during a soft handover in a BWA communication system.  
      It is another object of the present invention is to provide an apparatus and a method for signal processing for efficiently transmitting and receiving data in a handover in a BWA communication system.  
      In order to accomplish the above and other objects, according to one aspect of the present, there is provided a method for processing received signals in a handover of a Mobile Station (MS) in a Broadband Wireless Access (BWA) communication system including a plurality of Base Stations (BSs) capable of providing a service to the MS. The method comprises the steps of: receiving signals of different BSs; detecting preambles for each BS from the received signals; measuring channel quality information corresponding to each of the detected preambles; comparing the measured channel quality information with a preset threshold value; and processing the signals received from the different BSs based on a result obtained by comparing the measured channel quality information with the preset threshold value.  
      According to another aspect of the present, there is provided a method for processing received signals in a handover of a Mobile Station (MS) in a Broadband Wireless Access (BWA) communication system including a plurality of Base Stations (BSs) capable of providing a service to the MS. The method comprises the steps of: receiving signals of different BSs; detecting preambles for each of the BSs from the received signals; measuring a first measurement value and a second measurement value corresponding to each of the detected preambles; comparing each of the measurement values with a preset threshold value; processing all of the signals received from the BSs, when the first measurement value and the second measurement value exceed the preset threshold value; and selectively processing the signals received from the BSs according to a predetermined signal processing scheme, when the first measurement value and the second measurement value do not exceed the preset threshold value.  
      According to yet another aspect of the present, there is provided an apparatus for processing received signals in a handover in a Broadband Wireless Access (BWA) communication system including a plurality of Base Stations (BSs) capable of providing a service to the MS. The apparatus comprises a preamble detector for receiving signals of different BSs and detecting preambles for each of the BSs from the received signals; a channel quality information measurer for measuring channel quality information corresponding to each of the detected preamble; a signal processing determiner for comparing the measured channel quality information with a preset threshold value, and determining whether to process the signals received from the different BSs based on a result obtained by comparing the measured channel quality information with the preset threshold value; and a signal processor for processing the signals received from the different BSs, based on whether to process the signals.  
    
    
     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  illustrates the use of frequency resources in a BWA communication system;  
       FIG. 2  is a diagram illustrating an allocation of the same sub-channel in a same time slot by BSs and signal transmission based on the allocation according to an embodiment of the present invention;  
       FIG. 3  is a diagram illustrating an allocation of different sub-channels in a same time slot by BSs and signal transmission based on the allocation according to an embodiment of the present invention;  
       FIG. 4  is a diagram illustrating an allocation of different sub-channels in different time slots by BSs and signal transmission based on the allocation according to an embodiment of the present invention;  
       FIG. 5  is a flow diagram illustrating an operation method of a MS employed a soft combining scheme in handover in a BWA communication system according to one embodiment of the present invention;  
       FIG. 6  is a flow diagram illustrating an operation method of a MS in a soft combining scheme in handover in a BWA communication system according to another embodiment of the present invention;  
       FIG. 7  is a flow diagram illustrating an operation method of a MS in a selection scheme in handover in a BWA communication system according to further another embodiment of the present invention;  
       FIG. 8  is a flow diagram illustrating an operation method of a MS in a selection diversity scheme in handover in a BWA communication system according to further another embodiment of the present invention;  
       FIG. 9  is a block diagram illustrating a MS in a soft combining scheme in handover in a BWA communication system according to an embodiment of the present invention; and  
       FIG. 10  is a block diagram illustrating a MS in a selection diversity scheme in handover in a BWA communication system according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Preferred embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings. The same reference numerals are used to designate the same elements as those shown in other drawings. Additionally, in the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention.  
      The present invention proposes a signal processing scheme during a handover in a Broadband Wireless Access (BWA) communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. As such, a Mobile Station (MS) detects preambles from signals received from a plurality of Base Stations (BSs), compares channel quality information, e.g., Carrier-to-Interference and Noise Ratios (CINRs), corresponding to each of the preambles, with a preset threshold value, and processes the signals received from the BSs according to a result obtained from the comparison.  
       FIG. 1  illustrates the use of frequency resources in a BWA communication system. Referring to  FIG. 1 , an x axis denotes time and a y axis denotes frequency resources in the graph. The frequency resources denote one sub-channel including a plurality of frequency resources, and each sub-channel includes at least one sub-carrier.  
      A downlink  110  includes (n+1) sub-channels and an uplink  120  includes (m+1) sub-channels. The sub-channel may be constructed by a plurality of adjacent sub-carriers or by a plurality of non-adjacent sub-carriers.  
