Patent Publication Number: US-2013251059-A1

Title: Base station and mimo-ofdm communication method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/212,481, filed Sep. 17, 2008, now pending, which is a continuation of International Patent Application No. PCT/JP2006/305488 filed on Mar. 20, 2006, the contents of each are entirely incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a base station and MIMO-OFDM communication method thereof, and more particularly to a base station and MIMO-OFDM communication method thereof that transmits pilot signals together with data to a mobile station using Multiple Input Multiple Output (MIMO) communication from a plurality of antennas using a plurality of OFDM transmitters. 
     In many digital mobile communication systems, in order to compensate for distortion of a data signal in a fading channel, pilot signals are multiplexed and transmitted with a data signal from the transmission side. On the receiving side, the pilot signals that were transmitted from the transmission side are received, and the received pilot signals are compared with a known pilot signal to estimate channel distortion (channel estimation), then channel compensation is performed for the received data signal based on the channel estimation value. There are various methods for multiplexing the data signal and pilot signals, however, the case of using the OFDM (Orthogonal Frequency Division Multiplex) method is explained below. 
       FIG. 11  shows the construction of a transmitter in an OFDM communication system, where a data modulation unit  1  performs QPSK data modulation, for example, on the transmission data (user data and control data), and converts the data to a complex baseband signal (symbol) having a in-phase component and quadrature component. A time-division multiplexing unit  2  inserts pilot symbols into the data symbols by time-division multiplexing. A serial-to-parallel conversion unit  3  converts input data to parallel data of M symbols, and outputs the M symbols as M number of sub-carrier samples S 0  to S M-1 . An IFFT (Inverse Fast Fourier Transform) unit  4  performs IFFT (inverse Fourier transformation) on the parallel input sub-carrier samples S 0  to S M-1  and combines them, then outputs the result as a discrete-time signal (OFDM signal). A guard interval insertion unit  5  inserts guard intervals into the OFDM signal having M symbols that is input from the IFFT unit, and a transmission unit (TX)  6  performs DA conversion of the OFDM signal in which guard intervals have been inserted, then converts the frequency of the OFDM signal from a baseband signal to a radio signal, and after performing high-frequency amplification, transmits the signal from an antenna  7 . 
       FIG. 12  is a drawing explaining the serial-to-parallel conversion, and shows an example where a common pilot P is time-division multiplexed in front of one frame of transmission data. In a case where the common pilot p is 4×M symbols (=4 OFDM symbols) and the transmission user data is 28×M symbols (=28 OFDM symbols), the serial-to-parallel conversion unit  3  outputs M symbols of the pilot up to the first 4 times as parallel data, and after that outputs the M symbols of the transmission data 28 times as parallel data. As a result, in one frame period, the pilot can be time-division multiplexed in all subcarriers and transmitted 4 times, and on the receiving side it is possible to use that pilot to estimate the channel for each subcarrier and to perform channel compensation (fading compensation). Here, one OFDM symbol comprises M symbols. 
       FIG. 13  is a drawing that explains the insertion of guard intervals. When the IFFT output signal that corresponds to M number of subcarrier samples (=1 ODM symbol) is taken to be one unit, guard interval insertion is the copying of the end section to the front thereof. By inserting guard intervals GI, it becomes possible to eliminate the effect of multipath intersymbol interference. 
       FIG. 14  is a drawing showing the construction of an OFDM receiver. A signal that is output from the transmission antenna  7  passes through a fading channel (propagation path) and received by the receiving antenna  8  of the receiver, after which a receiving circuit (Rx)  9  converts the RF signal received by the antenna to a baseband signal and performs AD conversion to convert that baseband signal to a digital signal and outputs the result. An FFT timing synchronization circuit  10  detects an FFT timing from the time domain signal that is output from the receiving circuit  9 , and a symbol extract unit  11  removes the guard intervals GI and extracts the OFDM symbol at the FFT timing and inputs the result to an FFT unit  12 . The FFT unit  12  performs FFT processing for each extracted OFDM symbol to convert them to frequency domain subcarrier samples S 0 ′ to S M-1 ′. A channel estimation circuit  13  performs channel estimation for each subcarrier by calculating the correlation between pilot symbols received at fixed intervals and a known pilot pattern, and a channel compensation circuit  14  uses the channel estimation value to compensate for channel fluctuation of the data symbols. Transmission data that is distributed to each subcarrier is demodulated by the processing described above. After that, the demodulated subcarrier signals are converted to serial data and decoded. 
     The processing described above is for the case in which the pilot signals are densely arranged in the frequency direction, however, from the aspect of transmission efficiency of the data signal or the capability to follow channel fluctuation, scattered arrangement is known in which pilot signals are sparsely arranged in the time or frequency direction.  FIG. 15  shows an example of a scattered arrangement in which for a specified subcarrier, pilot signals P are embedded in a data signal DT across 4 adjacent OFDM symbols. Channel estimation using pilot signals in a scattered arrangement is performed as described below. 
     The receiver multiplies a received pilot signal after FFT by the complex conjugate of a known pilot signal to calculate the channel estimation value of the subcarrier in which the received pilot signal is embedded. Next, noise and an interference component are somewhat suppressed by averaging this channel estimation value over time among a plurality of adjacent symbols. After that, the channel estimation value of the subcarrier in which that pilot signal is embedded is used to find the channel estimation value of a subcarrier in which the pilot signal is not embedded by performing interpolation or extrapolation in the frequency direction. Similarly, for an OFDM symbol in which a pilot signal is not embedded, the channel estimation value of an OFDM symbol in which a pilot signal is embedded is used to interpolate and find that channel estimation value in the time direction. In the interpolation process mentioned above, linear interpolation is performed between subcarriers or OFDM symbols in which the pilot signal used for interpolation is embedded with the assumption that channel distortion fluctuates linearly. However, to be more precise, actual channel distortion does not fluctuate linearly, but rather has complex fluctuation due to high-speed movement of a mobile station in the time direction or due to delay spread of a multipath channel in the frequency direction, so error occurs in the channel estimation value after interpolation. 
