A Comparison of Pseudo-Noise and Conventional Modulation for Multiple-Access Satellite Communications

This paper compares pseudo-noise (PN) with conventional modulation techniques for multiple-access satellite communications of voice messages. The reference for all comparisons is the conventional frequency-division multiplex telephone system. The comparison study is concerned with theoretical channel parameters as well as practical considerations which are unique to satellite communications. For PN modulation, curves are presented which show the relationship between the intrinsic signal-to-noise ratio and the number of channels per megacycle for a given test-tone-to-noise ratio. It is concluded that high quality voice transmission can be achieved efficiently with PN-multiplexing. In particular, pulsed pseudo-noise transmission with some form of wide-deviation pulse-time message modulation and matched filter reception uses the down-link intrinsic signal-to-noise ratio and bandwidth as efficiently as the conventional single-sideband up and composite frequency-modulation with feedback down, provided that up-link power control is used. For lower quality communications, conventional modulation is more efficient in rf bandwidth utilization. Where rf bandwidth is not a significant factor, but the down-link intrinsic signal-to-noise ratio is important, then in the case of PN modulation, communications can be made thermal noise limited in the down-link. Here PN is, for all practical purposes, as efficient as orthogonal multiplexing.