DeepAI
Log In Sign Up

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

11/03/2019
by   Xiao Lu, et al.
0

With growing security threats to the evolving wireless systems, protecting user privacy becomes progressively challenging. Even if the transmitted information is encrypted and the potential wiretap channel is physically limited (e.g. through information-theoretic security approaches), the raw data itself, such as transmitter position and transmission pattern, could expose confidential information. In this context, covert communication that intends to hide the existence of transmission from an observant adversary by exploiting the physical characteristics of the wireless medium has been actively investigated. However, existing covertness techniques ineluctably consume additional resources such as bandwidth and energy, which burdens system deployment. In view of this concern, we propose an intelligent reflecting surface (IRS)-based approach to enhance communication covertness. The core idea is making use of a smartly controlled metasurface to reshape undesirable propagation conditions which could divulge secret messages. To facilitate the understanding of the proposed idea, we first provide an overview of the state-of-the-art covert communication techniques. Then, we introduce the fundamentals of IRS and elaborate on how an IRS can be integrated to benefit communication covertness. We also demonstrate a case study of the joint configuration of the IRS and the legitimate transmitter, which is of pivotal importance in designing an IRS-enhanced covert communication system. Finally, we shed light on some open research directions.

READ FULL TEXT VIEW PDF
01/19/2021

IRS-Empowered Wireless Communications: State-of-the-Art, Key Techniques, and Open Issues

In this article, we overview intelligent reflecting surface (IRS)-empowe...
10/27/2020

Random Shifting Intelligent Reflecting Surface for OTP Encrypted Data Transmission

In this paper, we propose a novel encrypted data transmission scheme usi...
03/30/2021

Intelligent Reflecting Surface for Wireless Communication Security and Privacy

Intelligent reflection surface (IRS) is emerging as a promising techniqu...
02/15/2021

Automatisierte Verwaltung von ITS Roadside Stations für den simTD Feldversuch

The simTD project is the first large-scale field trial for vehicle-to-ve...
12/18/2022

Perfectly Covert Communication with a Reflective Panel

Covert communication, a sub-field of information security, is focused on...
09/17/2020

Intelligent Reflecting Surface Aided Pilot Contamination Attack and Its Countermeasure

Pilot contamination attack (PCA) in a time division duplex wireless comm...
09/04/2020

Evolving Intelligent Reflector Surface towards 6G for Public Health: Application in Airborne Virus Detection

While metasurface based intelligent reflecting surfaces (IRS) are an imp...

Introductions

Provisioning secured communication becomes unprecedentedly challenging owing to the threat of technology integration. The ubiquitousness of access interfaces and utilization of shared spectrum in an open wireless medium makes the soaring volume confidential information (e.g. financial account, identity authentication, and business secret.) more exposed to malicious attackers, the goal of which is to intercept sensitive and private data. Therefore, ensuring the reliability and security of wireless data remains one of the most important tasks in developing future generation networks and has drawn increasing attention from wireless communities [1].

The current practice of wireless security mainly relies on application/transport-layer encryption. However, securing wireless communications with encryption faces the following challenges: 1) the standardized protocols adopted for public networks make a large number of entities confront the same threat; 2) the security level of encryption protocols is compromised if eavesdroppers have powerful computational capacities; and 3) distribution and management of cryptographic keys are difficult in decentralized networks with random access and mobility. To cope with these difficulties, physical-layer security (PLS) approaches have drawn significant research attention over the past years. Essentially, PLS approaches safeguard information by only exploiting the fundamental nature of the wireless medium (i.e. interference, noise, and fading), which avoids extra signaling and communication overheads incurred by encryption in the higher layer(s). There are two remarkable research tendencies on PLS, namely, information-theoretic secrecy (ITS) [2] and covert communication [3]. ITS approaches aim to achieve a positive secrecy rate (i.e. the rate difference of a legitimate channel and an eavesdropping channel), at which information can be conveyed confidentially. Nevertheless, merely preventing transmission from being deciphered is not sufficient from the perspective of privacy protection. There appear progressively more circumstances where revealing the position, movement, or even the existence of communication is crippling or even fatal. For example, exposure of business activities could bare commercial secrets. Moreover, divulging soldier-to-soldier communications on the battlefield could result in military operation failure. This raises the need for covert communication, also known as