     Therefore, in the example of scattered arrangement, the transmission efficiency of the data signal improves the wider the insertion interval of the pilot signals is, however, it becomes difficult for channel estimation to follow sudden channel fluctuation that occurs due to high-speed movement of a mobile station, and thus reception characteristics deteriorate. On the other hand, the transmission efficiency of the data signal decreases the narrower the insertion interval of the pilot signals is, and, since it becomes easy for channel estimation to follow sudden channel fluctuation, it becomes difficult for the reception characteristics to deteriorate. Therefore, when designing the arrangement of pilot signals, the geographical environment where the digital mobile communication system will be used, and the estimated speed of movement of the mobile station must be taken into consideration. 
     Here the case is considered in which in a multi-cell environment as shown in  FIG. 16 , a mobile station MS that is located near the boundary between cells A, B and C is performing communication with the base station BS A  in cell A. When the mobile station MS receives a downlink signal S A  from the base station BS A  in cell A, it also receives interference signals I B  and I c  from the base stations BS B  and BS C  in the adjacent cells B and C. 
     In the prior system, these interference signals are suppressed as follows. That is, the transmitter uses a spreading code (channelization code) for spreading transmit data, or uses repetition to copy data for one symbol to a plurality of symbols, after which it multiplies the result by cell unique scramble code and transmits the signal. The receiver multiplies the received signal by the same scramble code that the transmitter used, after which by performing de-spreading using spreading code, or in-phase addition to demodulate the signal, suppresses the interference signals from the adjacent cells to a certain extent. 
     However, pilot signals are transmitted at higher power than data signals, so even though the processing described is performed, the effect from the adjacent cells remains large. Particularly, as shown in  FIG. 17 , in an inter-base station synchronization system, the arrangement of pilot signals P A , P B  and P C  of the cells A, B and C is identical. Therefore, the pilot signals from each of the cells always interfere with each other, and the channel estimation accuracy deteriorates. In the case of an inter-base station asynchronization system as well, at a certain ratio in time, the pilot signals of each cell interfere with each other, and channel estimation accuracy deteriorates. 
     As a countermeasure for avoiding the problem described above, the pilot signals can be arranged in each cell so that they do not overlap. In the example shown in  FIG. 18 , the pilot signals of three adjacent cells are shifted in the frequency direction and arranged so that they do not overlap. In this case, it is not possible to make the interval in the frequency direction between pilot signals in a specified cell less than n subcarriers (n=number of cells−1). In the example shown in  FIG. 18 , n=2. 
     In the case of MIMO (multiple-Input Multiple-Output) multiplex transmission, it is necessary to obtain a channel estimation value for each transmission antenna, so orthogonal pilot signals are transmitted from each transmission antenna. As in the case of single antenna transmission, the arrangement of these pilot signals should be such that they do not overlap in each cell.  FIG. 19  is a drawing explaining a multipath environment in which base stations BS A , BS B , and BS C  in cells A, B and C use four antennas  1  to  4  to perform MIMO multiplex transmission, and  FIG. 20  shows an example of the pilot signal arrangement of the antennas  1  to  4  of each cell A, B and C. In this example, the pilot signals PA 0  to PA 3 , PB 0  to PB 3  and PC 0  to PC 3  for the four transmission antennas of each cell A, B and C are shifted in the frequency direction and arranged so that they do not overlap each other among the three adjacent cells. 
     However, in the example of pilot arrangement shown in  FIG. 20 , it is not possible to make the interval in the frequency direction between pilot signals at an arbitrary antenna in an arbitrary cell less than 11 (=3 cells×4 antennas−1) subcarriers. Therefore, when there is large channel fluctuation, the accuracy of channel estimation deteriorates due to interpolation. 
     When transmitting OFDM signals from a plurality of antennas, there is related art (refer to Japanese patent application 2003-304216) that deters interference in the propagation paths of the pilot signals. In this related art a pilot signal is transmitted from one antenna by a specified subcarrier, and no pilot carriers are output from other antennas, and subcarriers having the same frequency as the pilot sub carrier of the one antenna is taken to be a null signal. However, this related art is not intended for preventing the interference of pilot signals from other cells or other sectors. Moreover, this related art does not enable channel estimation with high precision while preventing interference of pilot signals from other cells or other sectors. 
     SUMMARY OF THE INVENTION 
     Taking into consideration the aforementioned problems, the object of the present invention is to enable channel estimation with high precision while preventing interference of pilot signals from other cells or other sectors that are not in communication with the mobile station. 
     Another object of the present invention is to sufficiently minimize interference between pilot signals and to enable channel estimation that is capable of following sudden channel fluctuation when the mobile station is at a cell boundary or sector boundary. 
     First MIMO-OFDM Communication Method 
     The present invention is a MIMO-OFDM communication method that uses a plurality of OFDM transmitters to transmit data and pilot signals to a mobile station from a plurality of antennas by multiple input multiple output (MIMO) communication, comprising steps of: transmitting data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station; and arranging the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified cell, and pilot signals that are used in channel estimation during single data stream transmission in adjacent cells do not overlap each other in the frequency direction and/or time axis direction. 
     The first MIMO-OFDM communication method of the present invention further comprises a step of making the power of pilot signals that are transmitted from a specified antenna and used for channel estimation during single data stream transmission greater than the power of pilot signals that are transmitted from other antennas. 
     The first MIMO-OFDM communication method of the present invention further comprises steps of receiving pilot signals that are transmitted from the base station, measuring the communication environment based upon the received pilot signals in the mobile station and feeding back data to the base station that indicates the communication environment; and deciding in the base station whether to perform multiple data stream transmission or single data stream transmission based on the data indicating the communication environment. 
