Research On 6G-oriented Ris Wireless Transmission Theories And Technologies

In recent decades,every upgrade of mobile communication has profoundly promoted or changed many aspects of human lifestyle and social development.The latest 5th Generation Mobile Communication Systems(5G)cannot meet the requirements of supporting the Internet of Things services in the future due to the high hardware costs and operating expenses brought by the adopted technologies such as massive multiple-input multiple-output(MIMO)and millimeter-wave(mm Wave).Thus,the exploration of the 6th generation mobile communication systems(6G)is on the ascendant all over the world.In the initial research and exploration stage of 6G,it is imperative to learn from the experience and lessons of 5G and develop key technologies with low hardware cost and low power consumption.Focusing on the low-cost reconfigurable intelligent surface(RIS)assisted 6G,this dissertation develops research on wireless transmission theory and key technologies to address the challenges of the complex electromagnetic physical modeling,high channel feedback overhead and high transmission scheme complexity,and forms the following six research innovations.First of all,considering the complex EM response of the RIS,the angle-dependent phase shifter model of the RIS is developed and its impacts on spectral efficiency(SE)is assessed.Starting from the equivalent circuit theory of the RIS unit cell,we show that when EM waves illuminate the RIS from different angles,the different projections of the RIS unit cell lead to different equivalent circuit parameters,which means that the reflection coefficient of the RIS unit cell is dependent on the incident angle of EM waves.Based on the angle dependency of the RIS,it is theoretically proved that the angle reciprocity of EM waves on RIS holds when the reflection phase differences of adjacent RIS unit cells are unchanged for the forward incident EM waves and reverse incident EM waves.Given that EM waves propagate through multipath in communication systems,the impacts of angle-dependent phase shifter model on the transmission performance are analyzed under the multipath channel.To reduce the impacts of the angle dependency,RIS’s configuration that utilizes the statistical channel state information(CSI)is proposed.Then,considering single-RIS-assisted frequency division duplexing(FDD)systems,adaptive feedback codebook and limited feedback bit partitioning are proposed.On the basis of the different channel characteristics of two sub-channels cascaded by the RIS,a dynamic cascaded codebook is developed for quantizing the path gain that dominates the feedback overhead.The different components of the proposed cascaded codeword can be dynamically adjusted according to the distribution of the line of sight(Lo S)and Non-Lo S path gain in each sub-channel.When the quantized CSI obtained by the cascaded codebook is utilized for transmission design,the upper bound of ergodic rate loss is derived.Based on the theoretical ergodic rate loss,the impacts of the limited feedback bit are analyzed and an adaptive bit partitioning strategy is proposed to minimize the upper bound of the ergodic rate loss.Numerical results show that the proposed cascaded codebook is robust against different environment and system parameters and the bit partitioning strategy can reduce the ergodic rate loss effectively.Subsequently,considering multi-RIS-assisted FDD systems,channel customization is proposed to reduce the channel feedback overhead and simplify the transceiver design.By configuring the phase shifters of the RISs with statistical CSI,we customize a sparse channel in rich-scattering environments,which significantly reduces the feedback overhead in designing the transceiver and RISs.Since the channel is customized in terms of singular value decomposition(SVD)with full-rank,the optimal SVD transceiver can be approached without a matrix decomposition and feeding back the complete channel parameters.The theoretical SE loss of the proposed transceiver and RIS design is derived by considering the limited CSI quantization.To minimize the SE loss,a bit partitioning algorithm that splits the limited number of bits to quantize the CSI is developed.Numerical results show that by customizing the channel via RIS’s configurations,the feedback overhead and transceiver design complexity can be reduced effectively.After that,considering that the characteristics of wireless channel determine the performance of transmission schemes,multi-timescale channel customization is proposed for multiRIS-assisted MIMO systems to promote the performance of different transmission schemes.In terms of SE improvement,we customize a high-rank channel for the spatial multiplexing(SM)transmission and a highly correlated rank-1 channel for the beamforming(BF)transmission by designing the phase shifters of the RIS with the statistical channel state information in the angle coherent time.With regard to reliability enhancement,we customize a fast-changing channel in the symbol timescale to compress the channel coherent time from the duration of the small fast fading to the duration of the symbol,which can achieve more diversity gain for the SM and BF transmissions.Numerical results demonstrate that the tradeoff between system throughput and reliability can be achieved by flexibly customizing the channel characteristic for the specific transmission scheme.Moreover,considering that the rank-deficient channel in strong Lo S mm Wave wireless systems cannot provide multi-stream transmission,a rank-controllable channel customization and a low-complexity multi-stream transmission are proposed for multi-RIS-assisted hybrid mm Wave systems.Based on the analysis of characteristics of the multi-RIS-assisted strong Lo S mm Wave channel,we theoretically show that a favorable channel with an arbitrary tunable rank and a minimized truncated condition number can be established by elaborately designing the reflection matrix,set segmentation,and deployment location of the RISs.With the channel customized in terms of the SVD,the closed-form expression between the singular value and reflection coefficient of RISs is derived and the optimal SVD transceiver can be obtained without matrix decomposition for the channel and for the digital and analog beamforming.During the transceiver design process,the required transmit power for realizing multi-stream transmission is derived.Numerical results illustrate that with the proposed channel customization,the rank deficiency in strong Lo S mm Wave systems can be addressed and the complexity of the hybrid transceiver design can be reduced.Finally,considering RIS-assisted multi-user mm Wave systems,a sparse array of sub-surface(SAo S)deployment is proposed to reduce the strong correlation between effective channels of multiple users by the RIS tile scheduling algorithm and the reflect coefficient design.Given that different users share the same channel from the base station to the SAo S,the autocorrelation and cross-correlation of effective channels are analyzed.Based on the closed-form expression for the correlation of the effective channel,an approximation and a lower bound of ergodic SE are derived in frequency division multiple access and in spatial division frequency division,respectively.On the basis of the above theoretical results,the RIS tile scheduling algorithm is proposed to maximize the sum ergodic SE by reducing the correlation between effective channels of multiple users.Numerical results show that the SAo S deployment with RIS tile scheduling algorithm can improve the sum ergodic SE effectively.