Adaptive Channel Estimation for Multiuser Underwater Communication Systems

Problem statement: Multiple Access Interference (MAI) signals and poor estimation of the unknown channel parameters in the presence of limited training sequences are two of the major problems that degrade the systems’ performance. Two synchronous multiuser receivers with Rake reception of Interleave Division Multiple Access (IDMA) and Code Division Multiple Access (CDMA) systems, in conjunction with channel estimation, are considered for communication over different short range shallow water acoustic channels. Approach: The proposed hard/soft chip channel estimation and carrier phase tracking are jointly optimized based on the Mean Square Error (MSE) criterion and adapted iteratively by the reconstructed MAI signal. This is generated from exchanged soft information in terms of Log-Likelihood Ratio (LLR) estimates from the single-users’ channel decoders. The channel parameters and error estimation are used to enable the chip cancellation process to retrieve an accurate measurement of the detrimental effects of Intersymbol Interference (ISI) and MAI. Results: The performance of the proposed receiver structures with small processing gain are investigated and compare with 2 and 4 synchronous users using memoryless Quadrature Phase-Shift Keying (QPSK) at an effective rate of 439.5 bps per user. Conclusion: The results demonstrate that the performance is limited by MAI and ISI signals and the IDMA performance outperforms long code CDMA and short code CDMA.