Unmanned Aerial Vehicle Communication Systems Research Articles

Envelope Level Crossing Rate and Average Fade Duration of a Generic 3D Non-Stationary UAV Channel Model

For the unmanned aerial vehicle (UAV) communication scenarios, the time-varying fading characteristics cased by the three-dimensional(3D) scattering environment and 3D trajectory have an important impact on establishing the reliable communication link. In this paper, a generic 3D non-stationary geometry-based stochastic model (GBSM) for UAV channels is developed. Different from the traditional 3D GBSMs, the proposed model takes the 3D arbitrary trajectories of UAV into account. On this basis, two important second-order statistical properties of fading envelope, i.e., the level crossing rate (LCR) and average fading duration (AFD), are investigated and derived in details. The obtained closed-form expressions are explicit functions of flight parameters and compatible with the results in the previous works. Numerical simulations show that the simulated results are in accordance with the theoretical and measurement data under 3D flight scenarios. The proposed model and statistical properties can be applied to the optimal design of channel coding and block interleave schemes for UAV communication systems.

Multi-Frequency Air-to-Ground Channel Measurements and Analysis for UAV Communication Systems

In order to implement unmanned aerial vehicle (UAV)-based wireless communications, a better understanding of wireless channels and the corresponding channel characterizations are critical. In this paper, air-to-ground (AG) channel measurements are carried out at some candidate frequencies, i.e., 1 GHz, 4 GHz, 12 GHz, and 24 GHz. With measurement data, the crucial channel parameters are comprehensively analyzed. Firstly, based on the channel model in the 3rd Generation Partnership Project (3GPP), the essential coefficients for modeling path loss, including path loss exponents (PLEs) and height-dependent factors, are obtained for AG channels. Then, a novel autocorrelation model for shadow fading is proposed. Besides, the small-scale fading is statistically analyzed, where the log-logistic distribution is found as the best fit among popular distributions. Moreover, the second-order statistical characteristics, including the level crossing rate (LCR) and average fade duration (AFD), are extracted to describe in-depth the fading behavior. Overall, the results and findings in this paper are essential for realizing reliable communications in AG wireless systems.

Hybrid-Duplex Communications for Multi-UAV Networks: An Outage Probability Analysis

In this letter, the outage probability of an unmanned aerial vehicle (UAV) network with hybrid-duplex (HBD) UAV communications is investigated in a stochastic geometry framework. We demonstrate that the HBD UAV communication system (HBD-UCS) can concurrently support more UAVs while achieving higher reliability than the half-duplex (HD) UCS (HD-UCS). Specifically, at low transmit power regimes, it is shown that the HBD-UCS attains lower uplink and downlink outage probability than an HD-UCS, as the UAV operating altitude is increased.

Optimal 3D-Trajectory Design and Resource Allocation for Solar-Powered UAV Communication Systems

In this paper, we investigate the resource allocation algorithm design for multicarrier solar-powered unmanned aerial vehicle (UAV) communication systems. In particular, the UAV is powered by the solar energy enabling sustainable communication services to multiple ground users. We study the joint design of the 3D aerial trajectory and the wireless resource allocation for maximization of the system sum throughput over a given time period. As a performance benchmark, we first consider an off-line resource allocation design assuming non-causal knowledge of the channel gains. The algorithm design is formulated as a mixed-integer non-convex optimization problem taking into account the aerodynamic power consumption, solar energy harvesting, a finite energy storage capacity, and the quality-of-service requirements of the users. Despite the non-convexity of the optimization problem, we solve it optimally by applying monotonic optimization to obtain the optimal 3D-trajectory and the optimal power and subcarrier allocation policy. Subsequently, we focus on the online algorithm design that only requires real-time and statistical knowledge of the channel gains. The optimal online resource allocation algorithm is motivated by the off-line scheme and entails a high computational complexity. Hence, we also propose a low-complexity iterative suboptimal online scheme based on the successive convex approximation. Our simulation results reveal that both the proposed online schemes closely approach the performance of the benchmark off-line scheme and substantially outperform two baseline schemes. Furthermore, our results unveil the tradeoff between solar energy harvesting and power-efficient communication. In particular, the solar-powered UAV first climbs up to a high altitude to harvest a sufficient amount of solar energy and then descends again to a lower altitude to reduce the path loss of the communication links to the users it serves.

