  Large-scale antenna arrays employed by the base station (BS) constitute an
essential next-generation communications technique. However, due to the
constraints of size, cost, and power consumption, it is usually considered
unrealistic to use a large-scale antenna array at the user side. Inspired by
the emerging technique of reconfigurable intelligent surfaces (RIS), we firstly
propose the concept of user-side RIS (US-RIS) for facilitating the employment
of a large-scale antenna array at the user side in a cost- and energy-efficient
way. In contrast to the existing employments of RIS, which belong to the family
of base-station-side RISs (BSS-RISs), the US-RIS concept by definition
facilitates the employment of RIS at the user side for the first time. This is
achieved by conceiving a multi-layer structure to realize a compact
form-factor. Furthermore, our theoretical results demonstrate that, in contrast
to the existing single-layer structure, where only the phase of the signal
reflected from RIS can be adjusted, the amplitude of the signal penetrating
multi-layer US-RIS can also be partially controlled, which brings about a new
degree of freedom (DoF) for beamformer design that can be beneficially
exploited for performance enhancement. In addition, based on the proposed
multi-layer US-RIS, we formulate the signal-to-noise ratio (SNR) maximization
problem of US-RIS-aided communications. Due to the non-convexity of the problem
introduced by this multi-layer structure, we propose a multi-layer transmit
beamformer design relying on an iterative algorithm for finding the optimal
solution by alternately updating each variable. Finally, our simulation results
verify the superiority of the proposed multi-layer US-RIS as a compact
realization of a large-scale antenna array at the user side for uplink
transmission.
