Multipath beam steering for OFDM systems

Abstract

This thesis focuses on the use of a multi-channel receiver for spatial signal processing for underwater acoustic OFDM communications systems. OFDM systems prove effective in combatting the frequency selectivity caused by multipath propagation in underwater acoustic channels. Coherent symbol detection requires estimation of the channel response at each receiving element, typically performed using tap-based models (Least Squares) or path-based models (Path Identification). This thesis presents a spatial processing design which uses a multi-channel receiver to isolate individual multipath returns, both easing the channel estimation requirements and increasing the SNR for symbol detection. The beam steering algorithms are considered in both a coherent and differential OFDM system context and include narrowband and broadband beamforming to the principal, stable path, as well as narrowband and broadband null-steering. These spatial processing algorithms are then extended to an iterative implementation, which approaches the theoretical performance for a beamformer with full multipath channel knowledge. The performance of these algorithms is compared to their single-channel equivalents using both the LS and PI algorithms for coherent detection. Results are illustrated through simulation as well as experimental data processing.--Author's abstract