Underwater optical wireless communications in turbulent conditions: from simulation to experimentation

Underwater optical wireless communication (UOWC) is a technology that aims to apply high speed optical wireless communication (OWC) techniques to the underwater channel. UOWC has the potential to provide high speed links over relatively short distances as part of a hybrid underwater network, along with radio frequency (RF) and underwater acoustic communications (UAC) technologies. However, there are some difficulties involved in developing a reliable UOWC link, namely, the complexity of the channel. The main focus throughout this thesis is to develop a greater understanding of the effects of the UOWC channel, especially underwater turbulence. This understanding is developed from basic theory through to simulation and experimental studies in order to gain a holistic understanding of turbulence in the UOWC channel. This thesis first presents a method of modelling optical underwater turbulence through simulation that allows it to be examined in conjunction with absorption and scattering. In a stationary channel, this turbulence induced scattering is shown to cause and increase both spatial and temporal spreading at the receiver plane. It is also demonstrated using the technique presented that the relative impact of turbulence on a received signal is lower in a highly scattering channel, showing an in-built resilience of these channels. Received intensity distributions are presented confirming that fluctuations in received power from this method follow the commonly used Log-Normal fading model. The impact of turbulence - as measured using this new modelling framework - on link performance, in terms of maximum achievable data rate and bit error rate is equally investigated. Following that, experimental studies comparing both the relative impact of turbulence induced scattering on coherent and non-coherent light propagating through water and the relative impact of turbulence in different water conditions are presented. It is shown that the scintillation index increases with increasing temperature inhomogeneity in the underwater channel. These results indicate that a light beam from a non-coherent source has a greater resilience to temperature inhomogeneity induced turbulence effect in an underwater channel. These results will help researchers in simulating realistic channel conditions when modelling a light emitting diode (LED) based intensity modulation with direct detection (IM/DD) UOWC link. Finally, a comparison of different modulation schemes in still and turbulent water conditions is presented. Using an underwater channel emulator, it is shown that pulse position modulation (PPM) and subcarrier intensity modulation (SIM) have an inherent resilience to turbulence induced fading with SIM achieving higher data rates under all conditions. The signal processing technique termed pair-wise coding (PWC) is applied to SIM in underwater optical wireless communications for the first time. The performance of PWC is compared with the, state-of-the-art, bit and power loading optimisation algorithm. Using PWC, a maximum data rate of 5.2 Gbps is achieved in still water conditions

Effects of Turbulence Induced Scattering on Underwater Optical Wireless Communications

This paper presents a comprehensive description of the relative effect of optical underwater turbulence in combination with absorption and scattering. Turbulence induced scattering is shown to cause and increase both spatial and temporal spreading at the receiver plane. It is also demonstrated that the relative impact of turbulence on a received signal is lower in a highly scattering channel. Received intensity distributions are presented confirming that fluctuations in received power from this method follow the commonly used Log-Normal fading model. The impact of turbulence induced scattering on maximum achievable data rate in the underwater channel is investigated.Comment: 9 pages, 10 figures and 3 table

Exploiting polarization state for beyond 10 Gbps underwater optical wireless data transmission in hostile channel conditions

This paper experimentally demonstrates the performance of subcarrier intensity modulation with polarisation division multiplexing (SIM-PDM) in a range of different water conditions. Underwater optical wireless communication (UOWC) is an emerging technology that offers high speed, low latency links over link distances in the order of metres. However, the effects of the UOWC channel present a challenge when designing a reliable link. These include: turbulence induced fading, which causes fluctuations in the received signal amplitude; particulate absorption, which causes an attenuation in the received optical power; and scattering, which causes spatial and temporal dispersion in the received signal. The SIM technique offers a resilience to turbulence compared to the state of the art on-off keying scheme, whilst additionally offering the potential for multi-level modulation orders – and therefore increased data rates – by encoding data on the signal phase as well as amplitude. In this work, PDM is used in conjunction with SIM to increase the spectral efficiency by separately modulating data across two orthogonal polarisation states. As long as these signals propagate identical channels, the polarisation states are maintained. Here, two orthogonally polarised laser beams are independently modulated with quadrature amplitude modulation (QAM), implemented via SIM to form the QAM-SIM-PDM technique. The performance of this technique is evaluated in terms of bit error rate and the maximum achievable data rate in clear, turbulent, and turbid water conditions. It is shown that data rates in excess of 10 Gbps are achievable using the QAM-SIM-PDM technique