low probability of detection (LPD) communication

or undetectable communication, the objective of which is to shelter the presence of a legitimate transmission from a vigilant adversary while maintaining a certain covert rate at the intended user111In this article, covert communication refers to physical-layer techniques that hide wireless transmission over covert channels. This is different from the concept of covert information techniques that conceal a secret message in a cover medium (e.g. text, image and audio/video message) instead of masking the transmission behavior. .

Notably, covert communication offers three major advantages as follows: First, covertness techniques guarantee a stronger security level compared to ITS. If a communication link is hidden from an adversary, the information carried is immune from interception. Secondly, in contrast to encryption, the performance of covert communication does not rely on the adversary’s competence. In other words, the achievable security level will not be degraded even if the adversary has powerful information processing capability. Thirdly, covertness techniques can be implemented either as alternative or complementary solutions for upper-layer security and privacy techniques, such as steganography and encryption.

Generally, the existing covertness techniques can be classified into two categories in terms of the effects on adversaries. One is to mitigate the information signal leakage (e.g. through spread spectrum), and the other is to enlarge interference dynamics (e.g. via artificial noise (AN) generation) to cover the signal leakage. These approaches unavoidably consume additional system resources, such as bandwidth (e.g. for spread spectrum) and energy (e.g. for AN generation), and sacrifice the communication performance at legitimate users. To tackle this issue, we introduce an intelligent reflecting surface (IRS)-based solution to facilitate covert communication. The core technology of IRS is to have full control of electromagnetic behavior of the impinging waves by leveraging programmable metamaterials. Empowered by the IRS, the proposed approach has the potential to safeguard transmission from malicious detection by changing the propagation environment.

This approach is radically different from the existing ones since recycling the environment resources has not been previously considered for covert communication. An outstanding merit of the IRS that motivates the use of it for covert communication is its compatibility with the existing systems. An IRS can work in conjunction with existing techniques for covertness without the necessity to redesign the corresponding protocols and hardware. Meanwhile, an IRS can be jointly configured with the existing system for performance optimization.

Although IRS-based wireless communication has attracted a lot of research attention recently, existing literature on developing IRS-based security techniques is scarce. Recently, the authors in [4, 5] have studied IRS configuration problems for ITS under different system settings. However, to the best of the authors’ knowledge, none of the existing works has investigated IRS in the context of covert communication. This article aims to introduce the IRS as an enabling technology to advance communication covertness. We first introduce the fundamental principles of covert communication and review the existing techniques for covert communication. Then, we elaborate on the basics of the IRS and discuss in detail how the IRS can be applied to benefit communication covertness. Subsequently, we perform a case study in which the IRS and the legitimate transmitter are jointly configured to maximize the covert rate, i.e. the achievable rate at which covertness requirement can be satisfied. Finally, we discuss a few promising research directions and conclude the work.

Overview of Covert Communication

To facilitate communication stealthiness, a variety of techniques have been developed to obscure the detection at adversaries. This section first introduces the principles of covert communication and then presents an overview of the existing techniques.

Understanding Covert Communication

Consider a general point-to-point communication scenario where a legitimate transmitter (Alice) intends to deliver a message wirelessly to the target receiver (Bob) without being detected by an adversary (warden Willie). Willie monitors the wireless channel with the aim to detect whether Alice is on transmission or not. Hence, Willie faces a binary decision between null hypothesis

that Alice is mute and the alternative hypothesis

that Alice is transmitting. For such a purpose, Willie can perform statistical hypothesis testing based on the average power received in a time slot denoted as

. contains the received interference power and noise power in the case of and additionally contains the received signal power from Alice in the case of . Let and denote the decisions of Willie in favor of and , respectively. The decision of Willie follows a threshold-based rule which advocates and when is smaller or greater than a predefined threshold , respectively. According to this rule, erroneous decision occurs in two circumstances: 1) Willie sides with when is true, i.e. false alarm, and 2) Willie sides with when is true, i.e. miss detection. The total probability that Willie makes erroneous decisions (i.e. including false alarm and miss detection) can be interpreted as the covert probability for transmissions from Alice to Bob. It is evident that the covert probability is influenced by the uncertainties of , , and .

