Unmanned Aerial Vehicles (UAVs) or Unmanned Aircraft Systems (UAS) are expected to become an important part of 5G or Beyond 5G (B5G) communications, including their applications within cellular architectures (5G UAVs). In such architectures, small UAV systems are typically used to assist wireless broadcasting and point-to-point transmission. At least in the near term, to enable UAV systems to operate in airspace alongside commercial, cargo, and other manned aircraft, dedicated protected aviation spectrum may be required, while regulators should also adapt to the use of UAV systems. Command control (C2) or Command and Non-Payload Communication (CNPC) links can provide critical security information for UAV control in both line-of-sight and beyond-line-of-sight satellite communication link environments. The control and non-payload communication links may be applied in 5G and satellite systems. This article summarizes the control and non-payload communication links, describes their fundamental concepts and challenges faced. The article will also introduce emerging technologies that may be used for UAV command control and payload communication, such as millimeter-wave systems, while also addressing navigation and surveillance challenges and briefly discussing UAV-to-UAV communication and hardware issues.
1 Introduction
The size of UAVs varies from a few centimeters to several meters, and they are widely used in various scenarios, including consumer entertainment flying, military needs, crop monitoring, railway inspections, and more. One of the key requirements for using UAVs is to provide command and non-payload communication data connections, also known as command communication. Non-payload communication links are dedicated to safe and reliable communication between remote pilot ground control stations and aircraft to ensure safe and effective UAV flight operations. This link can be a line-of-sight air-ground link between two entities or a beyond-line-of-sight link using another platform (such as satellites or High Altitude Platforms (HAP)). The data rates of these links are expected to be moderate (e.g., the maximum rate for compressed video is 300kbps and will not be continuously used).
Payload communication links are typically used for data applications and generally require high throughput. The type of payload communication depends on the application type (e.g., agriculture, public safety, etc.), leading to a wide variety of types. Although interruptions in payload links can cause inconvenience, the issue is not very serious, while interruptions in control and non-payload communication links can have severe consequences. The functions of control and non-payload communication are related to different types of information, such as remote control commands, non-payload telemetry data, navigation assistance support, air traffic control (ATC) voice relay, air traffic service data relay, target tracking data, airborne weather radar downlink data, non-payload video downlink data, and so on. The cellular mobile industry is also very interested in using UAVs to expand its capacity, providing low-cost wireless connections to devices that are not covered by existing infrastructure. There may also be other cellular applications, such as serving as user devices or relays.
This article will focus on the broader applications of UAVs in scenarios where communication is achieved through ground infrastructure and satellite systems, as shown in Figure 1. Completely different links (line-of-sight and beyond-line-of-sight) represent different channel conditions and operating frequencies, as well as different latencies and communication distances, increasing the challenges of achieving the extremely high reliability required for control and non-payload communication links. In addition to existing cellular frequency bands (600MHz to 6GHz), the 5G realm is also considering the use of millimeter-wave frequency bands (24~86GHz). The free space and tropospheric attenuation of millimeter-wave frequency bands are significant, which will limit link distances. Therefore, if the millimeter-wave link is the only line-of-sight link, once it exceeds the line-of-sight millimeter-wave range, beyond-line-of-sight capability will be required. Of course, in remote areas beyond any ground station coverage, beyond-line-of-sight links are also needed. Although satellites are an obvious choice for beyond-line-of-sight communication, the choice of satellite orbit (e.g., Low Earth Orbit (LEO) or Geostationary Earth Orbit (GEO)) will significantly affect latency, link budget parameters, Doppler, and handover/cross-zone switching. To maximize frequency reuse, satellite operators are also planning to use narrower beams, which will increase the number of handovers, thus emphasizing connection reliability even more. Due to the much longer distances of beyond-line-of-sight links, achieving a link between UAVs and satellites for the currently planned beyond-line-of-sight frequency bands (above 5GHz) will very likely require the use of directional antennas and adaptive focusing beams, i.e., mechanically or electronically controllable antenna beams. The 5G millimeter-wave links also face similar issues, but the antenna gain requirements are much lower. Adaptive antennas, handover, and other issues make the system hardware and software more complex, increasing the size, weight, and power (SWaP) of the communication system.
