Unmanned aerial vehicle(UAV)has a rapid development over the last decade.However,an increasing number of severe flight collision events caused by explosive growth of UAV have drawn widespread concern.It is an important issue to investigate safety management approaches of UAVs to ensure safe and efficient operation.In this paper,we present a comprehensive overview of safety management approaches in large,middle and small scales.In large-scale safety management,path-planning problem is a crucial issue to ensure safety and ordered operation of UAVs globally.In middle-scale safety management,it is an important issue to study the conflict detection and resolution methods.And in small-scale safety management,real-time collision avoidance is the last line of ensuring safety.Moreover,a UAV can be regarded as a terminal device connected through communication and information network.Therefore,the enabling technologies,such as sensing,command and control communication,and collaborative decision-making control technology,have been studied in the last.

For the practical simulation and performance evaluation of unmanned aerial vehicle (UAV) multiple-input multiple-output (MIMO) Ricean fading channels,it is desirable to develop accurate UAV-MIMO channel simulation models for more realistic scenarios of non-isotropic scattering.In this study,using a twocylinder reference model to describe the distribution of scatterers,we propose new deterministic and stochastic simulation models.Analytical and numerical results indicate that both simulation models provide good approximations to the desired statistical properties of the reference model,and the stochastic simulation model results in a better performance under comparable computational complexity.

Nowadays,daily human life is closely intertwined with various networks.When natural disasters or malicious attacks break out,the failure of communication infrastructure due to direct destruction or indirect impact tends to cause a massive outage of communications.Emergency communication networks play a significant role in rescue operations.Recently,a flexible and efficient solution has been provided for emergency communications using unmanned aerial vehicles(UAVs).By means of their excellent characteristics,UAVs,serving as aerial base stations (ABSs),can be rapidly deployed to temporarily rebuild a damaged communication network to restore the users’ connectivity.In this study,we investigate the use of UAVs as ABSs for an emergency communication scene where user equipment is unevenly distributed and the communication infrastructure has completely failed due to a severe disaster.Effective communication probability(ECP),which integrates throughput coverage and connectivity,is used to evaluate the performance of a communication network.Through simulations,we analyze communication improvements that can be obtained by the flexible deployment of ABSs.The results show a noticeable increase in ECP when some ABSs are deployed in optimal locations.

In this paper,a cellular-connected unmanned aerial vehicle (UAV) mobile edge computing system is studied where several UAVs are associated to a terrestrial base station(TBS)for computation offloading.To compute the large amount of data bits,a part of computation task is migrated to TBS and the other part is locally handled at UAVs.Our goal is to minimize the total energy consumption of all UAVs by jointly adjusting the bit allocation,power allocation,resource partitioning as well as UAV trajectory under TBS’s energy budget.For deeply comprehending the impact of multi-UAV access strategy on the system performance,four access schemes in the uplink transmission is considered,i.e.,time division multiple access,orthogonal frequency division multiple access,one-by-one access and non-orthogonal multiple access.The involved problems under different access schemes are all formulated in non-convex forms,which are difficult to be tackled optimally.To solve this class of problem,the successive convex approximation technique is employed to obtain the suboptimal solutions.The numerical results show that the proposed scheme save significant energy consumption compared with the benchmark schemes.

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.

Due to the high maneuverability of unmanned aerial vehicles (UAVs),they have been widely deployed to boost the performance of Internet of Things (IoT).In this paper,to promote the coverage performance of UAV-aided IoT communications,we maximize the minimum average rate of the IoT devices by jointly optimizing the resource allocation strategy and the UAV altitude.Particularly,to depict the practical propagation environment,we take the composite channel model including both the small-scale and the large-scale channel fading into account.Due to the difficulty in acquiring the random small-scale channel fading,we assume that only the large-scale channel sate information is available.On this basis,we formulate an optimization problem,which is not convex and challenging to solve.Then,an efficient iterative algorithm is proposed using block coordinate descent and successive convex optimization tools.Finally,simulation results are presented to demonstrate the significant performance gain of the proposed scheme over existing ones.

Unmanned aerial vehicles (UAVs) have emerged as a promising solution to provide wireless data access for ground users in various applications (e.g.,in emergency situations).This paper considers a UAVenabled wireless network,in which multiple UAVs are deployed as aerial base stations to serve users distributed on the ground.Different from prior works that ignore UAVs’backhaul connections,we practically consider that these UAVs are connected to the core network through a ground gateway node via rate-limited multi-hop wireless backhauls.We also consider that the air-to-ground access links from UAVs to users and the air-to-air backhaul links among UAVs are operated over orthogonal frequency bands.Under this setup,we aim to maximize the common (or minimum) throughput among all the ground users in the downlink of this network subject to the flow conservation constraints at the UAVs,by optimizing the UAVs’ deployment locations,jointly with the bandwidth and power allocation of both the access and backhaul links.However,the common throughput maximization is a non-convex optimization problem that is difficult to be solved optimally.To tackle this issue,we use the techniques of alternating optimization and successive convex programming to obtain a locally optimal solution.Numerical results show that the proposed design significantly improves the common throughput among all ground users as compared to other benchmark schemes.

