  An intelligent reflecting surface (IRS), consisting of reconfigurable
metamaterials, can be used to partially control the radio environment and
thereby bring new features to wireless communications. Previous works on IRS
have particularly studied the range extension use case and under what
circumstances the new technology can beat relays. In this paper, we study
another use case that might have a larger impact on the channel capacity: rank
improvement. One of the classical bottlenecks of point-to-point MIMO
communications is that the capacity gains provided by spatial multiplexing are
only large at high SNR, and high SNR channels are mainly appearing in
line-of-sight (LoS) scenarios where the channel matrix has low rank and
therefore does not support spatial multiplexing. We demonstrate how an IRS can
be used and optimized in such scenarios to increase the rank of the channel
matrix, leading to substantial capacity gains.
