Abstract:
Methods, an apparatus, and a network manager for cooperative communication are provided. A method of communication by a serving base station performing a cooperative communication with a plurality of base stations includes selecting serving channel code information based on Channel Status Information (CSI) of the serving base station, receiving cooperating channel code information based on CSI of a cooperating base station from the cooperating base station performing the cooperative communication with the serving base station, determining a compensation parameter based on the serving channel code information and the cooperating channel code information, and determining a beamforming vector of the serving base station based on the compensation parameter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation application of a prior application Ser. No. 13/255,766, filed on Sep. 9, 2011, which is a National Stage application under 35 U.S.C. §371 of International application filed on Mar. 9, 2010 and assigned application number PCT/KR2010/001469, which claims the benefit under 35 U.S.C §119(a) of a Korean patent application filed on Mar. 10, 2009 in the Korean Intellectual Property Office and assigned Serial number 10-2009-0020099, the entire disclosure of which is hereby incorporated by reference 
    
    
     TECHNICAL FIELD 
     The present invention relates to a communication system and method and, in particular, to a base station cooperation system and method for providing a plurality of mobile stations with a communication service through cooperation among a plurality of base stations. 
     BACKGROUND 
     Typically, a communication is designed to provided communication services including voice and various multimedia services. In order to meet the high quality and high data rate requirements for multimedia services, recent communication systems employ Multiple Input Multiple Output (MIMO) technique. With the use of multiple antennas, it is possible to increase the channel capacity in the limited channel resource and thus provide high speed communication service. 
     Such a communication system is composed of a plurality cells neighboring with each other. The communication system includes a plurality of Base Stations (BS) for controlling their respective cells and a plurality of Mobile Stations (BS) that connect to one of the BSs to receive the communication service. In the communication between BS and MS, inter-cell interference may occurs at the cell boundary. This is because that the MS located at the cell boundary region receives the signals transmitted by a plurality of MSs. That is, the signals transmitted by the neighbor BSs can influence the MS as interferences. Accordingly, the MS experiences performance degradation at the cell boundary region. 
     In order to mitigate the inter-cell interference, various technique of cooperation among the BSs are being proposed. In such a communication system, the BSs share the channel information such that each BS can restrict or recommend the use of specific channel information. Through the cooperation among the BSs, it is possible to reduce the inter cell interference at the MSs. Each BS acquires Channel State Information (CSI) of the MS, selects a channel code from a previously stored codebook in match with the CSI, and sends the channel code to other BSs to share the channel information. Typically, the BS quantizes the analog channel state into a channel code which is transmitted. 
     SUMMARY 
     In the conventional communication system, however, when the channel state information is converted into the channel code, the quantization loss occurs between the channel state information and the channel code. The quantize loss causes BS cooperation performance degradation. 
     In order to solve the above problem, the present invention provides a BS cooperation system for providing a plurality of MSs with the communication service in cooperation among a plurality of BSs. 
     In accordance with an aspect of the present invention, a base station cooperation system for providing a plurality of mobile stations with communication service through a plurality of base stations includes a serving base station which provides a mobile station with communication service and at least one cooperating base station neighboring with the serving base station, wherein the serving base station includes a radio communication unit which acquires channel state information of the mobile station, a linear transformer which selects, when the channel state information is acquired, a serving channel code corresponding to the channel state information and produces linear transformation information using the serving channel code and the channel state information, a memory which stores a codebook having the serving channel code, and a control unit which transmits the serving channel code to the cooperating base station, producing, when cooperating channel code is received from the cooperating base station, serving weight information of the serving base station and cooperating weight information of the cooperating base station using the serving channel code and cooperating channel code and determines a beamforming vector by applying the linear transformation information to the serving weight information. 
     In accordance with another aspect of the present invention, a base station cooperation system for providing a plurality of mobile stations with communication service through a plurality of base stations includes a plurality of base stations which select, when channel state information of a mobile station, a channel code corresponding to the channel state information, produce linear transformation information using the selected channel code and the channel state information, and transmit the selected channel code; and a coordinator which produce, when the channel code is received from the base stations, weight information per base station using the channel code and transmits the weight information to corresponding base station, wherein the base station determines, when the weight information is received, a beamforming vector by applying the linear transformation information to the weight vector. 
     In order to solve the above problem, the present invention provides a BS cooperation method for providing a plurality of MSs with the communication service in cooperation among a plurality of BSs. 
     In accordance with another aspect of the present invention, a base station cooperation method for providing a plurality of mobile stations with communication service through a plurality of base stations includes selecting, at a serving base station when channel state information of a mobile station is acquired, a serving channel code corresponding to the channel state information from a codebook; producing linear transformation information using the serving channel code and the channel state information; producing, when a cooperating channel code is received from a cooperating base station, serving weight information of the serving base station and cooperating weight information of the cooperating base station using the serving channel code and the cooperating channel code; and determining a beamforming vector by applying the linear transformation information to the serving weight information. 
     In accordance with still another aspect of the present invention, a base station cooperation method for providing a plurality of mobile stations with communication service through a plurality of base stations includes selecting, at a base station when channel state information of a mobile station is acquired, a channel code corresponding to the channel state information from a codebook; producing linear transformation information using the selected channel code and the channel state information; transmitting the selected channel code to a coordinator; transmitting, at the coordinator when the channel is received, weight information per base station produced using the received channel code to the base station; and determining, at the base station when the weight information is received, a beamforming vector by applying the linear transformation information to the weight information. 
     Advantageous Effects 
