Use of Four-Level Phase Modulation for Digital Mobile Radio

The paper explains the advantages of and compares various forms of four-level phase modulation from the point of view of performance, implementation, and spectrum usage. The results are applicable to digital mobile radio systems employing a large number of transmissions in which cochannel and adjacent channel interference are experienced. Of particular interest is the extent to which the theoretical spectra may be realized with practical transmitters. Differences between theoretical spectra and those measured from practical power amplifiers are identified. A method of simulating the effects of power amplifier nonlinearities is described and results are presented which agree closely with measurements made on an actual power amplifier.