  In reconfigurable intelligent surfaces (RISs) aided communications, the
existing passive beamforming (PB) design involves polynomial complexity in the
number of reflecting elements, and thus is difficult to implement due to a
massive number of reflecting elements. To overcome this difficulty, we propose
a reflection-angle-based cascaded channel model by adopting the generalized
Snell's law, in which the dimension of the variable space involved in
optimization is significantly reduced, resulting in a simplified hierarchical
passive beamforming (HPB) design. We develop an efficient two-stage HPB
algorithm, which exploits the angular domain property of the channel, to
maximize the achievable rate of the target user. Simulation results demonstrate
the appealing performance and low complexity of the proposed HPB design.
