Wireless communications with user equipment mounted Reconfigurable Intelligent Surfaces
I. Zakir Ahmed, Hamid Sadjadpour
TL;DR
This work proposes a novel wireless architecture where small RIS panels (M ≤ 4) are mounted on user equipment (UE) and cooperatively assist a MaMIMO transmitter-receiver pair. By modeling the overall channel as $\boldsymbol{H} = \boldsymbol{H}_d + \sum_i \boldsymbol{Q}_i \boldsymbol{\Phi}_i \boldsymbol{G}_i$ and formulating a joint design problem for the precoders, combiners, and UE-RIS phase shifts, the authors develop an alternating-optimization framework augmented with IDBP to achieve near-optimal mean-squared error. The UE-RIS approach yields significantly simpler channel estimation due to on-device RIS, and simulations demonstrate that many distributed, low-element RISs can match or exceed the performance of a traditional large RIS, especially in dense 6G/NGWN scenarios with millimeter-wave and terahertz operation. The work suggests a practical path to capturing massive spatial multiplexing gains without heavy infrastructure deployment, by leveraging the density of UEs as dynamic RIS units and a cooperative network-layer optimization strategy.
Abstract
In traditional Reconfigurable Intelligent Surfaces (RIS) systems, the RIS is mounted on stationary structures like buildings, walls, or posts. They have shown promising results in enhancing the performance of wireless systems like capacity and MSE in poor channel conditions. The traditional RIS is a monolithic structure containing a large number of reflecting elements (passive or active). In this paper, we propose the idea of mounting a small number of RIS elements (usually between 2 to 4 ) on user equipment (UEs) like mobile phones, laptops, and tablets, to name a few. A joint coordinated optimization of phase shifts of all the passive RIS elements on the participating UEs is envisioned to enhance the performance of wireless communication between an intended transmitter and receiver in the MSE sense. Given that the RIS elements are mounted on the UEs, the challenging channel estimation problem with RIS is significantly simplified. For the case when there is a line-of-sight (LOS) channel and with a large number of participating RIS-mounted UEs, the LOS is converted into a multipath-rich-scattering channel even for millimeter wave and Terahertz operating ranges that enable higher spatial multiplexing gains, thereby significantly improving the MSE performance compared to traditional RIS channels. We support the above claims using simulations.