Characterization of non-conventional methods for alignment relaxation in underwater wireless optical communication systems

Abstract

The Internet of Underwater Things (IoUT) paradigm is expected to enable various practical applications such as environmental monitoring, underwater exploration, and disaster prevention. Supporting the concept of IoUT requires robust underwater wireless communication infrastructure. Optical wireless communication has the superiority of wide bandwidth, low latency, and high data capacity over its counterparts, namely, acoustic and radio-frequency. However, the transmission of the optical beam has inherent drawbacks in a harsh environment. Obstructions such as geometrical underwater terrains and underwater turbulence can pose a serious challenge to the alignment of the transmitter and the receiver.

Non-line-of-sight (NLOS) configuration is a generalized alignment scheme between the transmitter and the receiver such that the strict requirement of precise alignment (point-to-point) is no longer needed. In this dissertation, the effectiveness of NLOS to withstand challenging underwater turbulence is examined. Thermal gradients with a maximum temperature difference of 10 ◦C had a negligible effect on the received power. The presence of air bubble clouds caused an increase of 38% of the received power when the bubble area increased from 5.2 to 80 mm$^2$. Additionally, various salinity concentrations ranging between 30-40‰ are emulated. A gain of 32.5% in the signal-to-noise ratio is observed when the salinity gradient increased from 0.08 to 0.4‰·cm$^{−1}$. Moreover, a reduction of 2.35 dB/m of the pathloss is noticed. The bit-error ratio is used to examine the communication quality using on-off-keying modulation scheme.

In addition, this dissertation shows a practical wavelength-division multiplexing method based on large-area detection and wide field-of-view (FoV) photonic receiver. The dual-antenna is made of scintillating fibers with distinctive characteristics. An aggregated data rate of 1 Gbps is achieved. Two methods of wavelengths separation are demonstrated. Additionally a field deployment verification in an outdoor water pool is conducted at a maximum separation distance of 10m. The presented promising results pave the way for a robust underwater wireless optical sensor network that serves as a building block for achieving the goal of establishing IoUT.