Offset Quadrature Communications with Decision-Feedback Carrier Synchronization

In this paper, we focus our attention on the decisionfeedback approach to carrier synchronization of offset quadrature phase-shift-keyed (QPSK) and more generally offset quadrature amplitude-shift-keyed (QASK) systems. In particular, we present two decision-feedback loops for tracking offset QPSK and offset QASK, which are modifications of comparable loops previously given in the literature for carrier synchronization of QPSK and QASK respectively. The performance gains obtained using such synchronization techniques are discussed. In particular, we show that the use of offset-quadrature communications rather than conventional quadrature communications permits a doubled loop bandwidth at fixed data rate, signal-to-noise ratio and probability of error. However, the more efficient phase-tracking loop (decision-feedback as opposed to phase-locked loop) used reduces substantially the total signal-to-noise ratio required.