Adaptive power control for underwater acoustic communications

Experimental measurements of shallow water acoustic channels show that the received signal power varies over time as the channel experiences fading. This fact motivates the use of adaptive power (or rate) control as a way to improve the system performance or save the transmit power. In a system with adaptive power control, the transmitter adjusts its power so that the received power remains at a pre-specified level whenever possible, or it shuts down when the channel conditions deteriorate beyond some point. Adaptive power control relies on feedback by which the receiver informs the transmitter of the present channel state. The transmitter can then predict the next state and adjust its power accordingly. This procedure will be effective if the channel variations are slow enough for the feedback mechanism to be implemented. In this paper, we investigate the feasibility of adaptive power control for underwater acoustic (UWA) communications using data recorded during the Surface Processes Acoustic Communications Experiment (SPACE) which was conducted off the coast of New England in a shallow water channel over a range of 1 km. The slow variations of the locally-averaged received power are shown to exhibit properties of a log-normally distributed autoregressive (AR) process with a coherence time on the order of several seconds. Adaptive prediction of the gain is applied to experimental data, demonstrating the feasibility of power control and indicating that substantial savings in average power are available over extended periods of time (about 9 dB over several hours). These values are also confirmed by a theoretical analysis.