Fig. 1 illustrates the impacts of different parameters. The blue and orange lines represent the probability density function (PDF) of

with smaller variance and larger variance, respectively. For each case, the miss detection probability and false alarm probability can be represented by the left and right shadow areas, respectively. We can observe that, for a given

, it is possible to increase the miss detection probability by decreasing , and increase both the miss detection probability and false alarm probability by enlarging the variance of interference and noise. Thus, communication can be carried out more covertly with reduced signal leakage to Willie and/or with larger interference plus noise fluctuations.

Fig. 1: Illustration of the impacts of system parameters for covert communications.

Overview of Covertness Techniques

By exploiting the properties of covert communication, different approaches have been developed to enhance covertness performance, which are reviewed below.

Multiple Antennas

Taking advantage of spatial degrees of freedom, multiple-antenna techniques can be utilized to improve the stealthiness of wireless channels through directional transmission 

[6]

. This can be realized by means of beamforming to produce spatial selectivity. In particular, a beamformer adjusts the relative phase and amplitude of the signals on each element of an antenna array such that the superposed radiation pattern is constructive in the desired direction and destructive in the other directions. As a consequence, the transmitted signals can be concentrated towards the desired recipient to enhance the achievable rate and concurrently nulled at the adversary for LPD. Beamforming performance is largely dependent on the availability of instantaneous channel state information (CSI). The inaccuracy of CSI at a multi-antenna transmitter due to estimation error could result in a high probability of signal leakage to the adversary and thus degrading the covertness performance. However, the negative impact of imperfect CSI can be mitigated as the number of transmit antennas becomes massive, e.g. more than one hundred 

[7]. The highly correlated channels in massive antenna region render minimal CSI estimation errors and hence high beamforming resolution. Channel hardening effect that makes effective channel gains deterministic is an additional attribute of massive antennas that can be exploited to provide reliable transmission rate.

AN Generation

Random AN can be generated to increase interference dynamics, deliberately misleading the decisions of the adversary regarding the existence of any covert transmission. The key to a successful AN design is to avert the negative impact of jamming signals on legitimate channels [8]. For this, multiple-antenna techniques can be exploited to produce AN nulling in the directions of legitimate users. A more robust covert performance can be achieved if the position of the adversary is known so that the detectivity of the adversary can be corrupted to the largest extent through directional jamming.

An outstanding feature of the AN generation approach is its flexible implementation. In practice, AN can be generated by different entities. Some common ways of realizing AN generation are described as follows.

  • Cooperative jamming employs a third-party device (e.g. power beacon and drone radio transmitter) that functions as a helper to jam adversary’s channel. One or more friendly jammers can coordinate with Alice to disturb Willie’s channel while causing minimal impact on the legitimate transmission. Cooperative jamming incurs synchronization and communication overhead for transmit power control and may not efficiently work in the presence of mobility. The trustworthiness of a third-party jammer is also a practical concern. An unreliable jammer could function legitimately only occasionally and become malicious to detect covert transmission in conjunction with Willie.

  • Full-duplex jamming can support concurrent information reception and in-band AN generation with a full-duplex receiver. This approach surmounts the control the overhead, mobility, and trustworthiness issues of cooperative jamming at the cost of loopback self-interference from transmit to receive RF chains. Thanks to the recent advance of full-duplex techniques in multiple domains (e.g. antenna interface, analog baseband, and digital processing), self-interference can be suppressed to a tolerable extent with a viable expense. The jamming power needs to be carefully controlled for covertness requirements in consideration of legitimate information rate degradation by the residual self-interference.