       FIG. 1  illustrates only an exemplary case where one sub-channel is constructed by a plurality of adjacent sub-carriers, but this does not mean that the sub-channel is actually constructed in this manner. That is, in  FIG. 1  and other drawings, the sub-channel does not represent an actual location of a physical sub-carrier within a frequency band, but represents a set of sub-carriers selected through a specific method within the frequency band.  
      As illustrated in  FIG. 1 , the downlink  110  includes a transmission interval of a preamble  101  for a channel estimation of a MS, a BS detection, etc. Further, a guard time  130  is located between the downlink  110  and the uplink  120 . The guard time  130  is time for distinguishing the downlink  110  from the uplink  120 .  
      Hereinafter, a downlink soft handover method applied to a detailed embodiment of the present invention will be described. When a handover is performed, a soft combining scheme or a selection diversity scheme is applied in the downlink soft handover method. Further, a method for processing signals by using channel quality information extracted from a preamble will be described.  
      However, before describing the present invention, the soft handover method in a downlink will be described. Further, the soft handover method will be described in two embodiments, i.e., a case where cells or sectors of all BSs use the same sub-channelization method and a case where the cells or the sectors of all BSs use different sub-channelization methods. When all BSs use the same sub-channelization method, all sub-channels with the same index of each BS use the same sub-carriers. Further, when all BSs use the different sub-channelization methods in the downlink, it is noted that actual locations of used sub-carriers are different even though logical channel numbers between sub-channels are equal.  
      In the following description, even when cell A is replaced with a sector A and cell B is replaced with a sector B, it is noted that application of the present invention is possible. The sector A and the sector B exist in the same cell.  
      1. The Soft Handover Method when all BSs use the Same Sub-Channelization Method  
      A. Allocation of the Same Sub-Channel in the Same Time Slot  
       FIG. 2  is a diagram illustrating an allocation of the same sub-channel in the same time slot by BSs and signal transmission based on the allocation according to an embodiment of the present invention. Referring to  FIG. 2 , a BS of cell A allocates a sub-channel  211   a  to n MS during handover in a specific time slot  211  of multiple time slots  210  to  214  in a downlink frame. Further, a BS of cell B also allocates a sub-channel  221   a  to n MS during handover in a specific time slot  221  of multiple time slots  220  to  224  in a downlink frame.  
      The MS having received the same sub-channels in the same time slots has only to receive signals without distinguishing the signals of the BSs, similarly to when receiving the signals of one BS. That is, the BS of the cell A and the BS of the cell B broadcast the same allocation information (DL-MAP). Accordingly, if the MS receives the allocation information of either of the BS of the cell A or the BS of the cell B, the MS can receive all signals transmitted from the BS of the cell A and the BS of the cell B. Herein, the MS must recognize in advance that the same allocation information is being broadcasted from the BSs.  
      The MS can perform soft handover to one of the cells in consideration of intensities of signals from the cell A and signals from the cell B.  
      According to the sub-channel allocation and handover method as described above, latency is short, and inter-cell interference is reduced because the same sub-channel is used in the same time slot. Therefore, a coverage hole representing that a reception rate of n MS rapidly deteriorates can be reduced and channel estimation performance is better. Consequently, this method may also be applied to a broadcasting service.  
      B. Allocation of Different Sub-Channels in the Same Time Slot  
       FIG. 3  is a diagram illustrating an allocation of the different sub-channels in the same time slot by BSs and signal transmission based on the allocation according to an embodiment of the present invention. Referring to  FIG. 3 , a BS of a cell A allocates a specific sub-channel  311   a  to an MS during handover in a specific time slot  311  of multiple time slots  310  to  314  in a downlink frame. Further, a BS of a cell B allocates a specific sub-channel  321   n  to an MS during handover in a specific time slot  321  of multiple time slots  320  to  324  in a downlink frame.  
      The sub-channels  311   a  and  321   n  exist in the same time slot, but are different sub-channels. The different sub-channel denotes a sub-channel constructed by different sub-channelization methods or a sub-channel constructed by different sub-carriers.  
      Accordingly, only when the MS receives all allocation information transmitted from the BS of the cell A and the BS of the cell B, the MS can identify locations of the sub-channels  311   a  and  321   n  and receive data.  
      According to the method as described above, because data is transmitted in the same time slot, latency is short as described  FIG. 2  and performance improvement by frequency diversity can be also achieved.  
      The allocation information of the BS of the cell A and the BS of the cell B may also be separately transmitted according to each BS, the allocation information of all BSs may also be transmitted from a serving BS, or the allocation information of all BSs may also be simultaneously transmitted from all BSs. Accordingly, the MS can receive data in a handover region after identifying, in advance, one of various transmission methods of the allocation information as described above.  
      C. Allocation of Different Sub-Channels in Different Time Slots  
       FIG. 4  is a diagram illustrating an allocation of the different sub-channels in the different time slots by BSs and signal transmission based on the allocation according to an embodiment of the present invention. Referring to  FIG. 4 , a BS of a cell A allocates a sub-channel  411   a  to n MS in a specific time slot  411  of multiple time slots  410  to  414  in a downlink frame. Further, a BS of a cell B allocates a sub-channel  424   n  to an MS in a specific time slot  424  of multiple time slots  420  to  424  in a downlink frame. The time slot  411  of the cell A and the time slot  424  of the cell B are different time slots. The sub-channels  411   a  and  424   n  allocated to the MS are sub-channels having different frequency resources.  
      According to the method as described above, the MS receives signals from the different BSs or sectors through the different time slots and the different sub-channels. Further, because the two different BSs do not need to transmit data to be transmitted to the MS located in a handover region in the same time slot, flexibility occurs in a scheduling of each BS and there is no need for quick message transfer between a BS and a Base Station Controller (BSC).  
      Herein, the MS must receive all allocation information transmitted from each BS.  
      2. The Soft Handover Method when all BSs Use Different Sub-Channelization Methods  
      A. Allocation of Different Sub-Channels in the Same Time Slot  
      Each BS of each cell allocates a specific sub-channel to a MS. The sub-channel allocated to the MS by the BS exists in the same time slot, but is transmitted by means of the different sub-channels. The above-described method may be illustrated in the same manner shown in  FIG. 3 , except for the difference between the sub-channelization methods of the BSs._ This method has an advantage in that latency is short.  
      B. Allocation of Different Sub-Channels in Different Time Slots  
      Each BS of each cell allocates a sub-channel to a MS in a specific time slot of multiple time slots in a downlink frame. The MS receives signals through different time slots and different sub-channels. The above-described method may also be illustrated in the same manner shown in  FIG. 4 , except for the difference between the sub-channelization methods of the BSs.  
      According to this method, because the BSs do not need to transmit data in the same time slot, flexibility occurs in a scheduling of each BS. Further, there is no need for quick message transfer between a BS and a BSC. Therefore, this method may correspond to a generalized method of other methods.  
      In the downlink soft handover method as described above, a MS receives signals transmitted from a plurality of BSs and processes the received signals using a soft combining scheme or a selection diversity scheme.  
      According to the soft combining scheme, the MS receives the signals transmitted from the BSs, demodulates and combines the received signals, and outputs the combined signals to decoding channel codes. According to the selection diversity scheme, the MS receives the signals transmitted from the BSs, demodulates and decodes the received signals, and selects received signals with the best quality from the decoded data of the received signals.  
       FIG. 5  is a flow diagram illustrating an operation method of a MS in a soft combining scheme during a handover in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 5 , in steps  500  and  520 , the MS receives signals transmitted from a BS “A” and signals transmitted from a BS “B”. An operation process of the MS when the signals are received from the BS “A” is shown in steps  500 ,  502 ,  504 ,  506 , and  508 . Further, an operation process of the MS when the signals are received from the BS “B” is shown in steps  520 ,  522 ,  524 ,  526 , and  528 . The two kinds of signals received from the BSs “A” and “B” are different in that the BSs having transmitted the signals are different from each other. However, because the signals from the BS “A” and the signals from the BS “B” are processed through the same process, the following description will be given with stress on the signals received from the BS “A”. Accordingly, it is noted that the signals received from the BS “B” are processed through a process similar to that through which the signals received from the BS “A” are processed.  
      In step  500 , the MS receives the signals from the BS “A”. In step  502 , the MS detects a preamble from the received signals. The preamble represents information located in the first portion of the downlink as described in  FIG. 1 . In step  504 , the MS performs channel estimation and compensation. Herein, it is possible to perform the channel estimation and compensation by using the detected preamble. For example, the MS computes a CINR by using received preamble signals and performs the channel estimation and compensation. That is, because the MS have already known the preamble signals, the MS can perform the channel estimation and compensation by using the preamble.  
