Prerequisites & Notation

Before You Begin

Physical-layer security uses channel differences between legitimate users and eavesdroppers to deny secure information transfer to the eavesdropper — complementary to cryptographic security, which assumes computational hardness. Adding RIS gives the system designer active control over the channel geometry, dramatically expanding the secrecy-rate achievable region.

Wiretap channel, secrecy capacity(Review ch20)
Self-check: State Csiszár-Körner's secrecy capacity formula for the degraded broadcast channel.
Artificial noise for secrecy(Review ch20)
Self-check: How does artificial noise added in the null space of Alice-Bob channel degrade Eve?
RIS joint optimization framework (Ch. 5-7)(Review ch05)
Self-check: Recall AO + WMMSE for joint active-passive beamforming.
Robust optimization under channel uncertainty
Self-check: What does worst-case optimization guarantee vs. average-case?
Multi-user RIS (Ch. 7)(Review ch07)
Self-check: Recall the SOCP bisection for max-min RIS.

Notation for This Chapter

Security-specific notation. Alice is the transmitter, Bob is the legitimate user, Eve is the eavesdropper.

Symbol	Meaning	Introduced
$\mathbf{h}_B$	Alice-to-Bob effective channel (through RIS)	s01
$\mathbf{h}_E$	Alice-to-Eve effective channel (through RIS)	s01
$R_B, R_E$	Bob's and Eve's rates	s01
$R_s = [R_B - R_E]^+$	Secrecy rate: Bob's rate minus Eve's rate (non-negative)	s01
$C_s$	Secrecy capacity: supremum of $R_s$	s01
$\mathbf{z}$	Artificial noise signal added to jam Eve	s03
$\sigma^2_{E}$	Noise at Eve	s01
$\mathcal{U}_E$	Uncertainty set for Eve's channel / position	s04

← Ch 14 The RIS-Aided Wiretap Channel