1 Survey on Unmanned Aerial Vehicle Networks: A Cyber Physical System Perspective Haijun Wang, Haitao Zhao, Jiao Zhang, Dongtang Ma, Jiaxun Li, and Jibo Wei, Abstract—Unmanned aerial vehicle (UAV) networks are play- In these applications, one emerging trend is that the UAVs ing an important role in various areas due to their agility and work from individually to cooperatively by constituting a versatility, which have attracted significant attention from both reliable network, since that multiple UAVs can provide wider the academia and industry in recent years. As an integration of the embedded systems with communication devices, computation coverage, more flexibility and robustness through redundancy capabilities and control modules, the UAV network could build [16]. Accordingly, the challenges in a multi-UAV network a closed loop from data perceiving, information exchanging, are much more intractable compared with those for a single decision making to the final execution, which tightly integrates UAV. Therefore, researches on the multi-UAV networks have the cyber processes into the physical devices. Therefore, the attracted significant attention in recent ten years. Thanks to UAV network could be considered as a cyber physical system (CPS). Revealing the coupling effects among the three interacted the advances in wireless communication, high performance components in this CPS system, i.e., communication, computation computation and flight control areas, the UAV networks have and control, is envisioned as the key to properly utilize all obtained more powerful capabilities with respect to the com- the available resources and hence improve the performance of munication, computation and control. the UAV networks. In this paper, we present a comprehensive There appears a strong tendency to integrally design the survey on the UAV networks from a CPS perspective. Firstly, we respectively research the basics and advances with respect communication, computation and control modules toward- to the three CPS components in the UAV networks. Then we s intelligent UAV networks [17]. This is reasonable since look inside to investigate how these components contribute to these three cyber components play crucial roles in the UAV the system performance by classifying the UAV networks into networks, and they will affect and benefit from each other three hierarchies, i.e., the cell level, the system level, and the in a coupling way. For example, when the communication system of system level. Further, the coupling effects among link is unstable due to the shadow effect, the UAVs could these CPS components are explicitly illustrated, which could be enlightening to deal with the challenges in each individual aspect. alter their locations through flight control to pursue line New research directions and open issues are discussed at the of sight links, other than only adjusting the communication end of this survey. With this intensive literature review, we try parameters in vain. Thus, a cross-disciplinary viewpoint may to provide a novel insight into the state-of-the-art in the UAV bring inspirations to deal with the puzzles in each single area. networks. Index Terms—Unmanned aerial vehicle (UAV), cyber physical system (CPS), communication, computation, control. A. CPS can inspire the UAV networks The UAV network operates by building a closed loop, which consists of the initial data perceiving, information I. INTRODUCTION exchanging, decision making and the final execution. From Due to the characteristics of agility, versatility, low cost and this perspective, the complex UAV network can be considered easy-to-deploy, unmanned aerial vehicles (UAVs) are playing as a cyber physical system (CPS), which has attracted signif- an important role in both military and civilian areas. In military icant attention recently [45]. CPS achieves the tight coupling applications, the UAVs are envisioned as an indispensable part between the cyber domain and the physical domain by strictly of the future battlefield. They can not only capture different embedding the cyber processes into the physical devices. Thus, kinds of information on a large scale in terms of time and the reliable, real-time and efficient monitoring, coordination space proactively (e.g., border surveillance, intelligence re- and control to the physical entities can be conducted through connaissance), but also assist other unmanned/manned combat the closed loop [46]. For example, in the UAV networks, platforms to complete dangerous missions [1], [2]. The UAVs the data sensed by the sensors originates from the physical also flourish with the increasing demands of civilian applica- world (i.e., its mission circumstance), and the final decisions, tions, including agricultural plant protection [3], search and made by computation and conveyed by communication, are rescue [4], [5], environment and natural disaster monitoring translated into instructions and eventually take effects on the [6], [7], delivery of goods [8], communication relays [9], [10], physical world through the actuators. Besides, CPS has a aerial base stations [11]–[13], construction [14] and traffic broad definition, therefore the UAV networks consisting of surveillance [15]. a single UAV (the cell level), a UAV swarm (the system level) or multiple heterogeneous UAV swarms (the system of H. Wang, H. Zhao, J. Zhang, D. Ma, J. Li and J. Wei are all with College system level) all could be developed as CPS. And as the next of Electronic Science, National University of Defense Technology, Changsha, generation of systems, CPS is expected to be a key method to 410073, China (e-mail: haijunwang14, haitaozhao, zhangjiao16, dongtangma, lijiaxun, [email protected]). implement the artificial intelligence [45]. Thus, integrating a 2 Section I : Introduction CPS can inspire the UAV networks Existing surveys and motivations Paper organization Section II: Backgrounds and preliminaries Taking the UAV networks as CPS Autonomy in the UAV networks CPS component Communication Computation Control Section III: CPS components in the UAV networks Communication demands; Computation platforms; Flight controllers; Communication Decision making entity; Flight control challenges; Intelligent algorithms algorithms; Communication basics Section IV: UAV networks of three CPS hierarchies Key techniques Architecture UCL Interacting and Data processing Control and communicating and deciding execution Applications Key techniques Architecture USL Communication Computation Cooperative network coordination control Applications Key techniques Architecture Resource and service Flexible distributed USoS provision compuation Applications CPS hierarchy Section V: Coupling effects in the UAV networks Computation and communication Computation and control Communication and control Section VI: Challenges and open issues CPS model of the UAV Convergence of Computation offloading Resource scheduling networks heterogeneous networks Unwelcomed social Energy efficiency Security and privacy Performance evaluation impacts Section VII: Conclusion Fig. 1. The structure of the survey. 3 TABLE I A COMPARISON OF THE EXISTING SURVEYS ON THE UAV NETWORKS AND CPS Coupling UAV CPS Category Surveys Cyber issues Physical issues Hierarchy issue involved applications [18], 2018 [19], 2017 [20], 2016 Communication and Mobility, missions and No Yes System N/A [8], 2016 networking energy [21], 2015 [22], 2013 [23], 2016 Communication Mobility and missions No Yes Cell; System N/A UAV [24], 2017 network [25], 2016 Computation and control Autopilot platforms No Yes Cell N/A [26], 2015 [27], 2018 [28], 2015 Flight control algorithms Mobility No Yes Cell N/A [29], 2012 [30], 2017 Formation control algorithms Mobility No Yes System N/A [31], 2013 [32], 2014 None Mobility model No Yes Cell N/A [33], 2013 None Collective mobility model No Yes System N/A Communication infrastruc- [34], 2014 Communication Yes No System Smart grid ture and physical dynamics [35], 2016 Networking and security Wind turbine components Yes No Cell; System Smart grid Security, attacks and defens- [36], 2016 None No No System Smart grid es [37], 2016 Security and safety None No No System Medical devices CPS Traffic flow and mobility [38], 2016 Networking and protocols Yes No System Transportation models [39], 2017 Security and threat detectors None No No System General [40], 2016 Security, privacy and de- [41], 2017 None No Yes System General fenses [42], 2015 None UAVs as sensors No Yes System General Fixed-wing and multirotor Source seeking Contour mapping algorithm, [43], 2014 UAVs, mobility and mis- No Yes System and contour flight and formation control UAV- sions mapping enabled Communication network, Mobility, energy and UAV CPS [44], 2018 flight and formation control, No Yes Cell; System General testbeds and image analysis Communication, Autopilot platforms, com- Cell; System; This work computation and putation chips, mobility and Yes Yes System of sys- General flight/formation control missions tem CPS vision into the UAV networks is promising to significantly components, are not well excavated in these surveys, although improve the performance of the whole system. they can be instructive to deal with the problems from a cross- disciplinary perspective. On the other hand, most existing B. Existing surveys and motivations surveys on the CPS put particular emphasis on the design and implementation of the industrial systems, concerning energy The UAV networks and CPS have been extensively surveyed [34], [35], [48], transportation