      In step  506 , the MS demodulates and decodes a MAP. That is, the MS acquires MAP information by demodulating and decoding the MAP. Accordingly, the MS can understand a location of data allocated to the MS, i.e., data to be transmitted from a BS to the MS, in an entire frame from the MAP information. In step  506 , the MS recognizes a location of a frame allocated to the MS by demodulating and decoding data, and receives signals through a time slot and a sub-channel, through which data of the MS is transmitted, by means of the recognized location. In step  508 , the MS receives data through a sub-channel including data allocated to the MS, and demodulates the received data.  
      For the signals received from the BS “B”, the MS demodulates data through the process similar to that through which the signals received from the BS “A” are processed. Accordingly, because the operation of the MS for processing the signals from the BS “B” in steps  520 ,  522 ,  524 ,  526 , and  528  is similar to that of the MS for processing the signals from the BS “A” in steps  500 ,  502 ,  504 ,  506 , and  508 , the detailed description will be omitted here.  
      As described above, for the signals from the BS “A” and the signals from the BS “B”, the MS demodulates the data in steps  508  and  528 . Then, in step  510 , the MS performs a soft-combining for the data obtained by demodulating the received signals from the BSs, i.e., the BSs “A” and “B”. In step  512 , the MS decodes the data. As a result, the MS processes the signals transmitted from the different BSs through the process as illustrated in  FIG. 5 .  
      In order to improve system performance through the soft combining scheme as described above, precise channel estimation must be performed for the signals received from each BS. Accordingly, when the channel estimation is precise, i.e., stable, the system performance can be improved using the soft combining scheme. However, when the channel estimation is unstable, use of the soft combining scheme may deteriorate the system performance. When the MS determines that the channel estimation is unstable, detecting data by means of only BS signals, for which the channel estimation may be relatively stable, instead of the soft combining scheme, may improve the quality of received signals.  
       FIG. 6  is a flow diagram illustrating an operation method of an MS in a soft combining scheme in handover in a BWA communication system according to another embodiment of the present invention. Referring to  FIG. 6 , the operation is similar to that of the MS in the soft combining scheme as illustrated in  FIG. 5 , and a step of measuring channel quality information and comparing the information with a preset threshold value is inserted between a step of detecting a preamble from BS signals and a channel estimation and compensation step. Further, the MS processes the BS signals based on the comparison result. As compared with  FIG. 5 , steps  604 ,  624 ,  612 , and  614  are additionally performed in  FIG. 6 . That is, if these steps are omitted,  FIG. 6  shows a flow similar to that of  FIG. 5 . Accordingly, the following description will be given with emphasis on steps  604 ,  624 ,  612 , and  614  and the description on steps similar to those of  FIG. 5  will be omitted.  
      In step  600 , the MS receives signals transmitted from a BS “A”. In step  602 , the MS detects a preamble from the signals. In step  604 , the MS computes a CINR_A value. The CINR_A value denotes a CINR value for the signals received from the BS “A”. In step  604 , the MS determines if the CINR value for the signals received from the BS “A” is larger than a preset threshold value. This determination step inspects the reliability of the signals received from the BS “A”. That is, stability or instability of channel estimation is inspected by means of the CINR value.  
      As a result of the determination, when it is determined that the channel estimation is stable, i.e., when the CINR value for the signals received from the BS “A” exceeds the preset threshold value, step  606  is performed. Because steps  606 ,  608 , and  610  are equal to steps  504 ,  506 , and  508  of  FIG. 5 , the detailed description of these steps will be omitted.  
      However, when it is determined that the channel estimation is unstable, i.e., when the CINR value for the signals received from the BS “A” does not exceed the preset threshold value, step  612  is performed.  
      Further, the MS also performs the same operation for the signals received from the BS “B”. That is, in step  620 , the MS receives the signals from the BS “B”. In step  622 , the MS detects a preamble from the signals. In step  624 , the MS computes a CINR_B value. The CINR_B value denotes a CINR value for the signals received from the BS “B”. In step  624 , the MS determines if the CINR value for the signals received from the BS “B” is larger than the preset threshold value.  
      As a result of the determination, when it is determined that the channel estimation is stable, i.e., when the CINR value for the signals received from the BS “B” exceeds the preset threshold value, step  626  is performed. Again, because steps  626 ,  628 , and  630  are to the same as steps  524 ,  526 , and  528  of  FIG. 5 , the detailed description of these steps will be omitted.  
      However, when it is determined that the channel estimation is unstable, i.e., when the CINR value for the signals received from the BS “B” does not exceed the preset threshold value, step  612  is performed.  
      In step  612 , the MS determines if both the CINR_A value and the CINR_B value do not exceed the preset threshold value. When all of the CINR values do not exceed the preset threshold value, step  614  is performed. When all of the CINR values do not exceed the preset threshold value, this indicates that all of the signals received from the BSs are unstable.  
