Notes for differential demodulation of CPFSK signals

Abstract

This paper deals with differential demodulation ofCPFSK signals with modulation index 1/2 (MSK) and 1. Making use of the signal complex envelopes, we show thatthe existence of discrete lines with adequate phase in themodulated signal, can be used in a differential demodulationoperation as a local reference. This has advantages whendemodulating signals with intersymbol interference (ISI). Ingeneral because of its non-linear nature the differentialdemodulator increases the output ISI, unless the signalcarries a component that can be used as a reference. Thisanalysis allows us to understand the reasons of thesuperiority of the modulation index 1 relatively to MSKwhen demodulating through a delay and multiply structuretightly filtered versions of these signals.