Patent Publication Number: US-7590200-B2

Title: Receiver, a transmitter, a radio communication system and a channel estimation method

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to a receiver, a transmitter and a radio communication system, and a channel estimation method for a MIMO (Multiple Input Multiple Output) system in which both the receiver and the transmitter are provided with a plurality of antennas. 
   In conventional mobile communication systems, channel estimation generally has been performed using pilot signals, and channel compensation and equalization have been employed for detecting information symbols. In order to increase the channel estimation accuracy, more pilot signals are required. However, if more pilot signals are employed in frames, the actual transmittable information in frames becomes less. As a solution to this problem, an iterative channel estimation method is known, in which detected information bits are utilized for estimating channels. 
   A receiver performing such an iterative channel estimation is explained with reference to  FIG. 1 . 
   A receiver  10  performing such an iterative channel estimation comprises a plurality (M) of antennas  1 , a plurality of channel estimators  2  ( 2 - 1 ˜ 2 -M) and a plurality of channel updating (or renewing) units  4  ( 4 - 1 ˜ 4 -M) connected to the antennas  1 , an information signal detector  3  connected to the antennas  1 , a transmission symbol ‘s’ generator  9  receiving an output from the information signal detector and connected to the channel updating unit  4  ( 4 - 1 ˜ 4 -M), and a controller  5  connected to the information signal detector  3  and ‘s’ generator  9 . The information signal detector  3  is switch-ably connected to the channel estimators  2  ( 2 - 1 ˜ 2 -M) or the channel updating units  4  ( 4 - 1 ˜ 4 -M) via a switch  6 . Each of the channel estimators  2  ( 2 - 1 ˜ 2 -M) receives a pilot signal. 
   In operation, the antennas  1  receive signals and supply the received signals to the corresponding channel estimators  2  ( 2 - 1 ˜ 2 -M). Each of the channel estimators  2  uses the received signal and pilot signals included therein to estimate channels, and inputs the channel estimation value to the information signal detector  3 . 
   The information signal detector  3  uses the input channel estimation values and the received signals to detect information signals. The detected information signal such as information bits are output and supplied to the ‘s’ generator  9  also. The ‘s’ generator  9  uses the input information bits and performs a process the same as in the transmitter to generate estimated transmit symbols. 
   The estimated transmit symbols are input to each channel updating unit  4  ( 4 - 1 ˜ 4 -M). Each channel updating unit  4  ( 4 - 1 ˜ 4 -M) uses the input estimated transmit symbols instead of pilot signals to estimate channels. The channel estimators  2  ( 2 - 1 ˜ 2 -M), the information signal detector  3 , the channel updating units  4  ( 4 - 1 ˜ 4 -M) and the ‘s’ generator  9  are controlled by the controller  5 . 
   In this manner, channel estimation can be done using many received signals, and therefore the channel estimation accuracy can be improved. The channel estimation values can be used for detecting information in the information signal detector  3 , and therefore more highly accurate detection is obtained. 
   On the other hand, there exists a MIMO channel signal transmission system that can realize high frequency usage efficiency. In the MIMO channel signal transmission system, both transmitter and receiver use a plurality of antennas and have a plurality of channels between the transmitter and receiver to obtain parallel transmission and diversity advantages. One problem with this MIMO channel signal transmission system is that there are many channels to be estimated and therefore many pilot signals are required. 
   A scheme of combining the MIMO channel signal transmission system and the iterative channel estimation system is known as shown in Japanese Patent Laid-Open Application No. 2003-152603. A receiver according to this scheme is explained with reference to  FIG. 2 , in which a transmitter simultaneously sends N different information data streams over the same frequency. 
   The receiver  10  comprises M antennas  1 , a plurality of channel estimators  2  ( 2 - 1 ˜ 2 -M) connected to the corresponding antennas  1 , a receiving unit  7  connected to the antennas  1 , and s 1  generator  8 - 1 ˜s N  generator  8 -N connected to the corresponding channel estimators  2  ( 2 - 1 ˜ 2 -M). 
   The receiver  10  have M channel estimators  30  for M antennas and each of the M channel estimators has to estimate N values for N transmission streams. 
   The channel estimators  2  ( 2 - 1 ˜ 2 -M) use received data and pilot signals included in the received signals, estimate channels and input the channel estimation values to the receiving unit  7 . 
   The receiving unit  7  uses the input channel estimation values and the received signals, detects information signals, and outputs the detected information signals such as information bits (st 1 , . . . , stN). The receiving unit  7  supplies the information bits (st 1 , . . . , stN) to the s 1  generator  8 - 1 , . . . , the s N  generator  8 -N. 
   Each of the s 1  generator  8 - 1 , . . . , the s N  generator  8 -N generates an estimated transmit symbol s 1 , . . . , s N  from the input information bits, and inputs the generated estimated transmit symbols s 1 , . . . , sN to the corresponding channel estimator  2 - 1 , . . . ,  2 -M. 
