Modified decision feedback equalizer (DFE) for sparse channels in underwater acoustic communications

A modified adaptive decision feedback equalizer (DFE) is proposed to exploit fully the long sparse channels in high-speed underwater acoustic digital communications. Based on the relationships between the optimal feedforward filter, feedback filter (FBF) on the one hand and the channel impulse response on the other hand, influence of the precursor with different strength on the FBF is analyzed. The results show that only a few tap coefficients are significant in case of sparse channels. We give a fairly detailed theoretical analysis on how to predict the positions of these significant taps. Our modified DFE only works on the predicted significant taps. Comparing with the conventional DFE, the computational load of our modified DFE is greatly reduced with only a very small cost in performance. Our modified DFE has also another advantage of faster convergence since it needs only to work on considerably fewer tap coefficients. Computer simulations on sparse channel measured at sea confirm the above advantage of our modified DFE over the conventional DFE.