Ergodic Capacity and Error Performance of Spatial Diversity UWOC Systems over Generalized Gamma Turbulence Channels

In this paper, we study the ergodic capacity (EC) and average bit error rate (BER) of spatial diversity underwater wireless optical communications (UWOC) over the generalized gamma (GG) fading channels using quadrature amplitude modulation (QAM) direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM). We derive closed-form expressions of the EC and BER for the spatial diversity UWOC with the equal gain combining (EGC) at receivers based on the approximation of the sum of independent identical distributed (i.i.d) GG random variables (RVs). Numerical results of EC and BER for QAM DCO-OFDM spatial diversity systems over GG fading channels are presented. The numerical results are shown to be closely matched by the Monte Carlo simulations, verifying the analysis. The study clearly shows the adverse effect of turbulence on the EC & BER and advantage of EGC to overcome the turbulence effect

Multiplexed visible light communication systems using GaN-based sources

With the emergence of efficient semiconductor solid state lighting a new application space has emerged for communications, namely visible light communications (VLC). The high speed modulation capabilities of Gallium Nitride (GaN) based LEDs and laser diodes means these devices have the potential to supplement or replace existing radio wave standards such as Wifi, as well as creating new applications for optical communications such as underwater VLC. Given the ever increasing demand for information in modern society, it is desirable to continually increase the bandwidth capabilities of communication systems through both exploration of unused frequency spectrum, like the visible spectrum, and also the application of existing and developing multiplexing techniques. This thesis will focus on the investigation of quadrature amplitude modulation (QAM) based orthogonal frequency division multiplexing (OFDM) as applied to GaN based sources in various VLC systems. In this thesis investigation of complex modulation formats and advanced multiplexing techniques applied to novel m-LED devices has shown VLC system bandwidths of up to 655 MHz and data rates of up to 7.91 Gbit/s which, to the authors knowledge, is the highest data rate achieved using a single m-LED pixel for data transmission. The laser based VLC systems shown in this thesis have utilised both the simplest form of baseband modulation on off keying (OOK) as well as QAM based OFDM. These systems, at the time of publication, demonstrated the highest data rate achieved for each of these modulation types using commercially available devices. In addition to multiplexing using orthogonal frequencies the implementation of spatial multiplexing has become an area of great interest for free-space optical (FSO) communication links, particularly for its use in last-mile links within larger optical networks. Light carrying orbital angular momentum (OAM) has emerged as a potential candidate that could be utilised for multiplexing independent channels. The feasability of OAM multiplexing underwater has been investigated through analysis of inter-channel crosstalk for a set of 11 OAM modes propagating through 3 m of slowly flowing water, similar to that found in Oceanic conditions. At publication this was the first investigation of its kind where crosstalk effects induced by flowing water were measured

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

FSO通信システムの新高度化技術に関する研究

早大学位記番号:新7795早稲田大

Underwater optical communication systems

I present in this thesis the results of my research in the field of underwater optical communication system (UOCS). Recently, underwater free-space optical (FSO) communication has been one of the major interesting research subjects due to the high demand for underwater activities that require high-bandwidth and flexible solutions. An optical communication system consists of three main blocks: transmitter, medium and receiver. Therefore, I provide an introduction to the topic, outlining the physical reasons and the engineering challenges that are behind the following sections. Among others, the optical properties of ocean water, the optimum wavelength selection and a description of the main system components are provided so that the system link budget can be subsequently optimised. Various modulation schemes adopted in free-space optics communication are evaluated and compared with a focus on the most power-efficient modulation format that offers successful and reliable data transmission. The system performances are numerically investigated using statistical analysis techniques over a typical range of achievable SNR in an underwater scenario. The characterisation and the data performance of a commercial GaN-based laser diode operating at 450 nm are presented. The detrimental impact of the solar background power on the system performance and the strategy to minimise its contribution is also discussed for a conventional Silicon PIN detector and the novel Silicon PhotoMultiplier (SiPM) technology. The design trade-off between the performance improvement given by an optical narrow bandpass filter matching a Fraunhofer line and the field-of-view of the receiver is also presented. A flexible MATLAB model has been developed to simulate a range of different scenarios and evaluate the system performances in different situations and the necessary design trade-off. A novel free-space waveguiding method is also presented for laser-based underwater communication systems. The proposed Underwater Wireless Acousto-Optic Waveguide (UWAOW) generates a localised modification of the refractive index of seawater in response to an acoustic field. Two geometries and their modelling are provided in order to take advantage of the proposed technique. These results show the importance of this emerging, challenging and fascinating contemporary research field