Research on hybrid dual-hop radio frequency (RF)/free-space optical communication systems can improve the multipath fading robustness and communication coverage. However, since the co-channel interference problem in RF communication systems cannot be ignored, the adoption of a multi-user diversity scheme can improve the adverse interference effect and enhance the system performance. Meanwhile, optical reconfigurable intelligent surface (RIS) is introduced in the free-space optical (FSO) communication link to enhance the signal quality when the FSO communication cannot fulfill the conditions of line-of-sight communication.

To improve the co-channel interference in RF communication, we utilize the simultaneous transmission of multiple users to generate diversity gain and consider the problem that FSO communication cannot complete line-of-sight transmission. Additionally, RIS technology is introduced in the FSO link to put forward a scheme of RIS-assisted MUD-RF/FSO hybrid system under co-channel interference. The RF link with multi-user diversity obeys the independent homogeneous Rayleigh distribution, the optical RIS-assisted FSO link obeys the Gamma-Gamma distribution, and the decode-and-forward protocol is adopted at the electro-optical conversion relay node. Based on the probability density function (PDF) of the system's end-to-end instantaneous signal to noise ratio (SNR) and its cumulative distribution function (CDF), closed expressions for the system outage probability and average bit error ratio (BER) are derived. Simulation results show that RIS can significantly improve the performance of the MUD-RF/FSO hybrid system, and the increasing user number can bring diversity gain to the system, thus suppressing the undesirable effects caused by co-channel interference.

In strong turbulence conditions, the SNR of the FSO link is fixed at 50 dB to study the effect of different L and M on the outage probability of the system. The outage probability performance of the system is dramatically improved when the user number is changed from L=1 to L=2. This is attributed to the simultaneous transmission of multiple users generating a diversity gain (Fig. 2). Meanwhile, the SNR of the RF link is fixed to be 50 dB under strong turbulence to study the effect of different numbers of RIS reflection element surfaces and threshold values on the system interruption probability. When the threshold value is decreased from 4 dB to 1 dB with N=8, the system has a gain of about 3 dB under the interruption probability of 10^-3, indicating a great effect of different threshold values on the system performance. When the SNR of the FSO link is greater than 50 dB, the system interruption probability remains basically unchanged, which shows that the RF link plays a dominant role at this time. Under the same threshold, with the rising reflective elements on the optical RIS, the interruption probability of the system gradually decreases, which is because the increase in reflective elements can improve the channel quality of the FSO link and enhance the system performance (Fig. 3). The SNR of the RF link and FSO link are both fixed at 30 dB to compare the effect of different aperture radii and beam widths on the system outage probability. As the ratio of the aperture radius and the beam width increases, the system outage probability decreases. The system performance deteriorates when the turbulence intensity changes from weak turbulence intensity to strong turbulence intensity, indicating that the turbulence intensity can exert great influence on the FSO link (Fig. 4). To determine the effect of standard deviations of different pointing error angles and intelligent channel reconfigurable node (ICRN) deflection error angles on the average bit error rate of the system, we set the SNR of a fixed RF link as 20 dB in strong turbulence conditions. The smaller standard deviation of the pointing error angle and ICRN deflection error angle leads to a smaller corresponding value of the average bit error rate of the system, and the larger pointing error coefficient brings better system performance (Fig. 7).

We analyze the performance of the RIS-assisted MUD-RF/FSO hybrid system under co-channel interference. RF links obey Rayleigh distribution, RF links in the presence of CCI also obey Rayleigh distribution, decode-and-forward protocol is employed at relay nodes, and optical RIS-assisted FSO links obey Gamma-Gamma distribution. Meanwhile, the closed formulas for the system outage probability and average BER are derived and analyzed numerically. The simulation results show that the RF links play a dominant role at high SNR. The number of interfering signals and their corresponding SNRs have different degrees of influence on the system performance, while the increase in the user number can attenuate the adverse interference effects on the system. The system performance can be enhanced by increasing the number of RIS reflection units, improving the ratio of the aperture radius to the beamwidth, weakening the turbulence intensity, decreasing the threshold value, and lowering the standard deviation of the pointing error angle and the ICRN deflection error angle. Additionally, the best system performance is obtained by BPSK modulation.