Intelligent Reflecting Surface (IRS)-Enabled Covert Communications in Wireless Networks

INTRODUCTIONS

• Open wireless networks expose confidential information to malicious attackers, making reliable and secure wireless data a key concern for future networks.Source: INTRODUCTIONS · p. 1
• Encryption faces shared-protocol threats, potential compromise by powerful decryption, and difficult key management in decentralized networks with random access and mobility.Source: INTRODUCTIONS · p. 2
• Physical-layer security uses interference, noise, and fading to safeguard information without the extra signaling and communication overhead of higher-layer encryption; its two research tendencies are ITS and covert communication.Source: INTRODUCTIONS · p. 2
• ITS seeks a positive secrecy rate—the legitimate-channel rate minus the eavesdropping-channel rate—but preventing deciphering alone may not protect privacy when communication presence or location is exposed.Source: INTRODUCTIONS · p. 2
• Covert communication, also called low probability of detection or undetectable communication, hides legitimate transmission from a vigilant adversary while maintaining a covert rate for the intended user.Source: INTRODUCTIONS · p. 2

OVERVIEW OF COVERT COMMUNICATION

• Hiding a communication link makes its information immune from interception, and covertness does not depend on an adversary’s information-processing capability, unlike encryption.Source: OVERVIEW OF COVERT COMMUNICATION · p. 2
• Here, covert communication means physical-layer techniques that mask wireless transmission behavior, not techniques that conceal a message in text, images, or audio/video [3].Source: OVERVIEW OF COVERT COMMUNICATION · p. 2
• Covertness can complement or substitute for upper-layer steganography and encryption; the section introduces covert principles and surveys existing techniques.Source: OVERVIEW OF COVERT COMMUNICATION · p. 3

Understanding Covert Communication

• Alice sends to Bob while Willie monitors the channel and distinguishes H0, Alice is mute, from H1, Alice is transmitting.Willie aims to detect whether Alice is transmitting.Source: Understanding Covert Communication · p. 3
• Willie uses average received power in a time slot, comprising interference under H0 and Alice’s signal as well under H1, and compares it with threshold τ.He decides H0 below the threshold and H1 above it.Source: Understanding Covert Communication · p. 3
• Willie’s covert probability is the total probability of false alarm or mis-detection when deciding between H0 and H1.A false alarm selects H1 when H0 is true; a mis-detection selects H0 when H1 is true.Source: Understanding Covert Communication · p. 3
• For a given τ, reducing Alice’s received signal power SW can increase mis-detection, while enlarging interference-and-noise variance can increase both error probabilities.The covert probability is influenced by uncertainties in SW, IW, and σ2_W.Source: Understanding Covert Communication · p. 3

Overview of Covertness Techniques

• The overview frames covertness techniques as approaches that exploit covert communication’s properties to improve covertness performance.It introduces the techniques reviewed in the following subsections.Source: Overview of Covertness Techniques · p. 3

Multiple Antennas:

• Beamforming uses antenna-array phase and amplitude control to direct constructive radiation toward the recipient and null signals at an adversary for low-probability-of-detection (LPD).This transmitter-side approach can improve achievable rate and has high deployment scalability, but its performance depends largely on instantaneous channel state information (CSI).Source: Multiple Antennas · p. 4
• Massive antenna arrays can mitigate imperfect-CSI covertness loss, with more than one hundred transmit antennas given as an example.Highly correlated channels reduce CSI estimation errors and improve beamforming resolution, while channel hardening can support reliable covert rate.Source: Multiple Antennas · p. 4

AN Generation:

• Random artificial noise (AN) misleads an adversary about whether transmission exists, while spatial nulling can protect legitimate channels.Knowing the adversary’s position enables directional jamming to corrupt its detectivity to the largest extent.Source: AN Generation · p. 4
• Cooperative jamming uses a helper such as a power beacon or drone transmitter to disturb the adversary’s channel while minimally affecting legitimate transmission.It incurs synchronization and transmit-power-control communication overhead and sacrifices deployment scalability.Source: AN Generation · p. 5
• Full-duplex jamming enables a receiver to receive information and generate in-band AN concurrently, avoiding cooperative jamming’s control overhead and mobility issues.Its cost is loopback self-interference, which advances in antenna-interface, analog-baseband, and digital-processing techniques can suppress to a tolerable extent with viable expense.Source: AN Generation · p. 5
• AN injection lets the sender transmit information and AN simultaneously, ideally making AN orthogonal to the legitimate channel so only the adversary’s channel is affected.Its design balances covertness against information rate by optimizing jamming and information transmit powers; hybrid approaches can also be explored for performance enhancement.Source: AN Generation · p. 5

