Evaluation of Near-Field Fed RIS Illumination for Centimeter-Wave Gigantic MIMO Systems

Abstract

In this work, a Near-field Fed Reflective Intelligent Surface (NFED-RIS) System is designed and evaluated in preparation of the usage as an alternative beamforming system. The conceptual architecture of this smart antenna system has been developed and validated using a system model presented in this paper. Subsequently, a dual-polarized <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$8 \times 8$</tex> RIS array and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \times 2$</tex> Feeder array within the centimeter-wave band were designed and validated through an electromagnetic simulation prior to fabrication. The NFED-RIS system, which is based on these two patch arrays, will be experimentally evaluated and compared to the results of electromagnetic simulations, including potential deviations caused by imperfections not captured in the simulation model. Consequently, the RIS is regarded as a passive reflect array antenna that can be utilized as an analog beamformer. This is achieved by dynamically reflecting incident waves from the Feeder, resulting in steerable beams in the far field. The distance between the two arrays is critical to ensuring optimal system performance. One of the objectives of the experiments is to identify the optimal distance between RIS and Feeder through the analysis of the near field channel (S21 over-the-air measurements) across all antenna elements, excluding the RIS phase shifters. In addition to delivering deep insights and design trade-offs of the NFED-RIS system, the measurement provides the magnitude and phase distribution at the optimal distance on the RIS, which is crucial for determining the beamforming steering vectors for future far-field experiments.