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<p><span>The paper presents a BER analysis for hybrid Radio Frequency/multihop Free Space Optics, known as RF/FSO, communication system benefiting from Multiple Input Multiple Output (MIMO) configuration but impaired by Gamma-Gamma, known as GG, turbulence. Authors deploy Multiple Pulse Position Modulation, known as M-PPM, like a signaling technique. Furthermore, the exact as well as approximated amounts of Symbol-Error Rates, known as SERs, are both judged in this work. A closed form formula related to the whole Probability Density Function, called PDF, for our designed system is derived. Such interesting approach provides to us better quality of service by combating atmospheric turbulence.</span></p>

Atmospheric turbulence induced fading is one of the main impairments affecting the operation of free-space optical (FSO) communication systems. In this work, the average symbol error rate (SER) of M-ary pulse position modulation (PPM) is analyzed under Gamma-Gamma fading channel. Using the characteristics of Meijer G function, Closed-form expression for average SER is derived. In this paper, we analyze the performance of detect and forward relay or/and amplify and forward three-hop FSO systems over strong atmospheric turbulence channels. In our analysis, turbulence channels are modeled by Gamma-Gamma distribution and the AF transmission is based on optical amplify-and forward (OAF) technique. The DF relay refers to detect and forward protocol. We look for symbol-error probability (SEP) of dual-hop PPM modulated FSO systems taking into account the effects of various noises as well as path loss and geometric spreading of optical beam. The numerical results show that strong turbulence has a severe effect on the performance of FSO systems. we employ PPM modulation to improve the system's performance.

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