Blind equalization for underwater communications

Abstract

Underwater wireless (sensor) networks would vastly improve man's ability to explore and exploit remote aquatic environments. Despite underwater sensor and vehicle technology being relatively mature, underwater communications is still a major challenge. The most challenging characteristics of the underwater acoustic communication channel are its low and variable propagation speed, frequency-dependent attenuation and time-varying multipath propagation. Spatial and spectral signal processing techniques can be employed to mitigate the effects of the distortion caused by the underwater acoustic channel. In general, the underwater channel capacity is scarce and underwater transmitters have limited energy resources. To reduce energy consumption and to make more efficient use of the available capacity, this thesis elaborates on compensation of underwater channel distortion without employing training sequences. The latter is known as blind (adaptive) equalization. As a substitute for the missing training sequences, the constant modulus property of the transmitted signals is exploited. Deviations of the equalizer output from a constant modulus act as a reference for equalizer updates. Two blind spatial equalization methods, which both exploit structural properties of the signal-of-interest, are presented in this thesis. The first method, the extended CMA (E-CMA), is an algorithm capable of updating the directionality of the array to improve signal reception, while simultaneously correcting for phase offsets. Initial results from our underwater experiments demonstrate the E-CMA's promising performance. The second method for blind spatial equalization discussed in this thesis, is the angular CMA (A-CMA). In contrast to conventional adaptive methods, the A-CMA calculates steering angle updates instead of updates for the entire equalizer weight vector. Compared to the conventional CMA, the A-CMA provides faster convergence to a low mean square error level. Since the propagation speed of underwater acoustic pressure waves is variable, a direct-path acoustic wave can arrive later than a reflected/refracted wave. This phenomenon, which is known as nonminimum-phase behavior, complicates blind extraction of a channel's phase response. A method to improve and accelerate blind equalization of a channel's nonminimum-phase response, known as the all-pass CMA (AP-CMA) is presented. No standard synthetic underwater acoustic channel model exists. Therefore, real-life experiments are performed for true testing. Current commercially available systems for underwater acoustic signal processing make use of dedicated hardware. To implement other physical layer processing techniques (often) changes in hardware and/or proprietary firmware are required. Therefore, a flexible multi-channel underwater testbed has been developed and used in experiments, to evaluate the performance of (novel) blind equalizers. Overall, based on simulated and empirical data, this thesis indicates that blind spectral and blind spatial equalization are appealing means for mitigation of distortion experienced in the underwater channel.