The problems of spectrum limitations and co-channel interference (CCI) in radio frequency (RF) communication systems have limited the development of high-speed rate services. At the same time, free-space optical (FSO) communication is complementary to RF communication because it does not require a spectrum license and is highly resistant to interference. However, FSO communication is susceptible to atmospheric turbulence and pointing errors, which can deteriorate the communication link and even cause disruptions. In order to improve the performance of FSO systems, traditional solutions use spatial diversity, aperture averaging, and FSO-RF hybrid systems. Based on the FSO-RF hybrid system, we introduce reconfigurable intelligent surface technology in the FSO link to solve the problem of FSO being unable to carry out line-of-sight communication. Considering the CCI problem in the RF link, we propose a reconfigurable intelligent surface (RIS)-assisted FSO-RF hybrid system scheme under CCI, which further improves the performance of the system compared with the traditional FSO/RF scheme.

We present a scheme for a hybrid FSO-RF system under CCI. Firstly, reconfigurable smart surface techniques are introduced in the FSO link for the case where line-of-sight transmission is not possible for FSO communication, and CCI signals in the RF link are considered. The FSO link obeys the Gamma-Gamma distribution, and the RF link obeys the Rayleigh distribution. In addition, a decode-and-forward protocol is used at the optoelectronic switching relay node. Then, based on the probability density function of the system end-to-end instantaneous signal-to-noise ratio (SNR), closed-form expressions for the system outage probability and average bit error rate (BER) are derived, and Monte Carlo simulation is used to verify the accuracy of the results. Finally, the corresponding conclusions are drawn based on the diversity order of the system.

With a fixed average SNR of the FSO link, the outage probability performance of the system deteriorates as the H and CCI SNR increase, i.e. when the number of interfering signals at the destination node and the corresponding SNR increase. Moreover, under the same parameter conditions, the RIS-assisted system can achieve an outage probability of 10^-4, while the conventional hybrid system can only achieve an outage probability of 10^-2, indicating that the performance of the RIS-assisted two-hop hybrid FSO-RF system under CCI is much better than that of the traditional FSO/RF system (Fig. 2). The performance of the proposed FSO-RF hybrid system is better than that of the traditional FSO/RF hybrid system under the same parameters with fixed average SNR of the RF link and for different pointing error coefficients on the outage probability of the system. For the RIS-assisted hybrid system, when the pointing error coefficient is reduced from 1.23 to 0.73, the outage probability of the system increases significantly, indicating that the pointing error coefficient is the dominant factor affecting the outage probability of the system (Fig. 3). The effect of five modulation methods on the average BER of the system is analyzed for a fixed FSO link average SNR. The two modulation methods, differential binary phase shift keying (DBPSK) and noncoherent binary frequency shift keying (NBFSK), are close in effect, but DBPSK performs better. The RIS-assisted hybrid system can significantly reduce the average BER compared with the traditional FSO/RF system with the same modulation, i.e. both using BPSK. The results are consistent with the simulation results, indicating the accuracy of the obtained formulae (Fig. 5). The effect of the five modulation methods on the average BER of the system is analyzed for a fixed RF link average SNR, with low BER. DBPSK>NBFSK>OOK>BFSK>BPSK when γˉSR is less than 30 dB. NBFSK>DBPSK>OOK>BFSK>BPSK when γˉSR is higher than 30 dB. In comparison with the traditional FSO/RF system, the two BERs can largely coincide under high SNR conditions considering the effect of strong pointing errors, but for low SNR conditions, the performance of the RIS-assisted hybrid system is significantly improved (Fig. 6).

In this paper, the performance of RIS-assisted FSO-RF hybrid systems under CCI is investigated. The FSO link under RIS assistance obeys the Gamma-Gamma distribution, and the RF link in the presence of CCI obeys the Rayleigh distribution. The decode-and-forward protocol is used at the relay node, and closed-form expressions for the system outage probability and the average BER are derived. In addition, to obtain more significant conclusions, an asymptotic analysis of the outage probability is also provided in this paper, and Monte Carlo simulations are performed to analyze them numerically. The simulation results show that HD detection outperforms IM/DD detection among the different light detection methods and that this detection method is effective in combating the effects of atmospheric turbulence on the system. Compared with the traditional FSO/RF system, the performance of the proposed system solution in this paper is greatly improved with the assistance of RIS, even in the presence of CCI.