The effect of discrete multipath on the reception of mobile satellite signals

In some areas, mobile satellite signals experience significant differential delay due to reflection from surrounding mountains and man made objects. The delay can be large enough to cause severe frequency selective fading, even at relatively low bit rates. This paper examines the sensitivity to discrete multipath of two modulation techniques-MSK and GMSK - which are currently being considered for mobile satellite use, when detected differentially and with a discriminator. The approach is semi-quantitative; to approximate the degradation, we identify dangerous regions in the parameter space and calculate the probability of occupying them. We find that GMSK degrades less than MSK at all values of delay.