PERFORMANCE COMPARISON OF ADAPTIVE EQUALIZERS IN UNDERWATER COMMUNICATION WITH CHANGE OF DEPTH

Abstract

The world has entered the information age of rapid development, due to the rapid development of communication technology, underwater acoustic communication is also fast into the people's field of vision. However, the underwater acoustic channel is a typical time varying multipath fading channel. After passing the channel transmission, the transmitted signal can be deemed to have a different path to reach, with the superposition of multiple components of different time delay and amplitude, it will cause the inter-symbol interference. Underwater sound velocity is very slow than the one of electromagnetic wave, on air-ground radio wireless communication, underwater acoustic channel has a higher sensitivity to the relative motion between the transmitter and receiver, thus also observed Doppler effect in underwater acoustic communication. To improve communication speed and reduce the error rate is a major problem on underwater phase coherent communication system,. One is to reduce the inter symbol interferences, the other is dynamic mechanism of the underwater multipath propagation. In order to compensate for the effects of multipath effects, many techniques have been adopted, and one of them is the channel equalizer. In this work, we used four types of adaptive equalizers that are a feed forward equalizer, a decision directed equalizer, a decision feedback equalizer and a combination of DFE with DDE with the normalized least mean square algorithm and the recursive least square algorithm to reduce ISI so that we compared the performance of four kinds of adaptive equalizers. Using these equalizers with NLMS and RLS algorithm at 100 meters and 400 meters transmission distance and water depth is 14.7m, 15.7m and 16.7m carry out simulations, respectively. As a result, the average mean square error of the RLS algorithm is smaller than NLMS algorithm, and a shorter time to get to steady state. Eventually, the equalizers would compensate interferences from where complexity of in underwater acoustic channel. The nonlinear equalizer is better than linear equalizers at most cases and a combination of DDE with DFE got the best results among all the acoustic equalizers.