CHAPTER ONE

1.0 INTRODUCTION

1.1 Brief Overview of Free-Space Communication

Telecommunication is a process whereby information is encoded and transmitted over a distance through a channel or a medium by a sender to one or more receiver electronically (Advance English Dictionary). From the above definition the word that is to be emphasis is “distance” gotten from the Greek prefix “tele” meaning “distant”. Distance is a major point to consider when building a telecommunication device or system, therefore scientist over the years have worked and are still working on different means to enhance the efficiency of the distance and data-rate of telecommunication systems. Examples of such works are in the area of Radio and Microwave communications, Light Amplification by Stimulated Emission of Radiation (LASER) or optical communication systems and wired communication technology among others [5].

Optical communication systems are in various form and have been used for thousands of years. Among the early users of this system are the Ancient Greeks, they polish their shields to sends signals during battle [6]. In modern era, semaphores and wireless solar telegraphs called heliographs have been developed using coded signal to communicate with the receiver.

In the past ten to twenty years free-space optical communication (FSO) has attain a state of interest as an alternative to radio frequency communication [1]. Free-space optics also known as free-space photonics (FSP) is a branch of optical communication system that deals with the transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain data or broadband communication. Some of the sources of these beams used are LASER and non-lasing sources like light-emitting diodes or IR-emitting diodes (IREDs) [4]. This technology is useful in areas with high electromagnetic interference (EMI) and therefore can be used for the most demanding industrial and domestic applications [5].

The growing need for high-speed and tap-proof communication systems to create links to satellites, ground stations, deep-space probes, unmanned aerial vehicles (UVAs), high altitude platforms (HAPs), aircrafts and other mobile or static communication systems makes the improvement and works on free-space optics a very high priority for telecommunication standards societies and space agencies such as; International Telecommunications Union (ITU) and National Aeronautics and Space Administration (NASA) among others. Links created by this technology can be used in both military and professional fields [1]. For example, it will take Radio frequency (RF) systems nine years to transmit a 30cm resolution map of the entire Martian surface (at one bit per pixel), but with an FSO system (such as LASERCOMM) this can be achieved in nine weeks.

FSO links between much distanced buildings have long been developed and established in developed countries such as the United State of America, but this technology just seems to be lacking in African countries like Nigeria among others [6]. If this technology is implemented in Nigeria it will greatly increase data-rate, and “Last Mile Challenges” can be reduced or completely eliminated.

Furthermore, free-space optic (FSO) systems are license-free access technologies with high frequency and very short wavelength therefore allowing higher data transmission, with less transmit power. It is a potential alternative or complement solution to common physical layers like fiber optic links. But in spite of the major merits of the FSO technology and diversity of its applications, works is on-going on ways to reduce or eliminate some of the major limitations of the technology such as fading resulting from ‘index-of-refraction turbulence (IRT) and link blockage by clouds, snow and rain [4].

In this project, a model will be designed to transmit and receive four bit numerical integers over a reasonable distance in free space to illustrate the working principle of the FSO systems.