Selected Publications

Abstract: Physical-layer security (PLS) is a promising technique to complement communication security in beyond-5G wireless networks. However, PLS developments in current research are often based on the ideal assumption of infinite coding blocklengths or perfect knowledge of the wiretap link's channel state information (CSI). In this work, we study the performance of finite blocklength (FBL) transmissions using a new secrecy metric - the average information leakage (AIL). We evaluate the exact and approximate AIL with arbitrary signaling and fading channels, assuming that the eavesdropper's instantaneous CSI is unknown. We then conduct case studies that use artificial noise (AN) beamforming to thoroughly analyze the AIL in both Rayleigh and Rician fading channels. The accuracy of the analytical expressions is verified through extensive simulations, and various insights regarding the impact of key system parameters on the AIL are obtained. Particularly, our results reveal that allowing a small level of AIL can potentially lead to significant reliability improvements. To improve the system performance, we formulate and solve an average secrecy throughput (AST) optimization problem via both non-adaptive and adaptive design strategies. Our findings highlight the significance of blocklength design and AN power allocation, as well as the impact of their trade-off on the AST.

Abstract: Short-packet communication (SPC) and unmanned aerial vehicles (UAVs) are anticipated to play crucial roles in the development of 5G-and-beyond wireless networks and the Internet of Things (IoT). In this paper, we propose a secure SPC system, where a UAV serves as a mobile decode-and-forward (DF) relay, periodically receiving and relaying small data packets from a remote IoT device to its receiver in two hops with strict latency requirements, in the presence of an eavesdropper. This system requires careful optimization of important design parameters, such as the coding blocklengths of both hops, transmit powers, and UAV's trajectory. While the overall optimization problem is nonconvex, we tackle it by applying a block successive convex approximation (BSCA) approach to divide the original problem into three subproblems and solve them separately. Then, an overall iterative algorithm is proposed to obtain the final design with guaranteed convergence. Our proposed low-complexity algorithm incorporates 3D trajectory design and resource management to optimize the effective average secrecy throughput of the communication system over the course of UAV-relay's mission. Simulation results demonstrate significant performance improvements compared to various benchmark schemes and provide useful design insights on the coding blocklengths and transmit powers along the trajectory of the UAV.

Abstract: Unmanned aerial vehicles (UAVs) are envisioned to be extensively employed for assisting wireless communications in the Internet of Things (IoT). On the other hand, terahertz (THz)-enabled intelligent reflecting surface (IRS) is expected to be one of the core enabling technologies for forthcoming beyond-5G (B5G) wireless communications that promise a broad range of data-demand applications. In this article, we propose a UAV-mounted IRS (UIRS) communication system over THz bands for confidential data dissemination from an access point (AP) toward multiple ground user equipments (UEs) in IoT networks. Specifically, the AP intends to send data to the scheduled UE, while unscheduled UEs may behave as potential adversaries. To protect information messages from the privacy preservation perspective, we aim to devise an energy-efficient multi-UAV covert communication scheme, where the UIRS is for reliable data transmissions, and an extra UAV is utilized as an aerial cooperative jammer, opportunistically generating artificial noise (AN) to degrade unscheduled UEs detection, leading to communication covertness improvement. This poses a novel max-min optimization problem in terms of minimum average energy efficiency (mAEE), aiming to improve covert throughput and reduce UAVs’ propulsion energy consumption, subject to satisfying some practical constraints such as the covertness requirements for which we obtain analytical expressions. Since the optimization problem is nonconvex, we tackle it via the block successive convex approximation (BSCA) approach to iteratively solve a sequence of approximated convex subproblems, designing the binary user scheduling, AP’s power allocation, maximum AN jamming power, IRS beamforming, and both UAVs’ trajectory and velocity planning. Finally, we present a low-complex overall algorithm for system performance enhancement with complexity and convergence analysis. Numerical results are provided to verify the analysis and demonstrate significant outperformance of our design over other existing benchmark schemes concerning the mAEE performance.

Abstract: Unmanned aerial vehicles (UAVs) and Terahertz (THz) technology are envisioned to play paramount roles in next-generation wireless communications. In this paper, we present a novel secure UAV-assisted mobile relaying system operating at THz bands for data acquisition from multiple ground user equipments (UEs) towards a destination. We assume that the UAV-mounted relay may act, besides providing relaying services, as a potential eavesdropper called the untrusted UAV-relay (UUR). To safeguard end-to-end communications, we present a secure two-phase transmission strategy with cooperative jamming. Then, we devise an optimization framework in terms of a new measure − secrecy energy efficiency (SEE), defined as the ratio of achievable average secrecy rate to average system power consumption, which enables us to obtain the best possible security level while taking UUR’s inherent flight power limitation into account. For the sake of quality of service fairness amongst all the UEs, we aim to maximize the minimum SEE (MSEE) performance via the joint design of key system parameters, including UUR’s trajectory and velocity, communication scheduling, and network power allocation. Since the formulated problem is a mixed-integer nonconvex optimization and computationally intractable, we decouple it into four subproblems and propose alternative algorithms to solve it efficiently via greedy/sequential block successive convex approximation and non-linear fractional programming techniques. Numerical results demonstrate significant MSEE performance improvement of our designs compared to other known benchmarks.

