International Journal of Electrical and Computer Engineering (IJECE) Vol. 13, No. 6, December 2023, pp. 6435~6442 ISSN: 2088-8708, DOI: 10.11591/ijece.v13i6.pp6435-6442  6435 Journal homepage: http://ijece.iaescore.com Improving the performance of free space optical systems: a space-time orthogonal frequency division modulation approach Adedayo Olukayode Ojo, Isreal Esan Owolabi, Moses Oluwafemi Onibonoje Department of Electrical/Electronics and Computer Engineering, College of Engineering, Afe Babalola University Ado Ekiti, Ekiti State, Nigeria Article Info ABSTRACT Article history: Received Oct 7, 2022 Revised Jun 6, 2023 Accepted Jun 26, 2023 Free space optical (FSO) communication systems are known for high-level capacity and information security. The overall system performances of FSO systems are however significantly affected by atmospheric turbulence induced fading. This paper, therefore, proposes a space-time code (STC) technique to mitigate this effect through the introduction of an additional degree of error correction capacity by exploiting the spectral dimension in the coding space. A space-time trellis coded orthogonal frequency division modulation (OFDM) scheme was developed, simulated and evaluated for free space optical communication through a gamma-gamma channel. The evaluation of the coding gain obtained from the simulation results, the mathematical analysis and the truncation error analysis shows that the proposed technique is a promising and viable technique for improving the error correction performance on optical communication links. Keywords: Coding gain Coherent detection Free space optical communication Gamma-gamma channel Space-time trellis codes This is an open access article under the CC BY-SA license. Corresponding Author: Adedayo Olukayode Ojo Department of Electrical/Electronic and Computer Engineering, College of Engineering, Afe Babalola University Ado Ekiti PMB 5454, Km 8.5, Afe Babalola Way, Ado Ekiti. Ekiti State, Nigeria Email: ojoao@abuad.edu.ng 1. INTRODUCTION Performance expectations for new generational communication systems are always increasing. This is partly a consequence of users’ demand for high bandwidth, information security as well as high capacity. These demands have thus far served as the impetus for the design of communication systems that more efficiently meet users’ expectations [1]. In recent years, the radio frequency (RF) spectrum has come under heavy bandwidth, data security, power and licensing constraints, so much that communication systems designers are now looking to deploy optical communication solutions for high-speed communication links. Optical fiber communication is one of the first solutions that became widely accepted for backbone connectivity and other high-capacity point-to-point connections. However, its need for laborious cabling and layout is shifting attention to free space optical (FSO) in an attempt to establish wireless transmission of information in some applications. Free space optical communication entails the modulation of user information onto optical carriers, and the onward transmission of same over unguided free space channel to photoreceptors. FSO communication systems have proven to be up to this task by offering ultra-high data rates, enormous bandwidth, high data security, high immunity to interference and jamming, and avoidance of regulatory issues and spectrum licensing [2]. Broadly, FSO systems are often categorized based on their reception mechanisms. The most widely reported type of FSO system is the intensity modulation/direct detection (IM/DD) [3]. In this type of FSO communication system, the data to be transmitted is encoded by mapping the data signal to the intensity of the