Abstract
Underwater wireless optical communication (UWOC) has attracted increasing interest in various underwater activities because of its order-of-magnitude higher bandwidth compared to acoustic and radio-frequency technologies. Testbeds and pre-aligned UWOC links were constructed for physical layer evaluation, which verified that UWOC systems can operate at tens of gigabits per second or close to a hundred meters of distance. This holds promise for realizing a globally connected Internet of Underwater Things (IoUT). However, due to the fundamental complexity of the ocean water environment, there are considerable practical challenges in establishing reliable UWOC links. Thus, in addition to providing an exhaustive overview of recent advances in UWOC, this article addresses various underwater challenges and offers insights into the solutions. In particular, oceanic turbulence, which induces scintillation and misalignment in underwater links, is one of the key factors in degrading UWOC performance. Novel solutions are proposed to ease the requirements on pointing, acquisition, and tracking (PAT) for establishing robustness in UWOC links. The solutions include light-scattering-based non-line-of-sight (NLOS) communication modality as well as PAT-relieving scintillating-fiber-based photoreceiver and large-photovoltaic cells as the optical signal detectors. Naturally, the dual-function photovoltaic–photodetector device readily offers a means of energy harvesting for powering up the future IoUT sensors.
Highlights
O VER 70% of the Earth’s surface is covered by oceans
Studies of the physical layer of underwater acoustic communication have reached a certain level of maturity
While Underwater wireless optical communication (UWOC) offers high-speed data transfer and complements the existing RF and acoustic technologies, its ultimate performance is affected by the complex underwater environment
Summary
O VER 70% of the Earth’s surface is covered by oceans. Underwater oil exploration, oceanographic studies, and subsea military activities are examples of the growing need to explore the oceans for industrial, scientific, and military purposes. The recent development of high-speed power-efficient optoelectronic devices has offered the promise of OWC data rates of up to 100 Gbit/s [31] with transmission links of a few kilometers [32] Such devices include light-emitting diodes (LEDs) [33], superluminescent diodes [34], lasers diodes (LDs) [35], photodetectors [36], modulators [37], and the integration of these devices [38]. Lee et al demonstrated the performance enhancement experimentally by utilizing a near-infrared laser; they showed that the overall frequency response of the system gains an increment of up to a few tens of megahertz with increasing turbidity [47] These investigations led to the demonstration of real-time ultra-highdefinition video transmission over underwater channels with different turbidities [48]. By using highly sensitive PV cells as photodetectors, we show simultaneous energy harvesting and signal detection in an underwater environment, thereby providing solutions to the question of how to supply energy to an underwater data transceiver
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