   The channel estimators  2 - 1 , . . . ,  2 -M use the input estimated transmit symbols instead of the pilot signals to update (or renew) channel estimates. The channel estimators  2 - 1 ˜ 2 -M, the receiving unit  7  and s generator  8 - 1 ˜the s N  generator  8 -N are controlled by a controller (not shown). 
   The structure of the channel estimator  2 - 1  in the receiver  10  is explained with reference to  FIG. 3 . Other channels estimators  2 - 2 ˜ 2 -M are the same as the channel estimator  2 - 1  and therefore their explanations are omitted. 
   The channel estimator  2 - 1  comprises a channel estimator  2 - 11  receiving the received signal r 1 (t) and the pilot signal, a channel updating (or renewing) unit  2 - 12  connected to the channel estimator  2 - 11  and receiving the received signal r 1 (t) and the estimated transmit symbols (s 1 , . . . , s N ), and a multiplexer  2 - 15  switchably connected to the channel estimator  2 - 11  or the channel updating unit  2 - 12  via switches  2 - 13 . The channel estimator  2 - 1  further comprises a controller  2 - 14  connected to the channel estimator  2 - 11 , the channel updating unit  2 - 12  and the switches  2 - 13 . 
   In operation, the channel estimator  2 - 11  uses the received signal r 1 (t) and the pilot signal to estimate channels. Regarding the pilot signal, channel estimation can be comparatively easily done by making the pilot signals orthogonal among the streams. For example, it is possible to use a frame structure and channel estimation method disclosed in the following document. 
   “Turbo receiver with sc/simplified-MMSE (S-MMSE) type equalizer for MIMO channel signal transmission”, H. Fujii et. al., IEEE vtc 2003-Fall 
   On the other hand, during the data period, the received signals include a plurality of stream signals having no orthogonal relations. Then it is required to suppress interference between streams and estimate each channel. 
   The initial channel estimation values (h11, . . . , H1N) estimated in the channel estimator  2 - 11  are input to the channel updating unit  2 - 12  and the multiplexer  2 - 15  via the switch  2 - 13 . 
   The multiplexer  2 - 15  multiplexes the input initial channel estimators and outputs. 
   The channel updating unit  2 - 12 , based on the input initial channel estimation values and estimated transmit symbols (s 1 , . . . , s N ), estimates channels, and supplies the channels estimation values to the multiplexer  2 - 15 . The multiplexer  2 - 15  multiplexes the input channel estimation values and outputs. 
   The structure of the channel updating unit  2 - 12  is explained with reference to  FIG. 4 . 
   The channel updating unit  2 - 12  comprises a correlation vector calculator  2 - 121  receiving the received signals r 1 (t) and the estimated transmit symbols s 1 (t)˜s N (t), a correlation matrix calculator  2 - 122  receiving the estimated transmit symbols s 1 (t)˜s N (t), and a multiplier  2 - 123  connected to the correlation vector calculator  2 - 121  and the correlation matrix calculator  2 - 122 . 
   The channel from the transmit antenna n to the receiving antenna m is represented by hmn, and the vector Hm is represented by Hm=[hm1 hm2 . . . hmN] T , the estimated transmit symbol is represented by s n (t), that is the vector S(t) is represented by S(t)=[(s 1 (t) s 2 (t) . . . s N (t)] T , and the received signal is represented by rm(t). 
   The correlation vector calculator  2 - 121  calculates a correlation vector Rxd by an equation Rxd=Σ(rm*(t)S(t))/Nsmp, where 
   Nsmp means the number of received signals used for the channel estimation. 
   The correlation matrix calculator  2 - 122  calculates a correlation matrix Rxx by an equation Rxx=Σ(S(t)S(t) H )/Nsmp, where 
   H means conjugated transpose. 
   The correlation vector Rxd calculated by the correlation vector calculator  2 - 121  and the correlation matrix Rxx calculated by the correlation matrix calculator  2 - 122  are input to the multiplier  2 - 123 . The multiplier  2 - 123  obtains the channel Hn by an equation Hm=Rxx −1 Rxd. 
   However, the above explained related art examples have the following problems. 
   In the channel estimation method using MMSE (Minimum Mean Square Error), it is required to use the degree of freedom of the filter to suppress other streams, and therefore channel estimation accuracy is degraded especially when the number of the received signals is few. 
   When using the MMSE, it is required to obtain an inverse matrix of training signals, and therefore the amount of calculations becomes large. Even if a RLS (Recursive Least Square) algorithm is used to converge the channel estimation values, the amount of calculations is still large.
     [Patent Document #1]   

   Japanese Laid-open 2003-152603
     [Patent Document #2]   

   “Turbo receiver with SC/Simplified-MMSE (S-MMSE) type equalizer for MIMO channel signal transmission”, H. Fujii et. al, IEEE VTC2003-Fall 
   SUMMARY OF THE INVENTION 
   A general object of the present invention is to provide a receiver, a transmitter, a radio communication system and a channel estimation method which can improve channel estimation accuracy while reducing the amount of calculation. 
