This invention relates to a receiving apparatus in a communication system employing DMT modulation or filter bank modulation, etc. such as a communication system having several subchannels or a multicarrier communication system. More particularly, the invention relates to a receiving apparatus in a communication system that is based upon a maximum a posteriori probability estimation algorithm in which soft-decision target values that have been derived from a plurality of subchannels are employed to refine a soft-decision target value on a subchannel of interest.
Bit error rate (BER) in filter bank modulation, DMT modulation and FMT modulation, etc. multicarrier communication systems can be improved by utilizing a receive signal that contains distortion owing to inter-channel interference (ICI). ICI is produced by system malfunction or an unavoidable environment (e.g., loss of orthogonality between subchannels ascribable to frequency offset or the like) in a communication system, e.g., in OFDM-CDMA. ICI is caused by leakage of spectral energy and sometimes by leakage between subchannels referred to as crosstalk.
The main advantage of a turbo receiver in the present invention is that the behavior of ICI is handled as a zero-average Gaussian distribution probability variable (e.g., a Gaussian approximation used in non-patent reference K. Sathananthan and C. Tellambura, “Probability of error calculation of OFDM system with frequency offset”, IEEE Trans. Commun. Vol. 49, No. 11, Nov. 2001, pp. 1884-1888) and employs a finite-state discrete Markov process model. With such an ICI model, it appears that a simple Gaussian approximation is more realistic in view of the nature of ICI.
A turbo receiver according to the present invention is based upon a maximum a posteriori probability estimation algorithm. With this turbo receiver, information that has been derived from a plurality of subchannels after non-linear processing is employed to refine the estimated maximum a posteriori probability (soft-decision target value) of a subchannel of interest.
(a) Relationship between frequency offset and ICI
In a multicarrier communication system in which a band is split into a plurality of subbands, which are independent narrow bands, and transmit data is frequency-multiplexed subband by subband, sent and received, i.e., in a multicarrier communication system employing filter bank modulation, DMT modulation or FMT modulation, etc., selection of a filter set has traditionally been executed under the constraint that inter-symbol interference (ISI) and inter-channel interference (ICI) be completely eliminated.
In the case of an ideal transmission channel on which there is no Doppler shift, no offset frequency between transceivers and no occurrence of signal distortion, the above constraint assures error-free restoration of transmission symbols at the receiver. However, a frequency offset produced on each channel by inaccurate tuning of an oscillator or by a Doppler shift gives rise to a decline in BER ascribable to spectral leakage or ICI (non-patent reference K. Sathananthan and C. Tellambura, “Probability of error calculation of OFDM system with frequency offset”, IEEE Trans. Commun. Vol. 49, No. 11, Nov. 2001, pp. 1884-1888).
The only method of mitigating such a decline in BER is to make the frequency offset as small as possible, i.e., to maintain it within 1% of the subcarrier frequency interval. However, this method necessitates precise estimation of frequency offset. In addition, when a multicarrier signal mixed with noise is received, a problem is that the accuracy of the frequency offset estimation is lost when the noise level is high. Furthermore, this method does not operate correctly on a high-speed fading channel, i.e., on a high-speed fading channel on which the Doppler shift is not constant with respect to a transmission symbol and, moreover, varies with time.
Consider a subchannel ch0 of interest and first and second adjacent subchannels ch1, ch2 disposed above the subchannel of interest. FIGS. 1 and 2 illustrate the frequency responses of three subchannels in a case where the frequency offset is zero (FIG. 1) and in a case where the frequency offset is non-zero (FIG. 2). Signals of center frequencies f0, f1, f2 that correspond to first, second and third subchannels are indicated by vertical arrows in FIGS. 1 and 2. In FIGS. 1 and 2, subchannel ch0 indicates the channel of interest, and subchannel numbers ch1, ch2 indicate two subchannels placed above the channel of interest in terms of the frequency scale. If we let T represent the period of a DMT symbol, the frequency scale is normalized by a channel interval that is equal to 1/T. That is, one unit of the frequency scale is the channel interval. When the frequency offset (normalized by the channel interval) α is 0, as shown in FIG. 1, the transfer functions of the upper subchannels ch1, ch2 indicated by the dashed lines B, C in FIG. 1 impart infinite attenuation at the center frequency f0 of the subchannel of interest (solid line A). More specifically, if the frequency offset α is zero, the higher-order subchannels do not produce ICI in the subchannel of interest. In other words, if the frequency offset α is zero, the subchannels are orthogonal and absolutely no ICI exists.
If the frequency offset α is not zero, however, subchannel orthogonality is lost and ICI is produced. FIG. 2 illustrates the spectral characteristic of each subchannel when the frequency offset α is non-zero in a DMT system. Crosstalk signals from subchannels Ch1, Ch2 to the subchannel ch0 of interest have non-zero mutual gains indicated at α1,0, α2,0 in FIG. 2. According to this notation, the first index of α indicates the subchannel that is the source of interference, and the second index indicates the subchannel upon which the interference is inflicted. Thus, if the frequency offset α is non-zero, ICI (crosstalk) is produced in the non-zero mutual gains, i.e., between subchannels.
FIG. 3 is a general model for illustrating the mutual ICI among three subchannels in a DMT system having frequency offset. Here reference characters 10, 11, 12 represent transmitting devices on subchannels ch0, ch1, ch2, respectively; 2 a receiving device on the subchannel ch0 of interest, 3 a transmission path on the channel ch0 of interest; 41, 42 multipliers for multiplying subchannel signals D1, D2 by leakage transfer coefficients (interference coefficients) αi,0 of leakage from the subchannels ch1, ch2 to the subchannel ch0 of interest; 51, 52 combiners for combining crosstalk (ICI) from the subchannels ch1, ch2 with the subchannel of interest; 6 a source of noise; and 7 a noise combiner.
As evident from FIG. 3, the signal from the higher-order subchannel ch1 leaks into the subchannel ch0 of interest via the crosstalk coefficient α1,0, and the signal from the higher-order subchannel ch2 leaks into the subchannel ch0 of interest via the crosstalk coefficient α2,0.
(b) Technical Problems
Thus, it is necessary to so arrange it that the values (codes in case of binary numbers) of the receive signal on the subchannel ch0 of interest and transmit information symbols be decided correctly even if ICI occurs. For this purpose, the inventor of this application has proposed a receiving apparatus (a turbo receiver) that implements a turbo a posteriori algorithm for improving BER utilizing respective ones of ICIs for cases where the subchannels that impose crosstalk upon the subchannel of interest number one, two and n (patent references PCT/JP02/08763, PCT/JP02/08764, PCT/JP03/02537).
The more the number of subchannels to be considered is increased, the more the BER can be improved. A problem, however, is that the algorithm is complicated and so is the structure of the turbo receiver for implementing this algorithm.