     The first MIMO-OFDM communication method of the present invention further comprises steps of receiving pilot signals that are transmitted from the base station, measuring the communication environment based upon the received pilot signals in the mobile station and feeding back data to the base station that indicates the communication environment; and controlling the number of data streams by the base station during multiple data stream transmission based on the data indicating the communication environment. 
     Second MIMO-OFDM Communication Method 
     The present invention is a MIMO-OFDM communication method for a base station that divides a cell into sectors and uses a plurality of OFDM transmitters in each sector to transmit data and pilot signals to a mobile station from a plurality of antennas by multiple input multiple output (MIMO) communication, comprising steps of: transmitting data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station in a sector; and arranging the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified sector, and pilot signals that are used in channel estimation during single data stream transmission in adjacent sectors do not overlap each other in the frequency direction and/or time axis direction. 
     The second MIMO-OFDM communication method of the present invention further comprises a step of making the power of pilot signals that are transmitted from a specified antenna and used for channel estimation during single data stream transmission greater than the power of pilot signals that are transmitted from other antennas. 
     The second MIMO-OFDM communication method of the present invention further comprises a step of measuring in the mobile station the reception power of the sector antennas of each of the sectors, and deciding in the base station whether to perform multiple data stream transmission or single data stream transmission based on the difference in the reception power among adjacent sectors. 
     The second MIMO-OFDM communication method of the present invention further comprises a step of controlling the number of data streams used in said multiple data stream transmission based on the communication environment of the mobile station in each sector in the case of performing multiple data stream transmission. 
     Base Station 
     The base station of the present invention comprises construction for executing the first and second MIMO-OFDM communication methods described above. That is, a first base station of the present invention comprises: a communication method decision unit configured to decide whether to transmit data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station; and a pilot position control unit configured to control the position of the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified cell, and pilot signals that are used in channel estimation during single data stream transmission in adjacent cells do not overlap each other in the frequency direction and/or time axis direction. 
     A second base station of the present invention comprises construction for executing the second MIMO-OFDM communication method described above. In other words, the second base station comprises: a communication method decision unit configured to decide whether to transmit data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station in a sector; and a pilot position control unit that controls the position of the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified sector, and pilot signals that are used in channel estimation during single data stream transmission in adjacent sectors do not overlap each other in the frequency direction and/or time axis direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing giving an overview of the present invention. 
         FIG. 2  is a table showing the correspondence among the number of receiving antennas, number of data streams and used antennas when controlling the number of multiple data streams according to the communication environment of a mobile station, for example, the SIR. 
         FIG. 3  is a drawing showing the construction of the MIMO-OFDM communication system of the present invention. 
         FIG. 4  is a drawing showing in detail the construction of a base station. 
         FIG. 5  is a drawing showing in detail the construction of a mobile station. 
         FIG. 6  is an example of pilot signal arrangement in which pilots P 1  of each cell do not overlap at the same time in the frequency direction and time direction. 
         FIG. 7  is a drawing explaining the sector configuration. 
         FIG. 8  is an example of pilot signal arrangement of a second embodiment of the present invention. 
         FIG. 9  is a drawing showing the construction of a base station in a second embodiment of the invention. 
         FIG. 10  is a drawing showing the construction of a mobile station in a second embodiment of the invention. 
         FIG. 11  is a drawing showing the construction of a transmitter in an OFDM communication system. 
         FIG. 12  is a drawing explaining serial-to-parallel conversion. 
         FIG. 13  is a drawing explaining the insertion of guard intervals. 
         FIG. 14  is a drawing showing the construction of an OFDM receiver. 
         FIG. 15  is an example of scattered arrangement in a specified subcarrier in which pilot signals P are embedded in data DT across four adjacent OFDM symbols. 
         FIG. 16  is a drawing explaining a multi-cell environment. 
         FIG. 17  is a first example of conventional pilot signal arrangement. 
         FIG. 18  is a second example of convention pilot signal arrangement. 
         FIG. 19  is a drawing explaining a multi-cell environment in MIMO-OFDM communication. 
         FIG. 20  is an example of conventional pilot signal arrangement in MIMO-OFDM communication. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (A) Overview of the Present Invention 
     Cell Configuration and Pilot Arrangement 
       FIG. 1  is a drawing giving an overview of the present invention, where as shown in (A) of  FIG. 1 , base stations BS A , BS B  and BS C  in cells A, B and C use four antennas  1  to  4  respectively to perform MIMO-OFDM communication with a mobile station. As shown in (B) of  FIG. 1 , the pilot signal arrangement of the four antennas  1  to  4  of each of the base stations BS A , BS B  and BS C  is such that the pilot signals are densely arranged in the frequency direction, and the pilot signals are arranged so that they overlap in the time direction (in the first four OFDM symbols of a frame). The pilot signals of the antennas  1  to  4  of each of the base stations can be distinguished according to pattern, and as shown in (C) of  FIG. 1 , P 1  is the pilot signal for antenna  1 , P 2  is the pilot signal for antenna  2 , P 3  is the pilot signal for antenna  3  and P 4  is the pilot signal for antenna  4 , with the power of the pilot signals being set so that P 1 &gt;P 2 &gt;P 3 &gt;P 4 . It should be noticed that the pilots P 1  that are transmitted from the first antennas  1  of the base stations BS A , BS B  and BS C  of each of the cells A, B and C do not overlap in the frequency direction. 
     Switching Control Between Multiple Data Stream Transmission and Single Data Stream Transmission 
     In the present invention, in the event that the communication environment with the mobile station becomes poor, the base station switches from MIMO communication, which is multiple data stream transmission, to single input multiple output (SIMO) communication, which is single data stream transmission. For example, when the mobile station MS moves while communicating with the base station BS A , the base station BS A  transmits data and pilot signals to the mobile station MS from the four antennas  1  to  4  using MIMO-OFDM communication (multiple stream data transmission) as long as that mobile station MS is in an area AR 1  that is near the base station BS A  and the communication environment is such that the SIR (signal to interference ratio) is large. 