Abstract: Unmanned aerial vehicle (UAV) assisted wireless communication has recently been recognized as an inevitably promising component of future wireless networks. Particularly, UAVs can be utilized as relays to establish or improve network connectivity thanks to their flexible mobility and likely line-of-sight channel conditions. However, this gives rise to more harmful security issues due to potential adversaries, particularly active eavesdroppers. To combat active eavesdroppers, we propose an artificial-noise beamforming based secure transmission scheme for a full-duplex UAV relaying scenario. In the considered scheme, we investigate a UAV-relay equipped with multiple antennas to securely serve multiple ground users in the presence of randomly located active eavesdroppers. We formulate a novel average system secrecy rate (ASSR) maximization problem under some quality of service (QoS) and mission time constraints. Since the ASSR optimization problem is too hard to solve by conventional optimization methods due to the unavailability of the environment's dynamics and complex model, we develop some model-free reinforcement learning-based algorithms, i.e., Q-learning, SARSA, Expected SARSA, Double Q-learning, and SARSA(λ), to efficiently solve the problem without substantial UAV-network data exchange. Using the proposed algorithms, we can maximize ASSR via finding an optimal UAV trajectory and proper resource allocation. Simulation results demonstrate that all the proposed learning-based algorithms can train the UAV-relay to learn the environment by iterative interactions, thus finding an optimal trajectory, intelligently. Particularly, we find that SARSA(λ) based proposed algorithm with λ = 0.1 outperforms the others in terms of the ASSR.

Abstract: This paper investigates an average secrecy rate (ASR) maximization problem for an unmanned aerial vehicle (UAV) enabled wireless communication system, wherein a UAV is employed to deliver confidential information to a ground destination in the presence of a terrestrial passive eavesdropper. By employing an artificial noise (AN) injection based secure two-phase transmission protocol, we aim at jointly optimizing the UAV's trajectory, network transmission power, and AN power allocation over a given time horizon to enhance the ASR performance. Specifically, we divide the original non-convex problem into four subproblems, and propose a successive convex approximation based efficient iterative algorithm to solve it suboptimally with guaranteed convergence. Simulation results demonstrate significant security advantages of our designed scheme over other known benchmarks, particularly for stringent flight durations.

Abstract: Cooperative relaying is utilized as an efficient method for data communication in wireless sensor networks and the Internet of Things. However, sometimes due to the necessity of multi-hop relaying in such communication networks, it is challenging to guarantee the secrecy of cooperative transmissions when the relays may themselves be eavesdroppers, i.e., we may face with the untrusted relaying scenario where the relays are both necessary helpers and potential adversary. To obviate this issue, a new cooperative jamming scheme is proposed in this paper, in which the data can be confidentially communicated from the source to the destination through multiple untrusted relays. In our proposed secure transmission scheme, all the legitimate nodes contribute to providing secure communication by intelligently injecting artificial noises to the network in different communication phases. For the sake of analysis, we consider a multi-hop untrusted relaying network with two successive intermediate nodes, i.e., a three-hop communications network. Given this system model, a new closed-form expression is presented in the high signal-to-noise ratio (SNR) regime for the Ergodic secrecy rate (ESR). Furthermore, we evaluate the high-SNR slope and power offset of the ESR to gain an insightful comparison of the proposed secure transmission scheme and the state-of-arts. Our numerical results highlight that the proposed secure transmission scheme provides better secrecy rate performance compared with the two-hop untrusted relaying as well as the direct transmission schemes.

Abstract: In this paper, we consider an unmanned aerial vehicle (UAV) assisted communications system, including two cooperative UAVs, a wireless-powered ground destination node leveraging simultaneous wireless information and power transfer (SWIPT) technique, and a terrestrial passive eavesdropper. One UAV delivers confidential information to destination and the other sends jamming signals to against eavesdropping and assist destination with energy harvesting. Assuming UAVs have partial information about eavesdropper's location, we propose two transmission schemes: friendly UAV jamming (FUJ) and Gaussian jamming transmission (GJT) for the cases when jamming signals are known and unknown a priori at destination, respectively. Then, we formulate an average secrecy rate maximization problem to jointly optimize the transmission power and trajectory of UAVs, and the power splitting ratio of destination. Being non-convex and hence difficult to solve the formulated problem, we propose a computationally efficient iterative algorithm based on block coordinate descent and successive convex approximation to obtain a suboptimal solution. Finally, numerical results are provided to substantiate the effectiveness of our proposed multiple-UAV schemes, compared to other existing benchmarks. Specifically, we find that the FUJ demonstrates significant secrecy performance improvement in terms of the optimal instantaneous and average secrecy rate compared to the GJT and the conventional single-UAV counterpart.

