In this chapter, we described the fundamental system model, challenges, and technical solutions to THz physical layer security. Despite the good security performance provided by the high directivity of THz wireless communications, THz physical layer security still faces security challenges, including eavesdroppers inside beam sector, eavesdroppers in close proximity and THz covertness problem. In light of these challenges, three THz-specific security schemes were described. First, the DA-APM proposes to transmit signals at sides of molecular absorption peaks to enhance the wireless security and covertness. This scheme effectively combats eavesdroppers inside the beam sector and can even enhance the covertness of THz wireless communications. Second, the SIC-free RAN scheme takes the advantage of the distance-dependent temporal broadening effect to jam Eve in close proximity while not jamming Bob. This method handles the eavesdroppers in close proximity with a lower-complexity receiver AN mechanism. Third, we described the WSA and beamforming scheme to combat near eavesdropping without the aid of other nodes. The size rationality of practical THz WSA is justified.Despite the fruitful research progress on THz physical layer security, there are still open challenges remaining to be tackled, which are described as follows. First, since the optimal molecular absorption coefficients depend on the propagation distance, it is necessary to adjust the carrier frequency as the distance between Alice and Bob varies. How to efficiently and practically tune the carrier frequency to use proper sub-bands needs investigation. Second, the hybrid beamforming structure is usually adopted in THz communications. However, the hybrid beamformer design for the WSA is a non-convex optimization problem. It is necessary to investigate efficient and low-complexity solutions for the WSA beamformer design. Finally, active security attacks, for instance, eavesdroppers may send the jamming signal to the legitimate receiver while overhearing the secret message, need to be handled for THz physical layer security. In line with this, security mechanisms simultaneously combating active and passive eavesdropping are awaited to be explored as a future direction.
In this chapter, we described the fundamental system model, challenges, and technical solutions to THz physical layer security. Despite the good security performance provided by the high directivity of THz wireless communications, THz physical layer security still faces security challenges, including eavesdroppers inside beam sector, eavesdroppers in close proximity and THz covertness problem. In light of these challenges, three THz-specific security schemes were described. First, the DA-APM proposes to transmit signals at sides of molecular absorption peaks to enhance the wireless security and covertness. This scheme effectively combats eavesdroppers inside the beam sector and can even enhance the covertness of THz wireless communications. Second, the SIC-free RAN scheme takes the advantage of the distance-dependent temporal broadening effect to jam Eve in close proximity while not jamming Bob. This method handles the eavesdroppers in close proximity with a lower-complexity receiver AN mechanism. Third, we described the WSA and beamforming scheme to combat near eavesdropping without the aid of other nodes. The size rationality of practical THz WSA is justified.Despite the fruitful research progress on THz physical layer security, there are still open challenges remaining to be tackled, which are described as follows. First, since the optimal molecular absorption coefficients depend on the propagation distance, it is necessary to adjust the carrier frequency as the distance between Alice and Bob varies. How to efficiently and practically tune the carrier frequency to use proper sub-bands needs investigation. Second, the hybrid beamforming structure is usually adopted in THz communications. However, the hybrid beamformer design for the WSA is a non-convex optimization problem. It is necessary to investigate efficient and low-complexity solutions for the WSA beamformer design. Finally, active security attacks, for instance, eavesdroppers may send the jamming signal to the legitimate receiver while overhearing the secret message, need to be handled for THz physical layer security. In line with this, security mechanisms simultaneously combating active and passive eavesdropping are awaited to be explored as a future direction.
The terahertz (THz) band is expected to become a key technology to meet the ever-increasing traffic demand for future 6G wireless communications, and a lot of efforts have been paid to develop its capacity. However, few studies have been concerned with the transmission security of such ultra-high-speed THz wireless links. In this paper, we comprehensively investigate the physical layer security (PLS) of a THz communication system in the presence of multiple eavesdroppers and beam scattering. The method of moments (MoM) was adopted so that the eavesdroppers' channel influenced by the PEC can be characterized. To establish a secure link, the traditional beamforming and artificial noise (AN) beamforming were considered as transmission schemes for comparison. For both schemes, we analyzed their secrecy transmission probability (STP) and ergodic secrecy capacity (ESC) in non-colluding and colluding cases, respectively. Numerical results show that eavesdroppers can indeed degrade the secrecy performance by changing the size or the location of the PEC, while the AN beamforming technique can be an effective candidate to counterbalance this adverse effect.
The broadcast nature of wireless communication systems makes wireless transmission extremely susceptible to eavesdropping and even malicious interference. Physical layer security technology can effectively protect the private information sent by the transmitter from being listened to by illegal eavesdroppers, thus ensuring the privacy and security of communication between the transmitter and legitimate users. Thus, the main design goal of physical layer security is to increase the performance difference between the link of the legitimate receiver and that of the eavesdropper using well-designed transmission schemes. The development of mobile communication presents new challenges to physical layer security research. This paper provides a survey of the physical layer security research on various promising mobile technologies from secure key generation and keyless techniques, including secure key generation, directional modulation (DM), spatial modulation (SM), covert communication, and intelligent reflecting surface (IRS)-aided communication. Finally, the future topics and the unresolved technical challenges are presented in physical layer security for mobile communications.
