US Patent 7123668 Simple detector and method for QPSK symbols

A simplified Viterbi detector for maximum likelihood detection of nine-bit QPSK symbols, employing a difference metric and two comparators, instead of using two state metrics and four comparators. The detector is used to process the outputs of synchronously sampled matched filters. A multiplexer is employed to select the difference metric from amongst the input sample, the fed back value of the difference metric itself, or the inverses of either, depending on decision signals output from the comparators, which in turn compare the difference metric with the input sample and it's inverse. The even path history then represents the most likely value of the detected symbol.