   The above object of the present invention is achieved by a receiver ( 100 ) in a radio communication system transmitting MIMO channel signals, comprising a received signal replica generator ( 112 ) for generating received signal replicas based on previously obtained provisional channel estimation values and transmission signal estimation values; a canceller ( 111 ) for removing, from a received signal, received signal replicas of at least a part of streams that are not a channel estimation target stream; and a channel estimation value generator ( 113 ) for generating channel estimation values by determining a filter coefficient per stream based on outputs from the removing unit and the transmission signal estimation values. 
   In the receiver, the channel estimation value generator ( 113 ) may determine filter coefficients so as to reduce interference from streams that have not cancelled in the canceller ( 111 ). 
   In the receiver, the channel estimation value generator ( 113 ) may input the generated channel estimation values to the received signal replica generator as provisional channel estimation values. 
   The receiver may further comprise a blocking unit ( 117 ) for dividing a frame into plural blocks; an equalizing unit for separating the received signal to each stream; the channel estimation value generator obtaining a channel estimation value per block; the equalizing unit, based on the channel estimation value obtained per block, separating the received signal to each stream. 
   In the receiver, the channel estimation value generator ( 113 ) may determine a size of the block based on channel variation speed. 
   In the receiver, the received signal replica generator ( 112 ) may generate received signal replicas based on at least a part of transmission signal estimation values; and the canceller ( 111 ) may remove, from at least a part of received signals, received signal replicas of at least a part of streams that are not a channel estimation target stream. 
   According to another feature of the present invention, a transmitter ( 50 ) in a radio communication system transmitting MIMO channel signals comprises a pilot signal insertion controller ( 93 ) for inserting a pilot signal for channel estimation into at least a leading frame; and an information symbol controller for controlling the number of information symbols to be contained in a frame, based on the existence of the pilot signal. 
   According to another feature of the present invention, a radio communication system including a transmitter and a receiver ( 100 ) for transmitting MIMO channel signals is provided. The transmitter comprises a pilot signal insertion controller ( 93 ) for inserting a pilot signal for channel estimation into at least a leading frame; and an information symbol controller for controlling the number of information symbols to be contained in a frame, based on the existence of the pilot signal. The receiver comprises a received signal replica generator ( 112 ) for generating received signal replicas based on previously obtained provisional channel estimation values and transmission signal estimation values; a canceller ( 111 ) for removing, from a received signal, received signal replicas of at least a part of streams that are not a channel estimation target stream; and a channel estimation value generator ( 113 ) for generating channel estimation values by determining a filter coefficient per stream based on outputs from the removing unit and the transmission signal estimation values. 
   According to another feature of the present invention, a channel estimation method in a receiver in a radio communication system transmitting MIMO channel signals is provided. The method comprises the steps of generating received signal replicas based on previously obtained provisional channel estimation values and transmission signal estimation values; removing, from a received signal, received signal replicas of at least a part of streams that are not a channel estimation target stream; determining a filter coefficient per stream, based on the transmission signal estimation values; and generating a channel estimation value per stream, based on the signal from which the received signal replicas have been removed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows a block diagram of a receiver which performs iterative channel estimation; 
       FIG. 2  schematically shows a block diagram of a receiver in which the iterative channel estimation is applied to MIMO channel transmission system; 
       FIG. 3  schematically shows a block diagram of a channel estimator; 
       FIG. 4  schematically shows a block diagram of a channel updating unit; 
       FIG. 5  schematically shows a block diagram of a radio communication system according to an embodiment of the present invention; 
       FIG. 6  schematically shows a block diagram of a channel estimator according to a first embodiment of the present invention; 
       FIG. 7  schematically shows a block diagram of a channel estimator according to a second embodiment of the present invention; 
       FIG. 8  schematically shows a block diagram of a receiver according to a third embodiment of the present invention; 
       FIG. 9  illustrates frames sent by a radio communication system, (a) being frames in the prior art, (b) being frames in an embodiment of the present invention; 
       FIG. 10  schematically shows a block diagram of a transmitter according to a fourth embodiment of the present invention; 
       FIG. 11  schematically shows a block diagram of a channel estimator according to a sixth embodiment of the present invention; 
       FIG. 12  illustrates frame structure according to an embodiment of the present invention; 
       FIG. 13  illustrates frame structure according to an embodiment of the present invention; 
       FIG. 14  schematically shows a block diagram of a channel estimator according to a seventh embodiment of the present invention; and 
       FIG. 15  illustrates frames sent by a radio communication system according to an embodiment of the present invention, (a) showing low channel variation, (b) showing medium channel variation and (c) showing high channel variation. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following is a description of embodiments of the present invention, with reference to the accompanying drawings. 
   Throughout all the figures, members and parts having the same or similar functions are assigned the same or similar reference numerals or symbols, and redundant explanations are omitted. 
   A radio communication system according to an embodiment of the present invention employs a multi-input multi-output (MIMO) system. As shown in  FIG. 5 , this radio communication system comprises a transmitter  50  and a receiver  100 . The transmitter  50  has a plurality of transmit antennas # 1 ˜#N, each of which simultaneously transmits different information data streams using the same frequency. In this embodiment, the number of the transmit antennas equals to the number transmission streams, but the number of the antennas may be different from the number of the transmission streams. The receiver  100  has a plurality of reception antennas # 1 ˜#M, each of which receives all the information data streams transmitted from the transmitter  50 . The receiver  100  processes the received signal, estimates transmission symbols sent from the transmitter  50 , and outputs information bits st 1 ˜stN. 
