High-speed data transmissions are required for radio systems of the next-generation mobile communication. As a technology for realizing this high-speed data transmission, MIMO (Multiple Input Multiple Output) multiplexing has drawn attention, where data signals are transmitted from a plurality of transmission antennas using the same frequency, and where the data signals are demodulated using a plurality of reception antennas.
FIG. 1 is a diagram showing an exemplary configuration of a conventional MIMO transmission/reception system which employs MIMO multiplexing.
The MIMO transmission/reception system shown in FIG. 1 comprises transmitting apparatus 201 and transmission antennas 202-1-202-M on the transmission side, and reception antennas 203-1-203-N and receiving apparatus 204 on the reception side, where M is the number of transmission antennas (M is an integer equal to or larger than two), and N is the number of reception antennas (N is an integer equal to or larger than two). Different data signals are transmitted from a plurality of transmission antennas 202-1-202-M using the same frequency, and the data signals are received using a plurality of reception antennas 203-1-203-N, whereby a high-speed data transmission is achieved in proportion to the number of transmission antennas without increasing the transmission bandwidth. In receiving apparatus 204, the respective data signals transmitted from a plurality of transmission antennas 202-1-202-M must be demodulated (separated) from the signals received at a plurality of reception antennas 203-1-203-N.
While a variety of schemes have been proposed for the signal separation of MIMO, the simplest scheme, for example involves restraining interference from transmission antennas other than the transmission antenna subjected to demodulation, using a linear filter based on minimum mean square error (MMSE). However, the reception characteristics cannot be said to be good when this scheme is used. To improve the reception characteristics using this scheme, a scheme has been proposed for combining an MMSE filter with transmission antenna interference removal.
Also, a maximum likelihood detection (MLD) has been devised for generating replicas of all transmission antenna signals and selecting the most likely transmission antenna signal. This detection method, though exhibiting good reception characteristics, causes an exponential increase in processing amount in accordance with an increase in the number of transmission antennas and the modulation multiple-value.
On the other hand, in an uplink radio scheme of the next-generation mobile communication, a terminal must realize a high transmission power efficiency in order to expand a communication area, and a single-carrier (SC) scheme having a low peak to average power ratio (PAPR) is regarded as predominant. Also, when MIMO multiplexing is performed in the SC scheme, multipath interference constitutes a problem. Also, when an MMSE filter is used to separate signals of MIMO, an MMSE filter (two-dimensional MMSE filter) is required to simultaneously separate the MIMO signals and to restrain the multipath interference, i.e., in a spatial direction and a time direction. Then, a scheme which combines the two-dimensional MMSE filter with transmission antenna interference removal excels in characteristics, and is regarded as predominant.
FIG. 2 is a diagram showing an exemplary configuration of a conventional MIMO receiving apparatus.
In the MIMO receiving apparatus shown in FIG. 2, an increase in the processing amount is limited by equalizing two-dimensional MMSE and by removing transmission antenna interference through signal processing in a frequency domain. Also, a method of improving the characteristics by repeating the two-dimensional frequency domain equalization and reception processing of antenna interference removal has been devised (for example, see the article “Throughput Characteristic of SC-MIMO Multiplexing using Two-Dimensional MMSE weighting in Frequency Domain Repeating PIC,” Akinori Nakajima, Humiyuki Adachi, IEICE Technical Report, RCS2005-88, pp. 19-24, October 2005).
The MIMO receiving apparatus shown in FIG. 2 is a MIMO receiving apparatus which receives single-carrier MIMO signals, transmitted from M transmission antennas (M is an integer equal to or larger than two), at N reception antennas (N is an integer equal to or larger than two), and which separates the MIMO signals through processing in a frequency domain. This MIMO receiving apparatus comprises reception antennas 101-1-101-N, cyclic prefix (CP) removing parts 102-1-102-N, discrete Fourier transform (DFT) parts 103-1-103-N, reception filters 104-1-104-N, subtracting part 105, channel estimator 106, weight calculating part 107, two-dimensional frequency domain equalizer 108, inverse discrete Fourier transform (IDFT) parts 109-1-109-M, bit likelihood calculating parts 110-1-110-M, symbol replica generators 111-1-111-M, DFT parts 112-1-112-M, and antenna interference replica generators 113-1-113-M.
FIG. 3 is a diagram showing an exemplary radio frame format when frequency domain equalization is used.
The frame shown in FIG. 3 shows a radio frame signal transmitted from a certain transmission antenna. The radio frame signal comprises a plurality of blocks of pilot signals or data signals, and in the example shown in FIG. 3, there is a pilot signal block at the top, followed by a plurality of data blocks in sequence. CP is added to the top of each block in order to avoid multipath interference from the preceding block in the event of DFT processing. The CP is a signal generated by copying the last data in each block to the forefront. In MIMO, it is necessary to estimate a channel gain between a transmission antenna and a reception antenna, so that the pilot signals of respective transmission antennas are preferably orthogonal to one another. As a method of multiplexing pilot signals of the respective transmission antennas, frequency multiplexing using an IFDM (Interleaved Frequency Division Multiplexing), code multiplexing using a cyclically shifted CAZAC (Constant Amplitude Zero Auto-Correlation) code have been devised.