     When the mobile station MS moves to a cell boundary area and it receives interference from an adjacent cell and the SIR is small, the BER (bit error rate) increases even though MIMO-OFDM communication is performed. Therefore, in that case, the base station BS A  switches to single input multiple output (SIMO) communication (single data stream transmission) and transmits data and pilot signals from only one antenna. 
     When performing single data stream transmission, the base stations BS A , BS B  and BS C  of each of the cells transmit data and pilot signals to the mobile station from the first antenna  1 , however, as can be clearly seen from (B) of  FIG. 1 , in each of the cells A, B and C, the pilots P 1  that are transmitted from antenna  1  during single data stream transmission are arranged so that they do not overlap. That is, the pilot signals are arranged so that the pilot signals P 1  of a cell that are used for channel estimation during single data stream transmission and pilot signals P 1  of an adjacent cell that are used for channel estimation during single data stream transmission do not overlap each other in the frequency direction. 
     As a result, during single data stream transmission there is no interference between the pilot signals P 1  for channel estimation of one cell and the pilot signals P 1  of an adjacent cell, so even at the cell boundary, the SIR of the pilot signals is improved, and accurate channel estimation is possible. Moreover, in the example shown in (B) of  FIG. 1 , the pilot signals Pi (i=1 to 4) that are transmitted from a specified antenna can be arranged at 3-subcarrier intervals, so error in channel estimation that occurs during MIMO-OFDM communication due to interpolation is reduced, and accurate channel estimation becomes possible. 
     During single data stream transmission, the pilot signals P 1  for channel estimation of a cell over the pilots P 2  to P 4  of adjacent cells in the frequency direction, however there is little effect due to interference. Particularly, in the example of arrangement shown in (B) of  FIG. 1 , during single data stream transmission, the power of the pilots P 1  transmitted from antenna  1  is greater than the power of the pilot signals P 2  to P 4  that are transmitted from the other antennas  2  to  4 , so at the cell boundary pilot signals P 2  to P 4  other than the pilot P 1  arrive sufficiently damped, thus the interference from pilots P 2  to P 4  from adjacent cells can be considered to be minor. 
     Controlling the Number of Multiple Data Streams 
     Above, switching control was performed to switch transmission between multiple data stream transmission and single data stream transmission based on the communication environment of the mobile station, for example the SIR value. In the case of this switching control, only the power of the pilot signals P 1  need be made greater than the power of the other pilot signals P 2 , P 3 , P 4 , and there is no need to make the power between the other pilot signals different. However, in the case of controlling the number of multiple data streams according to the communication environment, for example the SIR value, the power of each of the pilot signals should be set differently such as P 1 &gt;P 2 &gt;P 3 &gt;P 4 . 
     In MIMO multiplex transmission, the signals from each transmission antenna arrive at the receiver while overlapping in the radio channel, so the distance between signal constellation points in the received signal becomes shorter the larger the number of data streams is, and the required reception SIR for achieving a fixed bit error rate BER increases. In other words, the smaller the number of data streams is, it is possible to achieve a fixed bit error rate even when the reception SIR is small. Therefore, in the present invention, the number of multiple data streams is controlled according to the communication environment, for example the SIR value. For example, as shown in  FIG. 2 , the communication environment is divided into four levels, where in the worst environment (communication environment  1 ), the number of data streams is 1, and the number of the antenna to be used is taken to be 1. In communication environment  2 , the number of data streams is 2, and the numbers of the antennas used are 1 and 2, in communication environment  3 , the number of data streams is 3, and the numbers of the antennas used are 1, 2 and 3, and in the best communication environment  4 , the number of data streams is 4, and the numbers of the antennas used are 1, 2, 3 and 4. 
     By controlling the number of multiple data streams according to the communication environment as described above, it is possible to take advantage of the merits of MIMO-OFDM communication as much as possible. 
     Example of Another Pilot Arrangement 
     In the example of the pilot arrangement shown in (B) of  FIG. 1 , the pilots P 1  that are transmitted by antenna  1  of each of the cells during single data stream transmission are arranged so that they do not overlap in the frequency direction, however, at the same time it is possible to arrange the pilots so that they do not overlap in the time direction or so that there is no overlapping in just the time direction. 
     As described above, with the present invention, pilot signals that are used for channel estimation by a mobile station that performs single data stream transmission can be arranged so that they do not overlap each other in adjacent cells in the time and/or frequency direction, and the mobile station that performs single data stream transmission, can receive a desired signal without receiving the effects of interfering signals from adjacent cells even at the cell boundaries. 
     (B) First Embodiment 
     (a) Construction of a MIMO-OFDM Communication System 
       FIG. 3  is a drawing showing the construction of a MIMO-OFDM communication system of the present invention, where BS indicates abase station, and MS indicates a mobile station. The base station BS is a base station of one of the cells A to C shown in (A) of  FIG. 1 , for example, cell A. All of the base stations of the cells A to C have the same construction. 
     In the base station BS, the same number of data streams D 0  to D M-1  as the number of transmission antennas M pass through specified processing by respective OFDM transmitters  11   0  to  11   M-1 , and are transmitted from transmission antennas  12   0  to  12   M-1 . The processing performed by the OFDM transmitters  11   0  to  11   M-1  include error correction encoding, data modulation, data/control signal/pilot multiplexing, scrambling, IFFT transformation, and GI insertion. 
