CLIPPING DISTORTION IN OPTICAL WIRELESS COMMUNICATION SYSTEMS: MODEL, SIMULATIONS AND STATISTICS Omar Falou and Xavier N. Fernando (ofalou, fernando@ee.ryerson.ca) Dept. of Electrical and Computer Engineering, Ryerson University, Toronto, Canada ABSTRACT Clipping distortion has been a subject of interest in many recent studies on frequency-division subcarrier multiplexing (SCM) used in cable television applications and wireless communication systems. Clipping occurs when the modulating signal current, which drives the laser diode, occasionally drops below the laser threshold current, turning the laser off. A model was built to investigate this phenomenon. A Rayleigh fading channel was used to simulate the wireless channel. It was found that, for channels with four fading paths, an increase in the number of these paths for the same fading channel will not increase the clipping percentage by as much as it was expect. Furthermore, an increase in the threshold current of the laser diode can significantly minimize the signal distortion. 1. INTRODUCTION Communications through optical fiber have become a major information-transmission system due to many advantages it provides. Networks based on optical fiber technology provides, fast, efficient and reliable means of transmitting information compared to other communication systems such as wireline and wireless systems [1]. A fiber optic transmission system consists of three basic elements: the optical transmitter, the fiber optic cable and the optical receiver. The optical transmitter converts electrical signal into an optical signal. The source of the optical signal can be either a Light Emitting Diode (LED) or a Laser Diode (LD). LDs have some advantages over LEDs since they can be focused to a very narrow range, allowing control over the angle of incidence. They also preserve the character of the signal over long distances, which make them ideal for long transmission distances [2].However, one of the disadvantages of LDs, is that signal distortion can occur, mainly the clipping distortion. Fig. 1. Relationship between optical output power and laser diode drive current [1] Clipping distortion has been a subject of interest in many recent studies on frequency-division subcarrier multiplexing (SCM) used in cable television (CATV) applications [3]-[5]. Clipping occurs when the modulating signal current, which drives the laser diode, occasionally drops below the laser threshold current, turning the laser off [3],[6]. In this project, we build a Simulink model to simulate an optical transmission system having a LD as its optical transmitter. Then, we use this model to investigate the effect of various Rayleigh fading channels on clipping distortion. 2. OVERVIEW OF CLIPPING DISTORTION For laser diodes, the relationship between optical output power and diode drive current is shown in Fig. 1. At lower drive currents, only spontaneous radiation is emitted, therefore, no increase in the power output is observed (Region 1). As the drive current reaches the lasing threshold, a sharply increase in optical power output occurs (Region 2). This region is called stimulated emission region or linear region. At high power outputs, the slope of this curve decreases due to junction heating and saturation is reached (Region 3) [1]. As depicted in Fig. 1, the laser exhibits a threshold current, I th , and the drive current is biased to a DC