  • AN injection is a sender-side technique capable of transmitting information signals and AN simultaneously. Ideally, AN is constructed to be orthogonal to the legitimate channel such that only Willie’s channel is affected. This approach avoids the impact of self-interference with the full-duplex jamming approach as well as addresses the aforementioned issues with the cooperative jamming approach. The crux of AN injection is to balance the trade-off between covertness and information rate by optimizing the transmit powers of jamming and information signals.

In addition, hybrid approaches can be explored for performance enhancement.

Cooperative Relaying

Cooperative relaying relies on cooperation from intermediate node(s) to facilitate undetectable communication. For legitimate communication, the access distance has a profound effect on covertness. For long-distance transmission, high transmit power is required to attain a target rate that unavoidably compromises the covertness. Cooperative relaying remedies this issue by multi-hop forwarding. The rationale is to shorten the access distance of each hop so as to keep the required transmit power low, rendering a low detection probability by Willie. It is worth mentioning that relaying protocols has a pivotal influence on covertness performance. For example, a relay node directly forwards the received signal with power amplification under amplify-and-forward (AF) protocol. By contrast, the relay node decodes the received signal and transmits a re-encoded version under decode-and-forward (DF) protocol. Both of the protocols result in different end-to-end signal-to-noise ratios, and thus different achievable rates. Moreover, to ensure a target rate requirement, DF and AF protocols need to use different transmit powers for the transmitter and relays which result in different covert probabilities.

Spread Spectrum

The spread spectrum approach facilitates covertness by suppressing the average power spectral density (PSD) of the transmitted signal below the noise floor level. Specifically, the information is modulated on a sequential noise-like wave, namely pseudo-noise sequence, which considerably spreads the transmission bandwidth compared to the one required by normal narrowband signals. As a result, it is difficult for an adversary to discriminate the information-bearing signals from noise, which significantly lowers the signal detectability. Typical modulation techniques adopted for bandwidth spreading include direct sequence which spreads the transmitted signal over multiple frequency channels and frequency hopping which randomly and speedily switches the transmission channel across a fairly wide frequency range. Generally, direct sequence is more immune to malicious detection as the PSD of the transmitted signal is continuously kept low. Frequency hopping is more exposed as it makes use of narrow-banded signals with high PSD on any frequency hop. In addition to LPD, spread spectrum can avoid an adverse effect of interference which is desirable in scenarios where jamming attack additionally exists. The frequency diversity empowered by spread-spectrum signal offers robustness of covert communication against fading. The cost incurred by spread spectrum techniques includes high hardware complexity (e.g. high-speed pseudo-noise generator), large signal processing overhead (e.g. pseudo-noise sequence acquisition) and bandwidth inefficiency.

Millimeter-Wave Communications

Operating at the frequency bands between 30-300 Gigahertz (GHz), millimeter-wave (mmWave) communication features steerable narrow beam, i.e. precise angular resolution can be realized by moderately small antenna dimensions. The directionality of the narrow beam naturally benefits covertness as signal leakage due to imperfect beam patterns towards the off-boresight directions can be suppressed. To intercept mmWave communication, an adversary can only detect the misaligned beam, which exhibits an on and off behavior where the bursty beam arrives intermittently [9]. This distinguishing beam pattern effectively disrupts the detectability of an adversary. Furthermore, the ultra-wide bandwidth of mmWave compared to microwave allows high flexibility in the frequency range of legitimate transmission. Scanning signals on a wide spectral ambit imposes a great amount of overhead for signal detection at an adversary. Additionally, as an antenna size shrinks with frequency, mmWave permits the implementation of mobile-friendly devices with lightweight and small form factor which is especially appealing for a secret operation in the tactical military environments. Although the directionality of short wavelength is desirable for covertness, The downside comes to weakened scattering and diffraction abilities which make mmWave attenuate acutely and susceptible to obstacles. Moreover, the Doppler shift of mmWave is strong even at walking speed. Thus, in contrast to the microwave counterpart, the covert rate of mmWave communication is vastly affected by the availability of line-of-sight channels and mobility.

Table I summarizes and compares the above-reviewed physical-layer techniques for covert communication.

