Low complexity multichannel adaptive turbo equalizer for large delay spread sparse underwater acoustic channel

A scheme of multichannel adaptive turbo equalizer for large delay spread sparse underwater acoustic channel is presented, with a complexity of single DSP chip implementation. Channel equalizer without input of decoder’s soft information is structured as multichannel feed-forward (FF) filter with phase lock loop (PLL), and during iterative processing with decoder, an intersymbol interference canceller (IC) filter is added in the channel equalizer structure to further utilize soft information of the decoder. FF filter and IC filter are sparselized based on magnitude truncation of full order equalizer filters. Multichannel waveforms in training period are used to calculate filter taps with non-sparse least mean square(LMS) adaptive algorithm, with an order larger than 1000 (for example: 4 receiving channels, 2 samples per symbol each channel, FF filter duration is 160 symbol, then the multichannel FF filter order is 1280, and real-time DSP chip implementation is possible with LMS algorithm). After sparse tap position selection, the order of equalizer filter is reduced to less than 200. Multichannel receiving waveforms of both training and data symbols are processed with fast self-optimized LMS (FOLMS) adaptive sparse equalizer with PLL. Turbo code, bit-interleaving coded modulation (BICM), and QPSK/8PSK with gray mapping are adopted as they perform well in fading channel. The information exchanges between equalizer and decoder are realized by symbol/bit converters. The scheme is robust to combat delay spread with a duration larger than hundred symbols and easily implemented on a single DSP chirp. Extrinsic information transfer chart (EXIT) is used to analysis the performances of sparse equalizer, channel decoder and their iteration. Shallow lake and ocean experiments verified the performance of the scheme, with PSK symbol rate 4k Baud, half rate turbo code. Communication distance was from 1 km to 5 km, and there was no error frame with iteration of turbo equalization, even when delay spread larger than 40 symbols and no dominant channel path.