Optical wireless, on the other hand, can easily transmit at 1 to 5 Mbit/s using light-emitting diodes (LEDs) and at gigabit data rates using diode lasers (see Fig. Underwater video is becoming increasingly important and is very challenging for acoustics, even when frame rates are reduced and compression algorithms are used. The velocity and attenuation of sound in water is dependent on frequency, with higher frequencies being strongly attenuated and intersymbol interference limiting commercial systems to <500 kbit/s. The traditional mode of communications between underwater vehicles has been acoustic. Recently, for short distances of a few meters, some modems can transmit at kilobit rates. However, the penetration of radio waves into water strongly depends on the conductivity and frequency, and traditionally has limited communication with submarines to very low frequency (VLF) and data rates on the order of 100 bits/s. In general, an optimized radio frequency (RF) communication system approaches about 1 bit/Hz of available bandwidth, suggesting the use of high carrier frequencies. To provide a frame of reference, telephone-quality audio generally requires approximately 3 kHz of bandwidth, while high-quality streaming compressed video needs around 500 Kbit/s. The bandwidth requirements for underwater operations can range from about 1 kHz for control signals to telemetry that may require 10 times as much bandwidth to transmit data or imagery. At North Carolina State University (NCSU), we focus on the practical considerations of building and implementing optical communication systems for underwater vehicles and robots, as well as underwater construction and subsea factories. Underwater free-space optical (FSO) communications using blue or blue-green lasers and light-emitting diodes (LEDs) provides niche, short-range (<150 m), high-bandwidth (megabits/second) solutions with ranges that are highly dependent on water quality. This creates intersymbol interference (signal distortion). Acoustics provide a long operational range (kilometers) at low data rates (kilobits/second), but typically have poor performance in shallow waters because of ocean noise and multiple acoustic reflections from the surface and bottom. ![]() Radio waves are exponentially attenuated by the high electrical conductivity of seawater. The underwater environment is challenging for all modes of communications, with distinct tradeoffs between range and data rate.
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