Open-water, real-time, high data-rate, underwater acoustic modem testing

This paper discusses trial results, design features and applications for an underwater acoustic Orthogonal Frequency Division Multiplexing (OFDM) modulator and demodulator operating in the 525 kHz to 825 kHz band with a throughput of 380 kbps over ranges extending to 100 m. The standard timing unit T is 16384 μs. Because the acoustic signaling is in the commercial AM radio band reception of the OFDM signal is affected by Radio Frequency (RF) interference. RF interference from in-band AM radio transmissions is particularly noticeable in fresh water, less so in seawater due to the much higher attenuation of the electromagnetic (EM) wave. This paper reports on work carried out to minimise such interference through the use of custom-designed hydrophones which incorporate a Faraday shield. The design of a Faraday shield is a compromise between conflicting electromagnetic and hydro-acoustic constraints. However, reducing the EM induced hydrophone signal through such screening has been found experimentally to permit a good acoustic SNR at the receiver input, in the presence of RF interference. Details of the RF shielding are described, including diagrams of the hydrophone construction and assembly methods. Finally, the performance of the screened hydrophone in an open water environment is illustrated.