High-frequency, highly directional short-range underwater acoustic communications

The underwater acoustic channel is a challenging environment for achieving high data rate communications due to multipath, attenuation, noise and propagation delay. Performing and maintaining adaptive channel equalization requires significant computational overhead, leading to costly and power-hungry devices. Here we describe a high-frequency acoustic modem design intended to mitigate the equalization requirement through highly directional, high frequency transmissions centered at 750 kHz. The high frequency additionally confers a data rate of up to 125 kbps between modems spaced on the order of meters apart. The modems are designed to record ambient noise before re-transmitting the recorded data in an encoded manner using high frequency transmissions through a separate, directional transducer. Such modems could be used to facilitate a cable-less benthic hydrophone array made from on-the-fly replaceable nodes. In order to determine inter-element spacing and to perform beamforming on the recorded data between modems, a synchronized yet independent Symmetricom® chipscale atomic clock (CSAC, model SA.45s) was incorporated into a custom signal conditioning and timing board inside each modem. Lab-based experimental results show the feasibility of high-frequency acoustic communication (ACOMMS) between these modems.