Radio communication system that uses a MIMO receiver

A receiver uses a transmission channel matrix to obtain a superior signal separation characteristic regardless of differences in levels of multipath signals. A multipath linear combining unit performs linear combining of the multipaths in received signals of the reception antennas by means of the transmission channel matrix between the plurality of transmission antennas and the plurality of reception antennas. A maximum likelihood detector compares signals in which multipaths have been combined by the multipath linear combining unit with reception replicas that have been found using the transmission channel matrix to estimate the transmitted signals of each of the transmission antennas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a receiver, a receiving method, and a radio communication system that are used in communication realized by MIMO, and more particularly to a receiver, a receiving method, and a radio communication system in which MIMO signals are demodulated by means of maximum likelihood detection (MLD) from signals that are received using a plurality of reception antennas.

2. Description of the Related Art

Radio communication methods in next-generation mobile communication demand high-speed data transmission. MIMO (Multiple Input Multiple Output) multiplexing is receiving attention as a technology for realizing high-speed data transmission. MIMO is a technology for transmitting a plurality of signals from a plurality of transmission antennas at the same frequency and the same time, receiving these signals using a plurality of reception antennas, and then demodulating (implementing signal separation of) the plurality of signals.

FIG. 1is a block diagram showing the configuration of a typical radio communication system that uses MIMO. In this case, the number of transmission antennas is M (where M is an integer equal to or greater than 1), and the number of reception antennas is N (where N is an integer equal to or greater than 1).

Referring toFIG. 1, the radio communication system includes transmitter81and receiver82. Transmitter81has a plurality of transmission antennas831-83M, and receiver82has a plurality of reception antennas841-84N.

Transmitter81transmits differing signals from each of a plurality of transmission antennas831-83Mat the same time and at the same frequency. Receiver82uses a plurality of reception antennas841-84Nto receive the signals that have been transmitted from transmitter81and, from these received signals, demodulates M signals by means of a signal separation process. According to this radio communication system, increasing the number of signals that are simultaneously transmitted and received in proportion to the number of transmission antennas enables the realization of high-speed data transmission without increasing the transmission bandwidth.

On the other hand, DS-CDMA (Direct Sequence-Code Division Multiple Access) is widely used as a mobile communication radio access method.

In DS-CDMA, time-spreading a transmission signal by a particular code can effectively reduce the interference of other cells in a multi-cell environment and enable one-cell repetition. In addition, separating multipath signals by means of despreading and then combining these multipath signals (rake combining) can obtain the path diversity effect.

In recent years, the possibility of enabling even higher-speed data transmission through the application of MIMO multiplexing to DS-CDMA (CDMA MIMO multiplexing) is being investigated. Various methods have been proposed as the signal separation process in CDMA MIMO multiplexing, examples including: Minimum Mean Square Error (MMSE), Vertical Bell Labs Layered Space-Time (VBLAST), and Maximum Likelihood Detection (MLD).

MLD has superior characteristics to MMSE and VBLAST but has the disadvantage that the increase in the number of transmission antennas and the number of modulation multivalues brings with it an exponential increase in computation load. As a solution to this drawback, a reduced-computation-load MLD is now being investigated that can greatly reduce the MLD computation load.

One example of a prior-art MIMO receiver that uses MLD to perform a CDMA MIMO signal separation process is next shown.FIG. 2is a block diagram showing the configuration of a MIMO receiver of the prior art. In this case, the number of transmission antennas provided in a transmitter (not shown) is M (where M is an integer equal to or greater than 1), and the number of reception antennas is N (where N is an integer equal to or greater than 1).

Referring toFIG. 2, the MIMO receiver of the prior art includes: reception antennas911-91N, despreaders9211-921L, . . .92N1-92NL, transmission channel estimation unit93, and MLD unit94.

Each of despreaders9211-921Ldespreads a respective path of each signal that is received at reception antenna911. Similarly, the received signals of each of the reception antennas are despread for each path by L despreaders, each of despreaders92N1-92NLdespreading the signal received by reception antenna91Nfor a respective path. The despread signals that are obtained as a result are provided to MLD unit94.

