Coded FQPSK and SOQPSK with iterative detection

We investigate the performance of Feher-patented quadrature phase-shift keying (FQPSK) and shaped-offset QPSK (SOQPSK) when serially concatenated with an outer code. We show that the receiver complexity for FQPSK and SOQPSK can he greatly reduced by viewing them as continuous phase modulation (CPM) waveforms. We use the pulse amplitude modulation (PAM) representation of CPM, which allows near-optimum detection of both modulations using a simple 4-state trellis. We compare the performance of the PAM-based approximation with another common approximation known as frequency/phase pulse truncation (PT). We use both of these reduced-complexity designs in serially concatenated coding schemes with iterative detection. In the end, we show that the PAM approximation has a slight performance advantage over PT, but both approximations achieve large coding gains in the proposed serially concatenated systems.