Distributed IRS Versus Centralized IRS: Which is Superior for Multi-Antenna Transmission?

In this paper, we investigate the capacity performance of a broadcast channel with a multi-antenna base station (BS) sending independent message to multiple users, aided by
$N$
IRS elements. In particular, two practical IRS deployment schemes, namely distributed IRS and centralized IRS, are con-sidered. Regarding the distributed IRS, the
$N$
IRS elements form multiple IRSs and each of them is deployed near one user; while for the centralized IRS, all the IRS elements are deployed in the vicinity of the BS. To theoretically compare the distributed IRS aided multi-antenna communication system and its centralized IRS counterpart, we derive the maximum achieved sum-rates for the two IRS deployment architectures under the symmetric channels assumption, respectively. Based on the derived closed-form expressions for the sum-rates, we compare the performance between the distributed IRS and centralized IRS, which demon-strates that the distributed IRS would be superior if the number of total IRS elements is large sufficiently.