      In step  614 , the MS processes data through one method of signal processing schemes, i.e., combining schemes. Hereinafter, the combining schemes proposed by the present invention will be described.  
      A first scheme: the MS processes only received signals having the largest value from among channel quality information, i.e., CINR values, measured from a preamble received from each BS, that is, the MS performs demodulation/decoding of a MAP and demodulation/decoding of data.  
      A second scheme: the MS processes all signals received from BSs, i.e., the MS performs demodulation/decoding of a MAP and demodulation/decoding of data.  
      A third scheme: the MS processes all signals received from BSs as reception error.  
      As a result of the determination in step  612 , when one of the CINR values exceeds the preset threshold value, the procedure is ended to prevent step  614  from being performed. However, when one of the CINR values does exceed the preset threshold value, the MS performs a signal processing for the signals having the CINR value exceeding the preset threshold value. That is, in step  604  or step  624 , when one of the CINR value for the signals received from each of BS exceeds the preset threshold value, the MS performs step  606  or step  626 . In steps  610  and  630 , the MS performs a data demodulation. In step  632 , the MS performs a soft combining for the demodulated data signals. In step  634 , the MS performs data decoding.  
      According to the method of  FIG. 6 , the MS measures the channel quality information, e.g., the CINR value, using the preamble detected from the signals transmitted from each BS. Further, the MS determines if the channel estimation is stable or unstable using the CINR value. Accordingly, the MS can determine whether to perform the soft combining for the signals transmitted from each BS. When it is determined that the channel estimation is unstable, the MS does not perform the signal processing, i.e., the MAP demodulation/decoding and data demodulation/decoding, for the unstable signals of the BS. Further, the MS does not perform the soft combining for the unstable signals of the BS as described above, thereby preventing the signals from deteriorating.  
       FIG. 7  is a flow diagram illustrating an operation method of an MS in a selection diversity scheme in a handover in a BWA communication system according to another embodiment of the present invention. Referring to  FIG. 7 , in step  700 , the MS receives signals transmitted from a BS “A”. In step  720 , the MS receives signals transmitted from a BS “B”. An operation process of the MS when the signals are received from the BS “A” is shown in steps  700 ,  702 ,  704 ,  706 ,  708 , and  710 . Further, an operation process of the MS when the signals are received from the BS “B” is shown in steps  720 ,  722 ,  724 ,  726 ,  728 , and  730 . The two kinds of signals received from the BSs “A” and “B” are different in that the BSs having transmitted the signals are different from each other. However, because the signals from the BS “A” and the signals from the BS “B” are processed through the same process, the following description will be given with emphasis on the signals received from the BS “A”.  
      As indicated above, in step  700 , the MS receives the signals from the BS “A”. In step  702 , the MS detects a preamble from the received signals. In step  704 , the MS performs channel estimation and compensation by means of the detected preamble. The channel estimation and compensation is performed as described in  FIG. 5 .  
      In step  706 , the MS demodulates and decodes a MAP. That is, the MS acquires MAP information by demodulating and decoding the MAP. Accordingly, the MS can understand a location of data allocated to the MS, i.e., data to be transmitted from a BS to the MS, in an entire frame from the MAP information. In step  706 , the MS recognizes a location of a frame allocated to the MS by demodulating and decoding data, and receives signals through a time slot and a sub-channel, through which data of the MS are transmitted, by means of the recognized location. In step  708 , the MS receives data through a sub-channel including data allocated to the MS, and demodulates the received data. In step  710 , the MS decodes the demodulated data.  
      For the signals received from the BS “B”, the MS demodulates data through the process similar to that through which the signals received from the BS “A” are processed. Accordingly, because the operation of the MS for processing the signals from the BS “B” in steps  720 ,  722 ,  724 ,  726 ,  728 , and  730  is similar to that of the MS for processing the signals from the BS “A” in steps  700 ,  702 ,  704 ,  706 ,  708 , and  710 , the detailed description will be omitted here.  
      In steps  710  and  730 , the MS decodes the data of the signals from the BS “A” and the signals from the BS “B”. The signals processed through the steps pass through step  712 . In step  712 , the MS performs selection diversity. Accordingly, the MS selects favorable data of the signals received from the BSs using the selection diversity scheme. The selection of the favorable data can be confirmed through a CRC check for checking if decoded data have been normally received.  
