A Simultaneous Information Transfer and Channel-Sounding Modulation Technique for Wide-Band Channels

A method of superimposing a pseudorandom channelsounding signal by amplitude modulation upon a conventional FM information-bearing signal is described. The important feature of the technique is the method of combining the pseudorandom waveform with the FM signal so that one component of the AM channel-sounding spectrum is stationary and unaffected by the frequency modulation. Coherent detection and cross correlation are used at the receiver to recover the "transfer function" of the medium, while the FM information signal is recovered by conventional FM detection methods. The technique is considered most applicable to a two-directional tropospheric scatter link in which a low data-rate feedback channel is available to command changes in parameters of the transmitted signal, based on the analysis at the receiver of the instantaneous transfer function of the medium, to optimize channel transmission. Parameters of the transmitter which might be varied are power, deviation, data rate, carrier frequency, etc. (It is assumed that the bandwidth of the channel-sounding signal is considerably wider than the information modulation bandwidth.)