Reconfigurable intelligent surface-assisted multi-user wireless communications systems

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Ur Rehman, Haseeb
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Intelligent reflective surfaces (IRSs) have recently emerged as a promising technology for 6G wireless communications as it can improve both the spectral and energy efficiencies of wireless systems at a low cost. In this thesis, we first tackle the problem of joint active and passive beamforming optimization for an intelligent reflective surface (IRS)-assisted multi-user downlink multiple-input multiple-output (MIMO) communication system under both ideal and practical IRS phase shifts. We aim to maximize the spectral efficiency of the users by minimizing the sum mean square error (MSE) of the users' received symbols. For this, a joint non-convex optimization problem is formulated under the sum minimum mean square error (MMSE) criterion. An alternating optimization and vector approximate message passing (VAMP)-based approach is presented to solve the joint problem under both ideal and practical constraints on the IRS phase shifts. Then, we introduce a novel approach of utilizing the IRS, now called modulating intelligent surface (MIS), for joint data modulation and signal beamforming in a multi-user downlink cellular network by leveraging the idea of backscatter communication. We present a general framework in which the MIS is used $i)$ to beamform the signals for a set of users whose data is modulated by the BS, and at the same time $ii)$ to embed the data of a different set of users by passively modulating the deliberately sent signals from the BS. By following the same VAMP-based optimization approach developed earlier, we optimize $i)$ the MIS phase-shifts for passive beamforming and data embedding for the BS- and MIS-served users, respectively, $ii)$ the active precoder and the receive scaling factors for the BS- and MIS-served users, respectively. Simulation results are presented to illustrate the performance of both the proposed IRS-based and MIS-based schemes under both perfect and imperfect channel state information (CSI). The results validate the superiority of the proposed method over the state-of-the-art techniques both in terms of throughput and computational complexity. The results also reveal that the proposed MIS-based approach outperforms the existing IRS-based schemes in terms of throughput while supporting a much higher number of users.
reconfigureable intelligent surface, VAMP, MIMO, IRS, modulating intelligent surface, MIS, RIS
Rehman, Haseeb Ur, et al. "Joint Active and Passive Beamforming Design for IRS-Assisted Multi-User MIMO Systems: A VAMP-Based Approach." IEEE Transactions on Communications (2021).