   Now the transmitter  50  according to this embodiment of the present invention is explained. 
   The transmitter  50  according this embodiment comprises a transmission signal generator  51  for receiving information bits, a plurality of pilot multiplexers  52 - 11 ˜ 52 - 1 N connected to the transmission signal generator  51 , a plurality of pilot signal generators  52 - 21 ˜ 52 - 2 N each connected to one of the pilot multiplexers respectively, and a plurality of antennas  53 . 
   The transmission signal generator  51 , based on the received information bits, generates N streams of transmission signals, each of which is input to one of the pilot multiplexers  52 - 11 ˜ 52 - 1 N. Each of the pilot signal generators  52 - 21 ˜ 52 - 2 N generates a pilot signal, which is input to a corresponding pilot multiplexer  52 - 11 ˜ 52 - 1 N. Each of the pilot multiplexers  52 - 11 ˜ 52 - 1 N multiplexes and transmits the input pilot signals and the transmission signals. 
   Next, the receiver  100  according to the embodiment of the present invention is explained. 
   In repeatedly estimating a channel, the receiver  100  according to this embodiment of the present invention cancels a reception signal replica of some highly reliable streams only, and suppresses other streams using a linear filter such as an MMSE filter. 
   The receiver  100  according to this embodiment of the present invention includes a plurality of antennas  101  # 1 ˜#M, a plurality of channel estimators  102 - 1 ˜ 102 -M each connected to one of the antennas respectively, a receiving unit  103  connected to channel estimators  102 - 1 ˜ 102 -M and antennas  101 , and s 1  generator  104 - 1 ˜s N  generator  104 -N each connected to the channel estimators  102 - 1 ˜ 102 -M. 
   First, after each antenna  101  receives the signals, its corresponding channel estimator  102  uses the received signals and pilot signals included therein, estimates channels, and supplies channel estimation values to the receiving unit  103 . 
   The receiving unit  103  uses the input channel estimation values and received signals to detect information signals, and outputs the detected information signals such as information bits (st 1 , st 2 , . . . , stN). These information bits (st 1 , st 2 , . . . , stN) are input to the corresponding s 1  generator  104 - 1 ˜s N  generator  104 -N. 
   Each of the s 1  generator  104 - 1 , . . . , the s N  generator  104 -N generates a transmission estimation value s 1 , . . . , s N  based on a corresponding input information bit respectively, and outputs the generated transmission estimation value to the channel estimators  102 - 1 ˜ 102 -M. 
   In the succeeding estimation process, each of the channel estimators  102 - 1 , . . . ,  102 -M uses the estimated transmit symbols s 1 ˜s N  instead of the pilot signals, to estimate a channel more accurately. The channel estimators  102 - 1 ˜ 102 -M, the receiving unit  103  and s 1  generator  104 - 1 ˜s N  generator  104 -N are controlled by a controller (not shown). 
   The structure of the channel estimators  102  ( 102 - 1 ˜ 102 -M) of the above mentioned receiver  100  is explained with reference to  FIG. 6 . 
   The channel estimator  102  includes a replica canceller  111  receiving a signal rm(t), a selector  118  connected to the replica canceller  111  and receiving a selection control signal, and a received signal replica rm 1  generator  112 - 1 ˜rmN generator  112 -N each connected to the selector  118  and receiving a provisional channel estimation value hm 1 ˜hmN and an estimated transmit symbol s 1 (t)˜s N (t), respectively. The channel estimator  102  further includes a plurality of filters  113 - 1 ˜ 113 -N each connected to the replica canceller  111  and each of the rm 1  generator  112 - 1  the rmN generator  112 -N. 
   The filter  113 - 1  is taken as one example representing all the filters  113 - 1 ˜ 113 -N. The filter  113 - 1  includes an adder  113 - 11  connected to the replica canceller  111 , a selector  113 - 12  connected to the rm 1  generator  112 - 1  and the adder  113 - 11  and receiving the selection control signal, a second selector  113 - 13  receiving the selection control signal and the s 1 (t)˜s N (t), a correlation vector calculator  113 - 14  connected to the adder  113 - 11  and the second selector  113 - 13 , a correlation matrix calculator  113 - 15  connected to the second selector  113 - 13 , a multiplier  113 - 16  connected to the correlation vector calculator  113 - 14  and the correlation matrix calculator  113 - 15 , and a third selector  113 - 17  connected to the multiplier  113 - 16 . 
   In the channel estimator  102  of the receiver  100  according to this embodiment of the present invention, before the received signal rm(t) is input to the filter  113  ( 113 - 1 ˜ 113 -N), the replica canceller  111  cancels the received signal replica corresponding to a part of stream signals from the received signal rm(t). 