Also, reception antenna 101-1-101-N shown in FIG. 2 receives a single-carrier MIMO signal. CP removing part 102-1-102-N receives each reception antenna signal, and removes part of the signal corresponding to the CP at a common timing. DFT part 103-1-103-N receives each reception antenna signal, from which the CP has been removed, and performs DFT at point:NDFT1(NDFT1 is an integer equal to or larger than two)  [Expression 1]to convert the reception signal to the frequency domain. Reception filter 104-1-104-N filters the reception signal in the frequency domain, and performs waveform shaping, noise suppression, user separation, and the like. Generally, raised cosine roll-off filters are used for reception filters 104-1-104-N. In the configuration shown in FIG. 2, reception filters 104-1-104-N perform signal processing in the frequency domain, but alternatively can perform signal processing in a time domain prior to DFT parts 103-1-103-N. Subtracting part 105 subtracts other transmission antenna interference replica while leaving a transmission antenna signal subjected to demodulation.
FIG. 4 is a diagram showing an exemplary configuration of subtracting part 105 for a DFT signal of reception antenna n.
Subtracting part 105 shown in FIG. 4 comprises replica combining parts 121-1-121-M, and subtractors 122-1-122-M. Replica combining parts 121-1-121-M combine transmission antenna interference replicas except for a transmission antenna signal subjected to demodulation. Subtractors 122-1-122-M subtract outputs of replica combining parts 121-1-121-M from the DFT signal of reception antenna n.
Assuming that reception signals at sub-carriersk(1≦k≦NDFT1)  [Expression 2]after DFT are given by:R(k)(R(k) is an N-row column vector)  [Expression 3]and i-th repetition interference replica of transmission antenna m is given by:Îm(i)(k)(Îm(i)(k) is an N-row column vector),  [Expression 4]
an equalizing signal for transmission antenna m after removal of the i-th repetition interference:Rm(i)(k)(Rm(i)(k) is an N-row column vector),  [Expression 5]is given by the following equation:
                    [                  Expression          ⁢                                          ⁢          6                ]                                                                                  R            m                          (              i              )                                ⁡                      (            k            )                          =                              R            ⁡                          (              k              )                                -                                    ∑                                                                    m                    ′                                    =                  1                                                                      m                    ′                                    ≠                  m                                            M                        ⁢                                                            I                  ^                                                  m                  ′                                                  (                  i                  )                                            ⁡                              (                k                )                                                                        (                  Equation          ⁢                                          ⁢          1                )            
Here, interference removal is not performed in the first reception processing (zero-th repetition), and a reception signal is used as it is. Specifically,[Expression 7]Rm(0)(k)=R(k)  (Equation 2)is established.
Channel estimating part 106 estimates a channel gain between a transmission antenna and a reception antenna in the frequency domain using a pilot signal inserted into each transmission antenna.
FIG. 5 is a diagram showing an exemplary configuration of channel estimating part 106 for finding a channel gain for transmission antenna m in reception antenna n.
Channel estimating part 106 shown in FIG. 5 comprises DFT part 131, transmission/reception filter 132, reference signal generator 133, correlation processing part 134, and noise suppressing part 135. DFT part 131 discrete Fourier transforms a pilot code of transmission antenna m for conversion to a frequency domain signal. Transmission/reception filter 132 passes the frequency domain signal of the pilot code through the transmission/reception filter 132. Reference signal generator 133 calculates a pilot reference signal, using the output of a transmission/reception filter 132, for use in correlation processing with a reception pilot signal. Correlation processing part 134 estimates a channel gain through correlation processing of the pilot reception signal in the frequency domain and the pilot reference signal. Noise suppressing part 135 suppresses noise of the channel gain estimated by correlation processing part 134 to improve the signal power-to-noise power ratio (S/N ratio) of a channel estimate which is the estimated channel gain. A method of averaging adjacent sub-carriers, a method of once converting a channel estimate to the time domain through IDFT and returning the same again to the frequency domain through DFT after removing a noise path, and the like have been contemplated for noise suppressing part 135. Channel estimating part 106 having the configuration shown in FIG. 5 performs signal processing in the frequency domain, but can also perform signal processing in advance in the time domain prior to DFT parts 103-1-103-N.
