Chapter 28: Reconfigurable Intelligent Surfaces

Learning Objectives

• Describe the RIS signal model including the cascaded BS-RIS-user channel, diagonal phase-shift matrix, and the relationship between RIS reflection coefficients and the received signal
• Analyse channel estimation challenges for passive RIS elements and describe structured estimation strategies based on element grouping, codebook design, and sparsity exploitation
• Formulate the joint active-passive beamforming optimisation problem and derive alternating optimisation, semidefinite relaxation (SDR), and manifold optimisation solution approaches
• Prove the N-squared SNR scaling law for RIS with N elements under optimal phase alignment and compare with N-scaling for relays and massive MIMO
• Evaluate practical RIS limitations including phase quantisation loss, wideband frequency selectivity, hardware imperfections, and the active-vs-passive RIS trade-off
• Compare RIS deployment against competing technologies (relays, massive MIMO, small cells) and identify scenarios where RIS provides the most benefit