The reconfigurable intelligent surface (RIS) is useful to effectively improve the coverage and data rate of end-to-end communications. In contrast to the well-studied coverage-extension use case, in this paper, multiple RIS panels are introduced, aiming to enhance the data rate of multi-input multi-output (MIMO) channels in presence of insufficient scattering. Specifically, via the operator-valued free probability theory, the asymptotic mutual information of the large-dimensional RIS-assisted MIMO channel is obtained under the Rician fading with Weichselberger's correlation structure, in presence of both the direct and the reflected links. Although the mutual information of Rician MIMO channels scales linearly as the number of antennas and the signal-to-noise ratio (SNR) in decibels, numerical results show that it requires sufficiently large SNR, proportional to the Rician factor, in order to obtain the theoretically guaranteed linear improvement. This paper shows that the proposed multi-RIS deployment is especially effective to improve the mutual information of MIMO channels under the large Rician factor conditions. When the reflected links have similar arriving and departing angles across the RIS panels, a small number of RIS panels are sufficient to harness the spatial degree of freedom of the multi-RIS assisted MIMO channels.

翻译：可重构智能表面（RIS）能有效提升端到端通信的覆盖范围和数据速率。与已被充分研究的覆盖扩展场景不同，本文引入多个RIS面板，旨在增强散射不足条件下多输入多输出（MIMO）信道的数据速率。具体而言，通过算子值自由概率理论，在存在直射链路与反射链路的情况下，基于Weichselberger相关结构的莱斯衰落信道中，得到了大规模RIS辅助MIMO信道的渐近互信息。尽管莱斯MIMO信道的互信息随天线数和信噪比（以分贝为单位）呈线性增长，但数值结果表明，为获得理论保证的线性增益，需要足够大的信噪比且该值与莱斯因子成正比。本文表明，所提出的多RIS部署方案在莱斯因子较大的条件下能有效提升MIMO信道的互信息。当反射链路的到达角和离开角在各RIS面板间近似相同时，少量RIS面板即可充分利用多RIS辅助MIMO信道的空间自由度。