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Unmanned Aerial Vehicles, abbreviated as UAVs, are aircrafts without any human pilot onboard, mainly controlled and managed remotely or via embedded autonomous computer programs. UAVs are also popularly known as drones. Unique features of UAVs pertaining to high mobility in three-dimensional space, autonomous operation, flexible deployment tend to find appealing solutions for wide range of applications including civil, public safety, Industrial IoT platforms (IIoT), security and defence sectors, cyber physical systems, atmospheric and environmental observation etc.
As soon as national legislations allow UAVs to fly autonomously, we will see swarms of UAV populating the sky of our smart cities to accomplish different missions: parcel delivery, infrastructure monitoring, event filming, surveillance, tracking, etc.
Applications of unmanned aerial vehicles (UAVs) have been fast growing during the past few years. However, traditional UAV systems mainly rely on simple direct communication between the UAV and the ground pilot over unlicensed spectrum (e.g., ISM 2.4 GHz), which is typically of low data rate, unreliable, insecure, vulnerable to interference, difficult to monitor and manage, and has very short operation range. UAVs are inherently mobile in nature and hence, require wireless support for communication needs.
To overcome the above limitations, there has been significant interest in integrating UAVs into cellular networks, e.g., 5G networks. On the one hand, UAVs with their own missions could be connected to cellular networks as new aerial users by exploiting advanced cellular technologies and the almost ubiquitous accessibility of cellular networks.
On the other hand, dedicated UAVs could be deployed as aerial base stations (BSs), access points (APs), or relays to assist 5G and beyond terrestrial wireless communications from the sky, leading to another paradigm known as UAV-assisted communications. UAV-assisted communications have several promising advantages, such as the ability to facilitate on-demand deployment, high flexibility in network reconfiguration, and high chance of having LoS communication links.
UAS are also critical for military intelligence, surveillance, and reconnaissance missions. Their roles have now expanded to areas including electronic attack, drone strikes, suppression or destruction of enemy air defense, network node or communications relay, combat search and rescue, and derivations of these themes. UAS-aided relaying, where UASs are deployed to provide wireless connectivity between two or more distant users or user groups without reliable direct communication links. The use of UAS as an aerial platform to extend coverage can be an option to meet the needs of small distributed forces operating in areas where line-of-sight communications are not possible.
The use of multiple UAS allows for beyond line of sight (BLOS) flight operations. In the mountainous or high-density urban regions, the communication between UASs and the ground control station can be easily interrupted, causing the ground control station to lose real-time data feedback from the UASs, leading to mission failure. In addition, if the mission requires long flight range, the communication might also be interrupted by the extended distance.
Drones integration into 5G/6G Networks
Cellular communication, which is used by mobile phones, is based on the transmission of information via radio waves. To create a seamless network, the coverage area (for example, a city) is divided into overlapping units, or cells, and a separate base station operates in each of them. These stations are capable of both transmitting and receiving radio waves from mobile phones. The main drawback is that the emissions from the base stations are flat (two-dimensional). That is why quality of communication varies at different heights.
Drones as Base Stations
Wireless communication systems that include unmanned aerial vehicles (UASs) promise to provide cost-effective wireless connectivity for devices without infrastructure coverage. Compared to terrestrial communications or those based on high-altitude platforms (HAPs), on-demand wireless systems with low-altitude UASs are in general faster to deploy, more flexibly re-configured. They are likely to have better communication channels due to the presence of short-range line-of-sight (LoS) links.
An emerging area for UAS is the ability for the aerial asset to support two-way communications between first responders. A reliable communication infrastructure is crucial for an efficient and successful emergency response system. Disruption of communication infrastructure may be caused directly by damage to cables and cellular towers, or indirectly, through shutdown of power and water.
In a disaster situation, first 48 to 72 hours are very crucial to perform any kind of mitigation to the damage or outage and to restore the normal state of the environment. The response time is the key in saving lives in the affected regions. The major problems in these initial hours are: lack of proper communication infrastructure, massive or often unpredictable losses of lives and property. Thus, the situation forces the first responder teams to implement and improvise the search and rescue (SAR) mission to be conducted quickly and efficiently.
Drones are relatively low-cost solutions that can efficiently extend next-generation connectivity in areas that are normally either unreachable via traditional infrastructure or momentarily unreachable due to, e.g., network disruptions. Drone BS are also attractive solutions for providing reliable, broadband and wide-area temporary wireless connectivity during special events or harsh scenarios, namely natural disasters, smart farming, sporting events, and many more scenarios where the permanent installment of cellular infrastructure is unnecessary and comes with considerable monetary overhead.
UAS technology is poised to provide cost-effective “just in time” services at the scene of an emergency. Latest advancements of UAVs and sensor networks are capable to meet this need in terms of disaster prediction, assessment, and fast recovery. UAVs can gather information (e.g., situational awareness, early warnings, person movement) during the disaster phase and this information are helpful for first responder teams to react efficiently. UAVs can re-establish the communication infrastructure (i.e., UAV-assisted paradigm) destroyed at the time of disaster.
Similarly, drone base stations (BSs) can increase the connection quality between an ad hoc/remote node and a given cellular base station.
