Non-line of sight underwater optical wireless communications

Underwater optical wireless communication (UOWC) has recently become a major research subject as it offers high data rates in the order of Megabits per second (Mbps) to Gigabits per second (Gbps) and lower latency due to the high speed of light in water than the incumbent acoustic technology. Presently, there is an increase in the number of unmanned devices such as remotely operated vehicles (ROVs) and the autonomous underwater vehicles (AUVs) deployed underwater which require high capacity and high bandwidth to transfer information underwater. Such AUVs and ROVs are capable of performing tasks at a depth inaccessible to divers due to the danger and risk associated deep down in the underwater world. This thesis concentrates on and explores the performance of Non-line of sight (NLOS) UOWC because it is a more realistic approach towards designing practical systems. This is because in practical scenarios, line of sight (LOS) communication links are not always possible due to obstructions from sea creatures, bubbles, large suspended particles and features of the seabed, especially in coastal and turbid water environments. LOS links are also unsuitable when the transmitter and receiver are non-stationary nodes. Following an introduction to and literature review of UOWC, the thesis considers the characteristics of a NLOS UOWC link with multiple scattering based on Monte Carlo (MC) simulation. For the first time, the channel response of an NLOS-UOWC system with different channel modulation schemes is then addressed. The resultant channel impulse (CIR) response varies with the type of water considered and the receiver field of view (FOV). Thus, the CIR for clear ocean, coastal water and turbid water for FOV values of 30° and 60° are investigated. Then, the CIR obtained using different modulation formats is determined in coastal water since this is a likely application medium. This is followed by evaluation of the bit error rate (BER) and throughput of the system, including variation in the receiver bandwidth. The system provides BER values of 10-4 or better and throughput of 2.1 Mbps. Furthermore, the impulse response of a NLOS UOWC link is determined using analytical models for the phase scattering from underwater propagation. These are used to obtain temporal dispersion results for NLOS UOWC links in coastal and harbour water environments. Curve fitting using the analytical models shows correlation coefficients of between 0.98 and 0.99, demonstrating the utility of the models employed. Finally, for the first time, the thesis presents the performance of a multiple-input multiple-output (MIMO) NLOS UOWC system employing continuous phase modulation (CPM), which is shown to offer sensitivity benefits of several dBs over on–off keying (OOK) without coherent reception. The CIR is obtained by using MC simulation. Turbulence is included by conditioning the CIR on log-normal statistics. To mitigate the resultant fading, spatial diversity with receiver side equal gain combining is employed. Shot noise is included by photon counting, followed by BER calculations using Saddle point and Gaussian approximations. The results show that spatial diversity offers performance improvements, for example an 8 dB sensitivity gain at 10-9 BER using 1 Gbps 3×1 multiple-input single-output (MISO) transmission over a 20 m link with 0.16 log-amplitude variance. The last part of the thesis also determines using an upper bound that Intersymbol Interference (ISI) has a significant impact at high bit rates, producing error floors for multiple-output arrangements