   In canceling the received signal replica from the received signal rm(t), only the received signal replica corresponding to a part of stream signals is subtracted, and other stream signals are suppressed by the liner filter  113 . In this case, each stream is input to the filter  113  stream by stream. In the filter  113 , a filter coefficient is different from stream to stream, and therefore a channel updating unit comprising the correlation vector calculator  113 - 14 , the correlation matrix calculator  113 - 15  and the multiplier  113 - 16  is provided to establish a filter coefficient per stream. 
   In the channel estimator  102  according to this embodiment of the present invention, each of the rm 1  generator  112 - 1 ˜the rmN generator  112 -N generates a received signal replica rm 1 ˜rmN, and inputs the generated received signal replica rm 1 ˜rmN to the selector  118  and the selector  113 - 12  in the corresponding filter  113  ( 113 - 1 ˜ 113 -N). The estimated transmit symbols s 1 (t)˜s N (t) are input to the second selector  113 - 13 . 
   The selector  118  selects only the stream to be cancelled. The selector  113 - 12  inputs the received signal replica to the adder  113 - 11  when the stream signal for channel estimation is cancelled, and input null symbols to the adder when the stream signal for channel estimation is not cancelled. The second selector  113 - 13 , from the input estimated transmit symbols s 1 (t)˜s N (t), selects the stream signal for channel estimation and the stream to be not-cancelled. 
   The stream to be cancelled may be a stream in which no error is detected in CRC (Cyclic Redundancy Check) determination process, or a stream whose signal strength is over predetermined threshold. In the above selection, using information regarding the reliability of each symbol obtained in error correction decoding process, the selection can be conducted symbol by symbol. 
   Information such as the CRC determination information can be input as selection control information to the selector  118 , the selector  113 - 12  and the second selector  113 - 13 . 
   The operation of the channel estimator  102  is now concretely explained. 
   The streams selected by the selector  118  as streams to be cancelled are represented by x 1 , x 2 , . . . , xC. The signal output r′m(t) from the replica canceller  111  after cancellation is represented by Equation (1): 
                   Equation   ⁢           ⁢     (   1   )       ⁢     
     ⁢       r   m   ′     ⁡     (   t   )       =         r   m     ⁡     (   t   )       -       ∑       n   ′     =       x   1     ⁢     x   2     ⁢   …   ⁢           ⁢     x   C           ⁢       r   mn   ′     ⁡     (   t   )                   (   1   )                 r   mn ′( t )=^ h   mn   ′^s   n ′( t ) 
   The output signal of the replica canceller  111  is input to the adder  113 - 11 . A case is explained where in the selector  113 - 12 , a stream for channel estimation represented by x is cancelled on the replica canceller  111 , and streams which are not selected as streams to be cancelled are represented by y 1 , y 2 , . . . , yC. 
   The selector  113 - 12  inputs the signal rmx(t) selected as streams for channel estimation to the adder  113 - 11  where the stream signal for channel estimation is cancelled. The adder  113 - 11  adds the cancelled signal to its own signal.
 
 r′mx ( t )= r′m ( t )+ rmx ( t ).
 
   The output signal of the adder  113 - 11  is a signal that is the received signal with the received signal replica subtracted of at least a part of streams other than the streams to which channel estimation is performed. 
   Supposing that S(t)=[sx(t) sy 1 (t) . . . syC(t)] T , wherein T means transpose of a matrix, the correlation vector calculator  113 - 14  calculates a correlation vector Rxd by an equation, Rxd=Σ(r′*mx(t)S(t))/Nsmp. * means complex conjugate. And the correlation matrix calculator  113 - 15  calculates a correlation matrix Rxx that is a filter coefficient by an equation, Rxx=Σ(S(t)S(t) H )/Nsmp. 
   H means Hermitian conjugate. 
   An output signal of the correlation vector calculator  113 - 14  and an output signal of the correlation matrix calculator  113 - 15  are input to the multiplier  113 - 16 . The multiplier  113 - 16  uses the correlation vector and the correlation matrix to perform multiplication of Rxx −1 Rxd. As a result, an output H from the multiplier  113 - 16  becomes Hm=[hx hy 1  . . . hyC], and this output signal is input to the third selector  113 - 17 . The third selector  113 - 17  selects and outputs the hx. Instead of selecting hx by the third selector, it is possible to omit an operation for obtaining hy 1 ˜hyC in the multiplier  2 - 123 . 
   As for streams which are not selected as a stream to be cancelled, S(t)=[sy 1 (t) sy 2 (t) . . . syc(t)] is the same for streams y 1 ˜yC, a correlation vector and correlation matrix can be calculated to collectively obtain channel estimations for these streams, Hm=[hy 1  hy 2  . . . hyc]. These output signals are input to the third selector  113 - 17 . The third selector  113 - 17  selects and outputs desired stream channel estimation. 
   As clear from the above equations, the channel estimations of the unselected streams can be collectively calculated and therefore the amount of calculations can be reduced. The flexibility of the filter can be used not for suppressing other streams but for strengthening streams for which channel estimation is performed. Further, by not removing a received signal in which an error has been found, but removing by filters, it becomes possible to increase channel estimation accuracy. The estimation values of the transmission symbols are used in this embodiment. However, these can be weighted with the reliabilities of the information bits constituting each symbol that is obtained in the error correction decoding process. In this case, the filter generating methods are described in patent document #1 and non-patent document #1. 
