Demodulators – Phase shift keying or quadrature amplitude demodulator
Patent
1995-12-22
1997-04-08
Grimm, Siegfried H.
Demodulators
Phase shift keying or quadrature amplitude demodulator
329306, 375325, 375328, 375329, 375332, 375341, H04L 27227
Patent
active
056191677
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a maximum likelihood coherent detection method which makes the maximum likelihood estimation of a reference signal for coherent detection and the maximum likelihood sequence estimation of a transmitted symbol sequence at the same time through the use of a plurality of received signal samples obtained by sampling a received phase-modulated digital signal with the symbol period.
PRIOR ART
Coherent detection and differential detection are widely used for the demodulation of phase-modulated signals. The coherent detection provides an excellent error rate performance as compared with the differential detection. To perform the coherent detection, it is necessary that the phase of the received carrier be known. To this end, the receiving side regenerates the carrier by some means, uses it as a reference signal to coherently detect the modulated phase of the received signal and decides the transmitted data. As a method of extracting an M-phase PSK reference signal, there is known a multiplying method according to which the received signal is M multiplied to remove therefrom the modulated phase, the multiplied signal is used to effect phase control of a voltage-controlled oscillator (VCO) to generate a signal of a frequency M times higher than the carrier frequency and the frequency of the signal is divided by M to regenerate the carrier. Another known method is, for example, an inverse modulation method according to which a detected data is used to inversely modulate the received signal to remove therefrom the modulated phase and then the carrier is similarly regenerated using the VCO. However, these methods are disadvantageous in that no fast carrier regeneration is possible because the carrier extracting and regenerating steps form a closed loop using the VCO.
Since the regenerated carrier has a phase uncertainty of 2.pi./M rad., a konwn signal sequence (several symbols, for instance) is transmitted periodically and used to avoid the uncertainty of the phase. This coherent detection is called absolute coherent detection in the sense of detecting the absolute phase. On the other hand, it is possible to exclude the influence of the phase uncertainty by differential encoding at the sending side and differential decoding at the receiving side, but the error rate in this instance would be about twice that of the absolute coherent detection scheme. Moreover, the bit error rate performance of the absolute coherent detection is superior to that of the differential detection. In the case of four-phase PSK, the difference between the differential detection and the absolute coherent detection in the ratio E.sub.b /N.sub.o of the received energy per bit necessary for securing a 0.1% bit error rate to the noise power density is as large as 2.5 dB or so.
Incidentally, there has been proposed a coherent detection scheme which does not involve the carrier regeneration of the above-mentioned methods (Literature 1: D. Divsalar and M. K. Simon, "Multiple-symbol differential detection of MPSK," IEEE Trans. Commum., vol. 38, pp. 300-308, March 1990 and Literature 2: P. Y. Kam, "Maximum-likelihood digital data sequence estimation over the Gaussian channel with unknown carrier phase," IEEE Trans. Commun., vol. COM-35, pp. 764-767, July 1987). The methods proposed in Literature 1 and 2 make the maximum likelihood phase sequence estimation of transmitted symbols through the use of received signal samples produced by sampling the received signal with the symbol period. A metric is calculated for each sequence composed of N symbols using N received signal samples and the sequence of the maximum metric is outputted. Hence, the carrier phase need not be regenerated. To obtain bit error rate performance close to that of ideal coherent detection, it is necessary to increase the number N of symbols for sequence estimation. Since the number of metric calculations for maximum estimation becomes M.sup.N, however, the amount of calculation increases exponentially as the number M of modulated pha
REFERENCES:
patent: 5291523 (1994-03-01), Bergmans et al.
patent: 5450445 (1995-09-01), Ushirokawa
Kam, "Maximum-Likelihood Digital Data Sequence Estimation Over the Gaussian Channel with Unknown Carrier Phase", IEEE Transactions on Communications, vol. COM-35, No. 7, pp. 764-767.
Divsalar, et al, "Multiple-Symbol Differential Detection of MPSK", IEEE Transactions on Communications, vol. 38, No. 3, pp. 300-308.
Grimm Siegfried H.
NTT Mobile Communications Network Inc.
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