Adaptive array processing for high-speed acoustic communication in shallow water

A novel multichannel signal-processing technique, capable of achieving stable high-speed underwater acoustic communication with a fairly low complexity of implementation, is presented. The approach is to split the space and time processing into two separate suboptimal processes to reduce computational complexity and instabilities associated with large tap vectors at large spread factors or BL products. The first suboptimal process starts with the eigen-spatial decomposition of the acoustic channel into multiple coherent orthogonal paths. Only the most energetic coherent paths are retained. For each selected path, individual synchronization is performed, the mean Doppler shift is estimated, and the message is decoded. The message binary information obtained for each individual path is recombined using maximal-ratio combining (MRC) to generate a decision binary sequence along with an array of quality factors, or metrics, associated with each bit. Each metric corresponds to the signal-to-noise-and-interference ratio (SNIR) associated with each bit. The second suboptimal process is a linear multichannel equalizer (LME). The mean Doppler estimate of the most energetic coherent path is used to resample each input signal of the array that go into the LME, to reduce the rate of fluctuation of the resulting effective channel. These resampled input signals appear as if they have passed through a channel with a lower B (Doppler spread), which allows the final space-time processor (the LME) to operate in a stable manner. The decision binary sequence and the array of quality metrics obtained at the output of the MRC are used as inputs in the decision process of the LME. If the metric associated with a decision bit exceeds a computed threshold, this bit, obtained at the output of the first suboptimal process, is used as a decision input to the LME. This technique is applied to achieve high-speed acoustic communication in very shallow water using coherent modulation techniques. As compared with coherent beamforming alone, the proposed technique causes a significant increase in SNIR, a significant decrease of the bit-error rate, and provides an estimate of the BL product. Experimental results demonstrate that stable acoustic communication can be achieved at 16000 coded bits per second (b/s) over 3.2 km in 40 ft of water and in sea-state 2 conditions. The fading properties of the channel are measured. Experimental results show that the BL product can vary by a decade in 116 ms and by two decades within minutes, from 0.001 to 0.1. The real-time analysis shows a strong correlation between the communication performance, the spread factor, and the spatial coherence of the channel.