Optimal Design of One-Way and Two-Way Coherent Communication Links

This paper is concerned with the optimal design of one-way and two-way communication systems which are of greatest interest in space applications. A two-way communication system is defined to be one composed of two radio links or channels, viz., an up-link channel and a down-link channel. The down-link RF carrier is derived from the carrier tracking loop located in the uplink receiver. Transmission of information which does not make use of the carrier derived in the up-link receiver is referred to as oneway communications. In particular, a design technique is presented in the form of a universal set of design curves which enable the communications engineer to make a "best" choice of parameters when faced with a given set of design constraints. This means that, given a certain desired error probability, a total transmitter power limitation, data rate, and carrier tracking loop bandwidth, the modulation index, i.e., the square root of the ratio of the power in the carrier to the total transmitted power, may be chosen so as to minimize the probability of error. As an alternate technique, and one which is not as difficult to carry out, the design may be predicted on the basis of selecting that value of modulation index which maximizes the signal-to-noise ratio at the output of the data demodulator. Design curves are given which allow one to employ this alternate criterion. Further, these results should prove useful when attempting to check experimentally a design based upon the criterion of minimum error probability.