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Large-scale antenna arrays employed by the base station (BS) constitute anessential next-generation communications technique. However, due to theconstraints of size, cost, and power consumption, it is usually consideredunrealistic to use a large-scale antenna array at the user side. Inspired bythe emerging technique of reconfigurable intelligent surfaces (RIS), we firstlypropose the concept of user-side RIS (US-RIS) for facilitating the employmentof a large-scale antenna array at the user side in a cost- and energy-efficientway. In contrast to the existing employments of RIS, which belong to the familyof base-station-side RISs (BSS-RISs), the US-RIS concept by definitionfacilitates the employment of RIS at the user side for the first time. This isachieved by conceiving a multi-layer structure to realize a compactform-factor. Furthermore, our theoretical results demonstrate that, in contrastto the existing single-layer structure, where only the phase of the signalreflected from RIS can be adjusted, the amplitude of the signal penetratingmulti-layer US-RIS can also be partially controlled, which brings about a newdegree of freedom (DoF) for beamformer design that can be beneficiallyexploited for performance enhancement. In addition, based on the proposedmulti-layer US-RIS, we formulate the signal-to-noise ratio (SNR) maximizationproblem of US-RIS-aided communications. Due to the non-convexity of the problemintroduced by this multi-layer structure, we propose a multi-layer transmitbeamformer design relying on an iterative algorithm for finding the optimalsolution by alternately updating each variable. Finally, our simulation resultsverify the superiority of the proposed multi-layer US-RIS as a compactrealization of a large-scale antenna array at the user side for uplinktransmission.

Compact User-Specific Reconfigurable Intelligent Surfaces for Uplink Transmission