Energy Consumption Minimization of Air-Water Visible Light Communications

As offshore industries and the internet of underwater things continue to expand, air-to-water communication is needed to connect various underwater sensors and things to the outside world. Long term energy-efficient air-to-water wireless communications have been increasingly attracting attention due to the need of military application, marine life studying, and climate monitoring. However, it is always a challenge to communicate directly across the air-water interface by a single type of wireless signal. Acoustic waves used underwater have a very low bandwidth and result in a high delay especially in the air, while electromagnetic waves have a high attenuation while travelling underwater and result in a short communication distance and a higher energy consumption. Traditionally, the communication between air and underwater requires an interface such as a floating base-station to serve as a relay. However, this solution can be costly, lack of responsiveness and even unacceptable in some situation. To overcome the air-water interface, visible light communication (VLC) has been considered for direct air-water communications. Compared with acoustic and radio frequency (RF) communications, VLC has higher bandwidth and lower energy consumption. As a result, VLC can achieve a higher data rate for air-water communication than acoustic or RF communications.

In this thesis, low-power air-water communications are investigated, while considering that the underwater nodes may be drifted by water currents within a certain range from their default positions. Therefore, diffused-line-of-sight VLC using light emitting diode (LED) is considered since it generally has a wider beam angle and results in an acceptable coverage area, and it has a lower transmission power consumption compared to lasers. This thesis studies two air-water VLC systems as follows. In the air-to-water VLC system, an LED transmitter carried by a unmanned aerial vehicle (UAV) hovering above the water transmits data of a certain volume to an underwater receiver in a deep water scenario. In the water-to-air VLC system, an UAV carrying a photodiode receiver hovers above the water and receives a certain amount of VLC signals from an underwater LED transmitter in a shallow water scenario.

To solve the light propagation path in order to obtain the light propagation distances and the coverage area, the air-water interface are mathematical modelled for both air-water VLC systems. In the air-to-water VLC system, Stokes’s third-order theory are used to model the water surface elevation for the deep water scenario, while in the water-to-air VLC system which consider the shallow water scenario, the Boussinesq equations and Korteweg-de Vries equation are adopted. The signal-to-noise ratio (SNR) at the receiver and the channel capacity are derived while considering the transmittance that represents how much light can penetrate the water surface, the receiving area of the receiver and the underwater noise terms.

To fulfil the requirement of long-term sustainable air-water VLC, in both air-water VLC systems, the energy consumption minimization problems are formulated and solved by the proposed algorithms. More specifically, in the air-water VLC system, a sequential quadratic programming based algorithm is devised to minimize the total energy consumption of the UAV by optimising the LED transmit power while guaranteeing that the average received SNR at the underwater receiver is above the required SNR for all possible receiver positions within the light coverage, while in the water-to-air VLC system, an algorithm is proposed to minimize the total energy consumption of the VLC system by jointly optimising the LED transmit power and the UAV height while guaranteeing that the average received SNR at the receiver is above the required SNR for all possible underwater transmitter positions that meet the light coverage requirement for the receiver.

For both work in air-water VLC, the proposed algorithm outperforms the other benchmark strategies of choosing the LED transmit power or the UAV height in minimizing the energy consumption while keeping the transmission time at the same level. Specifically, in the air-to-water VLC system, for the underwater receiver depth of 10 meters, the optimized LED transmit power can reduce the total energy consumption of the UAV by up to 47% as compared to that of LED transmitting at the maximum power, and in the water-to-air VLC system, for the underwater transmitter depth of 10 meters, the optimal values of the LED transmit power and the UAV height can save up to 32% of the total energy consumption of the system as compared with transmitting at a random power level and placing the UAV at a random height.