     The BS cooperation system and method of the present invention enables the BE to produce linear transformation information that is capable of minimizing the quantization loss between the channel state information and the channel code. In the BS cooperation system of the present invention, the BS uses the linear transformation information for beamforming to recover the quantization loss. With the quantization loss recovery, it is possible to improve the cooperation performance of the BS cooperation system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating architecture of the BS cooperation system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a configuration of the base station of  FIG. 1 ; 
         FIG. 3  is a flowchart illustrating the BS cooperation procedure according to an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating architecture of a BS cooperation system according to another embodiment of the present invention; and 
         FIG. 5  is a flowchart illustrating the BS cooperation procedure according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. The same reference numbers are used throughout the drawings to refer to the same or like parts. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. 
       FIG. 1  is a diagram illustrating architecture of the BS cooperation system according to an embodiment of the present invention. Although the description is directed to the BS cooperation system implemented with a MIMO technique, the present invention is not limited thereto. 
     Referring to  FIG. 1 , the BS cooperation system of this embodiment includes a plurality of cells. Here, some of the cells are neighboring with each other. The BS cooperation system includes a plurality of BSs  110  and  120  for controlling the respective sells. The BSs  110  and  120  provide the MSs with the communication service within their respective cells. 
     Each of the BSs can be the serving BS (B SA )  110  and simultaneously a cooperating BS (B SB )  120  cooperating with other BSs. Each of the MSs can be a serving MS (M SA ) connected to the serving BS  110  to receive the communication and simultaneously a cooperation MS (M SB )  121  connected to the cooperating BS  120  to receive the communication service. That is, the serving BS  110  and the cooperating BS  120  cooperate with each other to suppress the inter-cell interference to the serving and cooperating MSs  111  and  121 . 
     In the BS cooperation system, the serving and cooperating MSs  111  and  121  can be located at the cell boundary of the serving and cooperating BSs  110  and  120 . The serving and cooperating MSs  111  and  121  can receive the signals transmitted by the serving and cooperating BSs  110  and  120 . At this time, the serving BS  110  can transmit a signal to the serving MS  111  with a specific channel state information, e.g. H AA , and to the cooperating MS  121  with another specific channel state information, e.g. H BA . Also, the cooperating BS  120  can transmit a signal to the serving MS  111  with a specific channel state information, e.g., H AB , and to the cooperating MS  121  with a specific channel state information, e.g. H BB . 
     Each of the serving and cooperating MSs  111  and  121  checks the channel state informations and feeds back to the serving and cooperating BSs  110  and  120 . That is, the serving MS  111  can discriminate between H AA  and H AB . The serving MS  111  can feed back the H AA  and H AB  to the serving BS  110  or H AA  to the serving BS  110  and H AB  to the cooperating BS  120 . Likewise, the cooperating MS  121  can discriminate between HBA and HBB. Also, the cooperating MS  121  can feed back HBA and HBB to the serving BS  110  or HBA to the serving BS  110  and HBB to the cooperating BS  120 . When the channel state informations of the serving and cooperating MSs  111  and  121  are received, the serving and cooperating BSs  110  and  120  share the channel state information of the serving and cooperating MSs  111  and  121  via backhaul to cooperate with each other. 
       FIG. 2  is a block diagram illustrating a configuration of the base station  110  and  120  of  FIG. 1 . 
     Referring to  FIG. 2 , the base station is provided with a radio communication unit  210 , a control unit  230 , and a memory  220 . 
     The radio communication unit  210  is responsible for the communication function of the base station. The radio communication unit  210  includes a receiver for up-converting and amplifying the transmit signal and a receiver for low noise amplifying and down-converting the receive signal. The radio communication unit  210  can receive the channel state informations of the MSs  111  and  121  according to an embodiment of the present invention. 
     The memory  220  can include program and data memories. The program memory stores the programs for controlling general operations of the base station. The program memory can store the program related to the cooperation between BSs  110  and  120 . The data memory store the data generated while the programs are running. The memory  220  stores a codebook composed of a plurality of channels code according to an embodiment of the present invention. 
     The control unit  230  is responsible for controlling overall operations of the BS. The control unit  230  includes a data processing unit having a transmitter for encoding and modulating the transmit signal and a receiver for demodulating and decoding the receive signal. Here, the data processing unit can include a modem and a codec. The codec is provided with a data codec for processing packet data and an audio codec for processing audio signal including voice. 
     The control unit  230  acquires the channel state information of the MSs  111  and  121  by means of the radio communication unit  210  according to an embodiment of the present invention. The control unit  230  selects the channel code corresponding to the channel state information from the codebook stored in the memory  220  according to an embodiment of the present invention. The control unit  230  includes a linear transformer (not shown) for producing linear transformation information using the channel state information and the channel code according to an embodiment of the present invention. Also, the control unit  230  transmits the channel code by means of the radio communication unit  210  according to an embodiment of the present invention. In this manner, the BSs  110  and  120  can share the channels codes. Here, the channel code transmitted by the BS can be referred to as serving channel code. When the channel code is received by the radio communication unit  210 , the control unit  230  produces weight information per BS according to an embodiment of the present invention. Here, the channel code received at the BS can be referred to as a cooperating channel code. The control unit  230  determines a beamforming vector (beamforming matrix) by applying the linear transformation information to the weight information according to an embodiment of the present invention. In this manner, the control unit  230  provides the MSs  111  and  121  with the communication system using the beamforming vector according to an embodiment of the present invention. 
       FIG. 3  is a flowchart illustrating the BS cooperation procedure according to an embodiment of the present invention. In this embodiment, the base station cooperation procedure is performed through communication between the BSs  110  and  120 . Here, each serving BS can be the serving BS and simultaneously the cooperating BS  120 . 
     Referring to  FIG. 3 , in the BS cooperation procedure of this embodiment, the control unit  230  acquires the channel state information at step  311 . At this time, the control unit can acquire the channel state information using the sounding technique or the analog feedback technique. For example, the control unit  230  of the serving BS  110  can acquire H AA  and H AB  from the serving MS  111 . The control unit  230  of the serving BS  110  also can acquire the H AA  from the serving MS  111  and H BA  from the cooperating MS  121 . 
     If the channel state information is received, the control unit  230  selects a serving channel code corresponding to the channel state information from the codebook at step  313 . That is, the control unit  230  can select the channel code having the least difference to the channel state information among the channel codes of the codebook as the serving channel code. For example, the control unit  230  can selects C A  corresponding to H A . At this time, the control unit  230  can select the serving channel code using equation (1): 
     
