The effect of non-ideal interpolation on OQPSK carrier synchronization performance in digital modems

Discusses the carrier synchronization performance of a fully digitally implemented receiver, operating on a offset-QPSK (OQPSK) signal. OQPSK signals can be demodulated either by a parallel or a serial receiver structure. Both types of receiver structures are examined. The considered digital receivers operate on samples of the complex envelope of the received signal, taken by a free running clock which is not synchronized to the transmitter clock. The synchronized samples necessary for the carrier synchronization algorithm are computed by interpolating between the available non-synchronized samples. Because of finite memory, interpolation is non-ideal; hence, some amount of distortion is introduced, which affects not only the performance of the decision device but also the carrier synchronizer. By means of theoretical analysis, the authors investigate the tracking performance of a specific decision-directed feedback carrier synchronizer, assuming interpolation of orders zero, one and two