MSK and Offset QPSK Modulation

Minimum shift keying (MSK) and offset keyed quadrature phase shift keying (OK-QPSK) modulation techniques are often proposed for use on nonlinear, severely band-limited communication channels because both techniques retain low sidelobe levels on such channels, while allowing efficient detection performance. A more detailed performance comparison of the two techniques on such channels is, therefore, of interest. In this paper a Markov process representation is developed which is applicable to either the MSK or OK-QPSK waveform. This representation is employed to illustrate the similarity between the modulation processes and to obtain the autocorrelations and power spectral densities of the two waveforms. This Markov process representation may be similarly employed with other modulation waveforms of the same class. The autocorrelations and power Spectral densities of MSK and offset QPSK provide initial insight to expected performance on band-limited channels. The results of a digital computer simulation are presented. The simulation compares the bit error rates (BER's) of MSK and offset QPSK on nonlinear, band-limited double-hop links such as encountered in satellite communications. The simmulation results are presented as
E_{b}/N_{0}
degradation with respect to ideal detection versus channel noise bandwidth. The error probability was used as a performance metric, and equal adjacent channel interference as a constraint. For the channels simulated, MSK is found to provide superior performance when the channel noise bandwidth exceeds about 1.1 times the binary data rate. For narrower bandwidths, offset QPSK provides superior performance.