Improved performance of multiple-symbol differential detection for offset QPSK

In systems employing power amplifiers with non-linear characteristics, offset quaternary phase-shift keying (OQPSK) is preferred over conventional QPSK due to its reduced degree of instantaneous phase transitions. In addition, when pulse shaping filters are used on the transmitted signal, OQPSK results in lower peak-to-average power ratios than QPSK, and therefore allows for amplifiers to operate closer to saturation. Recently, it was shown for a multiple-bit differential detection of a particular preceding of OQPSK modulated data, the bound on bit error rate is 3 dB away from the corresponding bound on conventional QPSK. This paper refines that bound and extends the results to show that for a different, symbol-wise precoding, performance approaches that of multiple-symbol differential detection of conventional QPSK. In addition, results are presented for non-rectangular pulse-shaped OQPSK and a generalized likelihood ratio test detector. For that scenario, performance is within a fraction of a decibel of coherent performance for reasonable complexity.