The present invention relates to a frequency-domain-equalization method and apparatus for a single-carrier receiver, and more particularly, to a frequency-domain-equalization method and apparatus for a receiver in a single-carrier-transmission system that compensates for inter-symbol interference.
A single-carrier (SC: Single Carrier) transmission system is a transmission method that has been used for a long time (see H. Sari, G. Karam and I. Jeanclaude, “Frequency-Domain Equalization of Mobile Radio and Terrestrial Broadcast Channels”, Proc. Globecom 1994, San Francisco, Nov.-Dec. 1994, pp. 1-5), and with this system, data symbols are transmitted as a fixed-symbol-rate serial stream having a pulse for which the amplitude and/or phase has been modulated. A linear-frequency-domain equalizer (FDE: Frequency domain equalizer) performs reception filtering in the frequency domain in order to minimize the inter-symbol interference. That function is the same as that of a time-domain equalizer. However, from the viewpoint of difficulty in calculation, using a linear-frequency-domain equalizer that performs equalization for each data block is easier for a channel having a severe delay spread. In other words, since a linear-frequency-domain equalizer performs processing for each block, the computation load in a poor transmission path environment is less than in the case of a time-domain equalizer. In order for a frequency-domain equalizer, which performs Fourier transformation and inverse Fourier transformation, to operate with sufficient performance, there must be a guard interval between each data block. However, in a multi-path propagation environment having a delay time that is longer than the guard interval, it is not possible to remove all of the inter-symbol interference, and the transmission characteristics become poor.
FIG. 9 is a block diagram of a single-carrier-transmission system. In a single-carrier transmitter 10, a channel encoder (encoding unit) 11 encodes the data and pilots using convolution code or turbo code, for example, and a modulation unit 12 modulates the encoded data using QPSK and forms a block having a length of N modulated symbols. As shown in FIG. 10, a guard-interval-insertion unit 13 copies the end portion of the N-symbol transmission block onto the starting portion of each block as a cyclic prefix (guard interval). A digital-to-analog converter 14 converts the signal that is output from the guard-interval-insertion unit 13 to an analog signal, a radio-transmitting unit 15 performs up-conversion of the baseband signal to a radio frequency, and then amplifies the signal and transmits it from an antenna ATS. The signal that is transmitted from the antenna ATS is propagated along a multi-path propagation path (multi-path fading channel) 20 and received by a single-carrier receiver 30.
The length of the cyclic prefix that is inserted by the guard-interval-insertion unit 13 must be longer than the maximum delay spread so that no inter-symbol interference (ISI) is received. The cyclic prefix that is placed at the start of each block has mainly: (1) a function for removing distortion that is caused by inter-symbol interference from the previous block, and (2) a function for making it possible to see or detect the received block in cycle N.
In a single-carrier receiver 30, a radio-receiving unit 31 filters the signal that is received from the antenna ATR, and together with removing the unneeded frequency component, converts the frequency of the radio signal to a baseband frequency, an analog-to-digital converter 32 converts that baseband signal to a digital signal, and a guard-interval-removal unit 33 removes the guard intervals and inputs the signal to an S/P conversion unit 34 that constitutes a single-carrier frequency-domain equalizer (SC-FDE). The single-carrier frequency-domain equalizer comprises an S/P conversion unit 34, Fourier-transformation unit 35, channel-estimation unit 36, channel-compensation unit 37, inverse-Fourier-transformation unit 38 and P/S conversion unit 39.
The S/P conversion unit 34 converts N number of time-sequence data from which the guard interval has been removed into parallel data, and inputs the result into an N-point Fourier-transformation unit (DFT or FFT, it will be the same below) 35. The N-point Fourier-transformation unit 35 performs N-point Fourier transformation of the N number of time-sequence data, and outputs N number of sub-carrier components. The channel-estimation unit 36 uses the pilot symbols that are periodically sent, and by a well-known method estimates the channel characteristics of the N number of sub-carriers, then the channel-compensation unit 37 multiplies the N number of sub-carrier components that were output from the Fourier-transformation unit 35 by channel-compensation coefficients to perform channel compensation. The N-point inverse-Fourier-transformation unit (IDFT or IFFT, it will be the same below) 38 performs N-point inverse-Fourier transformation of the N number of channel-compensated sub-carrier data, and outputs N number of time-sequence data, then the P/S conversion unit 39 converts the N number of time-sequence data in order to serial data and outputs the result. A demodulation unit 40 performs QPSK demodulation of the signal for which frequency-domain equalization has been performed, and a decoding unit 41 decodes the encoded data and outputs the decoded received data.
FIG. 11 is a block diagram of a single-carrier CDMA transmission system. In a single-carrier CDMA transmitter 50, channel encoders for each user (encoder units) 511 to 51j encode transmission data using convolution code or turbo code, for example, and modulation units 521 to 52j modulate the encoded data using QPSK, for example. Spreading units 531 to 53j multiply and spread the data sequence that is output from the modulation units by multiplying the data sequence with spreading code that is orthogonal for each user, and a combining unit 54 combines the spread data that is output from each spreading unit. When the spreading factor is taken to be SF, the spreading code is a code string comprising SF number of chips.
FIG. 12 is a drawing explaining the data format. One frame comprises Nfi number of data, where Np number of pilot data are time multiplexed in front of Nd number of user data, so that Nfi=Nd+Np. Each of the data is multiplied by SF number of spreading codes, so that (Nd+Np)×SF number of multiplication results are output from the spreading units per frame, and then combined by the combining unit 54. The pilot data are used by the receiving side for channel estimation. The baseband transmission signal of the kth user can be expressed as shown below.
