Research Of Key MIMO Techniques For Next Generation Mobile Communication Systems

This thesis aims to empower next-generation communication systems based on multiple-input multiple-output(MIMO)technology,and tries to ob-tain three basic issues in the coming MIMO system:reliability(receiving end bit error rate/spatial diversity gain),and effectiveness(spectral efficiency/spa-tial multiplexing gain)and transmission security in practical scenarios,from the perspective of non-linear precoding in small-scale MIMO,the secrecy in physical layer security,and the new network architecture:cell-free massive MIMO.This thesis mainly considers the limiting non-ideal factors in practical scenarios.The framework and innovations are summarized as follows:First,we studied the application of non-linear vector perturbation precod-ing in practical transmissions.This thesis proposed a general power normaliza-tion factor quantization error model based on the rate-distortion theory,then,based on the proposed error model and considering the non-ideal feedback of the transmitter's channel state information,with the minimum mean square error criterion for practical scenario.The joint optimization problem of the optimal precoding matrix and the perturbation vector is studied,and a vector perturbation precoding scheme under non-ideal transmission scenarios is pro-posed.Simulation results have shown that the proposed scheme can effectively reduce the error floor at the receiving end with 1-3dB advantage over CVP,and is less sensitive to the non-ideal factors compared with the existing MMSE-VP schemes.Secondly,considering the application of VP in physical layer security of MIMO eavesdropping channels.First,this thesis analyzed the impact of ex-isting physical layer transmission schemes considering practical secrecy on the performance of the legitimate receiver and presented a secure transmission scheme based on vector perturbation,which can effectively improve the receiv-ing performance of legitimate users on the premise of ensuring actual security.Next,this thesis considers the security leakage problem introduced by QAM that may occur under the limited number of antennas and modulation order,and proposed a vector perturbation based secure transmission scheme,which can effectively curb security leaks in practical transmission scenarios without additional complexity.By redesigning the modulo parameters of the vector per-turbation precoding scheme,this thesis further proposed a secure transmission scheme with fixed/dynamic shift on modulo parameter to guard the security against eavesdropper who can eliminate perturbation vector.Simulation re-sults show that the proposed scheme can effectively improve the performance of legitimate users compared with AN-based schemes with 3-6dB gain,effec-tively avoid the security leakage and outage,and can be flexibly deployed in full load scenarios.Thirdly,as the existing precoding schemes in the cell-free massive MIMO network are not well compromised between spectral efficiency and scalability.This thesis proposed a precoding scheme based on distributed maximum ra-tio transmission and centralized zero-forcing.By selecting some access points flexibly through the use of joint signal processing:centralized zero-forcing,to eliminate the main interference in the network as much as possible,and the re-maining access points use the maximum ratio transmission precoding scheme to reduce the overall signalling overhead,complexity and front-haul overhead.This thesis presented the corresponding access point selection scheme based on the large-scale fading parameters,and derives the closed-form expression of spectral efficiency with corresponding to the proposed schemes at the same time.Based on the expression,this thesis gives the max-min power optimiza-tion scheme of the second-order cone optimization.In order to deal with the high complexity of power optimization schemes in large-scale networks,this thesis further provides a power allocation scheme based on first-order method optimization,which can greatly reduce the complexity of power al-location problems.Simulation results reveals that proposed scheme can gain 3-6 folds 95%likely UE spectral efficiency compared to distributed precod-ing when the number of antennas at the access point is small.Compared with centralized zero-forcing precoding scheme,proposed scheme has lower com-putational complexity and front-haul overhead.When the network is large,the proposed power allocation scheme can achieve far better performance than the uniform power allocation scheme,and when used in small-scale networks,the performance is close to the second-order cone power allocation scheme.