Research On Coherent Underwater Acoustic Communications And Adaptive Equalization Technique

Underwater acoustic (UWA) communications is a rapidly growing field of research, which once was exclusively military, is extending into commercial field, such as remote control in off-shore oil industry, pollution monitoring in environmental systems, speech transmission between divers etc. The needs for underwater wireless communications are increased.Since the underwater acoustic channel is temporal and spatial variability and multipath propagation, signal degradation is severe. In order to achieve high data rates on the severely band-limited UWA channels, band-width efficient phase-coherent modulation techniques should be employed, together with the adaptive equalization processing, to compensate for intersymbol interference and carrier phase fluctuation.The main content in this paper is the research of key techniques of phase coherent underwater communications. The main work is as follows:1. The techniques of adaptive equalization in coherent communications are studied. The performance of some adaptive algorithms is compared in the application of equalization with emulate, such as least mean square (LMS) algorithm, recursive least squares (RLS) algorithm, gradient adaptive lattice (GAL) algorithm, recursive least squares lattice (RLSL) algorithm.2. The method of carrier recovery in coherent communications is studied. Using the method of digital phase-locked loop (DPLL) embedded in adaptive equalization to adjust the phase of signal which can improve the performance of coherent UWA communications system. And the update algorithm in DPLL is studied.3. Doppler pre-processing is studied. When the relative motion existed in receiving and transmitting element, it can produce severe Doppler frequency shift. It is necessary to estimate and compensation Doppler frequency shift, and make the residual Doppler can be removed by adaptive equalizer. Block Doppler estimation and linear interpolation methods are feasible.4. Experiment in pond and lake is made to validate the performance of algorithms. Checking the performance of lattice decision feedback equalizer in UWA communications is main intension. The bit of error rates is commonly 10" in the experiment.5. Coherent communications system is realized in real-time.In all, phase-coherent underwater acoustic communications is studied deeply.