Antenna selection and space-time spreading methods for multiple-antenna systems

by Sudarshan, Pallav.

SUDARSHAN, PALLAV. Antenna Selection and Space-Time Spreading Methods for Multiple-Antenna Systems. (Under the direction of Prof. Brian L. Hughes.) The use of multiple antennas at the transmitter and receiver can significantly improve the performance of a wireless communication system. In recent years, there has been a lot of interest in deriving efficient receiver architectures and designing signalling and coding schemes that maximize the performance gains of a multi-antenna system. In this dissertation, we focus on two such issues: space-time spreading methods at the transmitter, and antenna selection techniques at the receiver. For a synchronous code-division multiple-access (CDMA) system that employs multiple transmit antennas, we characterize the asymptotic spectral efficiency in terms of the number of users, processing gain, signal to noise ratio (SNR), array size, etc. Using this formula, we design the linear space-time spreading methods that maximize the spectral efficiency. The strategy for optimal spreading sequence allocation across antennas, and across users is also addressed. We show that the system capacity per chip is maximized when each user employs all the spreading sequences allocated to it on each transmit antenna. We then study reduced complexity receiver designs for multiple-antenna systems. A RF pre-processing architecture, that processes the received signal at carrier frequency, followed by selection, and down-conversion is considered. Recent results show that this architecture can outperform conventional antenna selection with the same number of RF chains. We derive the optimum RF pre-processing that is based only on the large-scale parameters of the channel. For a correlated channel, we show that RF pre-processing using channel statistics gives good results, and that instantaneous channel knowledge is not required for pre-processing. A beam pattern based geometric intuition is also developed to justify the performance gains. To accommodate the practical design constraints imposed by current variable phase-shifter technology, a sub-optimal phase approximation is also introduced. We show that this scheme is extremely robust to RF imperfections, such as phase and quantization errors. The impact of imperfect channel estimates on the performance of RF pre-processing is also studied, and the scheme is shown to be robust to channel estimate imperfections, as well. Finally, we focus on antenna selection for multi-access channels. For a multi-user system, we derive the statistics-based selection criteria that maximizes tight bounds on ergodic capacity. Two different receiver architectures are considered, and the performance gain compared to sub-optimal selection is quantified. Antenna Selection and Space-Time Spreading Methods for Multiple-Antenna Systems by Pallav Sudarshan A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy Electrical Engineering Raleigh, NC 2004