In this case, if yn, lis the despread symbol of path l of reception antenna n, despread symbol vector y can be shown as shown in Equation (1).
y=[y0,0. . . y0,L-1y1,0. . . yN-1,L-1]T(1)

Transmission channel estimation unit93receives as input the signals that have been received at reception antennas911-91N, uses a known pilot signal that is included in these received signals to estimate for each path the transmission channel estimation value between the transmission and reception antennas. If the transmission channel estimation value of path l between transmission antenna m and reception antenna n is hm, n, l, then transmission channel matrix H can be represented by (N×L) rows and M columns as seen in Equation (2).

MLD unit94uses transmission channel matrix H that is obtained by transmission channel estimation unit93to generate reception replicas for the signals that have been received from all transmission antennas, calculates the error signals between the despread signals from each of despreaders9211-92NLand the reception replicas, and selects the transmission antenna signal that is most likely.

If the transmission symbol vector s is shown in Equation (3), and noise vector n is shown in Equation (4), transmission channel matrix H can be used to represent despread symbol vector y as shown in Equation (5):

In Equation (3), smshows the transmission symbol of transmission antenna m. In Equation (4), nn, lshows the noise in path l of reception antenna n.

FIG. 3is a block diagram showing the configuration of MLD unit94. Referring toFIG. 3, MLD unit94includes: transmission symbol candidate generation unit941, reception replica generation unit942, error signal calculation unit943, and bit likelihood calculation unit944.

Transmission symbol candidate generation unit941generates transmission symbol vector s, which is the combination of all transmission antenna symbols, and sends this transmission symbol vector to reception replica generation unit942.

Reception replica generation unit942generates all reception replicas {tilde over (r)}=Hs based on transmission symbol vector s from transmission symbol candidate generation unit941and transmission channel matrix H, and sends these transmission replicas to error signal calculation unit943.

Error signal calculation unit943finds the final error signalbased on reception replicas {tilde over (r)} from reception replica generation unit942and despread symbols y from despreaders9211-92NL, and sends this error signalto bit likelihood calculation unit944. At this time, error signal calculation unit943compares reception replica {tilde over ( )}rn, land despread symbol yn, las shown in Equation (6), and then adds each of the error signals as shown in Equation (7) to find the final error signal.

Bit likelihood calculation unit944receives as input error signalsthat correspond to all transmission antenna symbols s and calculates the likelihood for each bit that is transmitted from each transmission antenna. At this time, bit likelihood calculation unit944applies the bit likelihood as input to error correction decoder (not shown) (for example, turbo decoder) and restores the information bit sequence. One method for calculating bit likelihood is based on the difference between the minimum error signal of the symbol in which the target bit is +1 and the minimum error signal of the symbol in which the target bit is −1, as described in N. Maeda, K. Higuchi, J. Kawamoto, M. Sawahashi, M. Kimata, and S. Yoshida, “QRM-MLD Combined with MMSE-Based Multipath Interference Canceller for MIMO Multiplexing in Broadband DS-CDMA” (Proc. IEEE PIMRC 2004, pp. 1741-1746, September 2004 (Document 2)).

FIG. 4is a block diagram showing the configuration of another MIMO receiver of the prior art. The MIMO receiver shown inFIG. 4has a greatly decreased amount of MLD calculation compared to the device shown inFIG. 2(see Document 2).

QR decomposition unit95decomposes the transmission channel matrix H that is obtained in transmission channel estimation unit93into the product of the Q matrix and R matrix as shown in Equation (8), sends Q to QHconverter96, and sends R to reduced-calculation-load MLD unit97.

In this case, Q is a unitary matrix of (N×L) rows and M column, each column vector being orthogonal (QHQ=1), and the norm is 1. R is an upper triangular matrix of M rows and M columns.

QHconverter96multiplies despread symbol vector y with QHto convert y to an orthogonal coordinate system represented by Q. QHconverter96is of a configuration for realizing computation for multiplying QHby multipliers and adders. Signal vector z after coordinate conversion is represented by Equation (9).

In this case, noise n′ is noise n projected onto an orthogonal coordinate system represented by Q, and therefore uncorrelated with n at the same power.

Reduced-calculation-load MLD unit97uses the R matrix from QR decomposition unit95to generate reception replicas for the signals of all transmission antennas, calculates the error signals between the reception replicas and the signal vector z following coordinate conversion, and after cutting back symbol candidates, selects the most likely transmission antenna signal.

