Exploiting the immanent randomness of the wireless networks, physical layer security (PLS) is an promising technique that can overcome the limitations of conventional security schemes based on cryptography. Most of the existing PLS algorithms employ multiple antennas, which are used for digital beamforming. However, while it is challenging to adjust both amplitude and phase, because of significant energy consumption if the number of antennas is large. Thus, in this paper, we consider an analog beamforming-based PLS algorithm, which consumes considerably lower energy. In addition, motivated by the simple line-of-sight (LoS) channel assumption in prior work for analog beamforming-based PLS, we investigate the impact of the fading and scattering effects, assuming Rician fading channels, which has both LoS and non-LoS components. Through simulation, we show that secrecy rate decreases as the Rician K factor increases.
Exploiting the immanent randomness of the wireless networks, physical layer security (PLS) is an promising technique that can overcome the limitations of conventional security schemes based on cryptography. Most of the existing PLS algorithms employ multiple antennas, which are used for digital beamforming. However, while it is challenging to adjust both amplitude and phase, because of significant energy consumption if the number of antennas is large. Thus, in this paper, we consider an analog beamforming-based PLS algorithm, which consumes considerably lower energy. In addition, motivated by the simple line-of-sight (LoS) channel assumption in prior work for analog beamforming-based PLS, we investigate the impact of the fading and scattering effects, assuming Rician fading channels, which has both LoS and non-LoS components. Through simulation, we show that secrecy rate decreases as the Rician K factor increases.
Physical-layer security (PLS) is a promising alternative or complement to cryptography-based solutions, because security key management and distribution required in such schemes are challenging in future wireless technologies with a distributed nature (e.g., Internet-of-Things (IoT) and massive machine-type communication). Motivated by this fact, in this paper, we investigate a PLS algorithm using linear virtual antenna array (VAA) constructed by multiple single-antenna nodes in mobile ad hoc networks such as unmanned aerial vehicles (UAVs) and mobile robots. Digital cooperative beamforming (CB) has been extensively explored such as cooperative null-steering or jamming. However, in spite of their effectiveness, the digital CB-based schemes incur prohibitive overhead and complexity, which creates difficulty in networks with limited hardware (e.g., IoT). To overcome this issue, we consider an analog CB-based PLS with linearly distributed VAA, which can be implemented in a fully distributed manner. In the proposed scheme, the received signal at the intended receiver is co-phased, while other receivers capture noise-like signals through randomized radiation patterns using the linear VAA. Assuming Rician channels, we derive secrecy rate of the proposed algorithm in a closed-from expression for an arbitrary K-factor, which is not available in previous work on the analog CB-based PLS. Furthermore, we show that the random location perturbations of the VAA elements following Gaussian distributions can enhance the secrecy performance by resolving the angle-of-departure ambiguity issue in linear arrays.
This thesis focuses on the wireless security in the physical layer with beamforming technique. One of the emerging areas is the security enhancement in the physical layer, which exploits the intrinsic properties of the wireless medium. Beamforming, which has been proved to have many advantages, can also be applied to enhance the wireless security. One of the most common threats, i.e., passive eavesdropping, is addressed in this thesis. To reduce the risk of leaking information to the eavesdroppers, the physical region where the transmission is exposed to eavesdropping has been studied. In this thesis, the exposure region based beamforming technique is proposed to combat the threat from the randomly located eavesdroppers in a fading channel. In this system model, the large-scale path loss and a general Rician fading channel model are considered. The exposure region is established to describe the secrecy performance of the system, based on which the probability that the secrecy outage event occurs is defined to evaluate the security level of the exposure region. The antenna array is one of the most important factors that affect the secrecy performance of the exposure region based beamforming technique. The potential of using different array geometry and array configuration to improve security is explored. In this thesis, two common arrays, i.e., linear and circular arrays, are considered.Analytic expressions for general array geometry and for the linear and circular arrays are derived. Based on the empirical results, numerical optimization algorithms are developed to exploit the array configuration to enhance the system security level. In addition, experiments are carried out to study the performance of the beamformer with linear and circular arrays. Especially, the impact of the mutual coupling on the security performance is investigated.
Traditional cryptographic encryption techniques at higher layers require a certain form of information sharing between the transmitter and the legitimate user to achieve security. Besides, it also assumes that the eavesdropper has an insufficient computational capability to decrypt the ciphertext without the shared information. However, traditional cryptographic encryption techniques may be insufficient or even not suit- able in wireless communication systems. Physical layer security (PLS) can enhance the security of wireless communications by leveraging the physical nature of wireless transmission. Thus, in this thesis, we study the PLS performance in emerging wireless transmission systems. The thesis consists of two main parts. We first consider the PLS design and analysis for ground-based networks em- ploying random unitary beamforming (RUB) scheme at the transmitter. With RUB technique, the transmitter serves multiple users with pre-designed beamforming vectors, selected using limited channel state information (CSI). We study multiple-input single-output single-eavesdropper (MISOSE) transmission system, multi-user multiple-input multiple-output single-eavesdropper (MU-MIMOSE) transmission system, and massive multiple-input multiple-output multiple-eavesdropper (massive MI- MOME) transmission system. The closed-form expressions of ergodic secrecy rate and the secrecy outage probability (SOP) for these transmission scenarios are derived. Besides, the effect of artificial noise (AN) on secrecy performance of RUB-based transmission is also investigated. Numerical results are presented to illustrate the trade-off between performance and complexity of the resulting PLS design. We then investigate the PLS design and analysis for unmanned aerial vehicle (UAV)-based networks. We first study the secrecy performance of UAV-assisted relaying transmission systems in the presence of a single ground eavesdropper. We derive the closed-form expressions of ergodic secrecy rate and intercept probability. When multiple aerial and ground eavesdroppers are located in the UAV-assisted relaying transmission system, directional beamforming technique is applied to enhance the secrecy performance. Assuming the most general κ-μ shadowed fading channel, the SOP performance is obtained in the closed-form expression. Exploiting the derived expressions, we investigate the impact of different parameters on secrecy performance. Besides, we utilize a deep learning approach in UAV-based network analysis. Numerical results show that our proposed deep learning approach can predict secrecy performance with high accuracy and short running time.