Technique Effect on adversaries Computational complexity Deployment scalability
(Massive) Multiple antennas Weakening signal leakage High High
MmWave Weakening signal leakage High High
Cooperative relaying Weakening signal leakage Low Low
AN generation Increasing interference dynamics Low if using a third-party device and high otherwise Low if using a third-party device and high otherwise
Spread spectrum Weakening signal leakage High high
TABLE I: Comparison of existing techniques for covert communication

IRS-Enhanced Covert Communication

This section first elaborates the basics of IRS, including the principles, features, and differences from other related concepts, and then introduces the IRS-enhanced covert communication systems.

Fundamentals of IRS

An IRS is a software-controlled artificial surface that can be programmed to alter its electromagnetic response. The hardware realization of IRS is based on tunable metasurface, which is a thin and planar electromagnetic material consisting of discrete scattering particles spread over the structure, the electromagnetic characteristics (e.g. capacitances and resonances) of which can be digitally re-engineered without re-fabrication. This can be realized by leveraging electronically tunable meta-atoms, such as liquid crystal, varactor/PIN diodes, doped semiconductors, micro-electro-mechanical systems (MEMS) switches, and flexible plasmonics. Generally, there exist three approaches to change the electromagnetic properties of meta-atoms, namely, tunable resonator technique, guided-wave technique, and rotation technique, a detailed review of which can be found in [10].

Configuring the constitutional meta-atoms collectively enables the entire metasurface to synthesize a wide diversity of radiation patterns that are infeasible with natural materials. The meta-atoms can either be tuned uniformly or individually. The former can realize simple electromagnetic manipulations such as absolute absorption and passive reflection, while the latter can support more complicated operations such as wave polarizing, imaging, and holograms. With the striking advancement in fabrication techniques of metamaterials, modern IRSs are capable of fully reshaping the phase, amplitude, frequency, and reflecting angles of impinging signals in a full-duplex fashion. For instance, the authors in [13] implement a binary phase state IRS and demonstrate that for point-to-point transmission in an indoor environment the IRS can either boost the signal intensity at the receiver by an order of magnitude or totally cancel it.

Not to be confused with some related techniques that can also be applied to facilitate covert communication, we discuss their key differences and highlight the comparative advantages of IRS as follows.

  • A phased array utilizes an array of radiators with variable phase shifts to create different beam patterns. As each radiator is associated with a dedicated active RF chain, a phased array incurs high hardware cost and appears with a large form factor. Moreover, the performance of a phased array degrades at high frequency (e.g. GHz) as a result of reduced efficiency of the feed line. By contrast, an IRS features low-cost fabrications with nearly passive elements. Meta-materials such as ferroelectric films and graphene maintain good control of electromagnetic waves over a wide frequency range covering Terahertz and visible region [10]. Another desirable feature of the IRS is that the contiguous surface enables more fine-grained spatial resolution of electromagnetic control than that of the spaced antenna arrays with radiator separation.

  • Active metasurfaces [11] make use of active materials (e.g. epsilon-near-zero materials [12]) to generate an electromagnetic field on the entire surface. Although active metasurface provides exceptional controllability of signals, the operation is energy-consuming and the configuration usually incurs high computational complexity, e.g. due to signal processing. By contrast, an IRS entails considerably reduced computational complexity and lower energy profile due to its passive electromagnetic manipulation.

  • Full-duplex relays resemble IRSs in the aspects of full-duplex transmission and multipath diversity gain. Full-duplex relays can be either active or passive. An active relay employs active components (e.g. power amplifier, analog-to-digital converter (ADC), and mixer) for signal generation, which inevitably causes self-interference and signal processing latency. Instead, the passive electromagnetic operation of an IRS makes it free from self-interference. A passive relay, for which the electromagnetic responses (i.e. reflection coefficients) are pre-designed and fixed, contributes to multipath propagation through passively backscattering. By contrast, an IRS possesses greater flexibility in adjusting its electromagnetic response. Furthermore, an IRS is far more versatile than an information-forwarding relay as it can perform concurrent functions (e.g. beamsteering and interference cancellation) to satisfy heterogeneous quality-of-service (QoS) requirements.

