## 2 System and Channel Model
### One-Hop Hybrid FSO/RF Link Based on Selective Combination Scheme
Assume that the average SNR of each bit in the FSO and RF sub-links is equal, that is, \(\overline{\gamma}^{\text{FSO}}=\overline{\gamma}^{\text{RF}}\), and the distance of the direct link is 1 km for Figures 2-4. For strong turbulence, for the pointing errors parameter \(\xi=1\) and fading parameter \(m=2\), Figure 2 describes the relationship between the ABER performance and the SNR of the hybrid FSO/RF direct link under different modulation and detection schemes. It can be seen from Figure 2 that under the same detection scheme, the ABER performance of the hybrid FSO/RF direct link from high to low is: CBPSK, DBPSK, CBFSK, and NBFSK. Compared with frequency modulation, the ABER performance of the hybrid direct link under phase modulation is better, while the ABER performance of the hybrid direct link under coherent modulation is better than that under incoherent modulation. For example, when the heterodyne detection scheme is adopted and the SNR is 20 dB, the ABERs of the hybrid direct link with CBPSK, DBPSK, CBPSK, and NBFSK modulation are \(6.159\times 10^{-6}\), \(1.876\times 10^{-5}\), \(3.854\times 10^{-5}\), and \(1.158\times 10^{-4}\), respectively. The ABER of the hybrid FSO/RF direct link with the heterodyne detection scheme is lower than that with the direct detection scheme. For example, when the CBPSK modulation scheme is adopted and the SNR is 20 dB, the ABERs of the hybrid direct link under heterodyne detection and direct detection are \(6.159\times 10^{-6}\) and \(3.449\times 10^{-5}\), respectively. With CBPSK modulation and the fading parameter \(m=2\), Figure 3 depicts the relationship between the ABER and the SNR of the hybrid FSO/RF direct link under different turbulence intensity, detection schemes, and pointing errors. Comparing Figure 3a,b, it is easy to find that the ABER of the hybrid FSO/RF direct link with the heterodyne detection scheme is better than that of the direct detection scheme for any turbulence intensity and pointing errors. For example, for any turbulence intensity and pointing errors, the ABER of the hybrid direct link with the heterodyne detection is lower than \(10^{-5}\) when the SNR is 20 dB in Figure 3a, and the ABER of the hybrid direct link with the direct detection is higher than \(10^{-5}\) in Figure 3b. Although compared with only the FSO link, the performance of the hybrid FSO/RF link has been greatly improved, it can be seen from Figure 3 that the turbulence intensity and the pointing errors still have a significant impact on the ABER of the hybrid direct link. For example, in Figure 3a, with the heterodyne detection and 16 dB SNR, the ABERs of the hybrid direct link with the pointing errors parameter \(\xi=1\) and \(\xi=3\) under moderate turbulence are \(4.001\times 10^{-5}\) and \(1.445\times 10^{-5}\), respectively, and the ABERs of the hybrid direct link with the pointing errors parameter \(\xi=1\) and \(\xi=3\) under weak turbulence are \(3.879\times 10^{-5}\) and \(1.249\times 10^{-5}\), respectively. With CBPSK modulation and \(\zeta=1\), Figure 4 shows the relationship between the ABER and the SNR of the hybrid direct link under different turbulence intensity, detection schemes, and RF fading parameters. It can be seen from Figure 3 that the RF fading parameter has a greater impact on the ABER performance of the hybrid FSO/RF direct link. When this increases, the ABER of the system increases. For example, with a 20 dB SNR and moderate turbulence, the ABERs of the hybrid direct link with \(m=2\) and \(m=4\) under heterodyne detection are \(2.992\times 10^{-6}\) and \(2.13\times 10^{-8}\), respectively, and the ABERs of the hybrid direct link with \(m=2\) and \(m=4\) under direct detection are \(2.823\times 10^{-5}\) and \(1.228\times 10^{-7}\), respectively. Figure 3: Relationship between the ABER and the SNR of hybrid FSO/RF direct link under different turbulence intensity, detection schemes, and pointing errors. (**a**) Heterodyne detection (HD). (**b**) Direct detection (DD).