                                          s            k                    ⁡                      (            t            )                          =                              ∑                          i              =              0                                      Nd              +              Np              -              1                                ⁢                                    ∑                              m                =                0                            SF                        ⁢                                                            d                  k                                ⁡                                  (                                      t                    -                                          iT                      s                                                        )                                            ·                                                c                  k                                ⁡                                  (                                      t                    -                                          mT                      c                                                        )                                                                                        (        1        )            Here, dk(t), ck(t), Ts and Tc represent the modulation signal, spreading-code sequence, symbol period and chip period, respectively.
As shown in FIG. 12, in order to obtain a transmission signal that is free of ISI, a guard-interval-insertion unit 55 inserts G number of guard intervals into N number of data for each multiplication result, to form one transmission symbol. Here, N is the FFT size of the frequency-domain equalizer (FDE) on the receiving side. By taking ‘q’ to be the number of transmission symbols for each frame results in the following equation.(Np+Nd)×SF/N=q Depending on the guard-interval configuration of either a cyclic prefix or zero insertion, the transmission-symbol indirect wave for which the delay time is less than the guard interval does not distort other transmission symbols.
A digital-to-analog converter (D/A) 56 converts the signal that is output from the guard-interval-insertion unit 55 to an analog signal, and a radio-transmission unit 57 performs up-conversion of the baseband signal to a radio frequency, then amplifies the signal and transmits it from an antenna ATS. The signal that is transmitted from the antenna ATS propagates over a multi-path propagation path (multi-path fading channel) 60, and it is received by a single-carrier CDMA receiver 70.
By taking the channel-path response of the kth user to be hk(τ; t), the received signal becomes as the following.
                              r          ⁡                      (            t            )                          =                                                            h                k                            ⁡                              (                                  τ                  ;                  t                                )                                      ⊗                                          ∑                                  k                  =                  0                                                  K                  -                  1                                            ⁢                                                s                  k                                ⁡                                  (                                      t                    -                                          τ                      k                                                        )                                                              +                      n            ⁡                          (              t              )                                                          (        2        )            Here, τk is the propagation delay of the kth user, K is the number of users, n(t) is AWGN, and {circle around (x)} is the convolution integral.
In the single-carrier CDMA receiver 70, a radio-reception unit 71 filters the signal that was received from the antenna ATR, and together with removing the unneeded frequency component, converts the radio signal to a baseband frequency, and an analog-to-digital converter 72 converts the baseband signal to a digital signal, a guard-interval-removal unit 73 removes the guard intervals and inputs the result to the S/P conversion unit 74 that constitutes a single-carrier-frequency-domain equalizer (SC-FDE). The single-carrier-frequency-domain equalizer (SC-FDE) comprises a S/P conversion unit 74, Fourier-transformation unit 75, channel-estimation unit 76, channel-compensation unit 77, inverse-Fourier-transformation unit 78 and P/S conversion unit 79.
The S/P conversion unit 74 converts the N number of time-sequence data from which the guard interval have been removed to parallel data, and inputs the result to an N-point Fourier-transformation unit 75. The N-point Fourier-transformation unit 75 performs N-point Fourier transformation of the N number of time-sequence data, and outputs N number of sub-carrier components. The channel-estimation unit 76 estimates the channel characteristics of the N number of sub carriers by a well-known method of using the transmission symbols of the pilots that are periodically sent, and the channel-compensation unit 77 multiplies the N number of sub-carrier components that are output from the Fourier-transformation unit by channel-compensation coefficients to perform channel compensation. The N-point inverse-Fourier-transformation unit 78 performs N-point inverse-Fourier transformation of the N number of the channel-compensated sub-carrier data and outputs N number of time-sequence data, and the P/S conversion unit 79 converts the N number of time-sequence data in order to serial data, and outputs the result. An inverse-spreading unit 80 multiplies the serially input data by inverse-spreading code (same code as the spreading code) to perform inverse spreading, a demodulation unit 81 performs QPSK demodulation of the signal for which frequency-domain equalization and inverse spreading has been performed, and a decoding unit 82 decodes the demodulated data, and outputs the decoded received data.
As described above, in the single-carrier CDMA receiver, after the guard intervals have been removed, N-point Fourier transformation is employed, and channel distortion is compensated using prior technology such as the ZF method or MMSE method in the frequency domain. After channel distortion in both the amplitude and phase has been compensated, N-point inverse-Fourier transformation is employed. Next, the data is multiplied by inverse-spreading code, and finally the symbols are demodulated and decoded.
The SC-FDE transmission method is robust technology for multi-path and channel distortion. However, since guard intervals lower the transmission efficiency, it cannot be performed for a long time. Therefore, in several cases the guard-interval length becomes shorter than the maximum propagation delay, and in that case, the guard intervals become ineffective against inter-symbol interference (ISI).
A receiving method has been proposed that uses both a frequency-domain equalizer (FDE) and time-domain equalizer (FDE) with the purpose of obtaining good reception characteristics even in environments where there is multi-fading disturbance such as in a mobile receiving environment (refer to JP 2003-51802 A).
Moreover, the inventors of this invention have also proposed a receiving method and receiver for an OFDM receiver or OFDM-CDMA receiver that are more effective against propagation delay longer than guard intervals (refer to JP 15-998924 A).
The receiving method disclosed in patent document 1 performs time-domain equalization after performing frequency-domain equalization, after which it performs Fourier transformation and demodulation. However, this receiving method is less effective against propagation delay which is longer than guard intervals.
The receiving method disclosed in patent document 2 is more effective against propagation delay which is longer than guard intervals, however, it is for use in OFDM communication or OFDM-OCDMA communication, and can not be applied for use in single-carrier communication.