Abstract: This paper proposes a novel cooperative secure unmanned aerial vehicle (UAV) aided transmission protocol, where a source sends confidential information to a destination via an energy-constrained UAV-mounted amplify-and-forward relay in the presence of a ground eavesdropper. We adopt destination-assisted cooperative jamming as well as simultaneous wireless information and power transfer at the UAV-mounted relay to enhance physical-layer security and transmission reliability. Assuming a low-altitude UAV, we derive connection probability, secrecy outage probability, instantaneous secrecy rate, and average secrecy rate of the proposed protocol over Air-Ground channels, which are modeled as Rician fading with elevation-angel dependent parameters. Further, we analyze the asymptotic average secrecy rate performance of the proposed UAV-relaying scheme and derive high signal-to-noise ratio measures of the average secrecy rate to highlight the effect of various channel features on the system performance. By simulations, we verify our novel theoretical exact and approximate results and demonstrate significant performance improvement of our protocol, when compared to conventional transmission protocol with ground relaying and UAV-based transmission protocol without exploiting destination jamming. Finally, we evaluate the impacts of various system parameters, specifically, find the optimal UAV placement on the proposed protocol in terms of the aforementioned secrecy metrics.

Abstract: In this study, the authors investigate the security and the reliability performance of a two-way relay-based network, where a source–destination pair establishes secured communication through a wireless-powered untrusted amplify-and-forward relay while employing a friendly jammer. The relay utilises the harvested energy from radio-frequency signals sent by the sources to forward the received data; however, the friendly jammer employs that to generate and transmit noise-like signals to confuse the curious relay. Two power transmission protocols, namely variable power transmission (VPT) and constant power transmission (CPT) at the jammer and relay are adopted. As a benchmark to highlight the performance advantages of employing a jammer, they also considered the case of zero power transmission (ZPT), where there is no jammer in the network. For the three scenarios, the intercept probability (IP) and the connection outage probability (COP), as well-known secrecy criteria associated with the successful transmission are mathematically examined. Finally, the derived expressions are confirmed by comparison with Monte–Carlo simulations and furthermore, numerical examples are provided to demonstrate the effects of system parameters on the IP and the COP metrics. Specifically, they find that jammer's artificial noise distribution plays a paramount role in the secrecy performance of the considered system.

Abstract: In this paper, we propose a two-way secure communication scheme where two transceivers exchange confidential messages via a wireless-powered untrusted amplify-and-forward relay in the presence of an external jammer. We take into account both friendly jamming (FJ) and Gaussian noise jamming (GNJ) scenarios. Based on the time switching (TS) architecture at the relay, the data transmission is done in three phases. In the first phase, both the energy-starved nodes, the untrustworthy relay and the jammer, are charged by noninformation radio frequency (RF) signals from the sources. In the second phase, the two sources send their information signals and concurrently, the jammer transmits artificial noise to confuse the curious relay. Finally, the third phase is dedicated to forward a scaled version of the received signal from the relay to the sources. For the proposed secure transmission schemes, we derive new closed-form lower-bound expressions for the ergodic secrecy sum rate (ESSR) in the high signal-to-noise ratio (SNR) regime. We further analyze the asymptotic ESSR to determine the key parameters; the high SNR slope and the high SNR power offset of the jamming based scenarios. To highlight the performance advantage of the proposed FJ, we also examine the scenario of without jamming (WoJ). Finally, numerical examples and discussions are provided to acquire some engineering insights, and to demonstrate the impacts of different system parameters on the secrecy performance of the considered communication scenarios. The numerical results illustrate that the proposed FJ significantly outperforms the traditional one-way communication and the constellation rotation (CR) approach, as well as our proposed benchmarks, the two-way WoJ and GNJ scenarios.

Abstract: This paper presents a secrecy performance study of a wiretap communication system with finite blocklength (FBL) transmissions over Rayleigh fading channels, based on the definition of an average information leakage (AIL) metric. We evaluate the exact and closed-form approximate AIL performance, assuming that only statistical channel state information (CSI) of the eavesdropping link is available. Then, we reveal an inherent statistical relationship between the AIL metric in the FBL regime and the commonly-used secrecy outage probability in conventional infinite blocklength communications. Aiming to improve the secure communication performance of the considered system, we formulate a blocklength optimization problem and solve it via a low-complexity approach. Next, we present numerical results to verify our analytical findings and provide various important insights into the impacts of system parameters on the AIL. Specifically, our results indicate that i) compromising a small amount of AIL can lead to significant reliability improvements, and ii) the AIL experiences a secrecy floor in the high signal-to-noise ratio regime.

