Research On Precoding Method In IRS-Assisted Mmwave Systems

The millimeter wave(mm Wave)band has rich spectrum resources and can support the increasing high-data-rate broadband traffic.However,the serious path loss of mm Wave results in its poor penetration ability,which restricts its practical application.Addressing this blockage issue,the relays are employed to improve the coverage range and service quality of mm Wave communication.However,the energy consumption and hardware cost increase with relay deployment.Moreover,adding an excessively large number of active components such as relays in wireless networks can cause a more aggravated interference issue.Thus,it is imperative to improve the coverage of mm Wave multiple-input multiple-output(MIMO)communication at a lower cost.Intelligent reflecting surface(IRS)is a software-controlled surface comprised of a large number of reconfigurable passive elements.Moreover,the low-cost hardware and the small structure size of the reflecting elements can result in low-cost implementation and high flexibility for IRS installation.Thus,IRS has been introduced as a cost-effective solution to relay deployment in mm Wave communication.This dissertation focuses on the joint precoding of IRS-assisted mm Wave communication,and the main work and contributions are listed as follows:Firstly,the mixed timescale precoding scheme is proposed for IRS-assisted mm Wave MIMO communication.Specifically,We consider the single-antenna user scenario and conclude that the design of the IRS reflection coefficient matrix is independent of the hybrid precoders when only the line of sight(Lo S)path of BS-IRS link is considered.We further consider the joint base station(BS)-IRS precoding optimization problem using the mixed timescale CSI in the multiple-antenna user scenario and transform the optimization problem into a simpler form,where the update of the IRS reflection coefficient matrix designed according to the statistical CSI is independent of the hybrid precoders designed according to the instantaneous CSI of the overall channel.In other words,the IRS reflection coefficient matrix is only related to the statistical CSI and it requires to be updated only when statistical CSI of the channels changes,which significantly reduces the computational complexity and feedback overhead.Simulation results demonstrate the benefits of IRSs in terms of improving average spectral efficiency and requiring fewer high-cost radio frequency(RF)chains.Secondly,we propose a joint design of hybrid precoders and IRS reflection coefficient matrices in a multi-IRS-assisted mm Wave MIMO scenario with one user considered to maximize the ergodic spectral efficiency by exploiting the twin-timescale CSI,where only a reduceddimensional assist channel matrix requires to be estimated during each channel coherence time(CCT).We further transform the joint BS-IRS precoding optimization problem using the twintimescale CSI into a simpler form,where the update of the IRS reflection coefficient matrices is independent of the hybrid precoders,and the analog precoder is independent of the digital precoder.In other words,the passive IRS reflection coefficient matrices and the active analog precoder requires to be updated only when statistical CSI of the channels changes.Only the digital precoder need to be updated during each CCT.Simulation results demonstrate the effectiveness of the proposed algorithms.Thirdly,a joint precoding scheme using statistical CSI is proposed for IRS-assisted mm Wave multi-user multiple-input single-output(MU-MISO)system.A fractional programming based algorithm is proposed to design the IRS reflection coefficient matrix and the digital precoding matrix.Simulation results demonstrate the performance obtained in the cases with IRS is higher than that of the conventional system without the IRS.Finally,we provide the application scenes of the fully-connected and subarray-connected architectures and develop a practical partially dynamic subarrayconnected architecture to improve the transmission performance.In particular,the subarray-connected architecture can achieve better ergodic energy efficiency performance while the fully-connected structure get better ergodic spectral efficiency performance.To furtuer improve the spectral efficiency transmission performance for subarray-connected structure,a partially dynamic subarray-connected structure is proposed for wideband mm Wave MIMO systems.Numerical results verify that the partially dynamic subarray design algorithm offers one or two orders of computation time saving compared with the existing algorithms,and the hybrid precoding algorithm outperforms the existing algorithms in terms of spectral efficiency.Moreover,compared with the subarrayconnected structure,the proposed structure achieves spectral efficiency gain and energy efficiency gain.