FIG. 5is a block diagram showing the configuration of reduced-calculation-load MLD unit97. Referring toFIG. 5, reduced-calculation-load MLD unit97includes: transmission symbol candidate generation unit971, reception replica generation unit972, error signal calculation/symbol candidate reduction unit973, and bit likelihood calculation unit974.

As with transmission symbol candidate generation unit941ofFIG. 3, transmission symbol candidate generation unit971generates transmission symbol vector s, which is the combination of all transmission antenna symbols, and sends transmission symbol vector s to reception replica generation unit972.

Reception replica generation unit972generates all reception replicas {tilde over (r)}=Rs from transmission symbol vector s from transmission symbol candidate generation unit971and matrix R from QR decomposition unit95and sends the reception replicas to error signal calculation/symbol candidate reduction unit973.

Error signal calculation/symbol candidate reduction unit973reduces symbol candidates while finding error signalmfrom reception replicas {tilde over (r)}mand signal vector zmfollowing coordinate conversion by QHconverter96over a plurality of stages for the plurality of transmission antennas.

As an example, the reduction of symbol candidates is carried out successively starting from the largest transmission antenna number.

In each stage of the reduction of symbol candidates, error signal calculation/symbol candidate reduction unit973compares reception replicas {tilde over (r)}mand signal vectors zmas shown in Equation (10), and finds error signalmas shown in Equation (11). Error signal calculation/symbol candidate reduction unit973further reduces symbol candidates by selecting only a prescribed number from the symbol candidates having low error signalsm.

Bit likelihood calculation unit974calculates the likelihood for each bit that is transmitted from each transmission antenna based on the error signalsthat correspond to all transmission antenna symbols s that have been finally eliminated.

Nevertheless, the above-described prior art has the following drawbacks.

The MIMO receiver of the prior art shown inFIG. 2compares the reception replicas that are generated using the transmission channel estimation values of each path with the multipath signals that are separated in despreading to perform a signal separation process by MLD. At this time, the differences in levels of the multipath signals is not taken into consideration, and as a result, when there are differences in the levels of each multipath signal, the multipath interference sustained by each path differs, multipaths having a low level receiving the greatest multipath interference. In a MIMO receiver of the prior art, the influence of multipath interference is not taken into consideration in the calculation of error signals, and as a result, there is a disproportionately large addition of the error signal of multipaths having a low level and a consequent degradation of the signal separation characteristic.

On the other hand, the other MIMO receiver of the prior art shown inFIG. 4, performs the MLD process using signals in which despread signals undergo QHconversion, but because the transmission channel matrix is subjected to direct QR decomposition, the operation is principally equivalent to that of the MIMO receiver ofFIG. 2, and is therefore subject to the same problem.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a MIMO receiver, reception method, and radio communication system that can obtain a superior signal separation characteristic regardless of differences between the levels of multipath signals.

To attain the above-described object, the MIMO receiver of the present invention is a MIMO receiver for receiving signals that have been transmitted from a plurality of transmission antennas by means of a plurality of reception antennas and includes a multipath linear combining unit and a maximum likelihood detector. The multipath linear combining unit performs linear combining of multipaths in the received signals of the reception antennas by means of a transmission channel matrix between the plurality of transmission antennas and the plurality of reception antennas. The maximum likelihood detector compares the signals in which the multipaths have been combined by the multipath linear combining unit with reception replicas that have been found using the transmission channel matrix to estimate the transmission signals of each of the transmission antennas.

The present invention enables optimum combining of the multipath signals for the signal of each transmission antenna by means of a combining method that takes into consideration differences in the levels of multipath signals, and further allows a superior signal separation characteristic to be obtained regardless of the differences in level of multipath signals.

In addition, the MIMO receiver of the present invention may further include a whitening filter for whitening the noise in the signals in which multipaths have been combined by the multipath linear combining unit and for providing this whitened signal to the maximum likelihood detector.

The maximum likelihood detection process is therefore carried out after whitening noise after combining of the multipath signals, with the result that a superior signal separation characteristic can be realized in which the effect of noise is mitigated.

