An underwater optical communications system that implements forward error correction is demonstrated. The system uses a 405 nm diode laser to transmit on-off-keying (OOK) return-to-zero modulated data at a rate of 500 kbps to a photodiode receiver using a (255,129) Reed-Solomon code. The signal is digitized at the receiver and sent to a PC for post-processing. Experimental measurements collected in an indoor water tank suggest that the coded system can reduce the power required to achieve a bit-error-rate of 10-4 by approximately 8
dB relative to an uncoded OOK system.
As scientific progress demands more and varied data from the earth's oceans, and the military requires greater scrutiny of undersea traffic and threats, the need for reliable underwater communication links increases. The mobility requirements of submarines and autonomous underwater vehicles make tethered links infeasible, and radio frequency electromagnetic waves are highly attenuated in ocean water, preventing their widespread use. As sound waves undergo very little attenuation in the underwater channel, underwater acoustic
communications has been a topic of research for some time. However, due to issues of multipath interference and lack of bandwidth, acoustic data rates are limited. Acoustic links also exhibit very long propagation delays.
One potential solution for high bandwidth and low latency underwater wireless communications is the use of optical communication systems. Such systems have grown more popular in recent years with the creation of reliable, low-cost light sources, like LEDs and diode lasers that take advantage of the comparatively low attenuation of light in the 400 nm - 550 nm range in seawater. Communication links have been demonstrated at rates of 10 Mbps and 1 Gbps. Researchers have also investigated the benefits of different modulation techniques on underwater optical communication systems.
While researchers have examined the use of forward-error- correction (FEC) codes to improve acoustic underwater communication systems , these techniques have not been applied to an underwater optical communication system. In principle, the use of such coding should allow the system to operate at a lower signal-to-noise ratio (SNR) for a given bit-error-rate (BER). Since the received signal power directly corresponds to the amount of detected photons, absorption and scattering of photons is the main source of signal attenuation.
This paper describes the application of a Reed-Solomon (RS) FEC code to an underwater optical communication link, in order to reduce the power requirements and/or extend the range of such a link over a varying degree of water conditions.