Abstract
The challenge for next-generation underwater optical wireless communication systems is to develop optical transceivers that can operate with low power consumption by maximizing the transmission capacity according to the transmission distance between transmitters and receivers. This study proposes an underwater wireless optical communication (UWOC) system using an optical transceiver with an optimum transmission rate for the deep sea with near-pure water properties. As a method for actualizing an optical transceiver with an optimum transmission rate in a UWOC system, time-domain hybrid pulse amplitude modulation (PAM) (TDHP) using a transmission rate and distance-adaptive intensity modulation/direct detection optical transceiver is considered. In the TDHP method, variable transmission capacity is actualized while changing the generation ratio of two intensity-modulated signals with different noise immunities in the time domain. Three different color laser diodes (LDs), red, blue, and green are used in an underwater channel transmission transceiver that comprises the LD and a photodiode. The maximum transmission distance while changing the incidence of PAM 2 and PAM 4 signals that calibrate the TDHP in a pure transmission line and how the maximum transmission distance changes when the optical transmitter/receiver spatial optical system is altered from the optimum conditions are clarified based on numerical calculation and simulation. To the best knowledge of the authors, there is no other research on data-rate and distance adaptive UWOC system that applies the TDHP signal with power optimization between two modulation formats.
Highlights
Seafloor related research and developments have progressed in areas such as disaster prevention of tsunami and earthquakes, the off-shore oil industry, diver-submarine communications [1], and underwater environment monitoring systems
The optimum wavelength is the highest for dirty water and seawater at 520-570 nm, while pure water has the lowest optimum wavelength at 450-500 nm. These results suggest that the higher the water turbidity, the higher the optimum wavelength of the laser diodes (LDs)/light-emitting diode (LED) needed to overcome the attenuation effect
To understand further the optical properties of the seafloor, we reviewed previous research on the optical properties of deep-sea sites in the Mediterranean conducted by the Neutrino Mediterranean Observatory (NEMO) project in 1999 using a neutrino telescope deployed on the seafloor off the coast of Italy [23]
Summary
Seafloor related research and developments have progressed in areas such as disaster prevention of tsunami and earthquakes, the off-shore oil industry, diver-submarine communications [1], and underwater environment monitoring systems. VOLUME 9, 2021 because it maximizes the transmission capacity by flexibly setting two types of PAM-M signal ratios to the received SNR, which changes according to the water turbidity and transmission distance. As opposed to other research, we use the TDHP method to transmit data in an underwater channel using the UWOC system. As a contribution to the research field of Gigabit-class high-speed underwater optical wireless communication system, the transmission capacity and transmission distance are maximized according to the transmission environment by changing two parameters of our proposed TDHP by software control in the transceiver. One of the main factors affecting signal deterioration in the UWOC system is signal attenuation due to the absorption and scattering effects underwater. The factors affecting the PD are receiver aperture diameter D, the receiver field-of-view (FOV), and angle misalignment between the line-of-sight (LOS) and the optical axis of the receiver
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