同信道干扰下RIS辅助FSO-RF混构系统性能分析
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
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.
1 引言
射频(RF)通信系统存在的频谱局限和同信道干扰(CCI)问题,限制了高速率业务的发展。自由空间光(FSO)通信具有无需申请频谱许可、抗干扰能力强和易于部署的特点[1-2]。但是,FSO通信系统易受到大气湍流和指向误差的影响,从而导致通信链路恶化乃至中断。目前,研究人员利用多种技术方案来改善FSO通信系统的性能,例如空间分集技术[3]、孔径平均技术[4]、RF/FSO混构系统方案[5-6]等。
随着无线通信技术的不断发展,可重构智能表面(RIS)[7]技术成为当前的研究热点。RIS由许多无源和低成本元件组成,可以灵活地部署在无线通信传播环境中,并且能够控制撞击信号的幅度、相位和频率,而无需复杂的编译码过程。为了缓解RF通信系统中的干扰问题和扩大拥塞网络的覆盖范围,在RF链路中引入了RIS技术[8]。Yang等[9]分析了基于RIS的双跳混合FSO/RF系统的性能,发现在RF链路中引入RIS可以显著提升系统的性能。Li等[10]研究了RIS辅助RF/UMOC系统的性能,通过中断概率(OP)和平均误码率(BER)的渐进表达式,推导了系统的分集阶数,证明了RIS辅助系统能够有效提高混合双跳RF/UMOC系统的性能。Padhan等[11]分析了RIS辅助PLC/RF系统在智能电网应用中的性能,结果表明,RIS可以降低所考虑系统的BER。由于频谱资源的重复使用,CCI会严重降低接收信号的质量[12],因此分析其对通信系统的影响尤为重要。Petkovic等[13]研究了CCI影响下的混合RF/FSO系统的OP性能,并假定中继节点处存在多个CCI,其中RF链路遵循Nakagami-m分布,FSO链路考虑了具有大气湍流效应和指向误差影响的Gamma-Gamma分布,推导出准确的OP封闭表达式。Balti等[14]研究了多个CCI影响下混合RF/FSO系统的性能,推导了Meijer-G、单变量及多变量Fox-H函数的OP、BER和信道容量,并得出高信噪比下的渐进表达式。在此基础上,Sikri等[15]研究了多个CCI影响下的RIS辅助RF/FSO系统的性能,通过推导OP和BER新的闭合表达式评估所考虑系统的性能,仿真结果表明,即使存在CCI,基于RIS的混合RF/FSO系统与传统混合RF/FSO系统相比性能也显著增强。上述文献介绍了微波波段的RIS技术,而光波波段RIS的设计、制造和分析一直是纳米技术、材料科学和物理学学科中活跃的研究领域之一[16-18]。光学RIS由亚波长的光学天线阵列组成,这些天线会与传入的电磁辐射发生光-物质的共振作用,从而控制光的振幅、相位和偏振[19]。此外,与微波波段RIS部署类似,光学RIS可以反射入射光束,控制光束强度、相位、频率和极性,并根据用户的移动调制输出光束的方向[20]。文献[21]提出一种光学RIS辅助FSO链路的通信理论分析方法,描述了光学RIS的物理参数对端到端FSO信道质量的影响,但是该模型并没有考虑大气湍流的影响。在此基础上,Ndjiongue等[22]研究了光学RIS辅助FSO系统的性能,并推导了系统的OP和BER,其中FSO链路满足指向误差和大气湍流联合影响下的Gamma-Gamma模型的要求。Yang等[23]研究了多个光学RIS辅助FSO系统,FSO链路服从Gamma-Gamma分布,并根据力矩生成函数推导了OP、BER和信道容量的闭合表达式,仿真结果表明,随着光学RIS上元素数量的增加,系统的性能得以提升。然而,光学RIS在混构FSO/RF通信系统中的应用尚未得到深入研究。
为了解决FSO通信无法进行视距传输的问题,本文在FSO链路中引入RIS技术,并考虑RF信道由于频谱复用问题而受到多个CCI信号的影响,提出一种CCI影响下的RIS辅助FSO-RF混构系统方案。其中RIS辅助的FSO链路服从Gamma-Gamma衰落分布,CCI影响下的RF链路服从Rayleigh衰落分布。另外,在中继节点处考虑使用译码转发协议,基于系统端到端瞬时信噪比的概率密度函数(PDF),得到了系统的OP和BER,并进一步推导了系统OP的渐进表达式,导出系统分集顺序。仿真结果表明,RIS辅助下的FSO-RF系统相较于不存在RIS的系统性能得到了提升。
