Site diversity techniques for digital phase-modulated signals

This thesis presents several pre-detection antenna-spaced diversity techniques that are suitable for coherent deep space as well as satellite communications. Antenna diversity is used to improve the communication down-link performance. This is achieved by arraying together several ground antennas and then coherently combining the signals received at each of the array elements. In our approach, we assume first that the time delay is estimated, using known techniques, and is compensated for. Then in order to combine coherently before detection, the signals must all be co-phased together. For the co-phasing circuitry, we propose a bank of differential phase estimators that estimate the phase difference between each copy of the received signal and the reference copy. The differential phase estimator structure will be derived based on the maximum likelihood (ML) principles, when possible. After co-phasing all of the received signals, the signals can be combined at intermediate frequency (IF) in several ways. The method chosen, namely maximal ratio combining provides the maximum possible combined SNR. Next, the combined signal is processed by the appropriate coherent receiver that suits the modulation type being used. The modulation formats that are considered include Binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and staggered-QPSK (SQPSK). Two other modulation schemes, both of which are residual carrier modulations, are also considered. The first, known as PCM/BPSK/PM, is the modulation scheme currently used by the National Aeronautics and Space Administration (NASA) for deep space applications. The other modulation scheme, known as PCM/PM, is being proposed by NASA for future use in deep space as well as near-earth applications.; The ultimate judgement of any diversity system performance is the improvement in baseband SNR over that of a single-antenna (branch) system. Two measures are used to determine the performance of the end-to-end antenna diversity systems. These two measures are SNR degradation and symbol error rate (SER) loss. The performance of the end-to-end system is determined for L-diversity reception taking into account the impairments due to the combining errors as well as the synchronization errors due to the different loops of the receiver. In order to determine the impairments due to combining errors, the performance of the differential phase estimator bank that constitute the co-phasing circuitry is studied on detail. It is shown that the proposed differential phase estimators are asymptotically unbiased, asymptotically efficient, consistent in the mean square sense, and asymptotically Gaussian. Also, the asymptotic value of the IF imperfect combining degradation is determined for an array of equal size antennas as {dollar}L to infty{dollar}.; A basic result of this research is that each one of these pre-detection antenna diversity receivers has the capability to track the carrier when the signal is too weak to be tracked by a single antenna. (Abstract shortened by UMI.)...