Modelling and characterisation of short range underwater optical wireless communication channels

This thesis studies the modelling and characterisation of underwater optical wireless communication links, particularly short-range diffuse links, by using numerical Monte Carlo (MC) simulation. MC simulation provides a flexible, intuitive and accurate modelling of the underwater channel, which is severely affected by absorption and scattering processes. In diffuse Underwater Optical Wireless Communication (UOWC) links, scattering is expected to have a larger impact on communication link performance due to the wider beam divergence compared to collimated beams. Thus, this thesis will investigate the characterisation of path loss, spatial, temporal and angular dispersions of diffuse links in various types of water. Firstly, a detailed investigation on the path loss performance of diffuse beam in three types of water is presented. This includes the study on the contribution of unscattered and scattered components of light to the total received power and how they are attenuated. From the percentage of unscattered light that contributed to the total power reception, the distance at which the unscattered component drops to zero can be estimated. This distance is used to predict the transition point from minimal scattering to multiple scattering regime for diffuse beams in coastal and turbid water. In addition to this, the spatial dispersion effect is also studied at off-axis locations. To further understand the behaviour of scattering in diffuse links, the scattering order probability is evaluated for various beam sizes in various types of water. Currently, this kind of information cannot be obtained either analytically or experimentally. The information on the scattering order is used as the parameter to classify the links into three scattering regimes, namely minimal, intermediate and multiple scattering regimes. Further investigations into the transition regimes are conducted by investigating the impulse response and frequency response performance for temporal dispersion effects. From the impulse response and frequency response analysis, the bandwidth that can be supported by the channel can be predicted, which provides some insight into the potential and limits of the links. In addition to temporal dispersion, the angular dispersion performance is also evaluated. It is shown through the angle of arrival (AOA) distribution that diffuse beams exhibit significant angular dispersions, implying that a large receiver field of view (FOV) is needed for optimum power performance. The information on the AOA distribution is then used to study the impact of receiver FOV on the bandwidth. Finally, the effect of aperture on the power received and scattering order histogram is evaluated. As a conclusion, the numerical results presented in this thesis will provide an improved understanding of the effect of scattering on path loss, spatial, temporal and angular dispersions along with their relationships with each other