• Cooperative relaying uses intermediate nodes to facilitate undetectable communication.Source: Cooperative Relaying · p. 5
• Multi-hop forwarding shortens access distances, reducing required transmit power and helping keep Willie’s detection probability low.Long-distance transmission requires higher power to attain a target rate, which compromises covertness.Source: Cooperative Relaying · p. 5
• Using third-party devices as relays makes cooperative relaying’s deployment scalability relatively low.Source: Cooperative Relaying · p. 5

Spread Spectrum:

• Pseudo-noise modulation spreads transmission bandwidth and suppresses average signal power spectral density below the noise floor, reducing adversarial detectability.The noise-like signal makes information-bearing transmissions difficult to distinguish from noise.Source: Spread Spectrum · pp. 5–6
• Direct sequence continuously maintains low power spectral density and is more detection-immune than frequency hopping, whose narrowband hops have high power spectral density.Direct sequence spreads signals over multiple frequency channels, whereas frequency hopping rapidly switches channels across a wide range.Source: Spread Spectrum · p. 6
• Spread-spectrum frequency diversity improves robustness against fading, and its sender-side manipulation gives the approach high deployability.The frequency diversity is provided by the spread-spectrum signal.Source: Spread Spectrum · p. 6

Millimeter-Wave Communications:

• Millimeter-wave directionality suppresses off-boresight signal leakage, while an intercepted misaligned beam’s intermittent on-off pattern disrupts adversarial detection.The narrow beam enables precise angular resolution with moderately small antenna dimensions.Source: Spread Spectrum · p. 6
• Millimeter-wave ultra-wide bandwidth offers flexibility in legitimate transmission frequency, but its broad spectral scanning imposes substantial detection overhead on adversaries.The passage contrasts mmWave’s bandwidth with microwave communication.Source: Spread Spectrum · p. 6
• Millimeter-wave covert communication has low deployment stability because acute attenuation, obstacles, and walking-speed Doppler make covert rate depend heavily on line of sight and mobility.Its short wavelength weakens scattering and diffraction, while Doppler shift is strong even at walking speed.Source: Spread Spectrum · p. 6
• Existing covert techniques either mitigate information-signal leakage or enlarge interference dynamics, but consume bandwidth or energy and can reduce legitimate-user performance.Table I is described as summarizing and comparing the reviewed physical-layer techniques.Source: Spread Spectrum · p. 6

IRS-ENHANCED COVERT COMMUNICATION

• The IRS uses programmable metamaterials to control impinging waves and reshape propagation conditions, potentially hiding transmissions while reusing signals not received by intended receivers.The authors present this recycling of environmental resources as a novel approach for covert communication.Source: IRS-ENHANCED COVERT COMMUNICATION · p. 7
• An IRS can work alongside existing covertness techniques without redesigning their protocols or hardware because it serves as an auxiliary device that manipulates environmental signals.Source: IRS-ENHANCED COVERT COMMUNICATION · p. 7
• The IRS can also be jointly configured with the existing system to optimize performance, as illustrated by the article’s case study.Source: IRS-ENHANCED COVERT COMMUNICATION · p. 7

Fundamentals of IRS

• An IRS is a software-controlled artificial surface built from a thin tunable metasurface whose electromagnetic properties can be digitally re-engineered without refabrication.Source: Fundamentals of IRS · pp. 7–8
• Collectively configured meta-atoms synthesize diverse radiation patterns; uniform tuning supports absorption or passive reflection, while individual tuning enables polarization, imaging, and holograms.Source: Fundamentals of IRS · p. 8
• IRSs recycle environmental signals by reshaping phase, amplitude, frequency, and reflection angle; a binary-phase example boosted receiver intensity by an order of magnitude or canceled it.Source: Fundamentals of IRS · p. 8
• Unlike phased arrays with dedicated RF chains, IRSs use nearly passive, low-cost elements and a contiguous surface for finer-grained spatial control.Source: Fundamentals of IRS · pp. 8–9
• Compared with energy-consuming active metasurfaces, IRS reflection is virtually power-free and consumes power only when its units are reconfigured.Source: Fundamentals of IRS · p. 9
• IRSs avoid active relays’ self-interference and signal-processing latency while offering adjustable electromagnetic responses unlike passive relays’ usually fixed coefficients.Source: Fundamentals of IRS · p. 9
• Nearly passive IRS elements can have meager circuit power consumption and may be powered through microwave energy harvesting; one implementation used energy comparable to or below recycled microwave energy.Source: Fundamentals of IRS · p. 9
• Lightweight, ultra-thin IRS materials can coat objects to create rich scattering and more deterministic, self-sustainable propagation, with potential to serve heterogeneous QoS needs.Source: Fundamentals of IRS · pp. 9–10

