Characterization of signal response for surface water movements in underwater optical wireless communications

Underwater communications, aside from being used for marine archaeology and for search-and-rescue operations, aims to explore the activity of the ocean, especially how it responds during any seismic activity, like tsunamis, seaquakes, and earthquakes, for disaster prevention. There are already existing technologies that use this type of communication, particularly transmission using acoustic waves, like the autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs), but the concerns are its low data transmission rate, attenuation and refraction of signals, to name a few. Optical light within the blue to green band has been sought as an option for underwater transmission because of its low attenuation, high-speed and low-power transmission, especially for data-demanding applications like real-time video streaming. Characterizing the signal response of the transmitted noiseless and noisy signals, i.e., measuring the bit-error rates (BERs) for various signal-to-noise ratios (SNRs) transmitted, is explored for underwater optical communications, especially for surface-to-underwater transmission. The signal responses for air transmission are compared with the ones for surface-to-underwater transmission with surface water movements, with dust particles, and with milk powder mixtures. Since 5mm super bright LEDs allow only a transmission of 0.58 meter for blue and green colors for noiseless air transmission, transmission requires the use of a 5.5 centimeter diameter convex lens placed in front of the transmitter because of the convergence of light rays hence, allowing the transmission up to 1.9 meters. This concept paved way for a low-cost optical transmitter. For the noisy signal transmission in air, when the 500 kHz square wave signal, which represents 1 Mbps of binary data (alternating 0s and 1s), is mixed with 250 kHz of square wave noise signal, the signal cant be decoded at an SNR of 10 dB downwards, as 3 compared to the signal mixed with 3 MHz of noise, which allows data reception even at -10 dB because the 500 kHz signal is a first harmonic of the 250 kHz noise. The results confirmed Geiers article in [39] that decreasing SNR dB value means a decreasing data rate. Afterwards, transmission for surface-to-underwater is tested. It was confirmed that blue light attenuates less than green light for all SNR values, and faster surface water movements attenuate more, based on the received signals BER, although for some rare cases,increasing surface water movements doesnt necessarily translate to increased BER for certain SNRs and light colors, or even increasing SNRs does not translate to increased BER. These are due to the lens being held only in front of the transmitter, and the location of bubble movement and/or wave movements at the line of sight of the transmission, and also the bubbles possibly bringing the light rays back to the surface, which prevents light from reaching the receiver, causing BER. Turbidity and luminous intensity also play a role in signal transmission. A very small amount of particle (milk powder or dust) mixed to water results to a turbidity level of 10-25 NTU, and is enough for light not to pass through the receiver, even though the data being transmitted is a noiseless one