Joint beamforming optimization in RIS-aided MIMO wireless systems under mutual coupling and multiple reflections
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Date
2024-04-25
Authors
Wijekoon, Dilki
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Abstract
In a Reconfigurable Intelligent Surface (RIS), Mutual Coupling (MC) is unavoidable as a result of the sub-wavelength structure of its unit elements. In addition, MC inherently leads to multiple reflection effects that are ignored in conventional (approximative) RIS models.
In this thesis, we examine the problem of joint active and passive beamforming in a controllable multi-user reconfigurable intelligent surface (RIS)-assisted downlink and uplink wireless communication system, considering the MC among RIS elements. We formulate a problem to optimize active and passive beamforming jointly under the MMSE criterion. Given the non-convex nature of the problem, we use alternating optimization to decompose the problem into two sub-problems for downlink and uplink transmissions separately. In both transmissions, one sub-problem involves optimizing the phase-shift matrix of RIS. In downlink, the other sub-problem is the optimization of active precoding for the base station (BS), while the equivalent sub-problem in uplink is the optimization of the linear receiver matrix. We optimize the phase shift matrix under a physically-consistent model using the gradient descent algorithm for both transmissions. We use the Lagrange multiplier method to optimize active precoding in the downlink and apply the First Order Necessary Condition (FONC) to optimize the linear receiver in the uplink.
Simulation results are represented for both lossless and lossy RIS scenarios under perfect and imperfect channel state information. We discuss the impact of changing the number of RIS elements and the RIS element spacing on system performance. The results show that, with optimized phase shifts and active precoding, the inherent multiple reflection effect can improve the performance of RIS-aided wireless communications systems.
In our previous analysis, we focused on optimizing joint beamforming within a physically consistent environment by incorporating MC effects. However, we did not explicitly address the optimization of MC itself. This represents a critical aspect that warrants further investigation and consideration for a comprehensive and refined analysis. As mentioned earlier, MC emerges as an inherent feature in RIS particularly with sub-wavelength inter-element spacing. Alongside, the presence of electromagnetic (EM) transmit/receive radiation patterns is also inevitable. The simultaneous presence of these two factors naturally leads to the emergence of non-local RIS structures, which can be effectively described via non-diagonal phase shift matrices. As the second analysis, we focus on optimizing MC and radiation patterns in RIS-assisted multi-user MIMO wireless communication systems. We particularly formulate a novel problem to jointly optimize active and passive beamforming as well as MC and radiation patterns in a physically consistent manner considering reflective and transmissive RIS setups separately. To characterize the physically consistent model, we deploy scattering parameters and propose a novel approach to optimize MC and radiation patterns through an offline optimization method, rather than optimizing on the fly, using two distinct solution approaches for reflective and transmissive models. We present simulation results using both parametric and geometric channel modeling approaches. Our numerical results showcase that the system performance increases with the proposed MC and radiation patterns optimization, and this improvement is achievable without the need for optimizing them on the fly, which can be rather cumbersome.
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Keywords
Reconfigurable Intelligent Surface (RIS), joint active and passive beamforming, mutual coupling, multiple reflection effects, physically-consistent model, non-local RIS structures, radiation patterns, non-diagonal phase shift matrix, reflective and transmissive RIS