Adaptive modulation and power control for underwater acoustic communications

In this work we explore the feasibility of a cognitive acoustic communication system that exploits a dynamic closed loop between the transmitter and receiver with the goal of maximizing the information throughput. Specifically, we design a power control mechanism and couple it with an adaptive modulation method based on orthogonal frequency division multiplexing (OFDM). We propose two methods: the first method is adaptive overall power adjustment in which the transmitter modifies the power gain to provide a target SNR at the receiver by exploiting a feedback link in a time-varying channel. The second method adaptively adjusts the modulation level on each individual sub-carrier to achieve a target bit error rate (BER) at the receiver. Crucial to both of these methods is the ability to predict the acoustic channel and the signal-to-noise ratio (SNR) one travel time ahead, which enables adaptive adjustment of the transmit power and modulation level. The performance of the proposed algorithms is demonstrated using an in-air test bed and further verified with real-time at-sea experiments conducted off the coast of Kauai, HI, in June 2011. Experimental results obtained using real-time at-sea experiments show significant savings in power, as well as improvement in the overall information throughput (bit rate) as compared to conventional, non-adaptive modulation with the same power consumption and target BER.