     Signals, which are transmitted from the antennas  12   0  to  12   M-1  and are arranged so that they are not correlated with each other, pass through independent fading channels h nm  (m=0 to M−1, n=0 to N−1), and after being multiplexed in space, are received by N number of receiving antennas  31   0  to  31   N-1 . The signals that are received by the receiving antennas pass through processing by OFDM receivers  32   0  to  32   N-1  such as frequency down conversion, AD conversion, FFT timing detection, GI removal, FFT transformation, descrambling, and data/control signal/pilot separation, to generate y 0  to y N-1  reception data streams. Each of the reception data streams, are formed by M number of multiplexed transmission data streams, so a data processing unit  33  performs signal processing of all of the reception data streams to separate out and reproduce the transmission data streams D 0  to D M-1 . 
     Various methods are proposed for the algorithm for signal processing of separating out the transmission data streams D 0  to D M-1  from the reception signals, however, here the MLD (Maximum Likelihood Decoding) algorithm will be explained. When the transmission data streams are expressed by a M-dimensional complex matrix, and the reception data streams are expressed by a N-dimensional complex matrix, they are related by the following equations. 
     
       
         
           
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     The MLD algorithm estimates the transmission data stream (transmission vector) D by the following equation. 
         {circumflex over (D)}= argmin∥ Y−H·D∥   2  
 
     Here, by taking the number of arranged signal constellation points of modulated data that is input to the M number of antennas to be Q, then Q M  number of transmission vector combinations exist. In QPSK modulation, Q=4. In the MLD algorithm, Q M  number of transmission vector candidates (replicas) are generated, and calculation is performed according to the equations above, and the replica with the smallest result is estimated to be the input data. 
     (b) Construction of a Base Station BS 
       FIG. 4  shows the detailed construction of a base station BS for the case in which M=4, and is an example of controlling the number of multiple data streams according to the communication environment. The same reference numbers are used for parts that are the same as those shown in  FIG. 3 . 
     A data stream number decision unit  13  decides the number of data streams based on the communication environment with the mobile station MS as will be described later, and inputs the result to a control information mapping unit  14  and data stream dividing unit  15 . The control information mapping unit  14  maps the number of input data streams at a specified location as control information, an error correction encoding unit  16  performs error correction encoding processing the control information, and a data modulation unit  17  performs data modulation of the control data. 
     The data stream dividing unit  15  performs serial-to-parallel conversion of the data signal according to the number of data streams, and inputs the result to OFDM transmitters  11   0  to  11   3 . The OFDM transmitters  11   0  to  11   3  generate transmission signals for each of the transmission antennas  12   0  to  12   3  from the respective data streams D 0  to D 3 , and transmits those signals from the transmission antennas  12   0  to  12   3 . By taking the antenna numbers of the transmission antennas  12   0  to  12   3  to be 1 to 4, the number of data streams and the transmission antenna numbers used are as shown in  FIG. 2 . The OFDM transmitters  11   0  to  11   3  have nearly the same construction, where the only difference is that OFDM transmitter  11   0  transmits control signals, and the other OFDM transmitters  11   1  to  11   3  do not transmit control signals. 
     In the OFDM transmitter  11   0 , the error correction encoding unit  21  performs error correction encoding processing of the data of the first data stream D 0  that is input, and the data modulation unit  22  performs data modulation of the input data. A pilot signal generation unit  23   a  generates pilot signals P 1 , and a gain adjustment unit  23   b  adjusts the amplitude of the pilot signals P 1 . The method for adjusting the amplitude of the pilot signals will be described later. 
     Next, a data/pilot/control signal multiplexing unit  24  multiplexes the data signal, control signal and pilot signal and performs parallel output of subcarrier components of N samples. As shown in (B) of  FIG. 1 , the data/pilot/control signal multiplexing unit  24  maps the pilot signals P 1  on a specified subcarrier of a specified OFDM symbol. A scrambling unit  25  multiplies the subcarrier components of N samples by a cell unique scramble code, and an IFFT unit  26  performs IFFT transformation of the subcarrier components of the N samples to convert the frequency domain signal to a time domain signal. A GI insertion unit  27  inserts guard intervals (GI), and a transmission RF unit  28  performs D/A conversion, after which it performs frequency conversion to convert the signal from a baseband signal to a radio frequency signal, then amplifies the signal and transmits it from the transmission antenna  12   0 . 
     At the same time as the above, a reception RF unit  18  receives a signal from the reception antenna  19  that is transmitted from the mobile station MS, after which it performs frequency conversion to convert the frequency from a radio frequency to a baseband frequency, then performs AD conversion and inputs the signal to a control signal demodulation unit  20 . The control signal demodulation unit  20  performs demodulation to demodulate the control signal that was fed back from the mobile station, then extracts the downlink reception SIR information that is contained in that control signal. The downlink reception SIR indicates the communication environment of the mobile station MS, and is a reception SIR that is measured using the transmitted pilot signals that the mobile station MS receives from the base station BS. The data stream number decision unit  13  decides the number of data streams to transmit according to the table shown in  FIG. 2  based on the downlink reception SIR information, and inputs that number to the control information mapping unit  14  and data stream dividing unit  15 . 
     (c) Method of Deciding the Number of Data Streams 
     The fundamental characteristic of MIMO multiplex transmission is considered. In MIMO multiplex transmission, signals that are transmitted from each transmission antenna arrive at the mobile station while overlapping in a radio channel. Therefore, the larger the number of data streams is, the shorter the distance between signal constellation points of the received signals becomes, and the required reception SIR for achieving a constant bit error rate BER increases. In other words, the smaller the number of data streams is, the more possible it is to achieve a constant bit error rate BER even when the reception SIR is small. 
     Moreover, the reception SIR generally decreases as the distance between the base station and mobile station increases. Therefore, in order to maximize the throughput of data signals, by measuring the reception SIR and comparing that reception SIR with a specified threshold value, control can be performed so that when the reception SIR is high (when the mobile station is near the base station) the number of data streams is increased, and when the reception SIR is low (when the mobile station is near the cell boundary) the number of data streams is decreased. 