Safeguarding UAV Communications Against Full-Duplex Active Eavesdropper

Unmanned aerial vehicle (UAV) wireless communication has recently been recognized to be inevitable and prevalent in the fifth-generation (5G) wireless networks. In this paper, we propose a secure transmission scheme for a wiretap channel, where a source communicates with a legitimate UAV in the presence of an eavesdropper. We consider the full-duplex active eavesdropper, which performs both eavesdropping and malicious jamming simultaneously. The source transmits artificial noise (AN) signals, in addition to information signals, to confuse this eavesdropper. By considering the ground-to-UAV channel model, we analyze the hybrid outage probability, which takes both the transmission outage probability and the secrecy outage probability into consideration. We further provide the asymptotic hybrid outage probability in a more compact form, where both the transmit power at the source and the jamming power at the eavesdropper become large with a fixed ratio. Through the analysis and the numerical results, we determine the optimal power allocation factor between information signals and AN signals as well as the operating height of UAV that minimize the hybrid outage probability. We also provide the most harmful antenna configuration of the eavesdropper to the UAV communications, and this paper can be a useful framework for the design of confidential UAV communication system.

Performance Analysis of UAV Uplink Communication Technology

In order to meet the reliability and anti-jamming requirements of UAV communication system, the anti-jamming performance of the up-link communication technology is analyzed by comparing the influence of different jamming on the signal and the influence of Rice channel on the signal on the basis of in-depth study of the communication mechanism of the low-altitude UAV communication link. The simulation results show that the anti-jamming performance of the signal is better when the interference is added in the upstream communication technology, and the anti-broadband interference of the signal is better than other interference types when the channel is added.

Ultra-Reliable and Low-Latency Communications in Unmanned Aerial Vehicle Communication Systems

In this paper, we establish a framework for enabling ultra-reliable and low-latency communications in the control and non-payload communications (CNPC) links of the unmanned aerial vehicle (UAV) communication systems. We first derive the available range of the CNPC links between UAVs and a ground control station. The available range is defined as the maximal horizontal communication distance within which the round-trip delay and the overall packet loss probability can be ensured with a required probability. To exploit the macro-diversity gain of the distributed multi-antenna systems (DAS) and the array gain of the centralized multi-antenna systems (CAS), we consider a modified DAS (M-DAS), where the ground control station is equipped with the distributed access points (APs), and each AP can have multiple antennas. We then show that the available range can be maximized by judiciously optimizing the altitude of UAVs, the duration of the uplink and downlink phases, and the antenna configuration. To solve the non-convex problem, we propose an algorithm that can converge to the optimal solution in DAS and CAS, and then extend it into more general M-DAS. The simulation and numerical results validate our analysis and show that the available range of M-DAS can be significantly larger than those of the DAS and CAS.

Machine‐learning‐based prediction methods for path loss and delay spread in air‐to‐ground millimetre‐wave channels

The unmanned aerial vehicles (UAVs) have been widely applied in various fields due to their advantages like high mobility and low cost. Reliable communication is the premise to ensure the connectivity between UAV nodes. To provide reasonable references for the design, deployment, and operation of UAV communication systems, the precise prediction of radio channel parameters are required. In this study, the authors propose prediction methods for path loss and delay spread in air-to-ground millimetre-wave channels based on machine learning. Random forest and K-nearest-neighbours are the algorithms employed in the methods. Then, a feature selection scheme is proposed to further improve the prediction accuracy and generalisation performance of the machine-learning-based methods. Generally, machine learning algorithms require massive data for training purpose. However, measuring data is time-consuming and costly, especially when the scenario or frequency changes. Therefore, transfer learning methods are introduced to predict path loss with limited data. The proposed methods for path loss prediction are compared to Okumura-Hata and COST-231 Hata models. The lognormal distribution is the contrast model in delay spread prediction. Based on the data generated by ray-tracing software, the new methods have a smaller root mean square errors than contrast models.

A 3-D Geometry-Based Stochastic Model for UAV-MIMO Wideband Nonstationary Channels

In this paper, we propose a 3-D geometry-based stochastic model for multi-input multioutput wideband nonstationary channels between the unmanned aerial vehicle (UAV) and the ground user. The proposed model uses a single concentric-cylinders model, where the signal is a superposition of a line-of-sight component and single-bounced rays. In order to describe the nonstationarity of the channel, time-varying angels have been properly derived and added in the proposed model. At the same time, based on the reference model, we derive the expressions of the channel statistical properties of the space–time–frequency correlation function and space-Doppler power spectral density. In addition, corresponding deterministic and stochastic simulation models are developed based on the reference model. Moreover, we investigate the influence of some important UAV-related parameters on channel statistical properties as well as the channel nonstationarity. Some interesting observations and conclusions are obtained, which are helpful for the design of UAV communication systems. Finally, the great agreement between simulation models and the reference model demonstrates not only the utility of simulation models but also the correctness of the theoretical derivations and simulations.