       
         
           
             
               
                 
                   
                     
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     Here, m denotes the index of channel code, C(m) denotes the channel code corresponding to m, i denotes the index of BS, H i  denotes the channel state information, and C i (m′) denotes the serving channel code. Hi denotes the integrated channel state information in association with at least one MS that can communicate with the BS. For example, H i  of the serving BS  110  can be [H AA   T  H BA   T ] T  and H i  of the cooperating BS  120  can be [H AB   T  H BB   T ] T . Here, T denotes the transpose of matrix. 
     Next, the control unit  230  produces the linear transformation information unit the channel state information and the serving channel code at step  315 . That is, the control unit  230  produces the linear transformation information to minimize the quantization loss between the channel state information and the serving channel code. At this time, the control unit  230  converts the channel state information and the serving channel code linearly to produce the linear transformation using equation (2). For example, the control unit  230  can produce the linear transformation information using equation (3). In this manner, the control unit  230  can quantize the channel state information into the serving channel code such that the quantization loss between the channel state information and the serving channel code with the linear transformation information.
 
 F   i =min∥ H   i   F   i   −C   i ( m ′)+ε i ∥ 2   (2)
 
     Here, F i  denotes the linear transformation information, H i  denotes the channel state information, C i (m′) denotes the serving channel code, and ε i  denotes the channel state information error.
 
