  Intelligent reflecting surface (IRS) has recently received significant
attention in wireless networks owing to its ability to smartly control the
wireless propagation through passive reflection. Although prior works have
employed the IRS to enhance the system performance under various setups, the
fundamental capacity limits of an IRS aided multi-antenna multi-user system
have not yet been characterized. Motivated by this, we investigate an IRS aided
multiple-input single-output (MISO) broadcast channel by considering the
capacity-achieving dirty paper coding (DPC) scheme and dynamic beamforming
configurations. We first propose a bisection based framework to characterize
its capacity region by optimally solving the sum-rate maximization problem
under a set of rate constraints, which is also applicable to characterize the
achievable rate region with the zero-forcing (ZF) scheme. Interestingly, it is
rigorously proved that dynamic beamforming is able to enlarge the achievable
rate region of ZF if the IRS phase-shifts cannot achieve fully orthogonal
channels, whereas the attained gains become marginal due to the reduction of
the channel correlations induced by smartly adjusting the IRS phase-shifts. The
result implies that employing the IRS is able to reduce the demand for
implementing dynamic beamforming. Finally, we analytically prove that the
sum-rate achieved by the IRS aided ZF is capable of approaching that of the IRS
aided DPC with a sufficiently large IRS in practice. Simulation results shed
light on the impact of the IRS on transceiver designs and validate our
theoretical findings, which provide useful guidelines to practical systems by
indicating that replacing sophisticated schemes with easy-implementation
schemes would only result in slight performance loss.