     In order to do this, the data stream number decision unit  13  decides the number of data streams according to the value of the reception SIR. For example, the reception SIR (communication environment) is divided into four levels as shown in  FIG. 2 , where in the worst environment (communication environment  1 ), the number of data streams is 1, and the number of the antenna to be used is taken to be 1. In communication environment  2 , the number of data streams is 2, and the numbers of the antennas used are 1 and 2, in communication environment  3 , the number of data streams is 3, and the numbers of the antennas used are 1, 2 and 3, and in the best communication environment  4 , the number of data streams is 4, and the numbers of the antennas used are 1, 2, 3 and 4. 
     (d) Pilot Signal Amplitude 
     Next, the mapping position of the pilot signals by the data/pilot/control signal multiplexing unit  24 , and amplitude adjustment of the pilot signals will be explained. 
     When the mobile station is near a cell boundary, the reception SIR of the data signal is relatively low, so the data stream number decision unit  13  decides the number of data streams so that single data stream transmission is performed. Accordingly, from the standpoint of the mobile station that is at the cell boundary, it is sufficient that only the channel estimation value for single data stream transmission have a certain level of quality. Therefore, the power of the pilot signals P 1  that are transmitted from the transmission antenna  12   0  that are used for single data stream transmission is set sufficiently high. That is, the gain adjustment unit  23   b  of the OFDM transmitter  11   0  makes the amplitude of the pilot signals P 1  large before the signals are transmitted from the transmission antenna  12   0 , and the power of the pilot signals P 2 , P 3 , P 4  that are transmitted from the other transmission antennas  12   1 ,  12   2 ,  12   3  is lower than the power of the pilot signals P 1 . As shown in  FIG. 2 , when the number of data streams is control based on the communication environment, the pilot signal power is set as P 1 &gt;P 2 &gt;P 3 &gt;P 4 . 
     Even though a difference in power is set in this way, from the standpoint of the mobile station that is near the base station, the amount of damping of the pilot signal in the radio channel is relatively small, so there is little effect on the accuracy of channel estimation. 
     (e) Pilot Arrangement Method 
     (B) of  FIG. 1  is an example of the pilot signal arrangement in three cells A, B and C, where pilot signals are arranged in the portion of the first four OFDM symbols of a frame, and they are densely arranged in the frequency direction. 
     During single data stream transmission, the base stations BS A , BS B , BS C  of each of the cells transmit data and pilot signals P 1  to the mobile station from the first antenna  12   0 , however, in this example of pilot arrangement, the pilots P 1  from each cell A, B, C do not overlap in the frequency direction. That is, pilot signals P 1  in a cell that is used for channel estimation during single data stream transmission, and the pilot signals P 1  of adjacent cells that are used for channel estimation during single data stream transmission are arranged so that they do not overlap each other in the frequency direction. 
     For a mobile station MS at the cell boundary, only the power of pilot signals P 1  from other cells is large and dominant as interference. Therefore, it is necessary that there be no interference from other cells due to the pilot signals P 1 . Since transmission to the mobile station MS at the cell boundary is single data stream transmission, the pilot signals P 1  that are transmitted by single data stream transmission should not receive interference from pilot signals P 1  from other cells. In order to accomplish this, the positions of the pilot signals P 1  of each of the cells A, B, C are such that they are different from each other, and even though the pilot signals P 1  are arranged in the same positions as any of the pilot signals P 2  to P 4  from the transmission antennas  12   1  to  12   3  of other cells, there is no interference. 
     With the pilot arrangement shown in (B) of  FIG. 1 , the pilot signals P 1  for channel estimation of the cells A, B, C during single data stream transmission do not interfere with the pilot signals P 1  of adjacent cells, so the SIR of the pilot signals improves even at the cell boundary, and thus good channel estimation becomes possible. In addition, in the example shown in (B) of  FIG. 1 , pilot signals Pi (i=1 to 4) that are transmitted from a specified antenna are arranged at three-subcarrier intervals, so during MIMO-OFDM communication, the error in channel estimation that occurs due to the interpolation process becomes small, and good channel estimation becomes possible. In other words, since it is possible to keep the subcarrier interval between pilot signals from an arbitrary antenna of an arbitrary cell small, it is possible to improve the accuracy of the interpolation process, and thus channel estimation that is capable of following sudden channel fluctuation is possible. 
     (f) Construction of a Mobile Station 
       FIG. 5  is a drawing showing the detailed construction of a mobile station MS in the case where N=4, with the same reference numbers being used for parts that are the same as those shown in  FIG. 3 . 
     The signals received by each of the receiving antennas  31   0  to  31   3  are input to OFDM receivers  32   0  to  32   3 . All of the OFDM receivers  32   0  to  32   3  have the same construction, and a reception RF unit converts the frequency of the received signals from a radio frequency to a baseband frequency, then performs A/D conversion and inputs the result to a FFT timing detection unit  42  and GI removal unit  43 . The FFT timing detection unit  42  detects the FFT timing, and the GI removal unit  43  removes the GI according to the FFT timing, and after making the data N sampling parallel data, inputs the data to a FFT unit  44 . The FFT unit  44  converts the N sample signals in the time domain to N number of subcarrier signal components in the frequency domain, and a descrambling unit  45  multiplies the N number of subcarrier signal components by the same code as descramble code used by the base station, and extracts the signal received from the base station. A data/pilot/control signal separation unit  46  separates the data signal, pilot signal and control signal that are mapped at specified locations, and inputs the data signal to a MIMO demodulation unit  51  of a data processing unit  33 , inputs the pilot signal to a channel estimation unit  52 , and inputs the control signal to a control signal demodulation unit  53 . 
