Carrier synchronization of offset QPSK for deep space telemetry

Offset quadri-phase shift keying will be the telemetry modulation scheme for many future deep space missions, in accord with new CCSDS standards. The Block-V Receiver of the Deep Space Network has a carrier synchronization loop for offset QPSK, but this loop has four stable lock points within each cycle of carrier phase. In this paper, a method is proposed for reducing this four-fold phase ambiguity to two fold. This can be accomplished with the addition of simple signal processing at both the transmitter and receiver. The two-fold phase ambiguity is resolved by a unique synchronization word, just as is commonly done already with binary phase-shift keying (BPSK). This strategy of reducing the four-fold phase ambiguity to a two-fold phase ambiguity does require periodic bit inversions at the modulator. So this solution is not available for the general offset QPSK modulator that does not include the necessary periodic bit inversions.