Full spectrum combining of staggered-QPSK signals

Several antenna diversity techniques can be used at base stations (BS) to enhance the reliable transmission of signals from a “weak” single source. The services which can benefit from antenna arraying include the planned Motorola´s Iridium, power-limited satellites, and NASA´s next generation all digital receiver. A full spectrum combining (FSC) system is proposed for coherent staggered-QPSK (SQPSK) signaling and its performance is analyzed assuming imperfect carrier and symbol timing recovery. In order to align the signals for an array of L antennas, the combining system utilizes L-1 differential phase estimators. Each one of these phase difference estimators is shown to be an open loop maximum likelihood estimator (MLE). The performance of this estimator is derived and the variance of the estimate error is compared to the Cramer-Rao lower bound (CRLB). Next, the performance of this FSC technique is quantified based on the bit error rate loss. The results are verified by simulation for a scenario of two identical base stations arrayed together. The calculated and measured gain when operating at a data rate of 16 kbps and a BER of 10^-3 is 2.8 dB, compared to the ideal (i.e., no losses due to combining or imperfect references) gain of 3 dB in this case