Continuous phase modulation schemes, such as Gaussian minimum-shift keying (GMSK), are frequently used with limiter-discriminator (LD) detectors. This paper studies how the side information derived from the signal envelope can enhance the performance of a Viterbi algorithm (VA)-based receiver operating on the LD output of a GMSK scheme. By considering the joint probability density function of envelope and frequency, different approximations yield different novel metrics for VA, using the three-variables envelope and its derivative, and frequency error in different combinations. Simulation results confirm that such envelope-aided VA gives significant performance gains, and that envelope information complements the frequency information output by the LD detector in frequency-selective fading channels.

The conventional decision feedback equalizer (DFE) is a common solution to the frequency selective multipath propagation problem that appears in high data rate radio communications because of its simplicity and good performance. However, when limiter-discriminator (LD) is used for detection of GMSK signals, the global system has a nonlinear characteristic and the conventional DFE yield poor performance even for moderate delay-spread (DS) channels. An envelope-aided DFE is proposed that extends the use of GMSK with LD to high DS channels at a minimum increase in complexity and cost. The performance has been evaluated for both fractionally-spaced (FS) and T-spaced (TS) DFEs, using the exponential power-delay profile channel model.

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.