A Hybrid-Duplex System With Joint Detection for Interference-Limited UAV Communications

In this paper, the advantages of joint detection (JD) in a hybrid-duplex unmanned aerial vehicle (UAV) communication system (HBD-UCS) are investigated as a step toward addressing spectrum scarcity in UAV communications. Through extensive outage probability and finite signal-to-noise-ratio diversity gain analysis, we showed that the performance of JD is independent of the strength and the data rate of the inter-UAV interference signal. On the contrary, the successive interference cancellation (SIC) detector requires the data rate of the interfering UAV to be less than the ground station before meaningful performance can be seen. At the system level, it is observed that the half-duplex UAV communication system outperforms the HBD-UCS with JD at moderate and high SNR regimes, as the latter is constrained by self-interference at the full-duplex ground station. Finally, we investigated the multiplexing gain region and showed that the joint detector offers higher diversity gain over a wide range of multiplexing gains over the interference ignorant and the SIC detector.

A Power Series Approach for Hybrid-Duplex UAV Communication Systems Under Rician Shadowed Fading

A hybrid-duplex (HBD) UAV communication system (UCS), i.e., HBD-UCS, to improve spectrum utilization is investigated in this work. By considering the combined effect of fading and shadowing, a comprehensive outage probability analysis of the HBD-UCS under various inter-UAV interference and shadowing scenarios over Rician shadowed fading channels is conducted. It is demonstrated that the ground station (GS) in full-duplex (FD) mode operates at lower outage probability than in half-duplex (HD) mode. Furthermore, the joint detector is shown to achieve lower outage probability than the interference ignorant (II) detector and HD-UCS, even when severe shadowing is encountered. As such, utilizing joint detectors in an HBD-UCS enables multi-UAV networks to achieve high reliability when operating in urban environments.

Timing Synchronization and Ranging in Networked UAV-Aided OFDM Systems

The space-air-ground network architecture integrates satellite systems, aerial networks, and terrestrial networks, where the unmanned aerial vehicle (UAV) communication in the air segment has attracted particular interest and demonstrated tremendous potential in both military and civilian applications. Orthogonal frequency division multiplexing (OFDM) can be an effective solution to provide high rate and reliable transmission in UAV communication systems because of its high spectral efficiency and robustness against frequency selective fading. We herein focus our attention on two critical issues in our proposed UAV aided OFDM system, i.e., timing synchronization and ranging. Moreover, extensive simulations are conducted for evaluation. Finally, we design a real-world field test for verifying the effectiveness of our proposed multi-UAV OFDM communication system.

An Efficient SC-FDM Modulation Technique for a UAV Communication Link

Since the communication link of an unmanned aerial vehicle (UAV) and its reliability evaluation represent an arduous field, we have concentrated our work on this topic. The demand regarding the validity and reliability of the communication and data link of UAV is much higher since the environment of the modern battlefield is becoming more and more complex. Therefore, the communication channel between the vehicle and ground control station (GCS) should be secure and provide an efficient data link. In addition, similar to other types of communications, the data link of a UAV has several requirements such as long-range operation, high efficiency, reliability, and low latency. In order to achieve an efficient data link, we need to adopt a highly efficient modulation technique, which leads to an increase in the flight time of the UAV, data transmission rate, and the reliability of the communication link. For this purpose, we have investigated the single-carrier frequency division multiplexing (SC-FDM) modulation technique for a UAV communication system. The results obtained from the comparative study demonstrate that SC-FDM has better performance than the currently used modulation technique for a UAV communication link. We expect that our proposed approach can be a remarkable framework that will help drone manufacturers to establish an efficient UAV communication link and extend the flight duration of drones, especially those being used for search and rescue operations, military tasks, and delivery services.

Design of L-Shaped Open-Slot Antenna Used in UAV Airborne Communication System

According to the communication demands of communication system used in the UAV (Unmanned Aerial Vehicle) communication system, a ladder-shaped open-slotted antenna design is presented. The impedance effects and directional radiation performance of designed antenna are improved with adopting a pair of L-shaped narrow slot and a passive resonator among the antenna structures. The measured results show that the impedance bandwidth of the antenna is about 90%, the frequency range can cover from 3 GHz to 12 GHz, and the antenna has a relative small size just as 22 mm × 24 mm. The antenna is realized through adopting improved ground structure, and its electrical length is increased simultaneously. To improve its impedance matching effect, the ladder structure is adopted in the antenna. The measured results show that the proposed design scheme is useful and effective, the antenna size can be controlled within the size of 22 mm × 24 mm and the good impedance bandwidth can also be obtained, its frequency bandwidth is from 3 GHz to 13.2 GHz, and the designed antenna has good group delay characteristics at the same time.