       FIG. 8  is a flow diagram illustrating an operation method of a MS in the selection diversity scheme in handover in a BWA communication system according to another embodiment of the present invention. Referring to  FIG. 8 , the operation is similar to that of the MS in the selection diversity scheme as illustrated in  FIG. 7 , and a step of measuring channel quality information and comparing the information with a preset threshold value is inserted between a step of detecting a preamble from BS signals and a channel estimation and compensation step. The BS signals are processed based on the comparison result. As compared with  FIG. 7 , steps  804 ,  824 ,  814 , and  816  are additionally performed in  FIG. 8 . That is, if these steps are omitted,  FIG. 8  shows a flow similar to that of  FIG. 7 . Accordingly, the following description will be given with emphasis on steps  804 ,  824 ,  814 , and  816  and the description on steps similar to those of  FIG. 7  will be omitted.  
      In step  800 , the MS receives signals transmitted from a BS “A”. In step  802 , the MS detects a preamble from the signals. In step  804 , the MS computes a CINR_A value. The CINR_A value denotes a CINR value for the signals received from the BS “A”. In step  804 , the MS determines if the CINR value for the signals received from the BS “A” is larger than a preset threshold value. This determination step inspects the reliability of the signals received from the BS “A”. That is, stability or instability of channel estimation is inspected by means of the CINR value.  
      When it is determined that the channel estimation is stable, i.e., when the CINR value for the signals received from the BS “A” exceeds the preset threshold value, step  806  is performed. Because steps  806 ,  808 ,  810 , and  812  after step  804  are to the same as steps  704 ,  706 ,  708 , and  710  of  FIG. 7 , the detailed description these steps will be omitted.  
      However, when it is determined that the channel estimation is unstable, i.e., when the CINR value for the signals received from the BS “A” does not exceed the preset threshold value, step  814  is performed.  
      Further, the MS performs the same operation for the signals received from the BS “B”. That is, in step  820 , the MS receives the signals from the BS “B”. In step  822 , the MS detects a preamble from the signals. In step  824 , the MS computes a CINR_B value. The CINR_B value denotes a CINR value for the signals received from the BS “B”. In step  824 , the MS determines if the CINR value for the signals received from the BS “B” is larger than the preset threshold value.  
      When it is determined that the channel estimation is stable, i.e., when the CINR value for the signals received from the BS “B” exceeds the preset threshold value, step  826  is performed. Because steps  826 ,  828 ,  830 , and  832  are to the same as steps  724 ,  726 ,  728 , and  730  of  FIG. 7 , the detailed description of these steps will be omitted.  
      However, when it is determined that the channel estimation is unstable, i.e., when the CINR value for the signals received from the BS “B” does not exceed the preset threshold value, step  814  is performed.  
      In step  814 , the MS determines if both the CINR_A value and the CINR_B value do not exceed the preset threshold value. When all of the CINR values do not exceed the preset threshold value, step  816  is performed. When all of the CINR values do not exceed the preset threshold value, this indicates that all of the signals received from the BSs are unstable. In step  816 , the MS processes data through one method of signal processing schemes, i.e., selecting schemes, which have been described above.  
      Hereinafter, the combining schemes proposed by the present invention will be described.  
      A first scheme: the MS processes only received signals having the largest value from among channel quality information, i.e., CINR values, measured from a preamble received from each BS, that is, the MS performs demodulation/decoding of a MAP and demodulation/decoding of data.  
      A second scheme: the MS processes all signals received from BSs, that is, the MS performs demodulation/decoding of a MAP and demodulation/decoding of data.  
      A third scheme: the MS processes all signals received from BSs as reception error.  
      As a result of the determination in step  814 , when one of the CINR values exceeds the preset threshold value, the procedure is ended so as to prevent step  816  from being performed. However, when one of the CINR values does not exceed the preset threshold value, the MS performs a signal processing for the signals having the CINR value exceeding the preset threshold value. In other words, in step  804  or step  824 , when one of the CINR value for the signals received from each of BS exceeds the preset threshold value, the MS performs step  806  or step  826  performed.  
      In step  834 , the MS performs selection diversity for the decoded data using the selection diversity scheme. Therefore, the MS can use selectively received signals through the operation as illustrated in  FIG. 8  according to the reliability of the signals received from the BSs.  
