Underwater Acoustic Communications with Multi-Carrier Modulation

The underwater acoustic channel belongs to a class of stochastic communication channels that are frequency- and, at long range, energy-limited and doubly spread. The primary challenge in underwater communications is a very large delay and Doppler spread, which complicate the application of "optimal" methods of communication. Orthogonal Frequency Division Multiplexing (OFDM) (or more generally multi-carrier modulation, MCM) systems are well known as being attractive for communications through multi-path communications channels. The traditional approach expects the symbol duration of the transmitted signal to be larger than channel delay spread. This results in a low rate "sampling" of the channel impulse response (once per symbol in each sub-channel) and a sub-channel bandwidth that is less than the coherence bandwidth of the physical propagation channel. An alternative approach is proposed here that uses shorter pulses. This results in subchannels that have delay spreads of greater than one symbol but which are small enough in terms of symbols of delay spread to enable the use of soft decision multi-path decoders (e.g., the maximum-likelihood sequence detection (MLSD) algorithm). The expected result is a more robust and reliable system in doubly spread channels. A few approaches for MLSD receiver are considered. The comparison and numerical simulation of the approaches has shown the superior performance of the MLSD algorithm with a joint channel estimation and data recovery for each sequence of data symbols. The system has much better performance than the traditional decision feedback equalizer in a channel with severe frequency selective fading, and its energy efficiency is 3 dB better than that of a system that uses a pilot signal