Improving blind equalization algorithm for shallow water acoustic channels

Transmitting communication signals through band-limited shallow water acoustic channels is often corrupted by the widely existing ISI (intersymbol interference) and intrasymbol interference, which seriously affects the communication speed as well as the quality. Although baud-spaced equalizer (BSE) is quite effective for restraining ISI created by multipath effect within time dispersive underwater channels, it is ineffective for mitigating inherent intrasymbol interference effect of shallow water channels. Since the actual shallow water channels are always having serious frequency selective fading and time variability, the linear equalizers are not suitable for the compensations of these kinds of channels. Our aim in this paper is to present a nonlinear structural blind fractionally-spaced equalizer (FSE) that is more effective than BSE in meeting ever increasing demands on blind equalization processing for severe multipath fading shallow water acoustic channels. In the full paper we explain in much detail our CM-FSDFE (constant modulus fractionally-spaced decision feedback equalization) algorithm for FSE, which is different from CM-BSDFE (constant modulus baud-spaced decision feedback equalization) algorithm used in BSE. In the full paper we also explain in much detail the iterative procedure of CM-FSDFE algorithm. As convergence rate and mean square error (MSE) are two main factors in evaluating performance of blind equalization algorithm, we performed Monte Carlo simulations. The simulation results show that compared with CM-BSDFE algorithm, our CM-FSDFE algorithm does suppress more effectively not only ISI but also serious intrasymbol interference introduced by shallow water channels. Our new CM-FSDFE algorithm converges after only 800 iterations and obtains an improvement of about 7 dB reduction in residual MSE as compared with the MSE obtainable with CM-BSDFE algorithm, and therefore results in a great increase in both efficiency and reliability of shallow water acoustic communications.