Symbol Timing Recovery for SOQPSK

Abstract

Shaped offset quadrature phase shift keying (SOQPSK) is a highly bandwidth efficient modulation technique used widely in military and aeronautical telemetry standards. This work focuses on symbol timing recovery for SOQPSK. Continuous phase modulation (CPM) based detector models for SOQPSK have been developed only recently. The proposed timing recovery schemes make use of this recent CPM interpretation of SOQPSK, where SOQPSK is viewed as a CPM with a constrained (correlated) ternary data alphabet. One roadblock standing in the way of these detectors being adopted is that existing symbol timing recovery techniques for CPM are not always applicable since the data symbols are correlated. Here, we derive timing error detectors (TED) that are extended versions of existing non-data-aided (blind) and data-aided TED's for CPM, where the proposed extensions take the data correlation of SOQPSK explicitly into account. Further, for the nod-data-aided case, the merits of the modified TED are demonstrated by comparing its performance with and {\em without} taking the data correlation into account. A simple quantization scheme has also been discussed and implemented for the blind TED to yield an extremely low-complexity version of the system with only negligible performance losses. The S-curves of the proposed TED's are given, which rule out the existence of false lock points. Numerical performance results are given for the two versions of SOQPSK: MIL-STD SOQPSK and SOQPSK-TG. These results show that the proposed schemes have great promise in a wide range of applications due to their low complexity, strong performance and lack of false lock points; such applications include timing recovery in noncoherent detection schemes and false lock detectors.