Optimization of LDPC codes over the underwater acoustic channel

To combat severe intersymbol interference incurred by multipath propagation of sound waves in the underwater acoustic environment, we introduce an iterative equalization and decoding scheme by iteratively exchanging soft information between a low-density parity check (LDPC) decoder and a decision feedback equalizer. We apply extrinsic information transfer (EXIT) charts to analyze performance of LDPC codes over the acoustic multipath channel. Furthermore, using differential evolution technique, we develop an EXIT-aided method to optimize LDPC codes for the underwater acoustic channel. Design examples are presented for two different realizations of the underwater acoustic channel that are generated by an acoustic ray tracing model. Computer simulations show that the optimized LDPC codes outperform its regular counterpart or Turbo codes under the same coding rate and block length, with gains of 1.0 and 0.8 dB, respectively, at the bit error rate of 10-5.