Using Walsh-m Composite Sequence and LDPC-QPSK Joint Iteration to Reduce ISI in Long-range Underwater Acoustic Systems

Fang Fu, Dan-Feng Zhao

College of Information and Communication Engineering, Harbin Engineering University, Harbin 150001, China



Abstract:

To reduce inter-symbol-interference (ISI) in underwater acoustic (UWA) communication systems, a method based on LDPC-QPSK joint iteration and Walsh-m composite sequence is proposed in this paper. The method is intended for use in long-range and low signal-to-noise ratio (SNR) UWA communications. At the transmitter, Walsh-m composite sequence is introduced to resist multipath effect. At the receiver, a soft-input soft-output (SISO) module is implemented in a joint iterative process between QPSK demodulator and LDPC decoder. This method is demonstrated in three types of UWA channel models: positive, negative and invariable sound velocity gradients channels. It is shown that through contrastive simulation experiments, this method is more efficient than conventional methods based on independent decoding and demodulation. After two rounds of joint iteration, the proposed method can obtain 2.5 dB over conventional method at BER of 10-5. Numerical results verify that the proposed method is a good candidate for long-range underwater acoustic communication systems.ª¤

Key words:  underwater acoustic communication  joint iterative  LDPC decoding  QPSK demodulation  ª©Walsh-mªª composite sequenceª¤

DOI£º10.11916/j.issn.1005-9113.2013.01.016

Clc Number:TN929.3

Fund: