  Intelligent reflecting surface (IRS) is a new promising technology that is
able to reconfigure the wireless propagation channel via smart and passive
signal reflection. In this paper, we investigate the capacity region of a
two-user communication network with one access point (AP) aided by $M$ IRS
elements for enhancing the user-AP channels, where the IRS incurs negligible
delay, thus the user-AP channels via the IRS follow the classic discrete
memoryless channel model. In particular, we consider two practical IRS
deployment strategies that lead to different effective channels between the
users and AP, namely, the distributed deployment where the $M$ elements form
two IRSs, each deployed in the vicinity of one user, versus the centralized
deployment where all the $M$ elements are deployed in the vicinity of the AP.
First, we consider the uplink multiple-access channel (MAC) and derive the
capacity/achievable rate regions for both deployment strategies under different
multiple access schemes. It is shown that the centralized deployment generally
outperforms the distributed deployment under symmetric channel setups in terms
of achievable user rates. Next, we extend the results to the downlink broadcast
channel (BC) by leveraging the celebrated uplink-downlink (or MAC-BC) duality
framework, and show that the superior rate performance of centralized over
distributed deployment also holds. Numerical results are presented that
validate our analysis, and reveal new and useful insights for optimal IRS
deployment in wireless networks.
