Unmanned aerial vehicles (UAVs) have been instrumental in enabling many new applications and services, including military and rescue operations, aerial surveillance, civilian applications, precision farming, as well as providing extensive wireless network access in remote areas. They have also been used in other areas such as transmission line and oil rig monitoring, disaster recovery, etc. With their increasing payload ability and flight duration, using UAVs is a preferred choice for a multitude of upcoming wireless communication systems. However, because of their open operational nature, UAVs are highly vulnerable to severe security breaches via cyber attacks, eavesdropping on navigational and communication links, etc. Given their widespread applications, the requirement of secured UAV communications has become more and more critical since security failures can lead to detrimental consequences. Several works have examined the extent of privacy and security issues in UAV-assisted networks and introduced various mitigation techniques to address different security challenges. In this paper, a comprehensive survey is conducted that centers on the security issues related to UAV-aided networks. A descriptive taxonomy of various security intrusions on the UAV networks and commonly-used secrecy performance metrics are thoroughly reviewed. An in-depth discussion is provided on alleviating threats with proactive security techniques blended with key wireless communication technologies such as mmWave, non-orthogonal multiple access (NOMA), massive multiple-input multiple-output (MIMO), and cognitive radio. Moreover, several emerging topics like machine learning, software-defined networks, fog and edge computing, blockchain, are discussed in the context of UAV-aided secure communications.

Unmanned aerial vehicles (UAVs) have recently achieved sky-rocketed prominence in assisting various wireless communication scenarios due to their implicit characteristics like line-of-sight connectivity, three-dimensional mobility, on-demand deployment, and payload abilities. For realizing UAV-assisted communications in practical scenarios, one of the key challenges is the onboard power limitation of UAVs, which can affect their flight duration, maneuverability, and network lifetime. Exploiting energy harvesting (EH) techniques with UAV networks can improve operational time by addressing the UAV’s power constraints. In addition, UAVs can facilitate seamless connectivity for energy-constrained low-power devices in remote locations, disaster-struck areas, emergencies, and areas where human intervention is challenging and recharging or replacing batteries is problematic. In such scenarios, a low-altitude UAV, which flies flexibly, can serve as a radio-frequency (RF) energy transmitter to charge the nearby low-power devices efficiently and help enable their communications. Focusing on these aspects, this paper comprehensively surveys the current state-of-the-art EH techniques that can be utilized with UAV communications in various practical scenarios. Then, we highlight crucial challenges in integrating UAVs with prevailing wireless infrastructure and discuss different RF-EH receiver architectures that can be viable solutions for power-constrained UAV networks. Further, we show the channel modeling of UAV-assisted communications with EH and provide critical insights into the system performance. Besides, we offer an exhaustive review of recent works on RF-EH in UAV networks with next-generation paradigms and different problem-solving techniques/optimization methods. Finally, we suggest forthcoming trends and highlight open issues and future research directions.

This paper investigates the secrecy performance of an underlay cooperative cognitive relay network, wherein a secondary source vehicle communicates with a fixed secondary destination terminal via a direct link and with the assistance of a secondary amplify-and-forward relay vehicle in the presence of a passive secondary eavesdropper vehicle, taking into consideration the interference to primary user. We assume that the eavesdropper vehicle takes the advantage of both the relay and direct links. We consider that the vehicle-to-vehicle links are modeled as double-Rayleigh fading, while the vehicle-to fixed infrastructure links are modeled as Rayleigh fading. Such a scenario finds it relevancy in the vehicle-to-vehicle communication and/or vehicle-to-infrastructure communication under spectrum sharing heterogeneous cooperative vehicular networks. For such a realistic scenario, in particular, we derive a tight lower bound expression of the secrecy outage probability under mixed Rayleigh and double-Rayleigh fading channels. We also present an effective secrecy diversity order analysis and show that the considered system can achieve a secrecy diversity order of 2 for infinitely large average channel gain values of the main links. Finally, we demonstrate the accuracy of our analytical findings via numerical and simulation results and show the impact of channel conditions, primary interference constraints, and direct links on the secrecy performance of the considered system.

The authors investigate the secrecy performance of a cooperative relaying network, wherein a fixed source communicates with a fixed destination via an amplify‐and‐forward mobile relay in the presence of a passive mobile eavesdropper. They assume that the source‐to‐relay channel and relay‐to‐destination channel experience Nakagami‐ m fading, whereas relay‐to‐eavesdropper link follows double Nakagami‐ m fading. Under such a mixed fading environment, they derive the novel tight closed‐form expressions for the secrecy outage probability (SOP), probability of non‐zero secrecy capacity, and intercept probability of the considered system. They also deduce the asymptotic secrecy outage and intercept probability expressions in the high signal‐to‐noise ratio regime to highlight the impact of fading parameters and channel conditions on the secrecy diversity order. It is shown that the system can achieve secrecy diversity order of min( ) in terms of SOP, and in terms of intercept probability, where and represent the fading severity parameters between source and relay and between relay and destination, respectively. They also formulate and investigate analytically two optimisation problems, namely, optimal relay location with fixed power allocation and optimal power allocation with fixed relay location, to minimise the system SOP. They verify analytical findings via numerical and simulation results.

In this paper, we study the impacts of co-channel interference (CCI) and channel estimation error (CEE) on the performance of a cellular multiuser two-way relay network. The network comprises of one $N_{a}$-antenna base station (BS), M single-antenna users, and one L-antenna relay with $N_{r}$ number of interferers. In particular, by employing antenna selection at the relay, we propose beamforming and antenna selection transmission schemes at the BS with user scheduling in the presence of CEE and CCI. Then, we derive the tight closedform lower bound expressions for the overall outage probability under Nakagami-m fading. We also deduce the asymptotic outage expressions in the high signal-to-noise ratio regime, from which the impacts of CEE and CCI on the achievable diversity order are highlighted. Moreover, using the asymptotic expressions, three optimization problems to minimize the outage probability are formulated and solved: 1) optimal power allocation (OPA) with fixed relay location. Specifically, we develop two OPA approaches, namely, OPA I and OPA II. OPA I allocates the total power appropriately at each terminal, whereas OPA II allocates the fixed half of the total power at the relay and remaining power to base station and users, 2) optimal relay location with fixed power allocation, and 3) jointly optimal power allocation and relay location. Our results reveal that the OPA I outperforms OPA II, irrespective of different channel/system parameters, interferers, and relay locations. In addition, we demonstrate that the effect of interferers at the relay can be ignored if we allocate sufficient power to the base station and users. It is also shown that the CEE and CCI can severely degrade the outage performance, and even reduce the diversity order to zero. Further, it is highlighted that the impact of CCI can be alleviated by increasing the number of relay antennas. The numerical and simulation results validate our analytical findings, and also demonstrate that the antenna selection scheme offers comparable outage performance as beamforming scheme.