Decision feedback detectors for SOQPSK

We consider highly-simplified decision feedback detectors for shaped-offset quadrature phase-shift keying (SOQPSK), a highly bandwidth-efficient and popular constant-envelope modulation. In particular, we show that the state complexity can be reduced to a minimal level - two states - with asymptotically optimum performance, as demonstrated by performance analysis and confirmed by computer simulations. The complexity reduction is achieved by a novel manipulation of the differential encoder and the SOQPSK precoder, which are both part of the transmission model for SOQPSK. We give two possible architectures for achieving this complexity reduction: the pulse amplitude modulation (PAM) technique and the pulse truncation (PT) technique. We also formulate these detectors for coherent and noncoherent detection. The resulting family of detectors makes use of recent advances in SOQPSK technology based on a continuous phase modulation (CPM) interpretation of SOQPSK. The proposed simplifications are significant because they minimize the complexity of trellis-based SOQPSK detectors, which have become available only in recent years. Because trellis-based SOQPSK detectors are 1-2 dB superior to the widely-deployed family of symbol-by-symbol SOQPSK detectors, the proposed two-state detectors offer the simplest means of achieving these performance gains. Thus, these simple detection schemes are applicable in settings where high performance and low complexity are needed to meet restrictions on power consumption and cost.