5G cellular systems are likely to deploy millimeter-wave technology, bringing a large bandwidth for communication applications. The availability of large bandwidth can enable rapid command and may also transmit local map data, etc. The use of millimeter-wave links may also enable command and payload communications to be sent together on the same physical channel.
2 Future Control and Non-Payload Communication
In the United States, for medium to large UAVs, control and non-payload communication have adopted a standard developed by the Radio Technical Commission for Aeronautics (RTCA). This standard specifically addresses the L-band (~900~1000MHz) and a portion of the C-band allocated for aviation (5.03~5.091GHz). This standard is only applicable to air-ground links (line-of-sight), and RTCA is developing beyond-line-of-sight standards. By 2030, the estimated bandwidth requirements for ground-based line-of-sight control and non-payload communication for UAVs will be 34MHz, while the bandwidth demand for satellite-based beyond-line-of-sight control and non-payload communication links will be 56MHz. The RTCA standard does not include any 5G applications and primarily targets the lower three layers of the communication protocol stack. Nevertheless, this standard is applicable to any type of 5G application involving medium to large UAVs.
In the United States, the deployment planning for UAV system control and non-payload communication has two phases. The first phase supports ground networks (based on dedicated cross-zone switching capabilities) but does not target any industrial standard cross-zone switching capabilities, which will be addressed in the second phase. The frequency bands allocated for the first phase of control and non-payload communication are L-band and C-band. For the future control and non-payload communication defined in the second phase, i.e., beyond-line-of-sight control and non-payload communication, consideration will be given to using L, C, Ku, or Ka-band satellite communications, while also considering networked ground and C-band ground communications.
The development of standards related to small UAVs has been relatively slow. RTCA and the cellular mobile community (i.e., 3GPP) are studying this use case, and work is still ongoing. NASA has also initiated a Unmanned Aircraft System Traffic Management (UTM) project in collaboration with the Federal Aviation Administration (FAA) to develop low-altitude small UAV air traffic rules and technologies. Multiple concept validation field trials will be conducted over the next two to three years. Physical layer and media access control technologies will at least initially use commercial technologies such as cellular (LTE) and wireless local area networks (WiFi); however, these technologies are not optimal in many UAV operating environments, and for some control and non-payload communication links, the design is overly conservative and susceptible to interference and deception.
Therefore, work in this area is ongoing and is a theme of 5G UAV research.
One potential candidate for the UAV ground C2/control and non-payload communication link is Ultra-Reliable Low-Latency Communication (URLLC) services. The goal is to achieve an average latency of no more than 0.5 milliseconds through the evolution and revolutionary changes of the air interface, with a reliability exceeding 10-5. One such air interface is called 5G New Radio (5G-NR). Recently developed Dedicated Short-Range Communication (DSRC) for vehicle-to-vehicle (V2V) communication in 5G may also yield air interfaces and network technologies that can be used or adapted for single UAV air-ground links and UAV swarm air-to-air links.
Other alternatives for control and non-payload communication include several so-called “Long Range” (LoRa) communication solutions. These solutions are at least partially aimed at IoT applications and thus tend to use simplified protocols to support relatively low data rates (kbps). Before using these technologies in links for control and non-payload communication, their reliability may need to be improved.
3 Satellite Link Control and Non-Payload Communication
In many parts of the world, establishing a connection between UAVs and ground stations is very difficult, if not impossible. At this point, beyond-line-of-sight communication must be established. Using satellite connections can serve as a supplementary means or essential feature to improve or achieve coverage and reliability for commercial applications and tactical missions.
Table 1 provides some details comparing three levels of satellite system orbits. Notably, as orbital height increases, latency also increases: when using GEO orbits, latency can reach up to 0.5 seconds. This latency can affect the autonomous capabilities of UAVs. Therefore, due to the longer propagation delays of synchronous and medium Earth orbits, they may not be suitable for 5G, and thus, in 5G, Low Earth Orbit (LEO) constellations are garnering more attention.