Apart from its distinctive physical properties, IRSs are deployment-friendly. First, a metasurface can be fabricated with nearly passive elements (e.g. analog phase shifters) that do not rely on active components for transmission. Hence, the circuit power consumption of a metasurface is typically meager and can be powered through microwave energy harvesting. For example, the experimental results in [13] show that the energy consumption of the implemented IRS is typically comparable to or lower than the amount of microwave energy it can recycle. Second, thanks to the light-weight and ultra-thin footprint, metamaterials can be easily coated on the facade of environmental objects, e.g. walls, vehicles, and smart clothing, constructing a rich scattering environment. This is in contrast to the natural radio environment, the physical obstacles usually exert non-controllable negative effects of multipath fading, signal attenuation, and shadowing, conducing to probabilistic radio waves propagation behavior. With the exceptional abilities to tailor the property of electromagnetic waves, IRS-coated objects have the potential of delivering a more deterministic wireless propagation environment in a self-sustainable manner. This opens up board opportunities to satisfy heterogeneous QoS requirements for future generation networks (e.g. more stable connectivity, improved data rate, higher spectral efficiency) by only recycling existing environmental resources.

IRS-Aided Covert Communication Systems

By leveraging the powerful electromagnetic control of metasurface, an IRS can be carefully designed to improve the undesirable propagation conditions to facilitate covert communication. Generally, there are two functions of IRS that can be utilized to enhance transmission covertness. On the one hand, an IRS can reflect the desirable signals (e.g. information transmission) in phase with the ones at the intended receiver so as to strengthen the signals, referred to as signal intensification. On the other hand, an IRS can reflect the unwanted signals (e.g. information leakage and interference) in opposite phase with the ones at the unintended receiver, referred to as signal cancellation. Usually, there exists a trade-off in configuring the electromagnetic responses of the IRS elements to achieve the above two objectives simultaneously.

Fig. 2: IRS-enhanced covert communication systems.

Next, we elucidate how an IRS can be exploited to enhance covert communication in various system environments (Fig. 2). Fig. 2(a) illustrates a baseline system model where Alice intermittently transmits to Bob in the presence of Willie. In this scenario, an IRS can be employed to perform signal intensification at Bob and signal cancellation at Willie. Moreover, if the baseline system coexists with ambient interferers, as shown in Fig. 2(b), an IRS can be additionally configured to conduct interference cancellation at Bob and interference intensification at Willie to cover the signal from Alice. In addition to the malicious detection from Willie, legitimate users may confront other types of security attacks in practice. In this case, an IRS can be devised to realize multiple security targets. For example, Figs. 2(c) and 2(d) illustrates the scenarios where there appears a coexisting jamming attack and eavesdropping attack, respectively. Signal cancellation at both the eavesdropper and Willie and similar operations in Fig. 2(b) needs to be conducted in the former and latter cases, respectively, to handle the concurrent attacks. An IRS can be more contributive in scenarios with heterogeneous QoS requirements. For instance, Fig. 2(e) depicts a wireless-powered communication system where Bob first performs downlink wireless power transfer to Alice, and then Alice performs covert uplink transmission with the harvested energy. Other than achieving covert communication, the IRS additionally takes the role of facilitating wireless power transfer. Last but not least, as a complementary device, an IRS can work in conjunction with the existing techniques for covert communication reviewed in Section II.B. Furthermore, the electromagnetic responses of the IRS and the adopted covertness technique(s) can be jointly optimized to achieve the desired target. As such, an IRS can assist the existing techniques to form a well-integrated covertness solution for enhanced privacy protection. For example, Fig. 2(f) illustrates the use of an IRS with direct links to both Alice and Bob to address the impact of obstacles for mmWave covert communication. The beamforming weight at Alice and the phase and amplitude manipulations at IRS can be jointly designed to maximize the covert rate.

