High Frequency Coherent Acoustic Communications for the Networked Autonomous Littoral Surveillance System

The Networked autonomous littoral surveillance (NetALS) system is a wireless distributed bottom mounted sensor network being developed at DRDC Atlantic. NetALS will consist of a chain of bottom mounted sensor clusters that rely on ultrasonic communications in both synchronous time division and asynchronous frequency division schemes to transfer detection statistics from relativley cheap sensor nodes to more capable cluster heads where data fusion and decision making take place. A channel estimation based decision directed (CE-DD) algorithm for phase coherent demodulation of sensor data packets has been developed. This multi-channel CE-DD receiver relies on a time recursive use of conjugate gradient iterations to support computationally reduced channel estimation and is coupled to a Levin-son recursion for composite channel adaptive equalization. The algorithm was tested on shallow water data with source depth and bandwidth contraints as well as receiver ranges and apertures being consistent with a bottom mounted sensor network. Computational reductions beyond that of Levin-son´s O(M ²) for adaptive filter construction lead to a slight performance degradation. Error rates for communication at 2 and 4 bps/Hz are reported at ranges from 300 m to 800 m with receive phone signal to noise ratios between 8 and 22 dB. The effect of limited aperture on the acoustic communication link is demonstrated.