Abstract: Finite blocklength (FBL) communications are vital for mission-critical machine-type applications in beyond-5G (B5G) wireless networks. To realize secure communication with ultra-low latency, it is crucial to develop physical-layer security (PLS) schemes tailored to the specific requirements of the FBL. In this study, we present a novel physical security (PLS) metric---average information leakage (AIL)---to carefully investigate the secrecy performance of FBL transmissions. We evaluate the exact and approximate AIL with Gaussian signaling and arbitrary fading channels, assuming that the eavesdropper's instantaneous channel state information (CSI) is unknown.  Then, we reveal an inherent statistical relationship between the AIL metric in the FBL regime and the commonly-used secrecy outage probability (SOP) in conventional infinite blocklength (IBL) communications. We then conduct case studies that use artificial noise (AN) beamforming to thoroughly analyze the AIL in both Rayleigh and Rician fading channels. Through extensive simulations, we confirm the accuracy of our analytical results and obtain various useful insights.

Abstract: In this paper, we propose a secure short-packet communication (SPC) system involving an unmanned aerial vehicle (UAV)-aided relay in the presence of a terrestrial passive eavesdropper. The considered system, which is applicable to various next-generation Internet-of-Things (IoT) networks, exploits a UAV as a mobile relay, facilitating the reliable and secure exchange of intermittent short packets between a pair of remote IoT devices with strict latency. Our objective is to improve the overall secrecy throughput performance of the system by carefully designing key parameters such as the coding blocklengths and the UAV trajectory. However, this inherently poses a challenging optimization problem that is difficult to solve optimally. To address the issue, we propose a low-complexity algorithm inspired by the block successive convex approximation approach, where we divide the original problem into two subproblems and solve them alternately until convergence. Numerical results demonstrate that the proposed design achieves significant performance improvements  relative to other benchmarks, and offer valuable insights into determining appropriate coding blocklengths and UAV trajectory.

Abstract: In this paper, we propose a self-dependent two-way secure communication where two sources exchange confidential messages via a wireless powered untrusted amplify-and-forward (AF) relay and friendly jammer (FJ). By adopting the time switching (TS) architecture at the relay, the data transmission is accomplished in three phases: Phase I) Energy harvesting by the untrusted relay and the FJ through non-information transmissions from the sources, Phase II) Information transmission by the sources and jamming transmissions from the FJ to reduce information leakage to the untrusted relay; and Phase III) Forwarding the scaled version of the received signal from the untrusted relay to the sources. For the proposed system, we derive a new closed-form lower bound expression for the ergodic secrecy sum rate (ESSR). Numerical examples are provided to demonstrate the impacts of different system parameters such as energy harvesting time, transmit signal-to-noise ratio (SNR) and the relay/FJ location on the secrecy performance. The numerical results illustrate that the proposed network with friendly jamming (WFJ) outperforms traditional one-way communication and the two-way without friendly jamming (WoFJ) policy.

Abstract:  The application frameworks of unmanned aerial vehicles (UAVs), more commonly known as drones, have recently grown in popularity in public and civil domains. Drone technology, owing to its distinct characteristics, plays a significant role in establishing and improving the pervasive and seamless connectivity of devices in beyond fifth-generation (B5G) wireless networks. Drones can be employed to support terrestrial networks thanks to their rapid, on-demand, and efficient deployment, flexibility in maneuverability, and improved quality of service (QoS).  Although they can support applications for the Internet-of-Things (IoT) and cellular networks, the feasibility of such systems faces fundamental barriers. One of the major challenges is the security of such systems taking into account the inherent vulnerability to diverse malicious attacks that can breach confidentiality and privacy. Conventionally, network-layer cryptography algorithms have been used for security, but they require expensive cryptographic operations such as key generation and exchange. Since the energy-constraint aerial wireless platforms require low-complexity security techniques, one promising approach is the premise of low-complexity physical-layer security (PLS), which exploits randomness and impairments at the wireless physical layer without the need for an encryption key. 

In this thesis, we tackle the security problems for drone-based wireless communications by leveraging the PLS techniques. Our fundamental goal is to design and develop analytical frameworks for intelligent deployment, rigorous performance analysis, and optimizations of various secure and energy-efficient drone-enabled wireless communication systems. In summary, this thesis lays a solid foundation for safeguarding drone communications by proposing low-complexity solutions and efficient algorithms based upon convex optimization and artificial intelligence (AI).