On Half-Power Beamwidth of Intelligent Reflecting Surface

Half-power beamwidth (HPBW) is a key parameter to measure the performance of antenna array and the newly proposed intelligent reflecting surface (IRS) systems. In this letter, we first establish a system model for IRS in uniform linear array (ULA) and uniform rectangular array (URA) configurations, then the IRS HPBW in both cases are derived under the far-field condition without considering channel path-loss and fading. We find that in ULA and URA configurations, IRS HPBW is equal to antenna array’s HPBW at the same reflect/transmit angle with maximal ratio combining applied, while IRS HPBW is always greater than or equal to antenna array’s HPBW when all weights are equal. Besides, under certain incident angles, there will be abrupt changes in HPBW and the analytical expressions of the boundary values in both ULA and URA cases are provided. Unlike the traditional active antenna array, our conclusion shows that the IRS HPBW will change with the incident angles. Simulation results verify the correctness of our derivations.