   When updating the channels using the information symbols, the channel estimation values obtained using the pilot signals can be averaged or averaged with weighted (with the best values such as the number of pilot symbols, the number of information symbols, function such as SNR, etc.) to obtain finally updated channel estimation values. 
   Next, a radio communication system according to a second embodiment of the present invention is explained. 
   The radio communication system according to the second embodiment is the same as that illustrated in  FIG. 5  and therefore its explanation is omitted. 
   A structure of a channel estimator  102  ( 102 - 1 ˜ 102 -M) of a receiver  100  according the second embodiment of the present invention is explained with reference to  FIG. 7 . 
   The channel estimator  102  includes a replica canceller  111  receiving a signal rm(t), and a received signal replica rm 1  generator  112 - 1 ˜rmN generator  112 -N each connected to the replica canceller  111  and receiving a provisional channel estimation value hm 1 ˜hmN and an estimated transmit symbol s 1 (t)˜s N (t), respectively. The channel estimator  102  further includes a plurality of filters  114 - 1 ˜ 114 -N each connected to the replica canceller  111  and each of the rm 1  generator  112 - 1  the rmN generator  112 -N. 
   The filter  114 - 1  is explained as one example representing all the filters  114 - 1 ˜ 114 -N. The filter  114 - 1  includes an adder  114 - 11  connected to the replica canceller  111  and rm 1  generator  112 - 1 , a divider  114 - 12  connected to the adder  114 - 11  and receiving s 1 (t), and an averaging unit  114 - 13  connected to the divider  114 - 12 . 
   In the channel estimator  102  according to this embodiment of the present invention, each of the rm 1  generator  112 - 1 ˜the rmN generator  112 -N uses its corresponding (provisional) channel estimation value hm 1 ˜hmN and the transmission signal (symbol) estimation value s 1 (t)˜s N (t) and generates a received signal replica, and inputs the generated received signal replica to the replica canceller  111 . 
   Next, the replica canceller  111  subtracts all received signal replicas from the received signal rm(t). The received signal from which the received signal replicas have been subtracted is input by the replica canceller  111  to a respective filter  114 - 1 ˜ 114 -N which are prepared per each stream. 
   The operation of the filter  114 - 1  is explained as one example representing all the filters  114 - 1 ˜ 114 -N. 
   The adder  114 - 11  adds the received signal replicas of the channel estimation target streams to the received signal from which the received signal replica has been subtracted. The received signal obtained by this addition becomes a signal that all streams other than the channel estimation target streams are subtracted from. 
   
     
       
         
           
             
               
                 
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   In the divider  114 - 12 , this signal is divided by its corresponding training symbol. As a result, a channel estimation value can be obtained. The obtained channel estimation value is input to the averaging unit  114 - 12 . The averaging unit  114 - 13  averages the input plural channel estimation values. In this manner, a higher accurate channel estimation value can be obtained, and it is possible to omit generation of a filter where streams other than the channel estimation target streams are suppressed in the received signal. 
   A radio communication system according to the third embodiment of the present invention is explained with reference to  FIG. 8 . 
   The radio communication system according to this embodiment comprises the transmitter  50  explained above with reference to  FIG. 5  and a receiver  300  illustrated in  FIG. 8 . 
   The receiver  300  is explained. In the receiver  300  according to this embodiment, the above described channel estimator  102  is applied to a turbo equalizing receiver. 
   The receiver  300  according to this embodiment comprises a plurality of antennas  301  (# 1 ˜#M), a plurality of channel estimators  302 - 1 ˜ 302 -M each connected to the corresponding antenna # 1 ˜#M, a signal separator  303  connected to each of the antennas  301 , an information bit detector  304  connected to the signal separator  303 , and s 1  generator  305 - 1 ˜s N  generator  305 -N connected to the corresponding channel estimators  302 - 1 ˜ 302 -N. Output signals from the s 1  generator  305 - 1 ˜the s N  generator  305 -N are input to the signal separator  303  also. 
   In operation, the channel estimators  302  ( 302 - 1 ˜ 302 -M), based on the received signal and the pilot signal input to each channel estimator  301 - 1 ˜ 302 -M, perform channel estimation, and supply the channel estimation values to the signal separator  303 . The signal separator (equalizer)  303  separates the streams, and supplies its outputs to the information bit detector  304 . The information bit detector  304  performs de-mapping/decoding processes on the separated signal. As a result, provisional received signals are obtained (information bits st 1 , . . . , stN). The thus obtained information bits st 1 , . . . , stN are input to corresponding s 1  generator  305 - 1 , . . . , s N  generator  305 -N. Each of the s 1  generator  305 - 1 , . . . , s N  generator  305 -N generates an estimated transmit symbol s 1 , . . . , s N  from the input information bit, and inputs the generated estimated transmit symbol to the corresponding channel estimator  302 - 1 ˜ 302 -M. The generated transmission estimation values s 1 , . . . , s N  are input to the signal separator  303 . 
