Towards Sustainable IoUT Networks: Enhancing Self-Powered and Camera-Based Underwater Optical Wireless Communication Systems

Abstract

The Internet of Underwater Things (IoUT) has evolved into a robust technology capable of facilitating exploration across a wide range of vital applications. These applications encompass underwater surveillance for natural disaster management, monitoring water pollution, studying dynamic changes in underwater environments, and observing marine life. The IoUT anticipates supporting future networking systems that can bring tremendous improvements in data rates, connectivity, and energy efficiency. The IoUT framework integrates diverse underwater communication technologies, including optical, radio, and acoustic waves. This comprehensive approach contributes to expanding the capabilities of underwater communication systems, enabling a versatile and robust communication infrastructure. Underwater optical wireless communication (UOWC) possess several key attributes that set them apart from radio frequency (RF) and acoustic technologies, including wide coverage, high data rates, minimal latency, robust security, cost-effectiveness, and low energy consumption. Nevertheless, the aquatic channel presents a range of significant challenges for UOWC. These challenges include absorption, scattering, turbulence, alignment, multipath fading, and delay spread, limiting achievable communication ranges. Moreover, IoUT sensor nodes face the challenge of relying on battery power and lacking the ability to use solar energy for energy harvesting (EH). Additionally, the harsh and demanding environment of the ocean presents a formidable barrier to battery maintenance, making it a complex and costly task. Communication between sensor nodes is further complicated by the interaction of signal light with the particles in underwater environments and turbulence. The effects of scattering and turbulence result in signal degradation and increased energy consumption, which poses challenges to establishing reliable and efficient communication links. Addressing the energy constraints of IoUT devices is of paramount importance, and one promising solution involves the harvesting of energy from the surrounding environment. Visible light communication (VLC) has showcased a unique opportunity to utilize visible light signals for EH in situations involving energy-constrained IoUT devices. This technology also enables the establishing of high-speed data links while satisfying illumination requirements. VLC is an emerging technology in optical wireless communication (OWC) systems operating in the visible band (400-700 nm). The utilization of visible light beams for both information transfer and power delivery represents a revolutionary and transformative approach to addressing the energy constraints faced by future generations of IoUT devices. Simultaneous lightwave information and power transfer (SLIPT) introduces an efficient method for EH and information decoding (ID) utilizing solar cells as receivers, providing a practical solution to the energy challenges encountered in UOWC systems. Solar panels are already employed for EH purposes in autonomous underwater vehicles (AUVs) and underwater sensor networks (USNs), offering a practical motivation for underwater SLIPT. This approach allows the leverage of the existing infrastructure within underwater scenarios. Furthermore, i solar cells with expansive detection areas and lens-free operation have been shown to address link alignment issues in challenging underwater environments. This integration paves the way for implementing self-powered IoUT systems, presenting promising solutions to address the prevalent energy shortages in underwater systems. However, the significant light attenuation effect and limited solar energy in underwater environments lead to increased charging times when using artificial light sources. Another technology associated with VLC that employs a camera as an optical receiver is optical camera communication (OCC), covered by the IEEE standard 802.15.7. This inclusion supports the seamless integration and deployment of OCC within established communication standards, enhancing interoperability and promoting wider adoption. Additionally, OCC introduces a novel and cost-effective technology, utilizing existing lighting systems, such as light-emitting diodes (LEDs) as transmitters. The effective deployment of LEDs and cameras on underwater devices significantly enhances the capabilities of underwater optical camera communication (UOCC), further expanding its potential. Therefore, UOCC has been shown to be a promising technique for low-data rate applications. The low directivity of UOCC holds a significant advantage in underwater environments, where signals are prone to scatter. The wide coverage area resulting from the camera large field of view (FOV) allows it to capture a broad region of the scattered light, consequently enhancing the overall system performance. Furthermore, the UOCC system transmission robustness, diversity, and overall throughput can be significantly enhanced by employing camera spatial multiplexing techniques. Aside from the application in underwater links, establishing direct connectivity in the water-to-air (W2A) optical link is highly desirable for enabling real-time information transmission between AUV or remotely operated vehicle (ROV) and aerial devices, such as drones or airplanes. Subsequently, the data can be relayed from these aerial devices back to terrestrial stations using RF transmission technologies. In addition to the inherent challenges posed by UOWC, the dynamic and wavy nature of the water surface plays a pivotal role in determining transmission performance in vertical communication links. The behavior of the wavy surface significantly impacts communication performance and the overall robustness of the system in this water-air communication scenario. This thesis comprises three objectives aimed at