Weight calculating part 107 calculates a weight for two-dimensional frequency domain equalization using the channel estimate between the transmission antenna and the reception antenna. Generally, an MMSE algorithm is used for weight calculating part 107. An i-th repetition MMSE weight for transmission antenna m:Wm(i)(k)(Wm(i)(k) is an N-column row vector)  [Expression 8]is calculated using a channel estimate matrix:Ĥ(k)(Ĥ(k) is an N-row, M-column matrix)  [Expression 9]and noise power:σ2  [Equation 10]by the following equation:[Expression 11]Wm(i)(k)=ĤmH(k)[Ĥ(k)Gm(i)ĤH(k)+σ2I]−1  (Equation 3)where[Expression 12]Ĥ(k)=[Ĥ1(k), . . . ,Ĥm(k), . . . ,ĤM(k)]  (Equation 4)Also,Ĥm(k)(Ĥm(k) is an N-row column vector)  [Expression 13]is a channel estimate between transmission antenna m and the reception antenna.Gm(i)  [Equation 14]is an i-th repetition interference removal consideration matrix of transmission antenna m, and is given by the following equation:[Expression 15]Gm(i)=diag[gm,1(i), . . . ,gm,m′(i), . . . ,gm,M(i)]  (Equation 5)Here,gm,m′(i)  [Expression 16]is calculated using, for example, the average power of an i-th repetition soft decision symbol replica in the time domain of transmission antenna m:{circumflex over (d)}m(i)(t)  [Equation 17]by the following equation:
                    [                  Expression          ⁢                                          ⁢          18                ]                                                                      g                      m            ,                          m              ′                                            (            i            )                          =                  {                                                                      1                  -                                                            1                                              N                        SYMB                                                              ⁢                                                                  ∑                                                  t                          =                          1                                                                          N                          SYMB                                                                    ⁢                                                                                                                                                                                            d                                ^                                                                                            m                                ′                                                                                            (                                                                  i                                  -                                  1                                                                )                                                                                      ⁡                                                          (                              t                              )                                                                                                                                2                                                                                                                                          (                                                            m                      ′                                        ≠                    m                                    )                                                                                    1                                                              (                                                            m                      ′                                        =                    m                                    )                                                                                        (                  Equation          ⁢                                          ⁢          6                )            Here,NSYMB  [Expression 19]is the number of symbols in a data block.
Two-dimensional frequency domain equalizing part 108 receives the two-dimensional equalization weight calculated in weight calculating part 107 and the output of subtracting part 105, and multiplies them on a sub-carrier by sub-carrier basis to simultaneously separate MIMO signals and suppress multipath interference in the frequency domain. Assuming that the weight calculated in weight calculating part 107 is given by:Wm(i)(k)  [Expression 20]and the output of subtracting part 105 is given by:Rm(i)(k)  [Expression 21]an equalization signal of transmission antenna m two-dimensionally equalized in two-dimensional frequency domain equalizing part 108:{tilde over (R)}m(i)(k)  [Expression 22]is given by the following equation:[Expression 23]{tilde over (R)}m(i)(k)=Wm(i)(k)Rm(i)(k)  (Equation 7)
IDFT part 109-1-109-M which receives an equalization signal of each transmission antenna which is the output of two-dimensional frequency domain equalizing part 108, performs IDFT at point:NIDFT(NIDFT is an integer equal to or larger than two)  [Expression 24]to convert the equalization signal to the time domain. Outputs of an i-th repetition (i≧1) of IDFT parts 109-1-109-M constitute a final demodulated signal.
Bit likelihood calculating part 110-1-110-M calculates a likelihood for each bit transmitted from the equalization signal of each transmission antenna. Bit likelihood calculating parts 110-1-110-M also include hard decision of bits.
Symbol replica generating part 111-1-111-M generates a symbol replica of transmission antenna m from the likelihood of a bit transmitted from each transmission antenna. Used in symbol replica generating parts 111-1-111-M are a method of generating a hard decision symbol replica, a method of generating a hard decision symbol replica and multiplying it by a predetermined replica weighting coefficient (constant equal to or less than one), a method of generating a soft decision symbol replica, and the like. In particular, the method of generating a soft decision symbol replica exhibits good characteristics.
DFT part 112-1-112-M receives a symbol replica of each transmission antenna generated in symbol replica generating part 111-1-111-M, and performs DFT at point:NDFT2(NDFT2 is an integer equal to or larger than two)  [Expression 25]to convert the symbol replica to the frequency domain.
Antenna interference replica generating part 113-1-113-M generates a transmission antenna interference replica using a symbol replica signal in the frequency domain of each transmission antenna and a channel estimate. Assuming that a symbol replica signal in the frequency domain of transmission antenna m is given by:{circumflex over (D)}m(i)(k)  [Expression 26]and the channel estimate is given by:Ĥm(k)  [Expression 27]an i-th repetition interference replica of transmission antenna m:Îm(i)(k)  [Expression 28]is given by the following equation:[Expression 29]Îm(i)(k)=Ĥm(k){circumflex over (D)}m(i−1)(k)  (Equation 8)
As described above, the conventional MIMO receiving apparatus is characterized in that the processing amount is largely reduced as compared with signal processing in the time domain, and it is not affected by a timing error when the two-dimensional frequency domain equalization and antenna interference removal in the frequency domain, including the channel estimation, are performed.
However, the conventional MIMO receiving apparatus has problems in that repetitive reception processing for two-dimensional frequency domain equalization and antenna interference removal still continues linear processing in the time direction, a weight after convergence of the two-dimensional frequency domain equalization is an MMSE weight for a transmission antenna signal subjected to demodulation, and noise emphasis due to the equalization remains even if repetitions are made. Accordingly, the conventional MIMO receiving apparatus is inferior to the reception characteristics of MLD which has excellent characteristics.