Optimal Configuration for IRS-Enhanced Covert Communication under Noise Uncertainty: A Case Study

We show a case study of designing an IRS-enhanced covert communication system. We consider noise as the only cover medium with the aim to focus on showing the effects of IRS on the covertness performance.

System Model

Fig. 3: System model.

We consider an IRS-enhanced covert communication system where Alice intends to transmit to Bob with LPD by a warden Willie. An IRS is deployed to facilitate the covert transmission of Alice. We consider that Alice, Bob, and Willie are all equipped with a single antenna. Alice has a maximum transmit power budget denoted by . The IRS consists of passive reflecting units, each of which can generate an arbitrary phase shift of the incident signal wave independently. Similar to [5], we assume the CSI of all the channels are available at Alice and the IRS for the joint optimization, which yields the best system performance benchmark. It is worth noting that the CSI of Willie can be reasonably estimated when Willie is an active transmitter [14].

We consider a bounded uncertainty model for the noise observed by Willie , the PDF of which is given by [15, eqn. 3]: , if and , otherwise. Here, is the nominal noise power, is the uncertainty parameter. Similar to [15]

, the noise uncertainty at Bob is not considered as it does not affect the covertness performance. Specifically, the noise power at Willie is considered as Gaussian white noise with zero mean and variance

. Moreover, Willie is considered to know a priori distributions of and the received signals from Alice, however, is unaware of the existence and operation of the IRS. Thus, Willie can only set its detection threshold based on the available a priori knowledge.

We consider the optimization problems to maximize the covert rate by joint optimizing the configuration of the IRS units and the transmit power of Alice under the constraint that the sum of the false alarm probability and miss detection probability is smaller than a target threshold .

Numerical Results

Fig. 4: Covert rate as a function of (, , dBm, ).
Fig. 5: Covert rate as a function of (, dBm, ).

In the simulations, Alice, Bob, the IRS, and Willie are located at (0, 0), (, 0), (, 0), and (0, 15) in a two-dimensional area, respectively. Alice is considered to transmit with a probability of 50.

Fig. 5 depicts the covert rate as a function of the maximum transmit power . For the comparison purpose, we also present the results of without the use of an IRS. It can be found that can be dramatically improved with the aid of an IRS. Moreover, reaches a steady value at much larger in the case with an IRS compared to that without an IRS. Another observation is that greater noise uncertainty at Willie (represented by larger ) helps to improve . The performance gap between the cases with and becomes much more conspicuous with the increase of .

Fig. 5 demonstrates the impact of the number of IRS elements under varying transmission distance. The results show that larger renders better performance, especially when is large. For instance, the ratio of with to that with is when and is increased to when . This also implies that employing more IRS elements is an effective way to improve the covert transmission distance. For increase, if the target covert rate is 1 bps/Hz, the covert transmission distance is extended from about 8 m to about 11.7 m when is increased from to .

Concluding Remarks and Future Directions

With the integration of the IRS to covert communication systems, the previously unused environment resources can be recycled to enhance communication covertness. The article reviews the existing covertness techniques and envisions the use of the IRS to revolutionize covert communication systems in various aspects. A case study has also been presented to demonstrate that a considerable improvement of covert performance can be achieved through the joint configuration of the IRS and the covert communication system. We firmly believe that the emerging IRS technology will open up broad opportunities in designing and developing future wireless security, not limited to covertness techniques.

The scope of future research topics on IRS-enhanced covert communication is broad. Some open issues and research directions are as follows:

  • Channel estimation

    : It is apparent that the wave manipulation of an IRS is heavily dependent on the availability and accuracy of CSI. However, instantaneous CSI of the reflection channels is difficult to be acquired due to the nearly passive operation of an IRS. Even with the assistance of active components, such as embedded sensors throughout the IRS, channel estimation still suffers from detection errors and feedback outage. In this context, the machine learning-based approach that allows estimating channels without explicit feedback/detection is expected to be a feasible solution. In particular, devising machine learning-based channel estimation while maintaining the IRS as passive as possible is an intriguing research direction.