The above and other objects, features, and advantages of the present invention will become apparent from the following description with references to the accompanying drawings which illustrate examples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6is a block diagram showing the configuration of the radio communication system according to the first embodiment. This system uses MIMO (Multiple Input Multiple Output) multiplexing, and in this case, the number of transmission antennas is M (where M is an integer equal to or greater than 1) and the number of reception antennas is N (where N is an integer equal to or greater than 1).

Transmitter11transmits a different signal from each of the plurality of transmission antennas131-13Mat the same frequency and at the same time. Receiver12uses the plurality of reception antennas141-14Nto receive the signals that are transmitted from transmitter11, and demodulates M signals from the received signals by means of a signal separation process.

FIG. 7is a block diagram showing the configuration of the receiver according to the first embodiment. This receiver may be, for example, either a base station radio device or a mobile station radio device of a mobile communication system. Referring toFIG. 7, receiver12includes: reception antennas141-14N; despreaders2111-211L, . . . ,21N1-21NL; transmission channel estimation unit22; rake combining unit23; whitening filter calculation unit24; whitening filter25; and MLD unit26.

Each of despreaders2111-211Ldespreads each of the paths of respective signals received at reception antenna141. Similarly, the received signals of each of the reception antennas are despread by the L despreaders for each path, and the signals received at reception antenna14Nare despread for each path by despreaders21N1-21NL. The thus-obtained despread signals are provided to rake combining unit23.

Transmission channel estimation unit22takes as input the signals that have been received at reception antennas141-14N, and uses a known pilot signal that is contained in these received signals to estimate for each path the transmission channel estimation value between the transmission and reception antennas.

Rake combining unit23uses the transmission channel estimation values that are obtained at transmission channel estimation unit22to carry out optimum rake combining for each transmission antenna. In rake combining, maximum ratio combining (MRC) is typically carried out to maximize the signal-to-noise ratio (S/N) after combining, and the use of this maximum ratio combining is assumed in this case. Rake combining unit23is of a configuration for realizing the calculations for combining multipaths by means of multipliers and adders. The signal vector z following rake combining is represented by Equation (12).

Here, h′m, n, lis equal to hm, n, l/σ2m, n, l, and σ2m, n, lis the noise interference power. A correlation of noise n′ is generated by means of H′Hconversion, and direct use without alteration of the signal vector z following rake combining to perform the MLD process results in degradation of the signal separation characteristic. In the present embodiment, however, the noise of the signal vector z following rake combining is whitened.

Whitening filter calculation unit24calculates the coefficient of the whitening filter (linear filter) for whitening the noise of the signal vector z that follows rake combining, and provides this coefficient to whitening filter25. In this case, D is the whitening filter matrix. Whitening filter matrix D should satisfy Equation (14).
DHRinD=I(Rin=H′HH′)  (14)

If the characteristic value matrix of correlation matrix Rinisand the characteristic vector matrix is U, correlation matrix Rincan be decomposed as Rin=UUH, and whitening filter matrix D can be found by Equation (15).
D=UΛ−1/2(15)

Whitening filter25uses whitening filter matrix D that has been obtained by whitening filter calculation unit24to filter signal vector z that follows rake combining, finds signal vector z′ in which noise has been whitened as shown in Equation (16), and sends this result to MLD unit26.
z′=DHz(16)

MLD unit26uses transmission channel matrix H that is obtained by transmission channel estimation unit22and whitening filter matrix D that is obtained by whitening filter calculation unit24to generate reception replicas for the signals from all transmission antennas, calculates the error signal between signal vector z′ from whitening filter25and the reception replicas, and selects the most likely transmission antenna signal.

FIG. 8is a block diagram showing the configuration of MLD unit26. Referring toFIG. 8, MLD unit26includes: transmission symbol candidate generation unit31, reception replica generation unit32, error signal calculation unit33, and bit likelihood calculation unit34.

Transmission symbol candidate generation unit31generates transmission symbol vector s, which is the combination of all transmission antenna symbols, and sends this result to reception replica generation unit32.

Based on Equation (12) and Equation (16), signal vector z′ in which noise has been whitened can be represented as shown in Equation (17).
z′=DHH′H(Hs+n)=DHH′HHs+DHH′Hn=DH′HHs+n′(17)

Noise n′ is whitened and therefore uncorrelated.

Reception replica generation unit32generates all reception replicas {tilde over (r)}=DHH′HHs based on transmission symbol vector s from transmission symbol candidate generation unit31and transmission channel matrix H and whitening filter matrix D.

