Blind Beamforming for Multiple-IRS Assisted Wireless Transmission

Conventional beamforming methods for intelligent reflecting surfaces (IRSs) typically entail the full channel state information (CSI). However, channel acquisition is costly when IRSs are extensively deployed in the network. To overcome this difficulty, this paper proposes a novel strategy called blind beamforming that coordinates multiple IRSs by means of the received signal statistics without knowing CSI. Blind beamforming just requires measuring the received signal power at the user side with respect to a sequence of randomly generated phase shifts across all the IRSs. Its main idea is to extract the essential statistical information for beamforming by exploring only a small portion of the whole solution space of phase shifts. We show that blind beamforming guarantees a signal-to-noise ratio (SNR) boost of Θ(N ^2L ) under certain conditions, where L is the number of IRSs and N is the number of reflecting elements per IRS. The above result significantly improves upon the previous studies (including those with CSI) on multiple-IRS-assisted communication. Furthermore, we demonstrate the effectiveness of the proposed blind beamforming method through field tests at the commercial spectrum band of 2.6 GHz.