 F   i =( H   i   H   H   i ) −1   H   i   H   ·C   i ( m ′)  (3)
 
     Here, N T  denotes a number of transmit antennas of the BS, and N R  denotes a number of receive antennas of the MS. 
     Next, the control unit  230  transmits the serving channel code to the cooperating BS  120  at step  317 . If the cooperating channel code is received from the cooperating BS  120 , the control unit  230  detects the receipt of the cooperating channel code at step  319  and produces the weight information of the serving and cooperating BSs  110  and  120  at step  321 . For example, the control unit  230  can receive C B  corresponding to H B  from the cooperating BS  120 . The control unit  230  produces the weight information of the serving BS, i.e. serving weight information and the weight information of the cooperating BS, i.e. cooperating weight information, using the serving and cooperating channel codes. For example, the control unit  230  can produce the serving and cooperating weight information using equation (4): 
     
       
         
           
             
               
                 
                   
                     
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                   4 
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     Here, W A  denotes the serving weight information, and W B  denotes the cooperating weight information. 
     Finally, the control unit  230  applies the linear transformation information to the serving weight information to determine the beamforming vector at step  323 . That is, the control unit  230  applies the second linear transformation information to the serving weight information to produce the final weight information. At this time, the control unit  230  can produce the final weight information using equation (5). The control unit  230  determines the beamforming vector using the final weight information. Afterward, the control unit  230  provides the serving MS  111  with the communication service using the beamforming vector at step  325 . In this manner, the control unit  230  can recover the quantization loss between the channel state information and the channel code.
 
 W   i   ′=F   i   ×W   i   (5)
 
     Here, W i  denotes the serving weight information, F i  denotes the linear transformation information, and W i ′ denotes the final weight information. 
     Although the description is directed to the exemplary case where the BSs can communicate directly and the BS produce the per-BS weight information to determine the beamforming vector, the present invention is not limited thereto. that is, even when the direct communication between BSs is impossible, the present invention can be implemented. Such a case is described as another embodiment hereinafter. 
       FIG. 4  is a diagram illustrating architecture of a BS cooperation system according to another embodiment of the present invention. Although the description is directed to the BS cooperation system implemented with MIMO, the present invention is not limited thereto. 
     Referring to  FIG. 4 , the BS cooperating system of this embodiment is composed of a plurality of cells. Some of the cells are neighboring with each other. The BS cooperation system includes a plurality of BSs  410  and  420  for controlling the respective cells and a coordinator  430  for relaying the communication between the BSs  410  and  420 . The BSs  410  and  420  can communicate with each other via the coordinator  430 . The base stations  410  and  420  provide the MSs  411  and  421  with the communication service within their respective cells. 
     At this time, each BS can be the serving BS (BS C )  410  and simultaneously the cooperating BS (BS D )  420  which cooperate with other BS. Each MS can be the serving MS (MS C )  411  connected to the serving BS  410  for use of the communication service and simultaneously the cooperating MS (MS D )  421  connected to the cooperating BS  420  for use of the communication service. That is, the serving and cooperating BSs  410  and  420  cooperate with each other to suppress the inter-cell interference at the serving and cooperation MSs  411  and  421 . 
     In the BS cooperation system, the serving and cooperating MSs  411  and  421  can be located at the cell boundary of the serving and cooperating BSs  410  and  420 . The serving BS  410  can transmit a signal to the serving MS  411  with specific channel state information, e.g. H CC , and to the cooperating MS  421  with specific channel state information, e.g. H CD . 
     Each of the serving and cooperating MSs  411  and  421  can check the channel state information and feed back the check result to the serving and cooperating BSs  410  and  420 . That is, the serving MS  411  can discriminate between H CC  and H CD . The serving MS  411  also can feed back the HCC and H CD  to the serving BS  410  or H CC  to the serving BS and H CD  to the cooperating BS  420 . Likewise, the cooperating MS  421  can discriminate between HDC and HDD. The cooperating MS  421  also can feed back HDC and HDD to the cooperating BS  420  or HDC to the serving BS  410  and HDD to the cooperating BS  420 . In this manner, when the channel state information of the serving and cooperating MSs  411  and  421  are acquired, the serving and cooperating BSs  410  and  420  can share the channel state information of the serving and cooperating MSs  411  and  421  via the coordinator  430  to cooperate with each other. 
     Since the configuration of the BS according to this embodiment is similar to that of the above described embodiment, detailed description thereon is omitted herein. However, the BSs  410  and  420  of this embodiment do not share the channel codes directly but transmit the channel codes to the coordinator  430 . If the per-BS weight informations are received, each BS applies the linear transformation information to the weight information to determine the beamforming vector. The BSs  410  and  420  can provide the MSs  411  and  421  with the communication service using the beamforming vector. In this embodiment, it is not necessary for the BSs  410  and  420  to produce the per-BS weight informations. 
       FIG. 5  is a flowchart illustrating the BS cooperation procedure according to another embodiment of the present invention. In this embodiment, the BS cooperation procedure is performed through the communication between the BSs  410  and  420 , and the BSs  410  and  420  operate in the same manner. Here, each of the BSs  410  and  420  operates as the serving BS and simultaneously as the cooperating BS. 
     Referring to  FIG. 5 , in the BS cooperation procedure of this embodiment, the BS first acquires the channel state information at step  511 . At this time, the BS can acquire the channel state information using the sourding method or the analog feedback method. Here, the BS can acquire the channel state information in the form of analog signal. For example, the serving BS  410  can acquire H CC  and H CD  from the serving MS  411 . The serving BS  410  also can acquire H CC  from the serving MS  411  and H DC  from the cooperating MS  421 . 
     Upon receipt of the channel state information, the BS selects the serving channel code corresponding to the channel state information from the codebook at step  513 . That is, the BS selects the channel code having the least difference to the channel state information among the channel codes of the codebook as the serving channel code. For example, the serving BS  410  can select C C  corresponding to H C . The cooperating BS  420  can select C D  corresponding to H D . At this time, the BSs  410  and  420  can select the serving channel code using equation (6): 
     