Future low-orbit satellite providers are focusing on providing data links not only for ground users but also for remote cellular towers that cannot install ground cables due to high costs, serving as their transmission backbone. Although the propagation delays of low-orbit satellites are also significant compared to ground links, a low-orbit satellite can achieve a single-hop link between two points, providing coverage over hundreds of kilometers.
One possible way to smoothly integrate UAVs into 5G systems using low-orbit satellite beyond-line-of-sight links is the concept of a 5G relay node (RN) as shown in Figure 2. A low-complexity relay node on a UAV with satellite capability transmits 5G downlink/uplink waveform through a low-orbit satellite link connected to a real base station, which is the so-called donor node B at the UAV ground station. For airborne equipment (such as flight controllers needing control and non-payload communication or payload equipment requiring high throughput links), the relay node is akin to a ground station. When flying in swarms, UAVs equipped with relay nodes with satellite capabilities can act as “cellular towers” for the swarm. The relay node is transparent to the donor node B, which only sees many users. Although this approach is not optimal from a channel capacity perspective, it requires less communication infrastructure on the UAV and ground station side, simplifies the handover from 5G to satellite, and utilizes 5G technology.
4 Potential 5G Technologies for UAV Control and Non-Payload Communication
Cellular technology is an obvious alternative for UAV control and non-payload communication links, but it also has some drawbacks. The rapid maneuvering of UAVs and changes in antenna direction can cause strong fading. Therefore, modifications to the physical layer design may be necessary, such as modifying Orthogonal Frequency Division Multiplexing (OFDM) to Filter Bank Multi-Carrier (FBMC), Orthogonal Chirp Spread Spectrum (OCSS), or some other modulation techniques. FBMC modulation is more compact spectrally than OFDM, making it more practical in air-ground or satellite channels with no dispersion or slight dispersion.
Additionally, traditional single-carrier modulation is also suitable for air-ground links and is still used in most satellite communications today.
Another technology worth noting is MIMO systems at both ends of the link. These technologies are widely used in cellular and WLAN, but have not yet been applied in aviation (or satellite links). Part of the reason is due to aircraft mechanical integrity regulations, but it is also related to size, weight, and power constraints. As digital processing efficiency increases and frequency ranges widen, the application of MIMO in aviation will also increase.
Millimeter-wave systems may be used for UAVs. Due to their large available bandwidth, they may be used not only for control and non-payload communication but also for payload communication. The use of high-directional beamforming antennas will achieve higher bit rates and aggregated capacity. However, millimeter-wave UAV links have a well-known problem with extremely high path loss. However, in practice, for the same physical antenna size, shorter wavelengths can achieve greater antenna gain. In millimeter-wave UAV links, due to the movement of the UAV, the time constraints for beamforming training are stricter than for static ground millimeter-wave communication.
The high attenuation of millimeter-wave signals due to blockage is also a significant drawback. Using millimeter-wave links in UAVs may require precise flight algorithms to avoid blocked areas and maintain line-of-sight communication, but in certain locations, such as where UAV systems can see the entire street or rise above obstacles in urban areas, the reliability of millimeter-wave links may be superior to ground links. At the same time, the reliability of millimeter-wave links can be significantly enhanced by large antenna arrays that can improve directionality and reduce co-channel interference in millimeter-wave backhaul.
It is expected that ground 5G systems will provide at least 1Gbps data rates “anywhere”, offering a consistent data rate experience to all users; it can provide 5Gbps and 50Gbps data rates to high-mobility users and pedestrian users, respectively. The currently available frequency bands are 28GHz and 38GHz, with more than 1GHz of spectrum allocated. Based on channel measurements in ground environments, for these bands, the line-of-sight link distance does not exceed 200 meters, and non-line-of-sight (NLOS) links do not exceed 100 meters. Of course, link distances are greatly affected by transmission power, and in the future, the transmission power of large UAV base stations can be easily increased.