Physical layer security (PLS) has emerged as a new concept and powerful alternative that can complement and may even replace encryption-based approaches, which entail many hurdles and practical problems for future wireless systems. The basic idea of PLS is to exploit the characteristics of the wireless channel and its impairments including noise, fading, interference, dispersion, diversity, etc. in order to ensure the ability of the intended user to successfully perform data decoding while preventing eavesdroppers from doing so. Thus, the main design goal of PLS is to increase the performance difference between the link of the legitimate receiver and that of the eavesdropper by using well-designed transmission schemes. In this survey, we propose a conceptual, generic, and expandable framework for classifying the existing PLS techniques against wireless passive eavesdropping. In this flexible framework, the security techniques that we comprehensively review in this treatise are divided into two primary approaches: signal-to-interference-plus-noise ratio-based approach and complexity-based approach. The first approach is classified into three major categories: first, secrecy channel codes-based schemes; second, security techniques based on channel adaptation; third, schemes based on injecting interfering artificial (noise/jamming) signals along with the transmitted information signals. The second approach (complexity-based), which is associated with the mechanisms of extracting secret sequences from the shared channel, is classified into two main categories based on which layer the secret sequence obtained by channel quantization is applied on. The techniques belonging to each one of these categories are divided and classified into three main signal domains: time, frequency and space. For each one of these domains, several examples are given and illustrated along with the review of the state-of-the-art security advances in each domain. Moreover, the advantages and disadvantages of each approach alongside the lessons learned from existing research works are stated and discussed. The recent applications of PLS techniques to different emerging communication systems such as visible light communication, body area network, power line communication, Internet of Things, smart grid, mm-Wave, cognitive radio, vehicular ad-hoc network, unmanned aerial vehicle, ultra-wideband, device-to-device, radio-frequency identification, index modulation, and 5G non-orthogonal multiple access based-systems, are also reviewed and discussed. The paper is concluded with recommendations and future research directions for designing robust, efficient and strong security methods for current and future wireless systems.
Covert communication can prevent an adversary from knowing that a transmission has occurred between two users. In this article, we consider covert wireless communications in an Internet-of-Things (IoT) network with dense deployment, where an IoT device experiences not only the background noise but also the aggregates interference from other Tx devices. Our results show that in a dense IoT network with lower frequency AWGN channels, when the distance between Alice and the adversary Willie da,w= ω(n1/(2α)), Alice can reliably and covertly transmit O(log2√n) bits to Bob in n channel uses. In an IoT network with terahertz (THz) band, covert communication is more difficult because Willie can simply place a receiver in the narrow beam between Alice and Bob in order to detect or block their line-of-sight communications. We demonstrated that covert communication is still possible in this occasion by utilizing the reflection or diffuse scattering from a rough surface. From the physical-layer security perspective, covert communication can enhance the security of IoT network from the bottom layer.
In this work, the secrecy performance of a multiple-input–multiple-output (MIMO) terahertz (THz) communication system is investigated. To enhance physical layer security (PLS), a novel transmission protocol is proposed, which dynamically tunes the carrier frequency to align with the centers of molecular absorption loss (MAL) lines. This frequency-selection strategy increases atmospheric attenuation, thereby limiting the signal’s effective range and making it significantly more difficult for a distant eavesdropper (Eve) to intercept the transmission. To assess the secrecy performance, the exact statistical characteristics of the signal-to-noise ratios (SNRs) at both the legitimate user and Eve are derived. Using these statistical characteristics, a unified analytical framework is developed, which yields closed-form expressions for key secrecy metrics, such as the average secrecy capacity (ASC), the lower bound on the secrecy outage probability (SOP), and the probability of strictly positive secrecy capacity (SPSC). Asymptotic analyses in the high-SNR regime further reveal the secrecy diversity order and secrecy coding gain, providing deeper insight into the system’s robustness. Numerical results show that the proposed MAL-aligned frequency tuning can improve ASC by up to 25% and achieves nearly a twofold reduction in insecure area compared to conventional fixed-frequency transmission. These gains are achieved while imposing only a marginal reduction in the legitimate user’s capacity, thereby maintaining an efficient balance between security and communication performance.
As a complement to high-layer encryption techniques, physical layer security has been widely recognized as a promising way to enhance wireless security by exploiting the characteristics of wireless channels, including fading, noise, and interference. In order to enhance the received signal power at legitimate receivers and impair the received signal quality at eavesdroppers simultaneously, multiple-antenna techniques have been proposed for physical layer security to improve secrecy performance via exploiting spatial degrees of freedom. This paper provides a comprehensive survey on various multiple-antenna techniques in physical layer security, with an emphasis on transmit beamforming designs for multiple-antenna nodes. Specifically, we provide a detailed investigation on multiple-antenna techniques for guaranteeing secure communications in point-to-point systems, dual-hop relaying systems, multiuser systems, and heterogeneous networks. Finally, future research directions and challenges are identified.