       
         
           
             
               
                 
                   
                     
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     Here, m denotes the index of channel code, C(m) denotes the channel code corresponding to m, i denotes the index of the BS, H i  denotes the channel state information, and C i (m′) denotes the serving channel code. H i  denotes the integrated channel state information in association with at least one MS that can communicate with the BS. For example, H i  of the serving BS  410  can be [H CC   T  H DC   T ] T  and H i  of the cooperating BS  420  can be [H CD   T  H DD   T ] T . Here, T denotes the transpose of matrix. 
     Next, the BS produces the linear transformation information using the channel state information and the serving channel code at step  515 . At this time, the BS produces the linear transformation information to minimize the quantization loss between the channel state information and the channel code. At this time, the BS transforms the channel state information and the serving channel code linearly to produce the linear transformation as equation (7). For example, the BS can produce the linear transformation information as equation (8). In this manner, the BS can quantize the channel state information into the serving channel code and check the quantization loss between the channel state information and the serving channel code with the linear transformation information.
 
 F   i =min∥ H   i   F   i   −C   i ( m ′)+ε i ∥ 2   (7)
 
     Here, F i  denotes the linear transformation information, H i  denotes the channel state information, C i (m′) denotes the serving channel code, and ε i  denotes the channel state information error.
 
 F   i =( H   i   H   H   i ) −1   H   i   H   ·C   i ( m ′)  (8)
 
     Here, N T  denotes a number of transmit antennas of the BS, and N R  denotes a number of receive antennas of the MS. 
     Next, the BS transmits the serving channel code to the coordinator  430  at step  517 . If the channel codes are received form the BSs  410  and  420 , the coordinator  430  produces the weight information per BS at step  519 . For example, the coordinator  430  can receive C C  corresponding to H C  from the serving BS  410 . The coordinator  430  also can receive the C D  corresponding to H D  from the cooperating BS  420 . The coordinator  430  also produces the per-bs weight information using the channel codes of the BSs  410  and  420 . For example, the coordinator  430  can produce the weight information using equation (9). The coordinator  430  transmits the per-bs weight to the BSs  410  and  420  at step  521 . 
     
       
         
           
             
               
                 
                   
                     
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     Here, W A  denotes the serving weight information, and W B  denotes the cooperating weight information. 
     If the weight information is received, the BS applies the linear transformation information to the weight information to determine the beamforming vector at step  523 . That is, the BS produces the finally weight information by applying the second linear transformation to the weight information. At this time, the base station can produce the final weight information using equation (10). The BS determines the beamforming vector using the final weight information. Next, the BS provides the MSs  411  and  421  with the communication service using the beamforming vector. In this manner, the BS can recover the quantization loss between the channel state information and the channel code.
 
 W   i   ′=F   i   ×W   i   (10)
 
     Here, W i  denotes the serving weight information, F i  denotes the linear transformation information, and W i ′ denotes the final weight information. 
     According to the present invention, the BS cooperation system is capable of producing the linear transformation information that is capable of minimizing the quantization loss between the channel state information and the channel code. The base station applies the linear transformation to beamforming for providing the communication service so as to recover the quantization loss. This improves the cooperation performance in the BS cooperation system. 
     The embodiments disclosed in the specification and drawings aim only to help understand but not limit the present invention. Meanwhile, persons ordinarily skilled in the art would make modifications in terms of specific embodiments and application scopes without departing from the concepts of the present invention.