Compared to ground millimeter-wave cellular networks, using spatial division multiple access (SDMA) or beam division multiple access (BDMA) may be beneficial or even necessary in millimeter-wave UAV networks. The use of high-directional transmission enables different spatial beams to divide moving stations at different locations into different groups, as shown in Figure 3. Due to the large signal bandwidth and the use of SDMA, capacity may increase significantly. The main challenge of SDMA is how to ensure that different users in different groups (so-called “group users”) can access the base station simultaneously and at the same frequency without interference. A practical strategy is to group users by transmission angle, allowing only users from different spatial groups to access the channel simultaneously. It is noteworthy that in UAV cellular networks, the user grouping is not fixed due to the mobility of UAVs and ground users.
As shown in Figure 3, using mobile base stations is a significant advantage of UAV systems because these base stations can move and provide services to customers on demand. Mobile base stations are proposed as an effective solution for rural area users and emergency situations where ground base stations cannot be used, such as earthquake and flood disaster relief. In vehicular ad hoc networks (VANET) and the Internet of Vehicles, collaborative data exchange and relaying using UAV systems may become an effective solution for time-limited communication links between roadside units (RSU) and vehicles.
5 5G UAV Navigation and Surveillance
One area of concern for 5G is the theme of “critical communications,” including “UAVs and robots.” For positioning accuracy in the 5G era, the 3GPP navigation and surveillance performance requirements will be: in urban environments with a cellular size of about 200 meters, an accuracy of 0.5 meters and a capture time of 0.5 seconds. Below, the possible pathways for UAV navigation and surveillance methods in future 5G networks will be described. Figure 4 depicts an overall view of possible 5G UAV navigation and surveillance.
Recently, there have been many advancements in navigation and surveillance, including improved inertial navigation systems, the use of more than one Global Navigation Satellite System (GNSS), radar altimeters, LiDAR, and terrain databases. All of these have achieved precise performance in various military and civilian applications.
Due to size, weight, and power constraints, most UAV systems’ primary navigation functions only use GNSS. However, some UAV systems are embedding more precise tightly-coupled GPS/Inertial Navigation Systems (INS) for navigation and platform stabilization. In dense urban areas or indoor regions, satellite visibility is reduced, and GPS performance in these areas is quite poor. Moreover, interference, deception, and solar flares can all degrade GPS accuracy. Although it is relatively simple and inexpensive to interfere with GNSS signals, various technologies have been proposed to mitigate the effects of interference on received signals, such as zeroing filters, encryption, signal distortion detection, and direction of arrival sensing. Research has shown that combining different strategies can provide a reasonable security countermeasure for commercial deployment.
ADS-B is a surveillance technology in which aircraft periodically broadcast their position and receive messages from ground stations, making it one of the most popular technologies for freeing air traffic control from radar-based systems. The rapid development of UAV systems has far outpaced legislation, and current rules prohibit the installation of ADS-B Out (transmitter) systems on unregistered systems (most UAV systems). Due to the inability to ensure an acceptable level of aviation safety, many documents exclude small UAV systems from surveillance regulations and recommendations. However, the range, resolution, accuracy, and update rates of ADS-B are superior to other existing technologies.
Integrating ADS-B into standalone GNSS equipment enables operators (UAV pilots) to observe cooperating aircraft before they come into visual range. However, currently, the ADS-B bandwidth is only 1MHz, severely limiting its capacity. As the number of aircraft increases, ADS-B may become insufficient in areas where such information is most critical.
ADS-B’s limitations in supporting users in airspace below 400 feet (122 meters) pose another problem. Almost all UAV systems operate in the very low-level (VLL) airspace, defined by the FAA as below 500 feet (152 meters) in altitude. This issue may potentially be resolved in the near future by 5G/Beyond 5G network technologies, such as using existing LTE systems for ADS-B. However, using LTE alone has limitations, such as limited coverage in rural areas and link initialization times. The collaborative approach of ADS-B and 5G UAV systems can mitigate the co-channel interference risks of the 978MHz ADS-B frequency channel.
In 5G networks, the cellular coverage evolves from large base stations to small pico or nano base stations. By using more base stations with smaller coverage areas in a given area, capacity and spatial spectral efficiency can be increased; however, achieving such dense deployments comes at the cost of increased handover rates. This may lead to interruptions in data streams in user devices, becoming a significant challenge in 5G UAV user navigation and surveillance applications. In response, scholars have proposed implementing intelligent handover management modes in single-layer and two-layer downlink cellular networks.