      Referring to  FIG. 6  and  FIG. 8 , described preset threshold values can have each of identical value or different value. For example in step  604  and in step  624 ( or  step  804  and step  824 ), the preset threshold values can identical or different.  FIG. 9  is a block diagram illustrating an MS in a soft combining scheme in a handover in a BWA communication system according to an embodiment of the present invention. Referring to  FIG. 9 , the MS includes a preamble detector  900 , a CINR measurer  902 , a signal processing determiner  910 , and a signal processor  950 . The Signal processor  950  includes a first Fast Fourier Transform (FFT) unit  904 , a second FFT unit  920 , a first channel estimator/compensator  906 , a second channel estimator/compensator  922 , a first demodulator  908 , a second demodulator  924 , a combiner  926 , and a decoder  928 .  
      The preamble detector  900  receives signals from BSs and detects preambles from the received signals. The signals received through an antenna are signals that have passed through a Radio Frequency (RF) processing and an analog/digital conversion. In more detail, a first preamble detector  900 - 1  of the preamble detector  900  receives the signals from the BS “A” and detects the preamble from the received signals, and a second preamble detector  900 - 2  of the preamble detector  900  receives the signals from the BS “B” and detects the preamble from the received signals. Accordingly, the MS acquires frequency synchronization and time synchronization by detecting the preambles.  
      The first FFT unit  904  receives signals output from the preamble detector  900  and performs an FFT for the received signals. The first channel estimator/compensator  906  receives signals output from the first FFT unit  904  and performs channel estimation and compensation for the received signals. The first demodulator  908  receives signals output from the first channel estimator/compensator  906  and performs a data demodulation for the received signals.  
      The second FFT unit  920 , the second channel estimator/compensator  922  and the second demodulator  924  perform operations equal to those of the first FFT unit  904 , the first channel estimator/compensator  906 , and the first demodulator  908 , respectively. The difference is that the second FFT unit  920 , the second channel estimator/compensator  922 , and the second demodulator  924  process the signals received from the BS “B”.  
      The combiner  926  receives signals output from the first demodulator  908  and the second demodulator  924  and performs a soft combining for the received signals. That is, the combiner  926  combines the signals output from the demodulators by means of the soft combining scheme. The decoder  928  receives signals output from the combiner  926  and performs data decoding for the received signals.  
      Accordingly, the above-described MS can perform the method as illustrated in  FIG. 5 .  
      The CINR measurer  902  receives signals output from the preamble detector  900  and measures channel quality information, i.e., CINR values, from the preamble. Accordingly, the CINR measurer  902  may be referred to as a channel quality information measurer. More specifically, a first CINR measurer  902 - 1  of the CINR measurer  902  measures the CINR value from the preamble detected from the signals of the BS “A”, and a second CINR measurer  902 - 2  of the CINR measurer  902  measures the CINR value from the preamble detected from the signals of the BS “B”.  
      The signal processing determiner  910  receives signals output from the CINR measurer  902  and compares the CINR values with a preset threshold value. When the CINR values exceed the preset threshold value, the signal processing determiner  910  determines a signal processing. Further, the signal processing determiner  910  outputs control signals or operation signals to the first FFT unit  904  and the second FFT unit  920  based on the determination about whether to perform the signal processing, thereby controlling the processing for the signals of the BSs.  
      When all of the CINR values do not exceed the preset threshold value, as illustrated in  FIG. 6 , the signal processing determiner  910  may have a predetermined operation scheme or it is possible to set the signal processing determiner  910  to have the predetermined operation scheme.  
      Accordingly, when the CINR values measured from the preambles are larger than the preset threshold value, the signal processing determiner  910  may process only the received signals of the BS having the largest value from among the CINR values, process all signals received from the BSs, or process all signals of the BSs as error. Accordingly, the signal processing determiner  910  can determine if the signals of the BSs are proper for the soft combining and performs operations based on the determination.  
      The preamble detector  900  or the CINR measurer  902  may be constructed by separate modules as illustrated in  FIG. 9 , but may also be constructed by one module.  
      The first FFT unit  904 , the first channel estimator/compensator  906 , the first demodulator  908 , the second FFT unit  920 , the second channel estimator/compensator  922 , and the second demodulator  924  are also illustrated in  FIG. 9  in order to perform the operations for processing the signals of the BSs. Accordingly, modules performing the same operations may be constructed by one module or may be separately constructed. That is, the structure of the MS is not limited to that as illustrated in  FIG. 9 .  
       FIG. 10  is a block diagram illustrating an MS in a selection diversity scheme in a handover in a BWA communication system according to another embodiment of the present invention. Referring to  FIG. 10 , the MS includes a preamble detector  1000 , a CINR measurer  1002 , a signal processing determiner  1012 , and a signal processor  1050 . The Signal processor  1050  includes a first FFT unit  1004 , a second FFT unit  1020 , a first channel estimator/compensator  1006 , a second channel estimator/compensator  1022 , a first demodulator  1008 , a second demodulator  1024 , a first decoder  1010 , a second decoder  1026 , and a selector  1028 .  
