Soft decisions for DQPSK demodulation for the Viterbi decoding of the convolutional codes

The conventional soft decision algorithm for DQPSK uses only the differential angle between consecutive DQPSK symbols. However it is possible to improve the accuracy of the soft decision bits by taking the amplitude information of the two DQPSK symbols into account. This paper introduces novel soft decision algorithms based on this approach which give a performance improvement compared to the conventional methods equivalent to an SNR gain of up to 5 dB.