2 系统与信道模型
CCI影响下基于RIS辅助的FSO-RF混合系统如
2.1 RIS辅助FSO链路模型
第一阶段,即S-RIS、RIS-R通信阶段。中继节点R处的光检测器接收到的信号可以表示为
式中:
式中:
通过调整RIS的相位,使得信道相位被消除,即
假设S到RIS和RIS到R两个通道具有相同的指向误差
首先,根据文献[24],得到指向误差
式中:
然后,根据文献[25],大气湍流
式中:
最后,在给定天气和链路距离
对上述积分进行联立求解,最终得到闭合表达式:
式中:
式中:
式中:
根据文献[27]中的式(07.34.21.0084.01)可以计算出信噪比
式中:
2.2 RF链路模型
第二阶段,即R-D通信阶段。在RF链路中,中继节点R通过光电转换装置将光信号转换为电信号,通过RF信道传输到目的节点D,其中D周围存在H个CCI,用符号
式中:
式中:
进而可以求出
根据文献[15],
式中:
3 系统性能分析
3.1 OP分析
在所提系统中,根据S-RIS、RIS-R通信阶段和R-D通信阶段所得到的CDF,并考虑在中继节点处使用译码转发协议,则系统端到端的OP表示为
式中:
3.2 OP渐进分析
为了进一步得到信道参数对系统性能的影响规律,以下将对系统OP进行渐进分析。当
利用文献[27]中的式(07.34.06.0006.01),可将
式中:
因此系统的分集度主要受
3.3 BER分析
所考虑系统的BER为
式中:
将
对于CCI-RF链路,将式(18)代入
对
式中:
将式(
4 仿真分析
为了验证所推导OP和平均BER的准确性,采用蒙特卡罗仿真方法进行对比分析,其迭代次数设置为
不同的H和CCI信号的信噪比对系统OP的影响如
表 1. 仿真参数配置
Table 1. Configuration of simulation parameters
|
图 2. 不同H和CCI信号的信噪比对应的系统OP
Fig. 2. System OP corresponding to different H and signal-to-noise ratios of CCI signals
仿真参数设置为
指向误差系数
图 4. 不同湍流强度和光检测方式对应的OP
Fig. 4. OP corresponding to different turbulence intensities and light detection methods
图 5. 固定FSO链路平均信噪比时调制方式对系统BER的影响
Fig. 5. Effect of modulation methods on system BER for fixed FSO link average signal-to-noise ratio
图 6. 固定RF链路平均信噪比时调制方式对系统BER的影响
Fig. 6. Effect of modulation methods on system BER for fixed RF link average signal-to-noise ratio
5 结论
对CCI下RIS辅助FSO-RF混构系统性能进行分析,其中RIS辅助下的FSO链路服从Gamma-Gamma分布,存在CCI的RF链路服从Rayleigh分布,中继节点处采用译码转发协议。推导了系统OP和BER的闭合表达式,提供了对OP的渐进分析,并通过蒙特卡罗仿真进行数值分析。仿真结果表明,在不同光检测方式中,HD检测的效果优于IM/DD检测,并且该检测方式能有效对抗大气湍流对系统的影响。相较于传统的FSO/RF系统,在RIS的辅助下,即使存在CCI,所提系统的性能也有较大提升。下一步打算对多个光学RIS辅助的FSO-RF混构系统进行分析,并考虑不同大气信道模型的系统性能。
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