improving the EH rate, evaluating UOCC system performance in calm and turbulent conditions, and the feasibility of W2A OCC. The primary objective is to tackle the fundamental challenges faced in underwater environments related to the limited battery lifespan of IoUT nodes and data communication efficiency. This research seeks to develop innovative techniques and solutions that improve the EH rate while establishing a reliable and efficient communication framework within the context of UOWC SLIPT systems. The second objective of this research focuses on addressing a critical issue in both UOWC and UOCC systems, namely, underwater optical turbulence (UOT). UOT is mainly caused by two significant factors, namely, bubbles and temperature inhomogeneity. These factors create varying refractive indices along the propagation path. The light beams undergo single scattering, multiple scattering, and backscattering phenomena when interacting with regions of different refractive indices. The objective here is to understand thoroughly these phenomena effects and develop effective mitigation strategies. The final objective of this thesis pertains to W2A OCC link. This objective aims to overcome the challenges associated with transmitting optical signals across the water-air interface. The refractive index difference between water and air leads to optical beams being either reflected or refracted, causing misalignment and signal distortion. One of the most challenging aspects of W2A OCC is the continuous wavy water surface, which can significantly affect the transmission performance. Consequently, addressing the impact of surface waves on W2A OCC is crucial to developing a reliable and robust communication system. These objectives establish the essential basis for this research, and each one is precisely targeted at tackling specific challenges within the domain of UOWC. The findings presented in this thesis reveal significant advancements in addressing the challenges of UOWC. To address the first objective, a transmission and reception system for UOWC SLIPT is developed using power-splitting techniques, with the primary objective of reducing battery charging time while maintaining an adequate signal-to-noise ratio (SNR), enabling efficient IoUT nodes recharging and communication within reasonable timeframes. A novel transmission scheme is proposed in UOWC SLIPT, allowing a trade-off between EH rate and communication performance. This scheme involves adjusting direct current (DC) levels to adapt to channel conditions and battery status. Therefore, this transmission circuitry leads to a significant enhancement in the EH rate without compromising communication quality. The proposed technology demonstrates the potential for extended underwater device operation, contributing to the evolution of underwater communication and sensing applications. To address the second objective, the experimental evaluation of the UOCC system within regular and subpixel conditions is presented. Suppixel is an energy-efficient approach for low data rate IoUT applications, especially over extended link distances. The small dimensions of the LEDs in the image plane enable the camera to accommodate multiple transmitters, leading to a considerable increase in the overall link throughput. Furthermore, the impact of scattering and turbulence on regular and subpixel UOCC systems is evaluated. The results reveal that the area of illuminated pixels in the image plane is larger than that of a single pixel, even under subpixel conditions. This phenomenon holds independent of the water attenuation due to the scattering effects within the water medium. Therefore, considering the average value of several illuminated pixels caused by point spread function (PSF) can significantly improve the system SNR. Furthermore, turbulence in the underwater environment has a notable effect on light propagation. Turbulence leads to the spreading of light energy, resulting in an expanded projection of the light pattern on the captured image. Utilizing a camera as an optical receiver allows the capture of a considerable portion of the scattered light due to its large FOV. These features enhance the system robustness. The experimental results illustrate that the temperature inhomogeneity-induced turbulence results in higher illuminated pixel areas compared to the bubble case. Bubbles caused multiple scattering and backscattering, leading to some light beams being lost due to high beam deviation angles crossing the air-water interfaces in bubbles. The bubble-induced turbulence further affected the received signal by causing a blockage at times due to the significant size of the bubbles. System SNR measurements demonstrate that temperature inhomogeneity and bubble-induced turbulence negatively impact the system performance. However, similar to previous results accounting for the area of illuminated point conditions can improve the SNR. Additionally, the experiments indicate that bubbles have a higher impact on the received signal than temperature inhomogeneity-induced turbulence, resulting in more corrupted signals. The last experiment aligns with the third objective, concentrating on W2A OCC systems and the challenges arising from the dynamic water surface and misalignment. Experimental results provide valuable insights into the impact of surface waves on W2A OCC link performance. Surface waves lead to increased scattering, enlarging the area of illuminated pixels in the image plane compared to still water conditions. The SNR measurement for different LED positions along the camera vertical view on the LED strip reveals varying SNR values due to surface wave behavior and LED alignment. Statistical distributions are evaluated for predicting turbulence-induced fading caused by wavy surfaces in the W2A OCC link. The Generalized Gamma Distribution (GGD) exhibits a better fit to the measured data compared to Gaussian and Lognormal distributions, particularly in higher wave speeds.