Communication at Low Date Rates--Spectral Analysis Receivers

The purpose of this paper is to introduce a multiple frequency shift keying spectral analysis receiver based on the fast Fourier transform. A novel decision-directed automatic frequency control (AFC) for use with the fast Fourier receiver is mentioned. Not only is the system to be described better than a truncated autocorrelation receiver, the analysis required is unorthodox, being based on the fact that the spectral coefficients of the incoming waveform are chi-square (central or noncentral) distributed. The fast Fourier receiver, under typical conditions, is 1.5 dB better than the autocorrelation receiver, is more versatile (e.g., tone spacing to multiples of Hz, spectral window shaping very easy), easier to implement, less subject to signal-to-noise ratio degradation due to frequency offset, and provides fine frequency information to operate a decision-directed AFC.