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
1/T
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.