International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 06 Issue: 02 | Feb 2019 www.irjet.net p-ISSN: 2395-0072 © 2019, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 1995 Comparative Analysis of OOK, BPSK, DPSK and PPM Modulation Techniques for Intersatellite Free-Space Optical Communication R. PRADEEP 1 , B. VINOTHKUMAR 2 , M. UDHAYAKUMAR 3 , S. DHANALAKSMI 4 1,2,3,4 Assistant professor, KSR INSTITURE FOR ENGINEERING AND TECHNOLOGY --------------------------------------------------------------------------***---------------------------------------------------------------------------- ABSTRACT - The system of transmission of data through a wireless medium is called as Free Space Optical (FSO) communication, which uses modulated carrier wave as a laser beam. Free-space optical communication is the latest technology that has a lot of popularity as an alternative to radio frequency (RF) communication over the last two decades. Optical communication has lots of advantage over baseband or RF transmission systems which has high aggregate bit rates and/ or very long transmission distances. This paper intends to give an overview of optical modulation techniques such as On-Off Keying (OOK), Binary Phase-Shift Keying (BPSK), Differential Phase-Shift Keying (DPSK) and Pulse Position Modulation (PPM) which compares their characteristics Bit Error Rate(BER), Signal-to-Noise Ratio (SNR) and the receiver sensitivity. Keywords - Free Space Optics (FSO), On-Off Keying (OOK), Pulse Position Modulation (PPM), Binary Phase-Shift Keying (BPSK), Differential Phase-Shift Keying (DPSK) 1. INTRODUCTION The key element of the optical communication system is the optical source, which can easily be modulated. In this narrow wavelength band, the energy is concentrated and is capable of being modulated at very high data rates. The semiconductor laser is one of the primary sources of light in modern an optical system which is used as a carrier [1]. At the receiver, they are converted to electronic signals by photodetectors. There are many types of photodetectors, but the photodiodes are used almost in optical communication applications because of their size, suitable material, high sensitivity, and fast response time [2]. The two most commonly used photodiodes are the pin photodiode and the Avalanche Photodiode (APD) because they have good quantum efficiency. It is necessary to understand the characteristics of these photodiodes and the noise associated with optical signal detection for the design of receiver system. Figure 1 Block Diagram of FSO 1.1 PIN Photodiode The pin photodiode consists of p region and n regions separated by a lightly n-doped intrinsic I region. When an incident photon has energy, the photon can give up its energy and excite an electron. This generates free electron- hole pairs called photocarriers. The pin photodetector is designed, where most of the incident light is absorbed [1]. The performance of a pin photodiode is often characterized by its responsivity which is given by:    ( 1) Where I am the average photocurrent P optical power incident on the photodiode. 1.2 Avalanche Photodiode The primary signal photocurrent is multiplied internally by an avalanche photodiode. This increases the receiver sensitivity [1]. The pin photodetector and electronic amplifier have an advantage over APD because of its low cost [1]. For Free Space Optical (FSO) communication systems the required power under various modulation schemes can be derived from the Bit Error Rate (BER) expression for all modulation schemes and can be expressed as follows     √ (2) Where R is the responsivity of the photodiode.  Is the total noise power in the detector current. SNR is the signal-to-noise ratio. 2. OPTICAL INTERSATELLITE LINK Optical (inter-)satellite communication describes data transmission between satellites using laser sources in the near- infrared spectrum instead of the conventional radio frequency (RF). Fig. 1 illustrates a typical OISL scenario. As part of an earth observation process a satellite in a low- earth orbit (LEO) accumulates a high amount of data. This data should be transmitted to a ground station (GS) on earth in real time, e.g. in case of a fast tsunami forecast