# Performance Analysis of Hard-Switching Based Hybrid FSO/RF System over Turbulence Channels
## 1 Introduction
Another relay scheme was proposed in [7; 8], where the relay system was analyzed for two nodes with the dual hop between FSO and RF link. In [9], link adaptation technique was used for a hybrid system where RF-FSO dual link activated as the threshold of FSO link went below a certain threshold. Dual FSO threshold scheme was also discussed which is effective to avoid frequent switching when the weather conditions change rapidly. In [10], performance was evaluated using the selective combining (SC) technique for the hybrid system. Authors in [11] have proposed a rate less coding scheme using off-the-shelf Raptor code in hybrid FSO/RF system. They have compared the hybrid system with and without rate less coding to prove that hybrid system with rate less coding can fully utilize the available RF and FSO link resources. In this paper, a hybrid FSO/RF system is proposed to increase both the reliability and availability of the network. This system is analyzed assuming that FSO follows Gamma-Gamma fading whereas RF link follows Rayleigh fading. Intensity modulation with direct detection (IM/DD) using on-off keying (OOK) is implemented. The system under consideration is analyzed on the bases of outage probability (OP), bit error rate (BER) and channel's (average) ergodic capacity. To the best of our knowledge, the performance of the hybrid system with Gamma-Gamma fading for FSO and Rayleigh fading for RF has never been investigated before, by means of its OP, BER and ergodic capacity estimation. It is considered that information is transmitted with hard-switching either FSO or RF link depending on the circumstances, i.e., only one of the links will operate at each specific time. For the time at which the SNR of FSO link remains above a certain selected threshold, the system keeps working on FSO. However, when the quality of FSO link degrades to a level below the selected threshold, the system moves to RF link. This threshold for FSO link is given by \(\gamma_{th}{}^{FSO}\). There is another case when the system can completely go into outage and it's when the RF signal loses its SNR and goes below the specific threshold \(\gamma_{th}{}^{RF}\). For simplicity, it is assumed that at least one link remains active at a certain time and the system does not interrupt the total Hybrid FSO/RF link. As one link remains active at a time, the transmitter will have to consume less power and receiver design would be simpler as it will not require any combination techniques to extract data like in [9; 10]. Figure 1 portrays the basic idea of the hybrid system while Figure 2 shows the state transition diagram for this hard-switching between FSO and RF link of the Hybrid FSO/RF system under consideration. Weather condition can change rapidly and cause frequent transitions between FSO and RF link. It is assumed that the system does not experience abrupt changes in turbulence and the weather conditions are steady. The remainder of this work is organized as follows: Section 2 will provide a brief introduction of the system model and the derivation of performance analyzing parameters will be achieved in subsection A, B and C, respectively. Section 3 will present the numerical analyzation. Finally, Section 4 will conclude this paper.