【作者】 张妍；

【导师】 章嘉懿；

【作者基本信息】 北京交通大学 ， 信息与通信工程， 2023， 硕士

【摘要】 智能超表面(Reconfigurable Intelligent Surface,RIS)由于其具有智能调控无线通信环境以及提升通信系统性能的能力,被视为第六代(Sixth Generation,6G)移动通信系统的潜在关键技术之一。然而,受到实际各种非理想因素的制约,RIS辅助通信系统的性能分析与优化设计面临着诸多挑战,包括由于RIS硬件结构导致的幅度与相位之间的耦合、用户移动性导致的非理想信道状态信息(Channel State Information,CSI)与信道老化(Channel Aging)效应以及RIS单元密集排列导致的空间相关性等。针对以上挑战,本文的主要研究内容包括:第一,针对由于RIS硬件结构导致的幅度与相位之间耦合的实际条件,对RIS辅助通信系统进行准确的性能分析。具体而言,首先,在RIS幅相耦合模型下,利用概率论、信息论等工具推导RIS辅助通信系统中断概率与遍历容量的准确闭式表达式。其次,为便于分析关键参数对系统性能的影响,推导了中断概率以及遍历容量的近似表达式。最后,通过仿真验证结果的准确性。研究结果表明,系统中断概率以及遍历容量随着RIS反射单元数、发射信噪比、反射幅度最小值的增大以及反射幅度函数斜率的降低而得到改善。第二,针对用户移动性导致的非理想CSI影响下RIS部署方式选择问题,考虑在RIS集中式与分布式部署方式下对RIS辅助通信系统遍历容量进行比较。首先,利用Gamma近似方法,在非理想CSI下推导两种部署方式下系统遍历容量。其次,利用Jensen不等式推导系统遍历容量上下界。最后,仿真结果验证了公式的准确性,并揭示了增加发射功率、CSI的准确性以及反射单元的数量有助于提升系统遍历容量。此外,若集中式RIS部署在发射机或接收机附近,则集中式与分布式部署方式下系统遍历容量基本相同,其他情况下分布式部署优于集中式部署。第三,针对信道时变引起的信道老化效应以及空间相关信道下RIS辅助通信系统优化设计问题,考虑基站已知信道时间相关系数,设计低复杂度基站与RIS联合波束赋形算法。首先,在空间相关信道以及信道老化效应下,推导系统平均和速率闭合表达式。其次,建立以最大化系统平均和速率为目标的波束赋形优化问题。最后,利用交替优化算法对优化问题进行求解。仿真结果验证了推导公式的正确性。所提优化算法相比于RIS随机相位方案使系统平均和速率提升了2.6倍。综上所述,本文在非理想条件下对实际RIS辅助通信系统进行准确的性能分析,并设计波束赋形优化算法以提升系统性能。本文的研究成果为RIS在未来6G通信系统中的实际部署应用提供了理论依据。更多还原

【Abstract】 Reconfigurable Intelligent Surface(RIS)is considered as one of the potential key technologies for the sixth generation(6G)mobile communication systems due to its ability to intelligently control the communication environment and improve the performance of communication systems.However,due to various practical non-ideal factors,the performance analysis and optimization design of RIS-aided communication systems face many challenges,including the coupling between the amplitude and phase of the RIS elements caused by the hardware structure of the RIS,channel state information(CSI)and channel aging effects caused by user mobility,and spatial correlation caused by dense arrangement of RIS elements.To solve the above challenges,the main research content of this thesis includes:Firstly,based on the practical conditions of the coupling between amplitude and phase of the RIS caused by the hardware structure of the RIS,the performance of the RIS-aided communication system is analyzed accurately.Specifically,first,under the coupled amplitude and phase model of the RIS,the accurate closed form expressions of the outage probability and the ergodic capacity of the RIS-aided communication system are derived by using the tools of probability theory and information theory.Second,to facilitate the analysis of the impact of key parameters on the system performance,approximate expressions for the outage probability and the ergodic capacity are derived.Finally,the accuracy of the theoretical derivation is verified through simulation results.The results indicate that the outage probability and the ergodic capacity of the RIS-aided communication system improve with the increase of the number of RIS reflecting elements,signal-to-noise ratio,minimum values of the reflection amplitude,and the decrease of the steepness of the amplitude function.Secondly,considering the selection of RIS deployment methods under the impact of imperfect CSI caused by user mobility,the ergodic capacity of the RIS-aided communication system under the centralized and distributed deployment of the RIS is compared.First,the ergodic capacity is derived for the two deployment methods under imperfect CSI using Gamma approximation.Second,the upper and lower bounds of the ergodic capacity are derived by using Jensen’s inequality.Finally,the simulation results verify the accuracy of the theoretical derivation,and reveal that increasing the transmission power,the accuracy of CSI,and the number of reflecting elements improve the ergodic capacity of the system.In addition,a distributed RIS-aided system usually outperforms a centralized RIS-aided system,and that they provide almost the same ergodic capacity if a single RIS is deployed near the transmitter or near the receiver.Thirdly,considering the optimization design of the RIS-aided communication system under the impact of channel aging and spatially correlated channels,a low complexity joint beamforming algorithm of the base station and the RIS is designed based on the known time correlation coefficients.First,under the spatially correlated channels and channel aging effects,the closed-form expressions of the average sum rate are derived.Second,the joint base station transmit beamforming and the RIS reflecting phase shift design problem is formulated to maximize the average sum rate.Finally,the alternating optimization algorithm is used to solve the optimization problem.The simulation results verify the accuracy of the derived formula.Compared to the random phase design of the RIS,the proposed optimization algorithm improves the system average sum rate by 2.6 times.To sum up,this thesis analyzes the accurate performance of practical RIS-aided communication systems under non-ideal conditions and designs beamforming optimization algorithms to improve system performance.The research results of this thesis provide a theoretical basis for the practical deployment and application of RIS in future 6G communication systems.更多还原