     The channel estimation unit  52  first obtains a channel estimation value for the subcarrier of the OFDM symbol in which the pilot signals are embedded. In other words, the channel estimation unit  52  performs a correlation operation to correlate the pilot signals received from the receiving antennas  31   0  to  31   3  with a known transmission pilot signal, and performs in-phase addition of the obtained correlation value among 4 symbols, to obtain a channel estimation value for which interference signals from adjacent cells have been somewhat suppressed. Next, by performing interpolation or extrapolation of the channel estimation values for each of the antennas in the time or frequency direction, the channel estimation unit  52  obtains channel estimation values for subcarriers of OFDM symbols in which pilot signals are not embedded. After that, for each antenna, the channel estimation unit  52  inputs the channel estimation values for each of the subcarriers of each of the OFDM symbols to the MIMO demodulation unit  51  and control signal demodulation unit  53 . 
     The control signal demodulation unit  53  uses the channel estimation values to perform channel compensation of the control signals that were transmitted from the one transmission antenna  12   0  and received by the four receiving antennas  31   0  to  31   3 , then by performing diversity combining among the receiving antennas, demodulates the control information, extracts out the information about the number of data streams that were mapped at specified locations in the control signal, and inputs the result to the MIMO demodulation unit  51 . 
     The MIMO demodulation unit  51  uses the data signals received from each of the receiving antennas and the channel estimation values to perform a well-known MIMO channel separation process based on the number of data streams, and by performing P/S conversion of each of the data streams obtained from that MIMO channel separation operation and outputting the result, restores the data signal. 
     Moreover, a SIR estimation unit  34  uses the channel estimation values that are estimated by the channel estimation unit  52  to estimate the reception SIR. More specifically, the desired signal power S is regarded as the sum of the squares of both the real portion and imaginary portion of the channel estimation value (complex value) between a specified transmission antenna and receiving antenna, the interference signal power I is regarded as the variance of a plurality of symbols, and the estimated value of the reception SIR is computed by the ratio of S and I. A control information mapping unit  35  maps the SIR estimation value at a specified location as control information, and a control signal modulation unit  36  generates a control signal by performing a process such as error correction encoding and data modulation, then multiplexes the control signal, user data and pilot signals and transmits that multiplexed signal to the base station BS from a transmission antenna  37 . 
     (g) Another Example of Pilot Arrangement 
     The pilot signal arrangement shown in (B) of  FIG. 1  is an example in which the pilots P 1  of each of the cells do not over lap in the frequency di reaction, however, it is also possible for the pilots P 1  of each of the cells to not overlap at the same time in both the frequency direction and time direction. 
       FIG. 6  shows an example of pilot signal arrangement in which pilots P 1  of each cell do not overlap at the same time in both the frequency direction and time direction. According to this example of pilot signal arrangement, pilot signals are located every other sub carrier in the frequency direction in both the first four OFDM symbols and the next four OFDM symbols, and the subcarriers where the pilot signals are located are different between the first four OFDM symbols and the next four OFDM symbols. In addition, it is arranged that the pilot signals P 1  of the cells A, B, C do not overlap in the frequency direction each other, and the pilot signals P 1  of cell B and the pilot signals P 1  of cells A and C do not overlap in the time direction. 
     In the case of an inter-base station synchronization system, it is also possible to employ pilot signal arrangement in which the pilots P 1  of each cell do not overlap only in the time direction. 
     (h) Variation 
     In the first embodiment of the present invention, the number of data streams was controlled from 1 to 4 based on the communication environment (reception SIR) of the mobile station, however, configuration is also possible in which data and pilot signals are transmitted by switching between either MIMO communication, which is multiple data stream transmission, or single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment (reception SIR) of the mobile station. 
     (C) Second Embodiment 
     In the first embodiment, a method for reducing the effect of interference signals received by a mobile station at a cell boundary from adjacent cells was explained. Using a similar method, it is also possible to reduce the effect of interference signals received by a mobile station at a sector boundary from adjacent sectors in the case where a cell comprises a plurality of sectors. 
     (a) Sector Configuration and Pilot Signal Arrangement 
       FIG. 7  shows the presumed sector configuration. A cell is divided into three sub-areas, each having a center angle of 120°, such that sectors A, B and C are formed by antennas A 1  to A 4 , B 1  to B 4  and C 1  to C 4  having directionality such that they cover the respective sub-areas. In the case where the mobile station MS is in a area CAR near the center of sector A, signals are received from the base station BS only from sector antennas A 1  to A 4 , however when the mobile station MS moves near the boundary between sectors A and B, the mobile station MS receives signals from sector antennas B 1  to B 4  as interference signals. 
     Normally, these interference signals are suppressed as described below. That is, at the base station BS, after spreading using spreading code or copying the information of one symbol to a plurality of symbols by repetition, the resulting signal is multiplied by a sector-unique scramble code and transmitted. At the mobile station MS, the signal is multiplied by the same scramble code as used by the base station BS, and then de-spreading using the spreading code, or in-phase addition is performed, thereby the interference signals from the adjacent sectors are suppressed somewhat. However, in regards to the pilot signals, the pilot signals are transmitted at relatively high power, so even though the processing described above is performed, they continue to interfere with adjacent sectors, and have a large effect on the adjacent sectors. Particularly, when the arrangement of pilot signals of each sector is common, the pilot signals of each sector interfere with each other, and the accuracy of channel estimation deteriorates. 
     In a second embodiment of the invention, as in the first embodiment, the pilot signals P 1  from the transmission antennas A 1 , B 1  and C 1  that are used in single stream transmission are transmitted at a sufficiently high power, and the pilot signals P 2 , P 3 , P 4  from the transmission antennas A 2  to A 4 , B 2  to B 4  and C 2  to C 4  are transmitted at a lower power than the pilot signals from the transmission antennas A 1 , B 1  and C 1 . In other words, the transmission power of the pilot signals P 1 , P 2 , P 3  and P 4  is set so that P 1 &gt;P 2 &gt;P 3 &gt;P 4 . Moreover, as shown in  FIG. 8 , the arrangement of the pilot signals is such that, as in the first embodiment, the positions of the pilot signals P 1  from the transmission antennas A 1 , B 1  and C 1  of the sectors A, B and C are different from each other in the frequency direction. 
     By doing this, as long as a mobile station at a sector boundary performs single stream transmission, the effect on channel estimation of pilot signals from other sectors becomes minor, so stable reception can be performed. Therefore, by switching the number of data streams transmitted when a mobile station is at a sector boundary to a single data stream, stable reception can always be performed regardless of the position of the mobile station within a sector. 
     (b) Base Station Construction 
       FIG. 9  is a drawing showing the construction of a base station of this second embodiment of the invention, and it differs from the construction of the first embodiment shown in  FIG. 4  in that downlink reception SIR information and a sector boundary judgment bit are mapped in a control signal that is fed back from the mobile station MS, with the same reference numbers being used for parts that are identical. 
     A control signal demodulation unit  20  demodulates control information that is sent from a mobile station MS, and inputs downlink reception SIR information and a sector boundary judgment bit to a data stream number decision unit  13 . A sector boundary judgment bit of ‘0’ indicates that the mobile station MS is in the center of the sector, and a sector boundary judgment bit of ‘1’ indicates that the mobile station MS is at the sector boundary. The data stream number decision unit  13  decides the number of data streams based on the reception SIR information and the sector boundary judgment bit. More specifically, first, when the sector boundary judgment bit is ‘1’, the data stream number decision unit  13  decides the number of data streams to ‘1’. However, when the sector boundary judgment bit is ‘0’, the data stream number decision unit  13  decides the number of data streams by comparing the reception SIR information with a specified threshold value as in the first embodiment. 
     (c) Mobile Station Construction 
       FIG. 10  is a drawing showing the construction of a mobile station of this second embodiment, where processing from the FFT unit and later differs from that of the first embodiment. Also, the construction of OFDM reception units  32   0  to  32   3 , and data processing unit  33  are different. That is, the OFDM reception units  32   0  to  32   3  comprise a reception RF unit  41 , FFT timing detection unit  42 , GI removal unit  43  and FFT unit  44 . The data processing unit  33  comprises a first sector signal processing unit  33   a , second sector signal processing unit  33   b  and third sector signal processing unit  33   c . In the first sector processing unit  33   a , descramble units  45  that correspond to the OFDM reception units  32   0  to  32   3  multiply the N number of subcarrier signal components that are output from the FFT units  44  by the same code as the scramble code for sector A, then extracts and outputs the signals received from sector A. Next, data/pilot/control signal separation units  46  separate out the data signals, pilot signals and control signals that are mapped at specified locations, and input the respective data signals to a MIMO demodulation unit  51  of the data processing unit  33 , input the pilot signals to a channel estimation unit  52  and input the control signals to a control signal demodulation unit  53 . After that, as in the first embodiment, the channel estimation unit estimates the channel of sector A where the mobile station MS is located, the control signal demodulation unit  53  demodulates the control signal and inputs the number of data streams to the MIMO demodulation unit  51 , and the MIMO demodulation unit  51  demodulates and outputs the data signal. 
     On the other hand, the second sector processing unit  33   b  multiplies the N number of subcarrier signal components that are output from the FFT units  44  of the OFDM reception units  32   0  to  32   3  by the same codes as the scramble code of sector B, then extracts and outputs the signals received from sector B. Next, the second sector processing unit  33   b  separates the pilot signals that are mapped at specified locations and estimates the channel of sector B as in the first embodiment. Similarly, the third sector processing unit  33   c  performs channel estimation of sector C. 
     The SIR estimation unit  34  uses the channel estimation value that was estimated by the channel estimation unit  52  to estimate the downlink reception SIR from sector A by the same method as in the first embodiment. A sector boundary judgment unit  38  uses the channel estimation values from the first to third signal processing units  33   a  to  33   c  to generate a sector boundary judgment bit. More specifically, taking the sum of the squares of the real portion and imaginary portion of the channel estimation values (complex values) to be the desired signal power, when the difference between the desired signal power of the first and second sector signal processing units  33   a ,  33   b , or the difference between the desired signal power of the first and third sector signal processing units  33   a ,  33   c  is less than a threshold value, the sector boundary judgment unit  38  determines that the mobile station is near the sector boundary and sets the sector boundary judgment bit to ‘1’. For all other cases, the sector boundary judgment unit  38  sets the sector boundary judgment bit to ‘0’. A control information mapping unit  35  maps the SIR estimation value and the sector boundary judgment bit as control information, and a control signal modulation unit  36  generates a control signal by performing processing such as error correction encoding and data modulation, after which it multiplexes the control signal, user data and pilot signals and transmits the resulting signal from the transmission antenna  37 . 
     Effect of the Invention 
     With the present invention, pilot signals that are used for channel estimation during single data stream transmission in a specified cell, and pilot signals that are used for channel estimation during single data stream transmission in adjacent cells do not overlap each other in the frequency direction and/or time axis direction, so interference from pilot signals of other cells with which a mobile station is not communicating can be prevented, and thus channel estimation can be performed with high precision. 
     Moreover, with the present invention, pilot signals that is used for channel estimation during single data stream transmission in a specified cell, and pilot signals that are used for channel estimation during single data stream transmission in adjacent cells do not over each other in the frequency direction and/or time axis direction, and the power of the pilot signals that are used in channel estimation and that are transmitted from a specified antenna during single data stream transmission is greater than the power of pilot signals that are transmitted from other antennas, so when a mobile station is at the cell boundary, the inference from pilot signals is sufficiently small, and thus it is possible to perform channel estimation that is capable of following sudden channel fluctuation. The same effects are obtained even when the cell is divided into sectors. 
     Furthermore, with the present invention, the interval between pilot insertion subcarriers from a specified antenna can be shortened, so during MIMO-OFDM communication, it is possible to perform channel estimation that is capable of following even when sudden channel fluctuation occurs.