Multi-carrier modulation for high-rate underwater acoustic communications

The current state-of-the-art in high-rate acoustic communications is represented by adaptive multi-channel equalization of single-carrier wideband signals, which is used in real-time acoustic modems and as a benchmark for coherent communications performance. An alternative technique with significant potential for achieving high bit rates over multipath-distorted (frequency selective) channels is multi-carrier modulation (MCM). MCM has many technical advantages for acoustic communications including an increased tolerance of time synchronization errors made possible by the use of a set of narrow-band channels. In addition, each frequency channel can be transmitted using relatively narrow-band high efficient transducer and low-power amplifier. Each of MCM frequency channels can have relatively low delay spread when measured in units of the baud period and can be equalized with low-complexity algorithms and simplified array processing. The reduction of the inter-symbol interference (ISI) ISI span in each sub-channel allows for the application of sophisticated soft decision decoders, such as maximum-likelihood sequence detectors (MLSD) and adaptive BCJR MAP algorithm. These algorithms are optimal for joint channel estimation and data recovery and are the principal part of the proposed MCM system and their properties are the general objective of the paper. The adaptive form of MAP algorithm has been considered. More practical MLSD algorithm with dual loop feedback and pre-survivor (PSP) processing was tested in SPACE08 and RACE08 shallow water acoustic communications experiments and yielded excellent results.