IRS Element Grouping With Element-Wise Reflection Coefficients

To reduce the overheads of channel estimation and beamforming caused by a large number of intelligent reflecting surface (IRS) elements, a strategy of grouping the IRS elements, in which the same reflection coefficient is shared by all IRS elements within a group, has been used. However, this approach results in very low beam gains at specific directional angles of a channel. To address this issue, we propose a novel scheme that employs a variation pattern of the reflection coefficients within a group. The patterns are designed to differ among the groups, while being fixed regardless of a dynamic channel. We present several sets of patterns that increase the minimized beam gain over the directional angles of a channel and enhance the worst-case performance metrics such as the outage capacity and coverage probability. Some of the pattern sets are generated via heuristic methods, while others are crafted using a systematic iterative sub-optimization technique. Numerical examples demonstrate that the proposed scheme outperforms the conventional method in terms of the outage capacity.