  A conceptual example is first analyzed to show that efficient wireless
communications is possible, when user equipment (UE) receiver, BS transmitter
or/and the scatter (reflector) in wireless channels employ the required channel
state information (CSI) to remove the randomness of signal phase. Then, the
principles and optimization of three reflective intelligent surface (RIS)
assisted mmWave (RIS-mmWave) models are introduced. The first model assumes one
BS, one RIS and one UE; the second one assumes one BS, one RIS and multiple
UEs; while the third RIS-mmWave model assumes one BS, multiple RISs and
multiple UEs. Furthermore, the optimization of BS precoder and RIS phase-shifts
is addressed in the context of the massive RIS-mmWave scenarios, where the
number of BS antennas and that of RIS reflection elements are significantly
larger than the number of supported UEs. The analyses demonstrate that, while
the deployment of RISs with mmWave is capable of solving the blockage problem
and has the potential to significantly improve efficiency, finding the
near-optimum solutions for RIS phase-shifts is highly challenging in practice.