   Each channel estimator  302 - 1 , . . . ,  302 -M uses the received transmission estimation values instead of pilot signals to estimate the channel, and inputs the channel estimation values to the signal separator  303 . 
   The signal separator  303  uses the input estimated transmit symbols and the channel estimation values to separate the streams again. It is possible to previously obtain the reliabilities of the provisional detection bits and separate the streams on reliabilities. 
   As described above, the receiver  300  according to this embodiment estimates channels from the detected information bits by repeatedly equalizing and decoding, and utilizes the updated channel estimation values to perform the next equalizing process. In this manner, highly accurate channel estimation can be obtained. 
   Next, a radio communication system according to a fourth embodiment of the present invention is explained. 
   The structure of the radio communication system according to this embodiment is the same as that explained above with reference to  FIG. 5 , and its explanation is omitted. 
   A transmitter in the radio communication system according to this embodiment is explained below. 
   First a frame structure of frames sent by the transmitter is explained and then a structure of the transmitter is explained. 
     FIG. 9  shows a prior art frame structure and a frame structure according to this embodiment of the present invention. 
   In the prior art, even when one user occupies the frame continuously, pilot signals for channel estimation are inserted per frame because of channel variation, as shown in  FIG. 9(   a ). 
   In this embodiment of the present invention, as shown in  FIG. 9(   b ), a pilot signal is necessary in the leading frame, but no pilot signal for channel estimation is necessary in the second frame and after because that data signal in the preceding frame can be used to estimate the channel to continue communication. 
   The transmitter  90  is explained with reference to  FIG. 10 . 
   The transmitter  90  can send the frames as shown in  FIG. 9(   b ). The transmitter  90  comprises a transmission signal generator  91  for receiving information bits, a plurality of pilot multiplexers  92 - 11 ˜ 92 - 1 N connected to the transmission signal generator  91 , a pilot signal insertion controller  93  connected to the transmission signal generator  91  and the pilot multiplexers  92 - 11 ˜ 92 - 1 N, a plurality of pilot signal generators  92 - 21 ˜ 92 - 2 N each connected to a corresponding pilot multiplexer  92 - 11 ˜ 92 - 1 N, and a plurality of antennas (# 1 ˜#N). 
   In the frames sent by the transmitter  90  according to this embodiment of the present invention, the leading frame and the succeeding frames contain different numbers of information symbols. Then, in the transmitter  90 , the insertion of the pilot signal is controlled and the numbers of information symbols contained in frames are also controlled. 
   The operation of the transmitter  90  is explained. 
   The transmission signal generator  91 , based on the received information bits, generates transmission signals, and inputs the generated transmission signals to each of the pilot multiplexers  92 - 11 ˜ 92 - 1 N. The pilot signal insertion controller  93  determines whether a pilot signal is necessary. Depending on the determination in the pilot signal insertion controller  93 , each pilot signal generator  92 - 21 ˜ 92 - 2 N generates a pilot signal and inputs the generated pilot signal to a corresponding pilot multiplexer  92 - 11 ˜ 92 - 1 N. Each pilot multiplexer  92 - 11 ˜ 92 - 1 N multiplexes the input pilot signal and information symbols, and transmits them. Meanwhile, the pilot signal insertion controller  93  sends instructions to change block sizes of an error correction coding process and interleave sizes in an error correction coding process to the transmission signal generator  91 . 
   The receiver  300  performs processing similar to that in the transmitter  90 . That is, based on control signals, the information bit detector  304  changes the number of information symbols to be detected. The channel estimators according to this embodiment can be applied to a turbo equalizer receiver. 
   In this manner, the amount of actually transmittable information can be increased. 
   A radio communication system according to the fifth embodiment of the present invention is explained. 
   A structure of the radio communication system according to this embodiment is the same as that explained above with reference to  FIG. 5 , and its explanation is omitted. 
   In the receiver  100  according to this embodiment, each of the above explained channel estimators  102  ( 102 - 1 ˜ 102 -M) estimates and updates (or renews) channel estimation values (hm 1 , hm 2 , hmN). The updated channel estimation values (hm 1 , hm 2 , . . . , hmN) are used as provisional channel estimation values and are input to the channel estimators  102  ( 102 - 1 ˜ 102 -M). The channel estimators  102  perform channel estimation again. 
   In this manner, more precise channel estimation values can be obtained and channel estimation accuracy can be improved. 
   The above described repetition is not only done twice but also may be done more than twice. The number of repetitions can be previously obtained as a function of the number of received signals and SNR (signal-to-noise ratio) to get the necessary channel estimation accuracy. 
   A radio communication system according to a sixth embodiment of the present invention is explained with reference to  FIG. 11 . 
   The structure of the radio communication system according to this embodiment is the same as that explained above with reference to  FIG. 5 , and its explanation is omitted. 
   In the above explained embodiments, the received signals contained in all the frames are used to estimate channels. In this embodiment, however, the receiver uses a part of the received signals to estimate channels. 
