Chapter Summary

Chapter Summary

Key Points

• 1.

  RIS enables physical-layer secrecy via channel engineering. In the wiretap channel, the secrecy rate is Bob's rate minus Eve's rate. The RIS can focus coherently on Bob (+$N^2$ gain) while nulling at Eve (-decoherent), giving an effective $2\log_2 N$ secrecy-rate boost over no-RIS baselines.

• 2.

  Joint secrecy rate maximization is AO-solvable. For fixed $\boldsymbol{\Phi}$, the active precoder is the dominant generalized eigenvector of $(\mathbf{h}_B\mathbf{h}_B^H, \mathbf{h}_E\mathbf{h}_E^H)$ — closed form. The passive subproblem is standard QCQP. AO converges cleanly with $\sim 15$ iterations.

• 3.

  Artificial noise + RIS: secrecy without Eve CSI. AN injected in Bob's null space doesn't reach Bob but jams Eve. Combined with a RIS that focuses AN in Eve's likely direction, the secrecy guarantee holds without knowing Eve's specific channel — only assuming her "genericity." Typical AN fraction: 30-50% of transmit power; secrecy rate: 3-7 bits/s/Hz under realistic parameters.

• 4.

  Robust design handles Eve uncertainty explicitly. Define $\mathcal{U}_E$ as Eve's channel uncertainty set (e.g., ball of radius $\rho$). Worst-case secrecy rate = Bob's rate minus max over $\mathcal{U}_E$ of Eve's rate. S-procedure converts the minimax to an SDP; solvable at moderate scale. Guarantees hold uniformly over $\mathcal{U}_E$.

• 5.

  The CommIT secure framework (Caire & Atzeni 2023). Robust SDP + real-time AO two-timescale design; graceful fallback to heuristics + AN. 2-4× higher worst-case secrecy than non-robust baselines. Deployment-ready for smart-city, enterprise, and government applications.