High rate acoustic link for underwater video transmission

A high bit rate acoustic link for video transmission over an underwater channel is investigated. The key to achieving this objective lies in two approaches: use of efficient data compression algorithms and use of high-level bandwidth-efficient modulation methods. Currently available video encoding standards allow video transmission at bit rates as low as 64 kbps. While this rate is still above the limit of commercially available acoustic modems, prototype acoustic modems based on phase coherent modulation/detection have demonstrated successful transmission up to 30 kbps over a deep-water vertical path. To bridge the final gap and provide acoustic transmission capability needed for near real-time video, we focus on the use of high-level bandwidth-efficient modulation methods. An experimental system, based on discrete cosine transform and Huffman entropy coding for video compression, and variable rate M-ary QAM was implemented. Phase-coherent detection is accomplished by decision-directed synchronization and adaptive equalization. System performance is demonstrated experimentally, using 25000 symbols/sec at a carrier frequency of 75 kHz over a short vertical path. Excellent results were obtained using modulation methods of 16, 32 and 64-QAM, thus achieving bit rates as high as 150 kbps, which are sufficient for real-time transmission of compressed video.