As the importance to guarantee confidentiality against security attacks by eavesdroppers has attracted significant attention, there has been much effort to enhance the security performance through cooperative beamforming (CB) in the context of physical layer security (PLS). In particular, analog CB (ACB), which requires vastly lower overhead compared to digital CB, can provide significantly higher secrecy rate compared to the PLS methods with co-located array. In this letter, we focus on ACB with open-loop architecture that requires low complexity, and consider phase offsets across multiple virtual antenna array (VAA) elements for ACB, which are caused by position estimation error of VAA. We investigate the impact of the phase offsets in ACB in terms of the secrecy throughput loss.
Physical layer security (PLS) has grown in popularity as it guarantees secure and reliable communication between wireless devices without restricting the ability of data analysts to eavesdrop. PLS benefits from the randomization property of the wireless channel, which improves immunity and protects against many dangers of information breaches from eavesdroppers. MIMO and Massive MIMO, which use numerous antennas at the transmitter and receiver ends, are fundamental technologies for improving efficiency in 5G and future network systems. The PLS still requires a significant amount of work before it can be employed in practical systems. The interception of eavesdroppers can be made difficult by adding artificial noise. This paper focuses on the physical layer security considering maximal ratio combining, and artificial noise with beamforming techniques to further enhance the secrecy rate. The proposed work demonstrates that the use of transmitter beamforming with artificial noise can significantly improve the degree of confidentiality of information being transmitted between a transmitter and an intended receiver while making it more difficult for eavesdroppers to intercept. The simulation results in terms of bit error rate show that the proposed method has a significant impact on the error rate performance of the eavesdropper.
Traditional methods to secure communications rely on algorithms at protocol stack level, which are based on mathematically derived cryptographic protocols, to encrypt transmitted signals. With the steep rise in the data traffic and storage each year, and the strong increase in computational power available for unauthorized decryption, traditional mathematical cryptographic measures might be compromised in the upcoming years. To protect communications at the lowest level, physical layer security (PLS) methods have been proposed recently. These techniques aim to ensure secure communication links by exploiting the known unique characteristics of the physical medium between communicating nodes in order to distort a transmitted signal in such a way that only the intended receivers are unaffected by the distortion, effectively reducing the eavesdropping capabilities of any unintended party. In this way, it is the channel itself which is secured. These techniques come at the expense of several trade-offs between transmission resources and the degree of secrecy achieved. Of our particular interest is a class of PLS methods which can be referred to as `security enhancing techniques', in which minimal priors about eavesdroppers are assumed. In this dissertation we introduce innovative, wave scattering-based concepts and algorithms for the successful protection of both wireless and wired communication channels at the physical layer. This new class of approaches for physical layer (PHY) secure communications involve analog, controllable and reconfigurable scattering media (termed "micromedia'") positioned near the communicating nodes. This implementation is achieved via truly physical, analog means by exploiting the use of unilateral pilot signals for channel estimations. Prior research in this nascent field has addressed the important case of wireless channels as well as the more manageable wired communication channel case, the latter in connection with applications to power line communications (PLC) which make use of the available smart grid and other existing infrastructure. On the other hand, within the wireless regime, most existing work has focused on the canonical free-space case scenario, or -~within mathematical, statistical information theoretic frameworks~- on the classical random matrix models such as those for fading and other channels. Little attention has been given to the important case of unknown, complex, fixed or possibly slow-fading (block-fading) wireless channels, where little prior information is available other than that facilitated by the parties through explicit, cooperative communication. In addition, within the wired channel scenario, attention has been given only to techniques already developed for the wireless case. Furthermore, in both cases, many of the proposed algorithms are implemented mathematically rather than physically or analogically. In contrast, our proposed scattering approaches involve analog reconfigurable media and are applicable not only to free space but also to rather complex and unknown media. Our particular interest are applications to communications in indoor channels, tunnels, caves, and similar media, including underwater channels. Within this wireless regime, we develop novel secure communication methodologies applicable to single-input-single-output (SISO), multiple-input-single-output (MISO) and multiple-input-multiple-output (MIMO) systems. The developed methods for the SISO case are subsequently extrapolated also to the transmission line wired channel. The presented results and computer simulations demonstrate the feasibility for achieving secure communication links by means of the proposed techniques as well as the relevant trade-offs between communication resources and secrecy. Importantly, the proposed techniques provide security for the SISO case which had so far not been covered as a security enhancing technique. In the wired case, we address the transmission-line-like channels and more complex circuit networks. Motivation is provided by applications to secure communication in PLC networks, including the smart grid, as well as near field communication systems, intra-chip communication, and even novel modems for secure communication via conventional wired RF and optical channels. In this work, particular attention is given to single-phase, one-dimensional transmission line systems and planar circuit-like networks. The ideas, as presented for those particular classes of systems, can, of course, be readily extended to more general wired systems, and thus have relevance also for potential application to PLC systems where one deals in general with multi-phase transmission lines, electric machines, and other components that can be modeled via suitable lumped or distributed system parameters--Author's abstract