Feasibility Evaluation of MIMO Based FSO Links Sangeetha A, Nalini Sharma, and Ipsita Deb Department of Electronics and Communication Engineering, VIT University, Vellore, Tamil Nadu, India Email: asangeetha@vit.ac.in; nalini.sharma2014@vit.ac.in; ipsita.deb2014@vit.ac.in Abstract —Free space optical communication (FSO) has evolved as a substitute to radio frequency communication over the few years. It gives a promising solution to the high-speed point-to-point communication. However, atmospheric absorption, scattering and turbulence cause considerable degradation of the wireless optical communication, which degrades system performance. Another major factor which concerns the system performance is the attenuation of signals due to the aforementioned atmospheric reasons. There are different factors in which the atmospheric turbulence conditions are observed using Gamma Gamma Model. Simulations have been carried out in the OptiSystem 14.0 software to study the impact of different weather conditions (clear, haze and fog) on the performance of the channel. Simulation results show that by implementing Multiple Input Multiple Output (MIMO) techniques for FSO systems, it is possible to reduce the BER for different range and achieve the accurate transmitted data at the receiver side. The performance improvements vis-a-vis, received power levels, bit error rate (BER), Q-factor and link distance range have been demonstrated in the presence of atmospheric turbulence conditions like haze, fog, clear sky etc. Index Terms—Bit Error Rate, FSO, MIMO, OptiSystem 14.0, Q-Factor, SISO. I. INTRODUCTION FSO communication is also known as optical wireless communication (OWC). These days many technologies are available such as coaxial cables, copper wires and optical fibers. These advances have a few constraints, i.e., congested spectrum and range, low information rate, security issues, costly authorization and high cost of establishment and installment [1]. FSO is a technique that transmits the data via the free space optical channel. Transmission of visible and infrared (IR) beams through the atmosphere is inferred from this channel. The motive of FSO is to eliminate the time, effort and cost of installing fiber optic cable, and it also retain the benefit of high data rates (up to 1 GB/s or beyond) for transmission of data, images, voice and video [1]. In this project, we have noticed the bit error rates and eye diagrams for different atmospheric conditions like haze, clear sky and fog. The main challenges we faced while designing this FSO model is to get optimal values which was achieved by using different models like SISO, MIMO and observe the results. Here we have used atmospheric turbulence model to analyze different conditions which is mentioned in this paper. For improvement in the system performance, Manuscript received May 25, 2018; revised February 21, 2019. Corresponding author email: asangeetha@vit.ac.in doi:10.12720/jcm.14.3.187-193 scintillation must be mitigated. A reduction in the spatial diversity and spatial coherence of the transmitted beam can be accomplished by employing multiple receivers and multiple transmitted beams. Lower scintillation at the cost of lower power at the receiver and a bigger divergence angle and is seen by the partially coherent beams with diminished spatial coherence [4]. The use of multiple light sources, allows the transmitter to produce a number of spatially separated channels which can be used to improve to improve spectral efficiency when coupled with multiple receivers or channel characteristics [1]. This spatial diversity makes the receiver to be capable of spatially separating multi-path components for the channel reliability to be enhanced [2]. Therefore, in order to handle the transmission under strong and weak atmospheric turbulence, the concept of MIMO and SISO must be constructed. In this paper, we analyze Multiple Input Multiple Output (MIMO) and Single Input Single Output (SISO) FSO communication system with Non Return to Zero (NRZ) and Avalanche Photodiode (APD) on the receiver side which is then connected to the generator to analyze the BER performance for different atmospheric turbulences. The objective is to design MIMO and SISO FSO link and simulate its performance. On increasing the number of transmitters and receivers in the MIMO system, the efficiency can be increased, which lowers the BER value. The remaining part of the paper is organized as follows. The mathematical model is presented in Section II. The simulation and the observations are made in Section III. Section IV includes discussion along with concluded remarks. II. THEORITCAL MODEL AND PROBLEM FORMULATION A. Free Space Optical (FSO) System FSO system consists of three main functional elements: transmitter-receiver and atmospheric channel as shown in the figure. At the transmitter, the data signal is modulated by modulator with the help of intensity modulation and the electrical signal is converted into an optical signal using optical sources like LED or LASER. For data trans- mission, light sources are used [2]. It propagates via free space to the receiver and transmits data at high data rates [3]. For optical communication, light sources adopted must have the appropriate wavelength, line- width, high modulation bandwidth and numerical aperture. Communication medium or channel is either free space or air or vacuum. Data transmits through this medium from the transmitter to the receiver. 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