      The preamble detector  1000  receives signals from BSs and detects preambles from the received signals. The signals received through an antenna are regarded as signals having passed through an RF processing and an analog/digital conversion. More specifically, a first preamble detector  1000 - 1  of the preamble detector  1000  receives the signals from the BS “A” and detects the preamble from the received signals, and a second preamble detector  1000 - 2  of the preamble detector  1000  receives the signals from the BS “B” and detects the preamble from the received signals. Accordingly, the MS acquires frequency synchronization and time synchronization by detecting the preambles. The first FFT unit  1004  receives signals output from the preamble detector  1000  and performs an FFT for the received signals. The first channel estimator/compensator  1006  receives signals output from the first FFT unit  1004  and performs channel estimation and compensation for the received signals. The first demodulator  1008  receives signals output from the first channel estimator/compensator  1006  and performs a data demodulation for the received signals.  
      The second FFT unit  1020 , the second channel estimator/compensator  1022 , the second demodulator  1024  and the second decoder  1026  perform operations equal to those of the first FFT unit  1004 , the first channel estimator/compensator  1006 , the first demodulator  1008 , and the first decoder  1010 , respectively. The difference is that the second FFT unit  1020 , the second channel estimator/compensator  1022 , the second demodulator  1024 , and the second decoder  1026  process the signals received from the BS “B”.  
      The selector  1028  receives signals output from the first decoder  1010  and the second decoder  1026  and performs selection diversity for the received signals. That is, the selector  1028  selects predetermined signals from the output signals of the decoders using the selection diversity scheme.  
      Accordingly, the above-described MS can perform the method as illustrated in  FIG. 7 .  
      The CINR measurer  1002  receives signals output from the preamble detector  1000  and measures channel quality information, i.e., CINR values, from the preamble. Accordingly, the CINR measurer  1002  may be referred to as a channel quality information measurer. More specifically, a first CINR measurer  1002 - 1  of the CINR measurer  1002  measures the CINR value from the preamble detected from the signals of the BS “A”, and a second CINR measurer  1002 - 2  of the CINR measurer  1002  measures the CINR value from the preamble detected from the signals of the BS “B”.  
      The signal processing determiner  1012  receives signals output from the CINR measurer  1002  and compares the CINR values with a preset threshold value. When the CINR values exceed the preset threshold value, the signal processing determiner  1012  determines a signal processing. Further, the signal processing determiner  1012  outputs control signals or operation signals to the first FFT unit  1004  and the second FFT unit  1020  based on the determination about whether to perform the signal processing, thereby controlling the processing for the signals of the BSs.  
      When all of the CINR values do not exceed the preset threshold value, as illustrated in  FIG. 8 , the signal processing determiner  1012  may have a predetermined operation scheme or it is possible to set the signal processing determiner  1012  to have the predetermined operation scheme.  
      Accordingly, when the CINR values measured from the preambles are larger than the preset threshold value, the signal processing determiner  1012  may process only the received signals of the BS having the largest value from among the CINR values, process all signals received from the BSs, or process all signals of the BSs as error. Accordingly, the signal processing determiner  1012  can determine if the signals of the BSs are proper for the selection diversity and performs operations based on the determination.  
      The preamble detector  1000  or the CINR measurer  1002  may be constructed by separate modules as illustrated in  FIG. 10 , but it may be constructed by one module.  
      The first FFT unit  1004 , the first channel estimator/compensator  1006 , the first demodulator  1008 , the first decoder  1010 , the second FFT unit  1020 , the second channel estimator/compensator  1022 , the second demodulator  1024 , and the second decoder  1026  are also illustrated in  FIG. 10  in order to perform the operations for processing the signals of the BSs. Accordingly, modules performing the same operations may be constructed by one module or may be separately constructed. That is, the structure of the MS is not limited to that as illustrated in  FIG. 10 .  
      As described above, the present invention enables a MS to process signals of BSs using a soft combining scheme and a selection diversity scheme in a handover in a BWA communication system. Further, according to the present invention, it is possible to determine a method for processing signals received from BSs using channel quality information, i.e., CINR values, of the signals.  
      Furthermore, the present invention enables a MS to flexibly operate based on channel conditions in handover, thereby efficiently processing received signals and preventing reception performance from deteriorating due to deterioration of the channel conditions.  
      While the present 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 present invention as defined by the appended claims.