As a promising candidate for millimeter wave (mmWave) multiple-input and multiple-output (MIMO) communications,hybrid precoding techniques can reap the benefit of large antenna arrays,yet with only limited number of radio frequency(RF)chains.In this paper,we investigate the problem of achieving the same performance of the fully digital system with hybrid precoding.Specifically,for the single user MIMO system,we propose a closed form hybrid precoding design that can achieve the optimal fully digital performance for both frequency-flat and frequency-selective channels,and only requires the number of RF chains to equal the number of paths of the channel.The design for the case with even less RF chains is also given.Furthermore,for the multiuser (MU) system with single antenna at each mobile terminal(MT),two MU beamforming schemes are considered,which are the directional beamforming and zero-forcing.We show that for both schemes,the fully digital performance can be achieved with our proposed hybrid precoding designs with the number of RF chains no less than the sum number of channel paths from the base station to all the selected MTs.Numerical results are provided to validate our analytical results and show the performance gain of the proposed hybrid precoding designs compared to other benchmark schemes.

Due to the potential capacity-boosting for wireless communications,radio vortex wireless communication (RowComm) over orthogonal states/modes of orbital angular momentum (OAM) has been paid much attention to in recent years.A uniform circular array (UCA),as an efficient and convenient antenna structure,can transmit/receive multiple OAM beams with different OAM-modes simultaneously when the transmitter and receiver are aligned.However,for high-order OAMmodes,the OAM beams are divergent accompanied by severe attenuation.Thus,it is difficult to directly use high-order OAM-modes to achieve high capacity for RowComms.To obtain high capacity potentially offered by OAM-modes,in this paper we transform the singular UCA into the concentric UCAs,where high capacity can be achieved using multiple parallel low-order OAM-modes instead of all high-order OAM-modes,to increase the capacity of transmitter-receiver aligned RowComms.In particular,we study two cases:1).concentric UCAs based RowComms without co-mode interference;2).concentric UCAs based RowComms with co-mode interference.We propose a co-mode-interference-free and a co-mode-interference-contained mode-decomposition schemes to recover transmitted signals for these two cases,respectively.Additionally,we develop optimal power allocation schemes to maximize the capacity for these two cases.Numerical simulations are presented to validate and evaluate that our developed concentric UCAs based low-order RowComms can significantly increase the capacity as compared with that of singular UCA based RowComms.

Vehicular ad hoc networks (VANETs) support safety- and non-safety-related applications that require the transmission of emergency safety messages and periodic beacon messages.The dedicated short-range communication (DSRC) standard in VANETs is used to exchange safety messages,and is involved in multi-hop data dissemination and routing.Many researchers have focused either on emergency data dissemination or routing,but both are critical.Routing protocols are commonly used for position-based routing and distancebased routing.This paper focuses on both emergency data dissemination and multi-hop routing,with the selection of the best data disseminator and trustworthy forwarder.To select the best forwarder,ring partitioning is performed,which segregates vehicles into rings based on the coverage area for routing.Each partition is selected with a best forwarder,which minimizes the hop count for data transmission.The work also includes effective video transmission for a user’s request.Video transmission in VANETs is involved in this work to provide efficient video delivery between rapidly travelling vehicles with reduced delay owing to the selection of good-quality channels.Video transmission is prioritized according to frame types,and they are then transmitted with respect to the preference of channels.The major issue in video streaming is the loss of packets,which is our focus to minimize it.Our proposed VANET environment is simulated in OMNeT++,and the results show remarkable improvements in terms of the packet delivery ratio,end-to-end delay,and reliability.

The passive optical network (PON) is the most promising candidate for supporting multiple service classes,i.e.,high priority (HP) traffic and best-effort (BE) traffic,in the next-generation broadband access networks.Providing an efficient dynamic bandwidth allocation (DBA) algorithm is the most important issue for supporting multiple service classes of traffic simultaneously.This paper presents a new DBA algorithm,which is the modified version of the hybrid slot-size/rate(HSSR) scheme,and is called the modified HSSR (MHSSR) algorithm.We have also modified the control message scheduling algorithm to fit it to the proposed scheme.In the proposed MHSSR scheme,each cycle time,i.e.,the length of a time cycle,is equally divided into two parts.In the 1st part of the cycle time,the bandwidth is dynamically allocated to the HP traffic of all the optical network units (ONUs).In contrast,in the 2nd part of the cycle time,the bandwidth is dynamically allocated to the BE traffic of one or multiple ONUs.Furthermore,to ensure guaranteed service for the HP traffic,the 1st part of the cycle time is extended while the 2nd part of the cycle time is compressed if the demand of the HP traffic is very high.However,the 1st part of the cycle time will never be compressed,even at the lightly loaded condition of the HP traffic.The modified control message-scheduling algorithm effectively coordinates the timing sequence between the two parts of the cycle time and ensures the synchronization between two Gate messages to each ONU.We have evaluated the performance of the proposed schemes through numerical simulations in terms of endto-end packet delay,bandwidth wastage,remaining bytes per time cycle,jitter,and throughput for different offered loads.From the comparative analysis,it can be seen that the proposed scheme provides better performance than the existing HSSR and delay variation-guaranteed schemes.