As shown in Equation (18), error signal calculation unit33compares signal zm′ in which noise has been whitened with reception replicas {tilde over (r)}mfrom reception replica generation unit32over the plurality of transmission antennas to find the error signals, and adds each of the error signals as shown in Equation (19) to calculate the final error signal.

Bit likelihood calculation unit34receives the error signalscorresponding to all transmission antenna symbols s and calculates the likelihood for each bit that is transmitted from each transmission antenna. At this time, bit likelihood calculation unit34applies bit likelihood to an error correction decoder (for example, a turbo decoder) (not shown in the figures) and restores the information bit sequence. Methods for calculating the bit likelihood include, for example, a method of calculation based on the difference between the minimum error signal of the symbol in which the target bit is +1 and the minimum error signal of the symbol in which the target bit is −1 as described in Document 2. Bit likelihood calculation unit34of the present embodiment can also apply any other known calculation method.

As described in the foregoing explanation, according to the present embodiment: rake combining unit23uses transmission channel matrix H to perform rake combining of the received signals of the plurality of reception antennas141-14Nfor each of transmission antennas131-13Mand thus performs combining that takes into consideration the influence of the differences in levels between multipaths; whitening filter25uses whitening filter matrix D to filter the signals following rake combining by rake combining unit23and thus whitens the noise; and MLD unit26uses transmission channel matrix H and whitening filter matrix D to determine the most likely transmission antenna signal for each transmission antenna from signals in which noise has been whitened by whitening filter25.

As a result, carrying out optimum combining of multipath signals for each transmission antenna signal to confer small weights to multipath signals having low levels allows a superior signal separation characteristic to be obtained regardless of differences in levels between multipath signals. In addition, whitening noise after combining of multipath signals to perform the MLD process reduces the influence of noise and enables the realization of a superior signal separation characteristic.

Although a case has been described as a preferable example in which the noise of signal vector z was whitened by whitening filter25and the MLD process then carried out, the present invention is not limited to this form. The receiver can also operate by carrying out the MLD process without whitening noise by means of whitening filter25. In such a case, whitening filter calculation unit24and whitening filter25inFIG. 7are unnecessary, and whitening filter matrix D need not be applied as input to MLD unit26.

Explanation next regards the second embodiment of the present invention with reference to the accompanying figures. The radio communication system according to the second embodiment is of the same configuration as in the first embodiment shown inFIG. 6.

Reception antennas141-14N; despreaders2111-211L, . . . ,21N1-21NL; transmission channel estimation unit22; rake combining unit23; whitening filter calculation unit24; and whitening filter25are all the same, as in the first embodiment shown inFIG. 7.

Whitening filter calculation unit24finds whitening filter matrix D shown in the above-described Equation (15). Whitening filter matrix D is provided to whitening filter25and to QR decomposition unit41.

In addition, whitening filter25uses whitening filter matrix D to filter signal vector z that follows rake combining by rake combining unit23, finds signal vector z′ in which noise is whitened as shown in Equation (16), and sends the result to QHconversion unit42.

QR decomposition unit41uses transmission channel matrix H from transmission channel estimation unit22and whitening filter matrix D from whitening filter calculation unit24to decompose matrix DHHHH into the product of the Q matrix and R matrix as shown in Equation (20); and sends the Q matrix to QHconversion unit42and the R matrix to reduced-calculation-load MLD unit43.

Here, the Q matrix is a unitary matrix of M rows and M columns. In addition, each column vector of the Q matrix is orthogonal (QHQ=I), and the norm is 1. The R matrix is an upper triangular matrix of M rows and M columns.

QHconversion unit42multiplies QHby signal z′ in which noise has been whitened and thus converts z′ to an orthogonal coordinate system represented by Q. QHconversion unit42is of a configuration for realizing calculation by multiplying QHby multipliers and adders. Signal vector z″ that follows conversion of coordinates is represented by Equation (21).

Here, noise n″ is noise n′ projected onto an orthogonal coordinate system represented by Q, and is therefore uncorrelated with the same power as n′.

Reduced-calculation-load MLD unit43uses the R matrix from QR decomposition unit41to generate reception replicas that correspond to all transmission antenna signals, calculates the error signals between the reception replicas and the signal vector z″ that follows coordinate conversion; and after reducing symbol candidates, selects the most likely transmission antenna signal.

