Spectra of Digital FM

Progress in analysis of multilevel frequency shift keying (FSK) has lagged behind that of binary. Inherent difficulties associated with an increase in the number of levels are partly responsible, but activity also has been inhibited by the general impression that multilevel FSK is inferior to differential phase modulation with the same number of levels operating in the same bandwidths. Recent work, Ref. 1, has evolved design principles showing a possibility of substantial improvements in multilevel FM performance over that formerly thought to be typical. Also there are many existing analog channels, e.g., in microwave radio relay, which operate by FM. Attempts to send digital data efficiently over such channels force consideration of the multilevel FSK problem. An important item in the statistical description of an informationbearing signal is the spectral density, which defines the average power density of the signal as a function of frequency. In addition to furnishing an estimate of bandwidth requirements, the spectral density is 1165