Effect of local oscillator phase noise in differentially detected CPFSK

The effect of oscillator phase noise on differentially detected CPFSK (continuous phase frequency shift keying) is considered. Using a typical oscillator phase noise model, the composite signal-to-noise ratio at the output of the differential detector is derived, Results from a typical microwave source are employed in the calculations and error rate performance indicates that when the channel bandwidth is equal to the transmission rate a modulation index equal to 0.5 is no longer optimum. In fact, when oscillator phase noise is particularly pronounced a modulation index of 0.6 can produce a marked improvement in performance. The results point towards a low cost oscillator implementation which lends itself to a high level of integration