Visible-light communications is the name given to a wireless communication system that conveys information by modulating light that is visible to the human eye. Communications may not be the primary purpose of the light; in many applications the light primarily serves as a source of illumination. Interest in VLC has grown rapidly with the growth of visible-light light emitting diodes (LEDs) for illumination. The motivation is clear: When a room is illuminated by LEDs, why not exploit it to provide communications as well as illumination? This sharing of resources can save electric power and raw materials.

VLC principle

A simple means for achieving visible-light communications is to switch the LED lighting on and off at a speed higher than is perceptible to the human eye. Eyes are organs that can detect changes in light brightness and power when these changes occur over a long time scale, but they cannot perceive light that is switched on and off rapidly, say at 200 Hz or more, depending on the eye. A photodiode, on the other hand, can easily recognize the the rapid on–off modulation. A photodiode is a photodetector that produces an electrical current that is proportional to the optical power that is incident on the photodetector surface. This simple principle makes possible visible-light communication technology that supports both illumination and wireless communication using an LED (see Figure 14.1).

The dual function of illumination and communications means that the user can always recognize a communications area.

Introduction

Wireless sensor networks have been an active area of research for the past few decades [1, 2]. The availability of low-power radio communications, microprocessors, and sensors at low cost has led to highly capable sensor “motes” that can be connected in wireless networks (see for instance [3]). Environmental monitoring [[2], monitoring of structures [4, 5], industrial process control [6], and mechanical systems [7] have all used wireless sensor networks, and such deployments are likely to become much more widespread. The next generation of appliances and objects will be part of the Internet of Things [8], and these will be connected wirelessly to each other and to an information infrastructure. Reduction in carbon emissions will require a smart electrical grid [9] that allows for local generation of power and control of its use within the home at the appliance level. This will in turn require large numbers of sensors and actuators which will both monitor and control generators and appliances.

In the field of healthcare the ageing population will lead to greater demands on resources, and home monitoring of patients has the potential to prolong life and quality of life whilst reducing demands on resources. Body area networks of sensors are being pursued as means to collect data to allow this [10].

A major challenge for sensor networks is that of prolonging sensor node lifetime, which is usually limited by battery capacity. An ideal sensor node would scavenge all the energy required from its environment, and therefore have an “infinite” operating life, barring failure.

Wireless optical communications underwater is enjoying a renewed interest from researchers due to the wide advances in laser sources and receivers, digital communications, and signal processing. Underwater free space optics (uFSO) fulfills several niche applications for wireless communications in ocean waters. While RF communications have become ubiquitous in our everyday lives above water, the RF portion of the electromagnetic spectrum exhibits high attenuation in seawater. Acoustics on the other hand have long enjoyed success for detection and communication underwater, given their ability to propagate long distances underwater (> km). However, for high-speed data transfer (> Mbps), acoustics are at a disadvantage, as it is well known that acoustic energy exhibits increasing attenuation with increasing frequency. Supported by enormous growth in the telecommunications industry over the past few decades, optical techniques are garnering serious consideration for underwater communications due to the high data rates they may provide. Additionally, as we will learn, the blue/green portion of the visible spectrum exhibits minimal absorption in seawater. Still, scattering of light by organic and inorganic particulates in ocean water can cause significant spatial and temporal dispersion, which may have a measurable impact on link range and available bandwidth.

This chapter serves as both an introduction to the field of light propagation under-water, as well as a survey of current literature pertaining to uFSO. We begin with a simple examination of a link budget equation. Next, we present an introduction of ocean optics in order to gain an appreciation for the challenges involved with implementing free-space optical links underwater.

Abstract: Free-space optical (FSO) communications is a practical solution for creating at three-dimensional global broadband communications grid, offering bandwidths far beyond those possible in Radio Frequency (RF) range. However, attributes of atmospheric turbulence and obscurants such as clouds impose perennial limitations on availability and reliability of optical links. To design and evaluate optimum transmission techniques that operate under realistic atmospheric conditions, a good understanding of the channel behavior is necessary.

In some prior works, the Monte Carlo ray tracing (MCRT) algorithm has been used to analyze the channel behavior. This task is quite numerically intensive. The focus of this chapter is on investigating the possibility of simplifying this task by a direct extraction of state transition matrices associated with standard Markov modeling from the MCRT computer simulations programs. We show that by tracing a photon's trajectory in space via a Markov chain model, the angular distribution can be calculated by simple matrix multiplications. We also demonstrate that the new approach produces results that are close to those obtained by MCRT and other known methods. Furthermore, considering the fact that angular, spatial, and temporal distributions of energy are interrelated, mixing time of Monte Carlo Markov chain (MCMC) for different types of aerosols is calculated based on eigen-analysis of the state transition matrix and possibility of communications in scattering media is investigated. We also consider in this chapter signal processing techniques for airborne FSO wireless communications through clouds. The FSO channel is known to be accompanied by multi-scattering, which causes severe inter-symbol interference in digital transmissions at high data rates.

Optical wireless communication is an emerging and dynamic research and development area that has generated a vast number of interesting solutions to very complicated communication challenges. For example, high data rate, high capacity and minimum interference links for short-range communication for inter-building communication, computer-to-computer communication, or sensor networks. At the opposite extreme is a long-range link in the order of millions of kilometers in the new mission to Mars and other solar system planets. It is important to mention that optical wireless communication is one of the oldest methods that humanity has used for communication. In prehistoric times humans used fire and smoke to communicate; later in history, Roman optical heliographs and Sumerians signalling towers were the communication systems of these empires. An analogous technology was used by Napoleonic Signalling Towers and “recently” by the light photo-phone of Alexander Graham Bell back in the 1880s.

Obviously, the data rate, quality of service delivered, and transceiver technologies employed have improved greatly from those early optical wireless technologies. In its many applications, optical wireless communication links have already succeeded in becoming part of our everyday lives at our homes and offices. Optical wireless products are already well familiar, ranging from visible-light communication (VLC), TV remote control to IrDA ports that currently have a worldwide installed base of hundreds of million of units with tens of percent annual growth. Optical wireless is also widely available on personal computers, peripherals, embedded systems and devices of all types, terrestrial and in-building optical wireless LANs, network of sensors, and inter-satellite link applications.