Computational simplified detection of digitally modulated radio signals providing a detection of probability for each symbol

摘要

The invention provides simple and reliable detection of &pgr; /4 shifted DQPSK modulated digital signals in a single-subscriber-unit, a multiple- subscriber unit (MSU) or a base transceiver station (BTS) of a fixed- wireless system, and is also directly applicable to other digital cellular or personal communication systems which utilizes a binary or M- ary PAM, FSK or PSK digital modulation scheme with differential or coherent encoding and time- and/or frequency-multiplexing. It offers great simplicity while providing soft-decision information for the later stage decoding of information bits encoded with an error correcting code. For each received sample z.sub.k+L and its estimated one z.sub.k+L, a Euclidean distance function is calculated. This Euclidean distance u(z. sub.k+L |v.sub.k+L, . . . , v.sub.k) is then added to the function derived from the previous iteration g(v.sub.k+L-1, . . . , v. sub.k), to yield a new Euclidean distance function f(v.sub.k+L, . . . , v. sub.k). Then a series of comparisons are carried out to find the minimum Euclidean distance with respect to each symbol within the channel memory span except v.sub.k. These minimum Euclidean distances are then added up to yield M values. The symbol corresponding to the minimum distance is the detected symbol. The same M Euclidean distance values are also used for soft decision derivation for use with an error detecting code. A simple measure of the accuracy of each symbol is calculated from the two shortest Euclidean distances. In particular, by taking the ratio of the difference to the sum of those two distances, the overall implementation of the demodulator becomes especially computationally efficient.