Adaptive detection for asynchronous uplink IDMA shallow-water acoustic channels

In this paper, a novel adaptive detector for uplink interleaved division multiple access (IDMA) transmissions is proposed. The conventional IDMA receiver mitigates multiple access interference (MAI) and inter-symbol interference (ISI) by exchanging iteratively extrinsic soft log-likelihood ratio (LLR) information between the Elementary Signal Estimation (ESE) and the a posteriori probability decoder (APP-DEC). In contrast, the proposed receiver jointly removes MAI and ISI effects by utilizing an adaptive, chip-level, centralized decision feedback equalizer (CDFE). The CDFE equalizer iteratively exchanges data symbols with the DEC. In decision-directed mode, for efficient estimation of the transmitted symbols, the CDFE equalizer depends on both a priori information of the user´s information symbols, which are fed back from the decoders, and the estimated channel impulse response. Simulation results using experimentally obtained shallow water impulse responses demonstrate the superior performance of the proposed CDFE receiver over the decentralized decision feedback equalizer (DDFE) and linear equalizer (LE).