摘要
为了解决射频(RF)通信中存在的同信道干扰问题,利用多用户的同时传输产生分集增益,并考虑自由空间光(FSO)通信无法完成视距传输的问题,在FSO链路中引入可重构智能表面(RIS)技术,提出一种同信道干扰下RIS辅助多用户分集(MUD)-RF/FSO混构系统方案。其中,具有多用户分集的RF链路服从独立同分布的Rayleigh分布,光学RIS辅助的FSO链路服从Gamma-Gamma分布,在电光转换中继节点处采用译码转发协议。基于系统端到端瞬时信噪比的概率密度函数及其累计分布函数,推导了系统中断概率和平均误码率的闭合表达式。仿真结果表明,RIS可以显著提升MUD-RF/FSO混构系统的性能。此外,用户数量的增加可以为系统带来分集增益,从而抑制同信道干扰所带来的不良影响。
Objective 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.Methods 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 MUDRF/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.Results and Discussions 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).Conclusions 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,decodeand-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.
作者
包朝园
曹阳
彭小峰
邢雯珺
Bao Chaoyuan;Cao Yang;Peng Xiaofeng;Xing Wenjun(School of Electrical and Electronic Engineering,Chongqing University of Technology,Chongqing 400054,China)
出处
《光学学报》
EI
CAS
CSCD
北大核心
2023年第22期108-116,共9页
Acta Optica Sinica
基金
重庆市教委科学技术项目(KJQN201901125)
重庆市教委基金(KJ120827)
重庆市科委社会事业与民生保障科技创新专项(cstc2017shmsA40019)
重庆市基础与前沿研究计划项目(cstc2019jcy-msxm X0233)
重庆市研究生科研创新项目(CYS23667)。
关键词
多用户分集
可重构智能表面
中断概率
平均误码率
同信道干扰
multi-user diversity
reconfigurable intelligent surfaces
outage probability
average bit error rate
co-channel interference
