Robust FM data transmission in multipath propagation environments

In a digital FM transmitter-receiver system for data communications, demodulation can be carried out by a simple and cheap non-coherent demodulator scheme consisting of a hard-limiter block plus an FM demodulator (discriminator, FD), which is simple and insensitive to modulation index changes. Its application field includes partial response continuous phase modulation (PR-CPM) schemes such as Gaussian frequency shift keying (GFSK) or its 0.5 modulation index version known as Gaussian minimum shift keying (GMSK). This modulation has gained interest in recent years because of its attractive spectral properties. GSM and DECT employ it. In this paper, a novel and effective post-processing method based on the particular distribution of the noise at the output of the FD and the behaviour of the instantaneous amplitude of the signal and its application to GMSK is presented.