The Impact of Vertical Salinity Gradient on Non-Line-of-Sight Underwater Optical Wireless Communication

Abstract

The non-line-of-sight (NLOS) underwater communication can offer a viable route in signal propagation and coverage, thus mitigating the pointing, acquisition, and tracking difficulties in line-of-sight optical communication. However, implementing the NLOS link is non-trivial. While the NLOS technique relies on light scattering, i.e., channel turbulence can facilitate NLOS communication, the associated path-loss (PL) can be significant. Signal fading can degrade link robustness, which arises due to ocean water temperature and salinity fluctuation and gradients. To evaluate the robustness of NLOS in natural waters, we systematically measure the link metrics, such as the bit error ratio, PL, and signal-to-noise ratio (SNR), of water bodies of uniform and nonuniform vertical salinity ranging from 30-40‰ (part-per-thousand). We found that salinity-induced turbulence can establish NLOS communication with PL reduction of 2.35 dB/m and SNR increase by 32.5% for dynamic water. Furthermore, a strong correlation was obtained between the strength of signal fluctuations and the received SNR. Finally, we obtained a Gaussian distribution of the statistical scintillation behavior. These results demonstrated the benefit of using the NLOS regime for underwater wireless sensor networks for aiding designers and engineers.

Current Trend in Optical Internet of Underwater Things

“…For example, the work in [102] utilized different water types to confirm the feasibility of NLOS communication, which otherwise would be challenging to perform in the natural environment, as shown in 3(a). In addition, the resilience of NLOS communication against turbulence effects such as thermal, salinity, and air bubbles was proven in prior studies [93], [103]- [105]. Another testbed study delivered a realtime full-duplex (FD) UWOC prototype utilizing a fieldprogrammable gate array (FPGA) module to achieve the video streaming feature in an indoor 10-m laboratory tank using frequency shift keying modulation [95].…”

“…4(b) that a strong linear correlation appears between the scintillation index, the normalized received power as the mean bubble area increases. Consequently, leading to the conclusion that NLOS is a robust modality against air bubbles-induced turbulence [93].…”

“…Salt elements in water, such as N aCl, M gCl, and M gSO 4 , make the principal contributions to the absorption of signal in the water [106]. Researchers hypothesized that salinity-induced turbulence enhances NLOS communication by predominantly enhancing the scattering events in the aquatic channel [93], [103]- [105]. Fig.…”

Security of Underwater and Air-Water Wireless Communication

“…Optical-based underwater WCNs utilize (visible or invisible) light to carry information whereby a laser diode (LD) and/or a light emitting diode (LED) are used as the sources for transmission [46][47][48][49]. The optical underwater WCNs recently received attention due to their favorable characteristics such as high speed, high data rates, and high bandwidth [47][48][49][50][51][52][53][54][55]. The effective communication range of optical-based underwater WCN is greater than its RF-based counterpart but quite lower than acoustic-based counterpart (typically, up to 100m).…”

“…It can potentially solve the problem of broadband and low-latency submarine WCNs [2,50,51]. The optical-based underwater WCNs can provide a high-speed data transmission rate (in Gbps) for a moderate transmission range [1,52,53]. Moreover, their high-speed transmission also guarantees that optical-based underwater WCNs can be used for many real-time underwater applications, e.g., underwater video transmissions [54][55][56].…”