Drone BSs at high altitudes are expected to provide long-term and cost-effective connectivity for rural areas. The integration of drone BSs with other physical layer techniques such as mmW and massive MIMO as well as cognitive radios is a promising solution for providing data-intensive services and is expected to create new challenges for next-generation flying. The optimal positioning of drone BSs is one of the critical challenges to be overcome in dense deployment scenarios. For that purpose, the optimal positioning of the drone BSs is an issue in need of tackling. Optimal positioning is one of the most critical challenges and must be addressed in dense 5G/6G deployment scenarios.
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The extension of terrestrial network coverage and capacity;
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The assistance of mobile ad hoc networks (MANETs);
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Beamforming applications.
UAV-Assisted Cellular Communication – In this paradigm, UAVs are realized as flying base stations, relays or localization anchors, that can intelligently reposition themselves to assist the existing terrestrial wireless communication system to improve the user perceivable Quality of Experience (QoE), spectral efficiency and coverage gains Due to dynamic mobility and repositioning, the integration of UAV brings many advantages to existing terrestrial communication system. Researchers have developed a model in which flying unmanned drones would serve as additional receivers-transmitters of radio waves and cover areas beyond the reach of conventional base stations. This will significantly improve the quality and reliability of the service.
The base station mounted on the UAV (flying base station or relays) could be provisioned on demand, which is an absolute appealing solution for disaster management, search and rescue or emergency response. The coverage and data rate of existing cellular networks can be improved by optimal 3D placement and coordination of flying base stations to cater the users need in hotspot areas.
From the communication viewpoint, the requirements of UAV can be classified into two broad categories. Control and Non-Payload Communication (CNPC) refers to the time critical control and safety commands to maintain the flight operations. CNPC includes the navigation, waypoint updates, telemetry report and air traffic control (ATC) updates to ensure secure and reliable UAV operation. usually demands highly secure and reliable communication with low data rate (few hundred Kb/s) requirements. The reliability requirement for CNPC is less than 10−3 packet error rate (PER).
The emerging unmanned aerial vehicles (UAVs) have been widely exploited for sensing purposes due to the larger service coverage compared with the conventional fixed sensor nodes. However, due to the limited computation capability of UAVs, real-time sensory data needs tobe transmitted to the BS/server for real-time data processing. Payload Communication refers to all the information dissemination activities between UAV and ground station pertaining to a UAV mission. For instance, in a surveillance operation, UAV needs to transmit real time video to the ground station/remote pilot via payload communication. Payload communication demands the underlying transmission medium to be capable of supporting high data rates (often higher in full HD video transmission or wireless backhauling).
Cellular-Assisted UAV Communication – This is also known as Cellular-connected UAVs. Flying UAVs are realized as new aerial User
Equipments (UEs) coexisting with terrestrial UEs that access the cellular network infrastructure from the sky. This paradigm has gained significant interest in recent times, because of the effective solution for establishing reliable wireless connectivity with ground cellular
stations. The communication channel mainly involves two types of links, namely Ground-to-UAV (G2U) link and UAV-toGround (U2G) link. In cellular-connected UAV, the G2U link serves the downlink purpose of control and command for proper UAV operations, whereas U2G link serves the uplink purpose of payload communication.
UAV-UAV Communication – In this paradigm, a group of UAVs reliably communicate directly with each other sharing the cellular spectrum with ground users in order to facilitate autonomous flight behaviours, cooperation in a UAV fleets, and collision avoidance
The UAVs are envisioned to be an essential part of Fifth Generation and Beyond Fifth Generation ( 5G/B5G) networks with potentials of supporting high data transmission (∼10 Gbits/s), stringent latency (1 ms round trip delay) and enhancements to radio access technologies (RATs). In this regard, the cellular networks are necessary to support the data transmission for UAVs, which is called as the Internet of UAVs. Very recently, 3GPP has approved a study item on enhanced support to seamlessly integrate UAVs into future cellular networks.
UAV cellular integration challenges
However, the utilization of highly mobile and energy-constrained UASs for wireless communications also introduces many new challenges. The aerial users typically fly higher than the BS antenna height and therefore, need 3D coverage suitable for varying UAV altitude. The base stations (eNodeBs) are typically designed and developed to provide optimal performance to the ground users. The current eNodeBs are downtilted to serve above purpose. Down-tilting the antennas produces radiation patterns that are not useful to serve aerial UEs, which are expected to be positioned at different altitudes with respect to the ground surface.
Unlike terrestrial cellular networks, UAV communications have many distinctive features such as high dynamic network topologies and weakly connected communication links. The mobility and handover characteristics of terrestrial cellular users are quite different from the 3D aerial mobility of cellular-connected UAVs. With increase in height, the radio environment changes and mobile UAVs face connectivity challenges. In this case, the performance of the system depends on the handover rates, including failed and successful handovers and radio link failures. Radio link failures occurs when the UAV is unable to maintain a successful connection with the serving cell.
