# Performance Analysis of Hard-Switching Based Hybrid FSO/RF System over Turbulence Channels
## 3 Numerical Results  
In this section, numerical examples are presented to illustrate our analysis. It is assumed that the transmitted power is equal to 320 mW for the FSO link and 16 mW for RF. Optical receiver responsivity \(R\) is fixed at a typical value of 0.5 \(A/W\) and the variance of shot noise for the FSO is \(\sigma_{FSO}=10^{-14}A^{2}\). For Gamma-Gamma distribution, typical values for strong atmospheric turbulence are selected to be \(\alpha=2.064\), \(\beta=1.342\), [6], while the typical value of refractive-index structure parameter is used for light haze, i.e., \(C_{n}^{2}=1.8\times 10^{-14}m^{-2/3}\). This value of \(C_{n}^{2}\) is considerable for visibility range of 4-10 km, so with our selected parameters, the distance between the transmitter and the receiver must not be more than 10 km, [12]. The threshold for both links of the hybrid communication system is fixed at \(\gamma_{th}=5\) dB. Additionally, it is assumed that RF link is always available and so the system never goes into complete outage. The antenna gains of transmitter \(G_{tx}\) and receiver \(G_{rx}\) are assumed to be 16 dBi and 30.4 dBi, respectively, which are typical values for such type of equipment. In Figure 3, using the obtained Equation (14), the OP is presented for three different cases as a function of the average electrical SNR for the FSO link at the receiver. The red curve shows the OP for FSO-only case which means that here RF link remains inactive for all the values of average SNR of FSO i.e., even when average SNR of FSO link goes below the selected threshold for switching. The pink and blue curves are for hybrid FSO/RF system for which average SNR of RF link is kept constant at 5 dB and 10 dB, respectively [19]. It can be seen from Figure 3 that the reliability of hybrid FSO/RF system surmounts the other case. We can also observe that for a hybrid system, the reliability improves by increasing the average SNR for the RF, i.e., performance is better for larger SNR values for the RF link. Figure 3: Probability as a function of average SNR of FSO link. In Figure 4, using the Equations (16), (19) and (20), shows the variations in average BER versus \(\overline{\gamma}_{FSO}\). It can be observed that by keeping the threshold fixed and with high quality of RF link, hybrid FSO/RF system shows improvement in performance as the value of \(\overline{\gamma}_{FSO}\) increases. For analyses, average SNR threshold is kept at 5 dB for FSO link. Improvement in BER can be seen because, in lieu of switching the transmission off, a feeble link is used for transmission and to keep the communication channel to be always available. With a better quality of radio frequency link, significant melioration in performance can be seen with varying SNR of FSO link. This effect can be observed from pink and blue curves of the graph in Figure 4 that high SNR link, i.e., \(\gamma_{RF}=10\) dB, [19], has less outage probability than low SNR link, i.e., \(\gamma_{RF}=5\) dB, [19]. Better quality of the backup link, i.e., RF, improves the overall performance of the hybrid system, as expected. The three curves in Figure 5, using Equations (21), (23) and (25), shows capacities of FSO-only, hybrid system with \(\gamma_{RF}=5\) dB and hybrid system with \(\gamma_{RF}=10\) dB, [19]. The result is as expected, the hybrid FSO/RF system shows improvement in the ergodic capacity of the system especially at low average SNR of FSO link \(\overline{\gamma}_{FSO}\). Over low \(\overline{\gamma}_{FSO}\) the capacity of only FSO link is far less than that of hybrid FSO/RF, but at higher \(\overline{\gamma}_{FSO}\) its capacity improves also. At higher \(\overline{\gamma}_{FSO}\) the system operates more on FSO link instead ofactivating available RF link. For this reason, the capacity shown by a red curve for FSO-only approaches the curve of hybrid FSO/RF system as the two cases converge. It should be mentioned here that for marginal cases of our work, our obtained results appearing in Figures 3-5, can be compared with previous studies [19; 12].