  • Information/communication theoretic models: With the signal intensification and cancellation capabilities of the IRS, an IRS-enhanced covert channel is expected to transport a larger volume of information bits. Hence, the conventional covert channel capacity needs to be revisited by taking into account channel programmability. Moreover, scaling laws of IRS-enhanced covert channel capacity needs to be derived for a fundamental understanding of achievable performance limits.

  • Impact of multiple IRSs: IRSs are anticipated to be deployed on the superficies of environmental objects located with perplexing spatial patterns. Therefore, it is a common scenario that the propagation environment is jointly shaped by multiple IRSs. The aggregated impact of the operation of ambient IRSs on IRS-enhanced covert communication worth to be investigated by considering their spatial distribution. Furthermore, the coordination among multiple cooperating IRSs to serve the same covertness objective under the impact of coexisting non-cooperative IRSs is an open issue.

References

  • [1] I. Ahmad, T. Kumar, M. Liyanage, J. Okwuibe, M. Ylianttila, and A. Gurtov, “Overview of 5G security challenges and solutions,” IEEE Communications Standards Magazine, vol. 2, no. 1, March 2018.
  • [2] Y. Liang, H. V. Poor, and S. Shamai, “Information theoretic security,” Foundations and Trends in Communications and Information Theory, vol. 5, pp. 355-580, June 2009.
  • [3] B. A. Bash, D. Goeckel, and D. Towsley, “Limits of reliable communication with low probability of detection on AWGN channels,” IEEE J. Sel. Areas Commun., vol. 31, no. 9, pp. 1921-1930, Sep. 2013.
  • [4] X. Yu, D. Xu, and R. Schober, “Enabling secure wireless communications via intelligent reflecting surfaces,” arXiv preprint arXiv:1904.09573.
  • [5] M. Cui, G. Zhang, and R. Zhang, “Secure wireless communication via intelligent reflecting surface”, IEEE Wireless Communications Letters, to appear.
  • [6] T. X. Zheng, H. M. Wang, D. W. K. Ng, and J. Yuan, “Multi-antenna covert communications in random wireless networks,” IEEE Transactions on Wireless Communications, vol. 18, no. 3, March 2019.
  • [7] E. Bjornson, et al., “Massive MIMO systems with non-ideal hardware: Energy efficiency, estimation, and capacity limits,” IEEE Transactions on Information Theory, vol. 60, no. 11, Nov. 2014.
  • [8] R. Soltani, et al., “Covert wireless communication with artificial noise generation,” IEEE Transactions on Wireless Communications, vol. 17, no. 11, pp. 7252-7267, Nov. 2018.
  • [9] J. G. Andrews, et al, “What will 5G be?,” IEEE J. Sel. Areas Commun. vol. 32, no. 6, pp. 1065-1082, June 2014.
  • [10] S. V. Hum and J. P. Carrier, “Reconfigurable reflectarrays and array lenses for dynamic antenna beam control: A review,” IEEE Transactions on Antennas and Propagation, vol. 62, no. 1, Jan. 2014.
  • [11] S. Hu, F. Rusek, and O. Edfors, “Beyond massive MIMO: The potential of data transmission with large intelligent surfaces,” IEEE Trans. Signal Process., vol. 66 , no. 10, May 2018.
  • [12] Y. C. Jun, J. Reno, T. Ribaudo et al., “Epsilon-near-zero strong coupling in metamaterial-semiconductor hybrid structures,” Nano Letters, vol. 13, no. 11, pp. 5391–5396, Oct. 2013.
  • [13] N. Kaina, et al., “Shaping complex microwave fields in reverberating media with binary tunable metasurfaces.” Scientific reports, 4, 6693. Oct. 2014.
  • [14] A. Mukherjee, S. A. A. Fakoorian, J. Huang, and A. L. Swindlehurst, “Principles of physical layer security in multiuser wireless networks: a survey,” IEEE Commun. Surveys Tuts., vol. 16, no. 3, pp. 1550-1573, Third Quarter 2014.
  • [15] B. He, et al, “On covert communication with noise uncertainty,” IEEE Communications Letters, vol. 21, no. 4, April 2017.