Research indicates that the minimum coverage of conventional air traffic control radar is 600 feet (183 meters). Its bandwidth or pulse repetition frequency is limited, only providing aircraft positions every 5 to 10 seconds. The minimum coverage capability of updated equipment may be as low as 100 feet (30.5 meters). Radar accuracy decreases as aircraft size decreases, making this technology nearly ineffective for small UAV systems operating below 500 feet (very low altitude). Another drawback of non-cooperative technologies (such as radar and visual systems) is that hardware implementation is more complex, and size, weight, and power are greater. State-of-the-art waveform and hardware designs can overcome size, weight, and power constraints.
Geolocation refers to various techniques aimed at “mobility prediction,” i.e., tracking and calculating the location of users or mobile terminals or their distance from them. Early studies considered time-based positioning techniques, such as Time of Arrival (ToA), Time of Arrival Difference (DoA), and Enhanced Observed Time Difference. All these techniques are accurate in line-of-sight scenarios, but in urban environments, many multipath components (MPC) in non-line-of-sight conditions can lead to performance degradation.
Research has proposed map-based positioning and tracking technologies based on received signal strength indication for GSM/3G networks. This technology measures the wireless signal attenuation, assuming the signal propagates in free space using omnidirectional antennas. This method has a well-known triangulation problem, where the accuracy of estimating the position is highly dependent on the number of measurements and the antenna layout. The study also proposed an enhanced time-of-flight tracking technology, which utilizes geographic information system map data and a predicted motion model to generate a series of alternative paths or shadow paths to improve the limitations of maps. The large available bandwidth in 5G can enhance time-delay resolution, thereby improving the positioning accuracy based on flight time methods. Another study proposed a cascade solution based on lossless Kalman filtering for joint ToA and DoA.
To address navigation problems in indoor areas where GNSS signals are weak or obstructed, researchers have proposed using Light Detection and Ranging (LiDAR). LiDAR will operate across the infrared to ultraviolet spectrum range, capable of measuring the distance, angle, speed, vibration, attitude, and even shape of illuminated targets. LiDAR uses emitters to send a known angle laser beam to targets and calculates the distance to the target by measuring the laser return echo to the photodetector sensor. The military has used LiDAR for years, and with design advancements, it is gradually becoming applicable for commercial use. Some researchers have proposed a LiDAR algorithm for navigation in robotic systems.
Dead reckoning is a navigation method used in signal blind spots, calculated based on previous positions, known and estimated speeds during operation, and headings at each speed to determine the current position of the UAV system. Some studies have used a Nonlinear Observer (NLO) and an External Kalman Filter (XKF) in experiments, where both estimators use the same Inertial Measurement Unit (IMU) sensor set (accelerometers, gyroscopes, and rate gyros), a camera, and an altimeter. The experiments employed a machine vision system and used optical flow to compute the UAV’s body-fixed linear velocity. The results indicated that compensating for additional deviations can reduce the NLO positioning error, while the XKF can further reduce errors by providing better speed estimates.
Another tool for UAV system navigation is the radar altimeter, which is usually not accurate enough. Using Kalman filtering to obtain optimal distance estimates is one way to improve altimeter performance.
The best way to address the challenges in UAV navigation and surveillance systems may be to combine several of the above methods. Researchers have integrated inertial navigation systems, GNSS, and low-cost LiDAR to generate high-quality and dense point cloud data with an accuracy of 1 meter. The combination of new radar navigation with GNSS and ADS-B is a promising solution for future UAV system surveillance and navigation in 5G.
