A non-proprietary, constant envelope, variant of shaped offset QPSK (SOQPSK) for improved spectral containment and detection efficiency

Shaped BPSK (SBPSK) and shaped offset QPSK (SOQPSK), as defined in various MIL standards, are widely employed on SATCOM links because of their combination of good spectral efficiency, constant envelope characteristics, and interoperability with legacy equipment. Because of the limited link margin available on most SATCOM links, spectral occupancy at levels of -30 dBc and lower has not received much attention. However, numerous terrestrial applications of OQPSK and similar waveforms (GMSK, Feher-patented FQPSK) have been proposed. In these applications, the power spectral density is relevant to levels of -70 dBc or lower, and these new waveforms are superior to the MIL-STD SOQPSK waveform in this regard. This paper describes a simple non-proprietary modification of the MIL-STD SOQPSK waveform which offers spectral containment and detection efficiency comparable to or better than FQPSK-B (Revision Al) and GMSK, while preserving a constant envelope characteristic and backward compatibility with existing equipment.