Branch-dependent metric for Viterbi based receivers for GMSK in frequency selective fading channel

Maximum likelihood sequence detection (MLSD) using the Viterbi algorithm (VA) is a widely employed detection technique for both coherent and non-coherent detection of continuous phase modulation (CPM) signals due to the inherently nonlinear behaviour of the digital channel. In particular, its application to limiter-discriminator (LD) demodulation schemes has received wide attention in the last years. Somehow, most of the VA proposed, recognize but neglect the effect of the non-Gaussian frequency noise at the output of the LD. As we show in this work this question is closely related to the signal envelope behaviour and hence becomes more important in the presence of frequency selective fading. Two simple schemes that provide a fair solution to that problem are proposed an evaluated in this contribution