Robust directional transmission beamforming techniques for smart antenna systems

Current developments in smart antenna technology include the use of adaptive antenna arrays to directively transmit and receive data from the mobile user. Implementing adaptive antennas only on uplink enhances the signal quality and provides considerable range extension for a cell site, but does not allow for significant increases in capacity. However, utilizing adaptive antennas for both uplink and downlink rejects co-channel interference to the degree that mobiles located at spatially distinct angles from the base station and mobiles located at shorter reuse distances can operate on the same frequency channel. Unfortunately, robust beamforming techniques for downlink have not been developed. Previous research studies have given theoretical performance comparisons of, primarily, DOA-based beamforming techniques for uplink transmissions. Very few RF experiments have been conducted which evaluate adaptive beamforming techniques in a real environment. In this thesis, experimental studies are conducted to evaluate adaptive beamforming techniques and determine a robust downlink transmission beamforming approach. Experimental data is presented which demonstrates the stability of strong specular DOAs over a 25 MHz frequency range, and the robust performance of a new integrated DOA-based beamforming approach for both uplink and downlink transmissions. The spatial signature beamforming approach is shown to perform better than DOA-based beamforming on uplink, but to be less robust on downlink for Frequency Division Duplex (FDD) systems. Additional research is needed to validate these results for larger frequency differences in both cellular and PCS bands