6 UAV-to-UAV Communication (UUC)
UAV-to-UAV communication is a subclass of air-to-air communication, bringing new challenges and opportunities. Air-to-air communication links are crucial for assessing the availability of control and non-payload communication links in any relay communication using multiple UAVs. The low-altitude flight of UAVs means they are close to ground objects, leading to more multipath components (MPC), which can degrade the availability of control and non-payload communication links if not compensated for. Research indicates that the Rician fading model is most suitable for UAV-to-UAV communication links, as this model has a dominant line-of-sight path and multiple non-dominant non-line-of-sight paths. Researchers have studied broadband channel models for airport parking and taxi environments, takeoff and landing scenarios, and in-flight scenarios. Additionally, some researchers used air-to-air communication models to assess the rate performance of multi-user (MU)-MIMO configurations and proposed a mathematical framework for multi-UAV communication in the millimeter-wave band with a central hub.High relative speeds between UAVs result in significant Doppler shifts, necessitating further in-depth research, such as appropriate waveform design for UAV-to-air communication links. Such waveform design studies should consider all the challenges of air-to-air communication links, such as multi-user interference (MUI), long distances, channel distortion, and high speeds.
Some UAVs flying at very low altitudes can benefit from occasionally accessing passing ground cellular networks, especially short-range 5G signals. Researchers have proposed a new concept of a three-dimensional cellular network with polyhedral shapes, which can effectively integrate UAV base stations and UAV users with cellular connections. Optimization algorithms indicate a significant reduction in latency, and the 3D UAV-enabled cellular network shows improved spectral efficiency compared to traditional networks based on signal-to-interference-plus-noise ratio (SINR). For future 5G millimeter-wave signals, the limitation of short propagation distances can be resolved by utilizing airborne relay node base stations. Figure 5 illustrates the future view of UAV-to-UAV communication, considering all the challenges mentioned above.
7 Flight Hardware Architecture and Development Trends
As the onboard systems market develops, the design and manufacturing costs of commercial and military UAVs are lower, and design cycles are shorter. This means that flight hardware will adopt Commercial Off-The-Shelf (COTS) principles. Along with the COTS trend, a combination of software-defined radio platforms based on FPGA and modular and model-based system design platforms (such as GNU Radio) is emerging.
The design of UAV communication hardware is the result of multiple related trade-offs, generally influenced by the following optimization criteria:
· SWaP-C Factors
· Instantaneous Bandwidth
· Distance (closely related to available RF power, receiver sensitivity, and antenna gain)
· Link Reliability, Integrity, and Continuity
Although not all criteria directly apply to control and non-payload communication (e.g., instantaneous bandwidth), these criteria are important to overall system design since the same hardware may also have payload processing capabilities.
Figure 6 illustrates an example of a flexible programmable communication system platform based on small form factor hardware modules. The choice of system architecture has a significant impact on control and non-payload communication. By using small modules, system designers can choose the required amount of hardware redundancy, thereby enhancing the reliability of control and non-payload communication links. Programmability and reconfigurability can achieve tight integration of control and non-payload communication systems with the actual systems to be controlled (such as flight controllers, navigation systems, or surveillance systems) on the same hardware platform (providing the appropriate amount of hardware redundancy). For example, dedicated onboard processing capabilities (such as arrival angle detection for navigation or image feature extraction and compression) can be placed in the same FPGA structure running control and non-payload communication modems, reducing latency and eliminating a series of potential sources of error caused by interfaces and cable connections between different hardware modules.
Advanced antenna systems can bring many advantages. For example, using multiple antenna diversity can mitigate the effects of fading, thereby enhancing link reliability. A more advanced example is using antenna arrays to form controllable beams, achieving higher antenna gains in directional adjustments compared to omnidirectional antennas. Furthermore, both transmitters and receivers using controllable beams can reduce the likelihood of radio activity being detected and interfered with, which may be crucial in certain applications. However, employing advanced antennas also brings system-level challenges, such as antenna array calibration, and beam adjustment requires position and attitude information.
8 Conclusion
This article briefly describes the UAV command/control (C2) (or control and non-payload communication) links for space and ground systems and discusses how to integrate UAVs into 5G using ground and satellite links, summarizing existing standards and research findings, and describing how satellite links may be used for these critical links. The article subsequently discusses the future potential control and non-payload communication technologies for 5G UAVs, emphasizing broadband millimeter-wave systems for short-range command. To achieve the extremely high reliability required for command links, multiple integrated navigation technologies must be employed. Finally, hardware challenges are discussed, which can be addressed using state-of-the-art hardware architectures utilizing multiple high-performance software radios and phased array antennas.