• IRS signal intensification strengthens desirable signals at intended receivers, while signal cancellation suppresses unwanted signals at unintended receivers; configuring both objectives involves a trade-off.Source: IRS-Aided Covert Communication Systems · p. 10
• In the baseline Alice–Bob–Willie system, an IRS can intensify signals at Bob and cancel them at Willie, increasing Bob’s transmission rate while lowering Willie’s detection probability.For wireless-powered covert communication, Alice can also transfer power or use simultaneous wireless information and power transfer to supply Bob with energy.Source: IRS-Aided Covert Communication Systems · p. 10
• For mmWave covert communication, an IRS with line-of-sight links to Alice and Bob can address blockages that impair the direct link.The passage identifies severe penetration losses and poor diffraction of non-line-of-sight links as sources of mmWave vulnerability.Source: IRS-Aided Covert Communication Systems · p. 10
• With co-channel interference, the IRS can cancel interference at Bob and intensify it at Willie; under concurrent eavesdropping and detection attacks, it can cancel signals at both adversaries.Source: IRS-Aided Covert Communication Systems · pp. 10–11

OPTIMAL CONFIGURATION FOR IRS-ENHANCED COVERT COMMUNICATION UNDER NOISE UNCERTAINTY: A CASE STUDY

• The case study considers noise as the only cover medium to focus on the effects of IRS on covertness performance.Source: IRS-Aided Covert Communication Systems · p. 11

System Model

• Alice transmits covertly to Bob under Willie’s observation; all three use one antenna, and an N-unit passive IRS provides independently adjustable phase shifts, with Alice limited by P_A.Each reflecting unit can impose an arbitrary phase shift independently.Source: System Model · p. 12
• All channels have power-law path loss with exponent α and block Rayleigh fading; Alice and the IRS know all channel state information for joint optimization, yielding the best-performance benchmark, as in.The passage notes that Willie’s channel state information can be reasonably estimated when Willie is active, but the sentence is incomplete.Source: System Model · p. 12
• Willie’s noise follows a bounded-uncertainty model with nominal power σ_n^2 and uncertainty parameter ρ ≥ 1; larger ρ means greater uncertainty in σ_W^2.Noise uncertainty at Bob is excluded because it does not affect covertness, as in.Source: System Model · p. 12
• Willie knows the a priori distributions of his noise power and Alice’s received signals, but not the IRS’s existence or operation, so he sets his detection threshold using only that prior knowledge.The noise at Willie is modeled as Gaussian white noise.Source: System Model · pp. 12–13
• The objective is to maximize Alice–Bob covert rate measured by Shannon capacity through joint IRS-phase and transmit-power optimization, with detection-error, phase-range, and power constraints.The sum of Willie’s false-alarm and mis-detection probabilities must exceed ξ; IRS phases lie in [0, 2π), and Alice’s transmit power must remain below P_A.Source: System Model · p. 13

Numerical Results

• An IRS dramatically improves covert rate versus no IRS and delays its plateau to higher Alice power, while greater Willie noise uncertainty increases R_AB, especially at higher power.The plateau occurs when the covertness constraint can no longer be maintained above a certain transmit-power level.Source: Numerical Results · p. 13
• With N = 64 versus N = 16 IRS elements, the RAB ratio is 163.6% at d = 5 and 523.2% at d = 10, indicating better performance especially at larger distances.Source: Numerical Results · p. 14
• For a 1 bps/Hz target covert rate, increasing N from 16 to 64 extends covert transmission distance from about 8 m to about 11.7 m.Source: Numerical Results · p. 14

CONCLUDING REMARKS AND FUTURE DIRECTIONS

• Integrating an IRS can recycle previously unused environmental resources, and the case study shows considerable covert-performance improvement through joint IRS and system configuration.The article also anticipates IRS technology creating opportunities in future wireless security beyond covertness.Source: CONCLUDING REMARKS AND FUTURE DIRECTIONS · p. 14
• Future work should develop feasible IRS-channel estimation, potentially using machine learning without explicit feedback or detection, because passive operation makes instantaneous reflection-channel CSI difficult to acquire.Covertness performance depends heavily on CSI availability and accuracy.Source: CONCLUDING REMARKS AND FUTURE DIRECTIONS · p. 14
• Future information-theoretic models should revisit covert-channel capacity under IRS programmability and derive scaling laws to establish achievable performance limits.IRS signal intensification and cancellation are expected to let covert channels transport more information bits.Source: CONCLUDING REMARKS AND FUTURE DIRECTIONS · p. 14
• Research should investigate how multiple IRSs jointly shape propagation, accounting for their spatial distribution and aggregated impact on IRS-enhanced covert communication.The paper identifies deployment on environmental-object surfaces in complex spatial patterns as a common scenario.Source: CONCLUDING REMARKS AND FUTURE DIRECTIONS · p. 15