Performance of Multilevel PCM

Up to now, the design of space communications systems has been dominated by the low received power which characterizes these systems. Two changing conditions will alter this emphasis. The first is the increase in available transmitter power caused by increased booster capability. The second is the reduction in available bandwidth due to a great increase in traffic requirements. Present systems use modulation systems which require minimum power at the expense of lavish use of bandwidth. Future systems must find a better compromise of characteristics. Multilevel pulse code modulation (PCM) is a modulation system offering such a compromise. Multilevel PCM is an extension of binary PCM to multilevel signals. In multilevel PCM the signal is sampled, quantized, and the value of the quantized sample is represented by a stream of symbols which may have any number of levels. Multilevel PCM was described and the approximate performance was computed in 1948 by Oliver, Pierce, and Shannon. Since then it has largely been ignored. This is the first exact computation of performance. The performance of multilevel PCM is compared with other systems on an equal bandwidth basis. It is found that multilevel PCM requires less input power than any commonly used modulation system of equal bandwidth. For example, to obtain 60-db signalto-noise ratio out and using only twice the information bandwidth, multilevel PCM requires 20 db less input power than any commonly used modulation system.