Demodulators – Phase shift keying or quadrature amplitude demodulator
Patent
1995-07-07
1996-05-21
Mis, David
Demodulators
Phase shift keying or quadrature amplitude demodulator
329307, 329310, 375261, 375324, 375327, 375344, H03D 300, H04L 27233
Patent
active
055193560
ABSTRACT:
In quadrature amplitude modulation, circular concentric decision regions capitalize on the observation that when the sample matrix is rotating, it is possible to identify samples more accurately by the radius of their orbit rather than their phase at any given time. The first embodiment provides that a scalar 1.sub.i is calculated for each constellation point so that the constellation point corresponding to the minimum 1.sub.i value is the symbol which the decision device decides was transmitted. The nearest constellation point having the minimum magnitude difference represents the decision. In the second embodiment, two complementary weighting factors are used to provide a weighted average of the two decision criteria in order to make the correct decision. .alpha. is the weight for representing standard rectangular decision regions, while (1-.alpha.) is the weight for representing the circular decision regions. The range for .alpha. is 0<.alpha..ltoreq.1. The variable .alpha. according to the present invention balances the effects of white noise versus constellation rotation. A carrier lock detection circuit is used to adjust .alpha.. When unlocked a low .alpha. near zero is used; when locked a high .alpha. at or near one is used. One alternative to running the calculations for each viable candidate constellation point q.sub.i is to use a lookup table. Quantizing the input to a reasonable level and supporting a small number of .alpha. levels obtains most of the benefit of the present invention at a reasonable cost. A combination of computation and storage balances the use processing and storage requirements.
REFERENCES:
patent: 5343499 (1994-08-01), Jasper et al.
patent: 5386202 (1995-01-01), Cochran et al.
patent: 5471508 (1995-11-01), Koslov
John A. C. Bingham, "The Theory And Practice Of Modem Design", John Wiley & Sons, Apr. 19, 1988, pp. 181-188.
Neil K. Jablon, "Joint Blind Equalization, Carrier Recovery, and Timing Recovery For High-Order QAM Signal Constellations", IEEE Transactions on Signal Processing, vol. 40, No. 6, Jun. 1992, pp. 1383-1397.
P. Herbig, "A New Acquisition Technique For Carrier Recovery With QAM Signals", Radio Relay Systems, Conference Publication No. 386, IEE, 11-14 Oct. 1993, pp. 316-321.
Mis David
National Semiconductor Corporation
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