Research On Cooperative Transmission Theory And Key Technologies Based On Reconfigurable Intelligent Surface

To meet the requirements of the next-generation wireless communications in multilayer,full-coverage,and diversification,and to alleviate the increasing contradiction between the communication requirements,such as large capacity,high rate and high energy efficiency,and the network structure,hardware cost and energy consumption,we focus on the studies of reconfigurable intelligent surface(RIS)technology,which has been envisioned as an effective solution.RIS is composed of a large number of sub-wavelength units with the ability of manipulating electromagnetic wave,such as absorption,scattering,and reflection.Since the characteristics of low energy-consumption,low cost,and manipulation of electromagnetic waves,RIS has been seen as one of the most potential technologies for next-generation wireless communications.The applications of RIS can form a controllable and programmable propagation environment in a passive transmission mode.Additionally,leveraging the existing technologies,such as multiple-input multipleoutput and beamforming,RIS collaborative transmission has great potential advantages in enhancing multiplexing gain,expanding coverage and improving capacity.This thesis mainly studies the theory and key technologies of RIS cooperative transmission.The contents of this thesis are listed as follows:Firstly,the capacity of the RIS-aided system is characterized and the RIS deployment strategy is discussed.We consider the extra degree of freedom offered by the rotation of the RIS and investigate its potential in improving the performance of RIS-assisted wireless communication systems.By considering radiation pattern modeling at all involved nodes,we derive the composite channel gain and present a closed-form upper bound for the system ergodic capacity over cascade Rician fading channels.Then,we reconstruct the composite channel gain by taking the rotations at the RIS plane,transmit antenna,and receive antenna into account,and extract the optimal rotation angles after investigating their impacts on the capacity.Moreover,we present a location-dependent expression of the ergodic capacity and investigate the RIS deployment strategy.Secondly,the achievable rate optimization of RIS-aided near-field wideband system is studied,as well as the phase shift design.By considering the near-filed effect of a highdimensional RIS and spatial bandwidth effect,we derive an accurate array manifold of the RIS in the near-field from the scattering point of view.Subsequently,we conceive a nearoptimal and non-uniform frequency based RIS phase designs for a single-user scenario to alleviate the beam-squint effect.As for the multi-user case,we provide a virtual-subarraybased phase shift design,which mitigates the beam-squint effect and mitigates deleterious effects of beam concentration.Thirdly,a phase shift codebook design scheme is studied,as well as a phase shift matching algorithm.Based on the traditional B-bit phase shift design,we design a discrete phase codebook via coding sequence with finite resolution.Specifically,by leveraging proper time-coding modulation,almost-full phase coverage with low-bit resolution can be achieved.Then,we propose a phase matching algorithm based on the sum of absolution errors minimization and length minimization criteria.Finally,an energy-sustainable RIS-aided system based on joint optimization of time switching and power splitting scheme is studied.Based on this scheme,the process is divided into two stages by a time-factor.In the first stage,with the power-split approach,part of the power is harvested by the RIS via a dedicated energy beam to realize the complete passivity of the RIS without any external energy supply.The remaining power is used for information transmission while guaranteeing a given quality-of-service(Qo S)at the user.In the second stage,the RIS is in the reflective state assisting the information transmission.After investigating the approximate expressions of the harvested energy at the RIS and achievable downlink data rate at the user,we analyze the system-level energy efficiency and deduce the optimal time-factor and power-factor jointly.In conclusion,this paper mainly studies the theory and key technologies of RIS cooperative transmission.Based on the theoretical analyses of RIS cooperative communication performances,we propose RIS deployment schemes,phase shift designs,and coordinated transmission schemes,which provide theoretical basis and technical support for realizing high-rate and energy-efficient RIS cooperative communications.