Fast OTA Diagnosis of RIS Using Only Harmonic Amplitude Introduced by Time Modulation

The electromagnetic wave manipulation capability of reconfigurable intelligent surfaces (RISs) critically depends on the proper functioning of the unit cells. However, unit failures may occur during RIS fabrication, transportation, or deployment. This article specifically addresses the diagnosis of non-reconfigurable failed units - defined as those whose operating states cannot be changed through control signals (e.g., permanently stuck at 0° or 180° phase states). Existing RIS diagnostic approaches face significant challenges, including reliance on complex measurement systems and excessive measurement overhead. To address these limitations, this article proposes a novel over-the-air (OTA) diagnosis method that utilizes only the amplitude of generated harmonics, enabling fast and hardware-efficient fault detection. The method requires only N harmonic amplitude measurements for the diagnosis of a RIS consisting of N units. The implementation of this OTA diagnostic method exploits the fundamental distinction: under periodic modulation, reconfigurable units generate identifiable harmonics while non-reconfigurable failed units produce no harmonic response. By excluding the fundamental frequency while utilizing multiple harmonic components, the signal-to-noise ratio(SNR) is improved and direct wave interference is reduced. This amplitude-only approach requires only a frequency sweeper and programmable source, eliminating need for phase measurements. Experimental validation using a 1-bit phase reconfigurable transmissive RIS ( $20{ \times }20$ units, millimeter-wave band) demonstrates rapid, automated and accurate fault identification.