Energy-efficient trajectory planning for UAV-aided secure communication

Wireless communication with unmanned aerial vehicles (UAVs) has aroused great research interest recently. This paper is concerned with the UAV's trajectory planning problem for secrecy energy efficiency maximization (SEEM) in the UAV communication system. Specifically, we jointly consider the secrecy throughput and UAV's energy consumption in a three-node (fixed-wing UAV-aided source, destination, and eavesdropper) wiretap channel. By ignoring the energy consumption on radiation and signal processing, the system's secrecy energy efficiency is defined as the total secrecy rate normalized by the UAV's propulsion energy consumption within a given time horizon. Nonetheless, the SEEM problem is nonconvex and thus is intractable to solve. As a compromise, we propose an iterative algorithm based on sequential convex programming (SCP) and Dinkel-bach's method to seek a suboptimal solution for SEEM. The algorithm only needs to solve convex problems, and thus is computationally efficient to implement. Additionally, we prove that the proposed algorithm has Karush-Kuhn-Tucker (KKT) point convergence guarantee. Lastly, simulation results demonstrate the efficacy of our proposed algorithm in improving the secrecy energy efficiency for the UAV communication system.

An enhanced direct anonymous attestation scheme with mutual authentication for network-connected UAV communication systems

In network-connected UAV (NC-UAV) communication systems, user authentication is replaced by platform identity authentication and integrity check because many NC-UAVs are operated without human intervention. Direct anonymous attestation (DAA) is an attractive cryptographic scheme that provides an elegant balance between platform authentication and anonymity. However, because of the low-level computing capability and limited transmission bandwidth in UAV, the existing DAA schemes are not suitable for NC-UAV communication systems. In this paper, we propose an enhanced DAA scheme with mutual authentication (MA-DAA scheme), which meets the security requirements of NC-UAV communication systems. The proposed MA-DAA scheme, which is based on asymmetric pairings, bundles the identities of trusted platform module (TPM) and Host to solve the malicious module changing attacks. Credential randomization, batch proof and verification, and mutual authentication are realized in the MA-DAA scheme. The computational workload in TPM and Host is reduced in order to meet the low computation and resource requirements in TPM and Host. The entire scheme and protocols are presented, and the security and efficiency of the proposed MA-DAA scheme are proved and analyzed. Our experiment results also confirm the high efficiency of the proposed scheme.

A secure wireless mission critical networking system for unmanned aerial vehicle communications

A privacy-preserving secure communication in ad hoc (without infrastructure) mission critical wireless networking system suitable for unmanned aerial vehicle communication systems is introduced and analyzed. It is expected that in a critical condition, few ad hoc (without infrastructure) mission critical wireless networking systems will work together. To make the simple and low cost privacy-preserving secure communication among the same network, each transmitting mobile node generates packets in such a way that its wanted receiving mobile nodes can read the message packets easily. On the other hand, the unwanted receiving mobile nodes from other networks cannot read those message packets. In addition, the unwanted receiving mobile nodes receive ‘jamming packets’ if they try to read them. This mechanism prevents the malicious receivers (readers from other networks) from reading the packets and obtaining information from this network. Results show that the throughput is very high and does not detect any jamming packets, if the receiving nodes of a network try to read packets transmitted by the nodes from the same networks.

Channel transmission strategy for mmWave hybrid UAV communications with blockage

The blockage is a tough hurdle for millimetre-wave (mmWave) unmanned aerial vehicle (UAV) communications. In this Letter, the authors propose an angle domain channel transmission scheme for mmWave hybrid UAV communication with blockage. Firstly, they investigate the angle domain channel transmission of mmWave UAV communication systems with massive array antennas. Then, they solve the blockage problem through the multi-UAV coordination. Specially, they provide a practical user scheduling algorithm to maximize the achievable sum rates. Finally, simulation results are provided to show the effectiveness of the proposed method.

Application of Adaptive Coded Modulation Technology in UAV Data Link

UAV data link has been considered as an important part of UAV communication system, through which the UAV could communicate with warships. However, constant coding and modulation scheme that UAV adopts does not make full use of the channel capacity when UAV communicates with warships in a good channel environment. In order to improve channel capacity and spectral efficiency, adaptive coded modulation technology is studied. Based on maritime channel model, SNR estimation technology and adaptive threshold determination technology, the simulation of UAV data link communication is carried out in this paper. Theoretic analysis and simulation results show that according to changes in maritime channel state, UAV can dynamically adjust the adaptive coded modulation scheme on the condition of meeting target Bit-Error-Rate (BER), the maximum amount of data transfer is non-adaptive systems three times.

Antenna for Wireless Communications on Unmanned Aerial Vehicles

A compact circularly polarized microstrip patch antenna was designed and fabricated based on the virtual load and limited airframe size of Unmanned Aerial Vehicles (UAV) applications. This antenna is used for ISM frequency band and to enhance the integration of sensors and circuits in UAV communication system. The paper presented the simulation and the measured results. This antenna Experimental results show the 3dB axial ratio bandwidth is 0.6%, the half power beamwidth is ±45º, and the gain is 4.5dBi. The antenna size reduction can be as large as 36%, as compared to that of the conventional CP design.