FIG. 10is a block diagram showing the configuration of reduced-calculation-load MLD unit43. Referring toFIG. 10, reduced-calculation-load MLD unit43includes: transmission symbol candidate generation unit51, reception replica generation unit52, error signal calculation/symbol candidate reduction unit53, and bit likelihood calculation unit54.

As with transmission symbol candidate generation unit31shown inFIG. 8, transmission symbol candidate generation unit51generates transmission symbol vector s, which is the combination of all transmission antenna symbols and sends transmission signal vector s to reception replica generation unit52.

Reception replica generation unit52generates all reception replicas {tilde over (r)}=Rs based on transmission signal vector s from transmission symbol candidate generation unit51and matrix R from QR decomposition unit41, and sends the result to error signal calculation/symbol candidate reduction unit53.

Error signal calculation/symbol candidate reduction unit53reduces symbol candidates while finding error signalsmbased on reception replicas {tilde over (r)}mand signal vector zm″ that follows coordinate conversion by QHconversion unit96over a plurality of stages for the plurality of transmission antennas.

As one example, reduction of symbol candidates is carried out successively starting from the largest transmission antenna number.

In each stage of cutting back symbol candidates, error signal calculation/symbol candidate reduction unit53compares reception replicas {tilde over (r)}mwith signal vector zm″ as shown in Equation (22), and finds error signalmas shown in Equation (23). Error signal calculation/symbol candidate reduction unit53reduces symbol candidates by selecting only a prescribed number from among symbol candidates for which this value is low.

Bit likelihood calculation unit54calculates the likelihood for each bit that is transmitted from each transmission antenna based on the error signalsthat correspond to all transmission antenna symbols s that are finally reduced.

As an example, a calculation load reduction algorithm based on QR decomposition is used in reduced-calculation-load MLD unit43of the present embodiment, but the present invention is not limited to this form. Any other known calculation load reduction algorithm may be applied in the present invention.

As described hereinabove, as in the first embodiment, the present embodiment allows a superior signal separation characteristic to be obtained despite differences in the level of multipath signals in a receiver that carries out a reduced-calculation-load MLD process.

The present embodiment also allows operation of a receiver in which the whitening of noise is not carried out by a whitening filter.

Explanation next regards the third embodiment of the present invention with reference to the accompanying figures. In the above-described first embodiment, an example was presented in which rake combining (MRC) was used as the method of combining multipath signals, but other methods of combining multipaths exist.

In CDMA, if the spreading rate is sufficiently great and if the code multiplexing number is small for the spreading rate, rake combining can allow multipath signals to be combined while adequately suppressing multipath interference by means of dispreading. However, if the code multiplexing number is large for the spreading rate, rake combining results in a serious degradation of characteristics due to multipath interference.

Minimum Means Square Error (MMSE) and Zero Forcing (ZF) are combining methods (equalizing methods) that take into consideration the suppression of multipath interference. In the third embodiment, an example is shown in which MMSE or ZF is used as the method of combining multipaths.

The radio communication system of the third embodiment is of the same configuration as in the first embodiment shown inFIG. 6.

FIG. 11is a block diagram showing the configuration of the receiver according to the third embodiment. Referring toFIG. 11, receiver12according to the third embodiment includes: reception antennas141-14N, transmission channel estimation unit22, equalizing weight calculation unit61, MMSE/ZF equalizer62, despreaders631-63M, whitening filter calculation unit64, whitening filter65, and MLD unit66.

Transmission channel estimation unit22takes as input the signals that are received at reception antennas141-14Nand uses a known pilot signal that is contained in these received signals to estimate the transmission channel estimation values between the transmission and the reception antennas for each path.

Equalizing weight calculation unit61uses the transmission channel estimation values that are obtained in transmission channel estimation unit22to calculate weights of an equalizing filter (linear filter) by the MMSE or ZF standards and provides these weights to MMSE/ZF equalizer62. Various algorithms exist for calculating weights, and equalizing weight calculation unit61of the present embodiment may use any already existing weight calculation algorithm.

