Indoor Dual-Band Experimental Evaluation of RIS-Aided Wireless Links at 5.2 GHz and 28 GHz under NLoS Conditions

Future wireless communication systems are envisioned to simultaneously operate in sub-6 GHz and millimeter-wave (mmWave) frequency bands, achieving high data rates, enhanced reliability, and wide-area coverage. However, sub-6 GHz bands are becoming increasingly congested, whereas mmWave links experience severe path loss and are highly susceptible to blockage in non-line-of-sight (NLoS) conditions. Reconfigurable Intelligent Surfaces (RIS) have emerged as a promising passive solution to reconfigure wireless propagation; yet, their frequency-dependent behavior remains insufficiently understood. This work presents an experimental study of commercially available RIS panels operating at 5.2 GHz and 28 GHz, conducted as an indoor evaluation at NTT Labs, Japan. An NLoS scenario is enforced by using an electromagnetic wave absorber frame that blocks the direct LoS path, ensuring that only RIS-reflected components reach the receiver (Rx) grid. The received power is measured across a uniform grid for both RIS-inactive and RIS-active cases. The results demonstrate average RIS-induced power gains of 8–10 dBm at sub-6 GHz and 15–20 dBm at mmWave frequencies, confirming the strong potential of RIS to mitigate NLoS losses and enhance link robustness across heterogeneous frequency bands.