Channel Statistical Distributions Analysis Of RIS-Assisted Wireless Communication Systems

The research of modern wireless communication technology shows that the randomness and uncertainty of free space are the key to the deformation and distortion of received signal.Reconfigurable intelligent surfaces(RIS)technology can realize intelligent and configurable wireless transmission environment,which provides a new idea for further improving wireless transmission performance.Early research on RIS mainly focused on the optimization algorithms of beamforming and channel estimation,and less on performance analysis;the main reason is that there is no approximate method that can accurately simulate the statistical distribution of channels in RISassisted wireless communication systems.Aiming at this problem,thesis proposes an accurate approximation scheme of channel statistical distribution for two different RIS-as sis ted wireless communication systems(RIS-based wireless relay,RIS-based RF transmitter).Specifically,this thesis accurately approximates the mathematical statistical distribution of the two channel models based on the small argument approximation(SAA)method,and uses the moment matching method to estimate the shaping parameters of the approximate probability density function.Based on the proposed approximation scheme,this thesis analyzes the outage probability and the average bit error rate(BER)of the system,and obtains the diversity gains under two different systems as N-1<G_d<N、N,respectively.Secondly,the exact expression and upper bound analytical expression of average channel capacity are derived,which proves that RIS can only provide diversity gain for the system.Finally,by comparing the numerical results of the SAA scheme and the central limit theorem scheme,it is proved that the former has higher accuracy both under high signal-to-noise ratio and the small number of reflection units.Aiming at the limited spectrum and low efficiency of communication systems,proposed a RIS-assisted uplink communication system.Specifically,the RIS is used as a virtual wireless relay to optimize the uplink signal,and the incident electromagnetic wave is secondary modulated to embed the information to be transmitted.Therefore,multiple users can share the same spectrum.Secondly,for the proposed uplink communication system,a complete modulation and demodulation scheme is presented,and an exact approximation of the probability density function of the Euclidean distance between the received signal constellation points is given.Finally,an analytical expression of the average BER performance of the system is derived based on the approximate results,which proves that using the RIS to construct the communication system is feasible.In order to improve the coverage and stability of the communication system,a RIS-assisted unmanned aerial vehicle(UAV)dual-hop communication system is proposed.As an air communication platform,UAV can build line-of-sight links between access points to effectively expand the communication range.However,when there are many obstacles in the transmission environment,the UAV needs to adjust the position flexibly to establish a stable connection.This not only leads to an increase in UAV power consumption,but also brings problems such as unstable connection and low communication rate.Research shows that using RIS to enhance UAV communication system is an effective means to solve the above problems.However,the unique hardware structure and three-dimensional spatial location lead to complicated channel statistical distribution of RIS-assisted UAV communication system.To solve these problems,this thesis firstly approximates the statistical distribution of RISassisted air to ground channels under Rayleigh-Rician fading channels.Based on the proposed mathematical model,the outage probability,average bit error rate and average channel capacity of the system are theoretically analyzed,and the asymptotic performance expression under high signal-to-noise ratio and the accurate upper bound closed-form solution for channel capacity are deduced.Secondly,through the numerical analysis results and simulation results,it is proved that RIS can significantly enhance the communication distance,coverage and link stability of the UAV communication system.Finally,the relationship between the UAV’s position and the system performance is revealed,and the analysis of the best position and the best height of the UAV is given,which provides a theoretical basis for optimizing the performance of the communication system.