Multi-array iterative receiver for underwater acoustic OFDM communications with EXIT chart evaluation

In this paper, a multi-array iterative receiver based on log-likelihood ratio (LLR)-combining detection involving joint sparse channel estimation and decoding is proposed for underwater acoustic OFDM communication. First, Extrinsic information transfer (EXIT) chart analysis is applied to evaluate the convergence behavior of the iterative receiver using the real data collected from the Kauai Acomms MURI 2011 (KAM11) experiment. This experiment was conducted in about 106 m-depth shallow water west of Kauai, HI, in June 2011, with a 20 kHz bandwidth (12–32 kHz) at range up to 3 km. It helps to explain the impact of different data configurations, detectors, and the diversity combinations in a highly inhomogeneous underwater environment and to predict the bit-error rate (BER) performance of the proposed receiver. Then the BERs as a function of the number of combined elements are illustrated to verify the prediction and analysis via the EXIT chart. Data transmission using 16QAM modulation achieves a BER of 10−4 at a data rate of 21 kb/s. The results provide guidance for the design of system parameters including the data configurations, the number of iterations for both iterative processing and low density parity check (LDPC) decoding, which are beneficial to achieve a good efficiency-performance tradeoff.