   The channel estimator  102  of the receiver  100  according to this embodiment includes a selector  115  receiving a received signal rm(t), a replica canceller  111  connected to the selector  115 , selectors  116 - 1 ˜ 116 -N  8  receiving corresponding estimated transmit symbols s 1 (t)˜s N (t), and transmission replica rm 1  generator  112 - 1 ˜transmission replica rmN generator  112 -N. Each of the transmission replica generators  112 - 1 ˜ 112 -N receives a corresponding provisional channel estimation value hm 1 ˜hmn, is connected to the corresponding selector  116 - 1 ˜ 116 -N, and is also connected to the replica canceller  111 . The channel estimator  102  further comprises a plurality of filter  114 - 1 ˜ 114 -N each connected to the replica canceller  111  and the corresponding rm 1  generator  112 - 1 ˜rmN generator  112 -N. 
   The filter  114 - 1  is explained as one example representing all the filters  114 - 1 ˜ 114 -N. The filter  114 - 1  includes an adder  114 - 11  connected to the replica canceller  111  and rm 1  generator  112 - 1 , a divider  114 - 12  connected to the selector  116 - 1  and the adder  114 - 11 , and an averaging unit  114 - 13  connected to the divider  114 - 12 . 
   The channel receiving unit  102  of the receiver  100  according to this embodiment has the selectors  115 ,  116 - 1 ˜ 116 -N selecting the received signals and the transmission signals respectively, in addition to the channel estimators explained above with reference to  FIG. 7 . 
   For example, in cases where transmission is continuous and a previous frame is used in the next frame to estimate channels as in the above embodiments, it is better to perform channel estimation for the received signal at the rear end of a frame as shown in  FIG. 12 , because the channel varies with time. In this situation, the selectors  115 ,  116 - 1 ˜ 116 -N select the received signals at the rear end of the frames. 
   On the other hand, when applied to the turbo equalizer, etc., it is desired to select information symbols from the whole frame for estimating channels and use averaged channel estimation values. In this situation, the selectors  115 ,  116 - 1 ˜ 116 -N select a part from the whole frame as shown in  FIG. 13 . 
   When estimating channels by using a part of received signals, it is necessary to determine the number of received signals to be used. The number of received signals to be used can be a certain fixed number or can be previously determined based on SNR. 
   In this manner, channel estimation accuracy can be improved, and it is possible to avoid channel estimation processing more than necessary. 
   A radio communication system according to a seventh embodiment of the present invention is explained. 
   A structure of the radio communication system according to this embodiment is similar to that explained with reference to  FIG. 5 , and its explanation is omitted. 
   As shown in  FIG. 14 , the channel estimator  102  of the receiver  100  according to this embodiment comprises a correlation vector calculator  113 - 14 , a correlation matrix calculator  113 - 15  and a multiplier  113 - 16  in each liner filter  113 - 1 ˜ 113 -N, in addition to the elements in the channel estimator shown in  FIG. 6 . The channel estimator  102  further comprises a blocking controller  117 . 
   In this embodiment, the blocking controller  117  divides a frame into some blocks, and channels are estimated block by block. When separating each block signal, or equalizing each block, channel estimation values obtained when equalizing each block are used. 
   The size of the block can be a previously determined size. Alternatively, the block size can be adaptively varied depending on channel variation. In this case, as the channels vary faster, the blocks should be divided into smaller pieces. For example, as shown in  FIG. 15 , as the channels vary higher, the block size should be smaller as shown in  FIG. 15(   a ) to  FIG. 15(   c ). In  FIG. 15 , fD means channel variation speed. Channel estimation values in each block can be obtained by weight averaging the channel estimation values of peripheral blocks. In this way, even when the channels vary, channel estimation accuracy is improved. 
   Alternatively, the channel estimator  102  can be provided with a plurality of blocking controllers  117  for each liner filter  113 - 1 ˜ 113 -N, each of which is connected a corresponding correlation vector calculator  113 - 14 , correlation matrix calculator  113 - 15  and multiplier  113 - 16 . 
   In the radio communication system according to this embodiment, interference from any other streams than the channel estimation target stream are previously cancelled or reduced and therefore the amount of calculation is decreased and channel estimation accuracy is improved. 
   By canceling any other streams than the channel estimation target stream, MMSE filter calculation can be omitted to reduce the amount of calculation. 
   For example, in a case where N (=4) streams and Sym (=10) data symbols are used for estimating channels, comparison is made regarding the number of division/multiplication calculation for one receiving antenna, as follows. 
   An amount of calculation in the prior art is
 
2× N 2× Sym+N 3+ N 2=400.
 
   An amount of calculation in the embodiments of the present invention is
 
2× N×Sym+N− 84.
 
   Therefore it is clearly understood that an amount of calculation can be reduce to ⅕ according to the embodiments of the present invention. 
   INDUSTRIAL APPLICABILITY 
   A radio communication system according to the embodiments of the present invention can be applied to MIMO (Multiple Input Multiple Output) receivers, MIMO transmitters and MIMO radio communication systems and their channel estimation methods. 
   The present application is based on Japanese Priority Application No. 2004-122163 filed on Apr. 16, 2004 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.