MMSE/ZF equalizer62uses the weights provided from equalizing weight calculation unit61to perform equalizing filtering of the received signals of reception antennas141-14Nand thus combine multipath signals while suppressing multipath interference for each transmission antenna. Each of the signals for each transmission antenna that have undergone multipath combining by MMSE/ZF equalizer62are provided to respective despreaders631-63M. When MMSE is used, combining is performed such that the ratio of signal power to noise interference power (S/(N+I)) following combination is maximized. In contrast, when the ZF method is used, combining is carried out such that the ratio of signal power to interference power (S/I) after combining is maximized.

Despreaders631-63Mdespread the signals for each transmission antenna from MMSE/ZF equalizer62.

If the equalizing filter matrix that is found in equalizing weight calculation unit61and used in MMSE/ZF equalizer62is W, the signal vector z that is supplied by despreaders631-63Mcan be represented by Equation (24).
z=WHy=WH(Hs+n)=WHHs+WHn=WHHs+n′(24)

Here, the size of transmission channel matrix H is matched to equalizing filter matrix W, and transmission channel matrix H is therefore defined by all despreading timing that corresponds to the equalizing window. In this case, “0” is placed in timings in which paths do not exist.

In addition, because correlation for noise n′ is generated by WHconversion, using signal vector z that follows despreading without alteration to carry out the MLD process, results in the degradation of the signal separation characteristic. In response to this problem, the present embodiment implements whitening of the noise of the signal vector z that follows despreading.

Whitening filter calculation unit64calculates the coefficient of a whitening filter (linear filter) that whitens the noise of signal vector z that follows despreading, and provides this coefficient to whitening filter65and MLD unit66. The operation of whitening filter calculation unit64is the same as that of whitening filter calculation unit24of the receiver shown inFIGS. 7 and 9. However, whitening filter calculation unit66of the present embodiment differs from whitening filter calculation unit24ofFIGS. 7 and 9in that correlation matrix Rinis set to Rin=WHW.

Whitening filter65uses whitening filter matrix D that is obtained in whitening filter calculation unit24to filter signal vector z that follows despreading, finds signal vector z′ in which noise has been whitened, and sends signal vector z′ to MLD unit66. The operation of whitening filter65is the same as that of whitening filter25of the receiver shown inFIGS. 7 and 9.

MLD unit66uses transmission channel matrix H, equalizing filter matrix W, and whitening filter matrix D to generate reception replicas for the signals from all transmission antennas, calculates the error signals between the reception replicas and signal vector z′ from whitening filter65, and selects the most likely transmission antenna signal.

The operation of MLD unit66is similar to that of MLD unit26of the receiver shown inFIG. 7. However, the operation differs from that of MLD unit26ofFIG. 7in that reception replicas {tilde over (r)} are found by means of Equation (25).
{tilde over (r)}=DHWHHs(25)

Although a time domain process was assumed in the preceding description of equalizing weight calculation unit61and MMSE/ZF equalizer62of the present embodiment, the present invention is not limited to this form, and a frequency domain process may also be applied.

In the present embodiment, moreover, a case was described of carrying out a normal MLD process, but a reduced-calculation-load MLD may also be applied as in the second embodiment.

As described in the foregoing explanation, according to the present embodiment, equalizing weight calculation unit61uses transmission channel estimation value H to calculate weights of an MMSE or ZF equalizing filter; MMSE/ZF equalizer62uses these weights to implement an equalizing filter process for the received signals of reception antennas141-14Nand thus carries out combining of multipath signals that takes into consideration the influence of the differences in levels between multipaths while suppressing multipath interference; despreaders631-63Mdespread signals for each transmission antenna; whitening filter65whitens the noise of signals after despreading; and MLD unit66uses transmission channel matrix H, whitening filter matrix D, and equalizing filter matrix W to determine the most likely transmission antenna signal for each transmission antenna from signals in which noise has been whitened. As a result, in a receiver that uses the combining method of the MMSE or ZF method that takes into consideration the suppression of multipath interference, a superior signal separation characteristic can be obtained regardless of differences in levels of multipath signals.

Further, the receiver in the present embodiment can also operate without carrying out whitening of noise by means of a whitening filter.

Explanation next regards the fourth embodiment with reference to the accompanying figures.