The risk of interference must also be considered when deploying UAS to supplement communications. Flying a UAS high above ground, i.e., above the terrestrial infrastructure and other obstacles, is likely to create higher levels of interference than engineers expected during the design and planning of the terrestrial network. Interference is higher because, as radio waves propagate through the air, signals from the UAS do not suffer as much attenuation or variations as signals transmitted for ground operations.
Although interference risks are much reduced when UASs are deployed in times of disaster, i.e., when the terrestrial infrastructure is partially down, the potential for disrupting device-to-device communications exists. As noted above, the (interference) risk is higher when multiple UASs are hovering in the same area, which may be the case if multiple agencies bring their own UAS to an incident scene.
Besides, they also suffer from some practical constraints such as battery power, no-fly zone, sensing requirements, etc. The battery life of a UAV is limited. During the mission, UAV must intelligently plans its trajectory from initial to destination location considering application
and use case. The key performance metrics, such as maximum allowed time to complete the mission, persistent cellular connectivity, QoS guarantees, energy consumption, etc. are some of the factors the UAV must respect during its mission. Hence, trajectory optimization is an essential aspect of UAV mission.
Energy Availability
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Energy consumed for the purpose of flying and hovering above a desired location;
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Energy consumed for communication and on-board processing;
It can be safely assumed that for the purpose of enabling long flight times, battery technology has to keep up with the newly introduced requirements set by 5G/6G communications.
Tethered UAVs
UAS vehicles come in a variety of types of configurations, the most common types of Unmanned Aerial Vehicles (UAVs) to be considered for use as an aerial platform for communications are Rotary Wing and Tethered. Rotary wing have the advantages of hovering capability, high maneuverability, and medium range for use in an incident such as a natural disaster or wildfire which covers a large geographical area.
Fixed Wing or unmanned airplanes generally have longer endurance but these systems must keep moving forward and require a larger take off, landing, and flight area. Their inability to hover over a location or object makes this solution more challenging for incidents with a smaller geographical footprint.
Tethered UAV aircraft are those that come equipped to be tethered to the ground and should be considered in those aerial operations where it is desired for the UAV to be in a single stationary fixed position at a particular altitude. Tethered units can operate indefinitely from a fixed location. Even tethered UAS systems require a remote pilot maintaining operations and a visual line of site. These types of UASs are limited to line of sight (LOS) operations and limited altitudes due to the weight of the tether.
Unleashing the potential of tethered drones to provide cellular network coverage in both urban and rural areas
The use of tethered unmanned aerial vehicles (TUAVs) has been modeled as a powerful new tool for improving cellular phone and internet networks. When employed as flying base stations with a cable connection, multirotor drones promise to quickly ramp up coverage, increase the efficiency of urban networks and provide much needed access in remote rural areas.
“Our aim has been to show that TUAVs offer a very appealing solution as a bridge between fixed base stations and free-flying drones,” says Mohamed-Slim Alouini, who has authored two papers on the subject with other KAUST mathematicians Mustafa A. Kishk and Ahmed Bader. “We hope our theoretical investigation will spur others to develop the idea to its full potential,” says Alouini.
In 2017, after Hurricane Maria devastated Puerto Rico, the U.S. telecommunications company AT&T showed how drones (UAVs) fitted with cellular transceivers could quickly be deployed to replace inoperable base stations and restore mobile coverage. Mathematicians have been calculating the optimal positioning of such “COWs” (cells on wings) ever since. Adding a cable for power and data has been an obvious development. A U.S. startup called Spooky Action, for example, has been experimenting with TUAVs to connect remote areas of Africa.
TUAVs on the market today can stay aloft for a month or more, much longer than their untethered counterparts, which must land to recharge every hour or so. With a fiber optic line running alongside their power connection, TUAVs can also “backhaul” their data to the core network with much greater efficiency. Their drawback is their restricted mobility, although the cable connecting currently available products can be as long as 150 meters.
“An interesting result was that with longer tethers, TUAVs will outperform free-flying UAVs in just about every scenario,” says Kishk. “Tomorrow’s 5G equipment is heavier and consumes more power than 4G today, so their advantage will become more apparent.” Eventually, the three KAUST researchers envisage TUAVs complementing fixed base stations in high-density urban networks. Tethered to tall buildings, they would offload data during peak hours and shift their position around the clock to cover varying traffic distribution throughout the day. In the low-density countryside, meanwhile, high-flying TUAVs promise a more viable alternative to expensive, tall towers needed to provide coverage to large but sparsely populated regions.
Cyber threats
While carrying out sensitive and real-time critical tasks, UAVs are prone to security threats and cyber physical attacks. Any malicious attempt to steal, misuse or control the UAV, can trigger undesirable situations and cause loss of confidential and private assets. Cellular-connected UAVs are usually equipped with a multitude of sensors that collect and disseminate data. This provides numerous opportunities to expose them to vulnerabilities. These flying platforms are prone to cyber physical attacks, with an intention to steal, control and misuse the UAV payload information by reprogramming it for undesired behaviour. For instance, in business use case such as goods delivery, the attacker can gain physical access to the customer package as well as to the UAV device. Existing information security measures are not well suited for cellular-connected UAVs, because these measures do not take into account possible threats imposed on numerous onboard sensors and actuator measurements of UAVs
