Chapter 4: Channel Estimation for RIS

Learning Objectives

• Formulate the RIS channel estimation problem and identify why passive elements cannot transmit pilots
• State and implement the ON/OFF subset-switching protocol; derive its pilot-overhead requirement
• Use the DFT codebook to decouple the $N$ element channels via orthogonal phase patterns
• Explain when compressed sensing is applicable (sparsity in angular/delay domain) and quantify the overhead reduction
• Derive the overhead-accuracy tradeoff: how much CSI error erodes the $\mathcal{O}(N^2)$ coherent gain