Multi-antenna underwater acoustic localization system

This work develops and investigates localization and beamforming techniques with multiple antennas for multiple access schemes in underwater acoustic (UWA) communication systems. To enhance the accuracy of node localization, a non-coherent ambiguity function (AF) approach has been developed based on matched field processing (MFP) for localization of a single-antenna UWA communication receiver to one or more transmit antennas. Firstly, we demonstrate that a non-coherent AF allows significant improvement in the localization performance compared to the coherent AF previously used for this purpose, especially at the high frequencies typically used in communication systems. Secondly, we propose a two-step (coarse-to-fine) localization technique. The second step provides a refined spatial sampling of the AF in the vicinity of its maximum found on the coarse space grid covering an area of interest (in range and depth), computed at the first step. This technique allows high localization accuracy and reduction in complexity and memory storage, compared to single step localization. Thirdly, we propose a joint refinement of the AF around several maxima to reduce outliers. Fourthly, the combination of different beamforming techniques with two-step localization in a UWA communication system with orthogonal frequency-division multiplexing (OFDM) has been investigated. For an approximate minimum mean square error (MMSE) transmit antenna precoder with subcarrier-by-subcarrier regularization and with diagonal loading, it is demonstrated that with a suitable assumed signal-to-noise ratio (SNR), the MMSE precoder can achieve a significant improvement in the detection performance compared to a previously proposed zero-forcing precoder. Numerical experiments for multiple communication scenarios and acoustic environments are run for validation of these proposed techniques.