A unified perspective on ARTM tier I waveforms - part I: common representations

The constraints placed on aeronautical telemetry channels require the use of bandwidth efficient, constant envelope modulations. Two such modulations have been adopted into the standard for telemetry channels: Feher-patented QPSK (FQPSK) and shaped offset QPSK (SOQPSK). These modulations are known as ARTM tier I modulations. In this paper we explore the similarities and differences between these two modulations. We show that both modulations can be represented as a cross correlated trellis coded quadrature modulation (XTCQM) as well as a continuous phase modulation (CPM). The XTCQM representation is exact for both FQPSK and SOQPSK and the CPM representation is exact for SOQPSK and a good approximation for FQPSK. The observation that these modulations are very similar suggests the possibility of a common detector which can detect either modulation with near optimal bit error rate performance. The bit error performance and complexity of three candidate common detector architectures are explored in the part II companion paper