On efficient spectrum utilization from the standpoint of communication theory

This paper reviews the concepts of spectrum efficiency, the parameters for assessing system performance, and the techniques for improving spectrum utilization. The rationale of possible bandwidth reduction and power savings by combined modulation techniques is discussed and special attention is given to combined amplitude and phase modulation (APSK) systems. It is believed, based on the available results of several analyses over additive Gaussian noise channels, that certain combined modulation systems can have significant power savings over single-parameter modulation systems when the number of bits per symbol of the signal is large and can achieve more efficient use of the spectrum for signal-to-noise ratios greater than certain levels.