We investigate the capacity of a broadcast channel with a multi-antenna base station (BS) sending independent messages to multiple users, aided by IRSs with N elements. In particular, both the distributed and centralized IRS deployment architectures are considered. Regarding the distributed IRS, the N IRS elements form multiple IRSs and each of them is installed near a user cluster; while for the centralized IRS, all IRS elements are located in the vicinity of the BS. To draw essential insights, we first derive the maximum capacity achieved by the distributed IRS and centralized IRS, respectively, under the assumption of line-of-sight propagation and homogeneous channel setups. By capturing the fundamental tradeoff between the spatial multiplexing gain and passive beamforming gain, we rigourously prove that the capacity of the distributed IRS is higher than that of the centralized IRS provided that the total number of IRS elements is above a threshold. Motivated by the superiority of the distributed IRS, we then focus on the transmission and element allocation design under the distributed IRS. By exploiting the user channel correlation of intra-clusters and inter-clusters, an efficient hybrid multiple access scheme relying on both spatial and time domains is proposed to fully exploit both the passive beamforming gain and spatial DoF. Moreover, the IRS element allocation problem is investigated for the objectives of sum-rate maximization and minimum user rate maximization, respectively. Finally, extensive numerical results are provided to validate our theoretical finding and also to unveil the effectiveness of the distributed IRS for improving the system capacity under various system setups.

翻译：本文研究了在多天线基站（BS）向多个用户发送独立消息的广播信道中，配备N个元件的智能反射面（IRS）辅助下的信道容量。具体而言，考虑了分布式与集中式两种IRS部署架构。对于分布式IRS，N个IRS元件构成多个IRS，每个IRS安装在一个用户集群附近；而对于集中式IRS，所有IRS元件均位于基站附近。为获取关键洞见，我们首先在视距传播和均匀信道设置假设下，分别推导了分布式IRS和集中式IRS所能达到的最大容量。通过刻画空间复用增益与无源波束赋形增益之间的根本权衡，严格证明了当IRS元件总数超过某一阈值时，分布式IRS的容量高于集中式IRS。基于分布式IRS的优越性，我们进一步研究了分布式IRS下的传输与元件分配设计。利用用户簇内与簇间的信道相关性，提出了一种基于空间域和时间域的高效混合多址接入方案，以充分挖掘无源波束赋形增益与空间自由度。此外，分别以总速率最大化和最小用户速率最大化为目标，研究了IRS元件分配问题。最后，通过大量数值结果验证了我们的理论发现，并揭示了分布式IRS在不同系统设置下提升系统容量的有效性。