Multiuser undersea acoustic communications in the presence of multipath propagation

Multipath propagation in underwater acoustic communications causes distortion to high-rate digital signals, but at the same time it provides diversity. Signals propagating from different locations to the same receiver undergo different channel effects, which provides a degree of separability. We exploit this fact the design of a multiuser communication system. Our goal is to isolate the signal of the desired user in the presence of multiuser interference without the use of typical direct sequence code-division multiple-access methods. Instead of assigning different spreading codes to the users, the available bandwidth expansion is used for channel coding. The same code is used by every source. Powerful coding, coupled with broadband array processing is shown to provide sufficient separability for a small number of sources. The intended application is an underwater network of several sources such as AUVs. Experimental results obtained with four sources moving at speeds up to 2 m/s in shallow water show excellent receiver performance at per-source information rate of 1400 bps within a bandwidth of 5 kHz. A 1/7 rate concatenated code is employed, together with a four-element receiver array and soft-decision decoding based on multichannel decision-feedback equalization.