In the above-described third embodiment, as an example in which the suppression of multipath interference is considered, a configuration was shown in which the MLD process was performed after implementing equalizing filtering based on the MMSE or ZF method for multipath interference. However, various other examples can also be considered as configurations that take into consideration the suppression of multipath interference. In the fourth embodiment, an example of a configuration is shown in which the results of first demodulating CDMA received signals (primary demodulation) are used to reproduce multipath signals, and signals obtained by eliminating the multipath interference from these multipath signals are then used to perform rake combining and the MLD process.

The radio communication system of the fourth embodiment is of the same configuration as in the first embodiment shown inFIG. 6.

FIG. 12is a block diagram showing the configuration of the receiver according to the fourth embodiment. Referring toFIG. 12, the receiver of the fourth embodiment includes: reception antennas141-14N; multipath signal reproduction unit71, multipath interference elimination units721-72N, despreaders2111-211L, . . . ,21N1-21NL; transmission channel estimation unit22; rake combining unit23; whitening filter calculation unit24; whitening filter25; and MLD unit26. Despreaders2111-211L, . . . ,21N1-21NL; transmission channel estimation unit22; rake combining unit23; whitening filter calculation unit24; whitening filter25; and MLD unit26are the same as in the first embodiment shown inFIG. 7.

Multipath signal reproduction unit71takes as input the received signals of reception antennas141-14N, performs primary demodulation of the transmitted signals, uses the results to reproduce the multipath signals for each reception antenna and moreover for each multipath signal, and then provides the results to multipath interference elimination units721-72N.

Various methods can be considered for the primary demodulation, and any demodulation method may be applied in the primary demodulation by multipath signal reproduction unit71of the present embodiment. As an example, there are primary demodulation methods that use MMSE or MLD as described in Document 2.

The demodulated signals may even be subjected to error correction decoding, whereby the reliability of the multipath reproduction signals can be further improved.

A method may further be considered in which configurations composed of multipath signal reproduction and interference elimination are connected in a multiplicity of stages to repeat the process and thus raise the reliability of the multipath reproduction signals.

Multipath interference elimination units721-72Neliminate multipath interference for each of reception antennas141-14N, and moreover, for each path.

The received signal rn, l(t) after elimination of multipath interference for path l of reception antenna n is found by subtracting multipath interference other than that of path l from received signal rn(t), as shown in

Here, τrindicates the timing of path l, and ln, l(t−τr) indicates the multipath signal of all transmission antennas of path l.

Despreaders2111-21NLtake as input the received signals from which multipath interference has been eliminated by multipath interference elimination units721-72Nand carry out despreading for each reception antenna and for each path.

Transmission channel estimation unit22takes as input the received signals from which the multipath interference has been cancelled and uses the known pilot that is contained in the received signals to estimate the transmission channel estimation values between the transmission and reception antennas for each path. In the present embodiment, transmission channel estimation unit22may also directly use the received signals of reception antennas141-14Nwithout using signals from which the multipath interference has been eliminated, but in such a case, the accuracy of the transmission channel estimation is degraded.

Rake combining unit23uses the transmission channel estimation values produced by transmission channel estimation unit22to perform optimum rake combining of the multipath signals for each transmission antenna.

Whitening filter calculation unit24calculates coefficients for the whitening filter (linear filter) based on the transmission channel estimation values.

Whitening filter25filters signal vector z that has undergone rake combining by rake combining unit23to find signal z′ in which noise has been whitened.

MLD unit26uses transmission channel matrix H and whitening filter matrix D to generate reception replicas for all transmission antenna signals, calculates the error signals between the reception replicas and each signal of signal vector z, and selects the most likely transmission antenna signal.

As described in the foregoing explanation, in a configuration according to the present embodiment, in which the results of first subjecting CDMA received signals to demodulation are used to reproduce multipath signals and signals, in which multipath interference has been cancelled from these multipath signals, then used to implement rake combining and an MLD process, the remaining multipath interference following cancellation of the interference that affects each path differs and an excellent signal separation characteristic can therefore be obtained regardless of differences in multipath signal levels.

Finally, configurations have been shown in the above-described embodiments in which linear combining of multipaths is carried out by means of rake combining, MMSE or ZF, whereby a superior signal separation characteristic can be obtained regardless of differences in levels between multipaths. However, the present invention is not limited to these rake combining and MMSE and ZF methods, and any configuration of multipath linear combining can be applied.