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Cellular-Connected UAV: Potentials, Challenges and Promising Technologies

Yong Zeng, Jiangbin Lyu, Rui Zhang

arXiv:1804.02217v1cs.IT

TL;DR

UAVs need high-rate, low-latency, and ultra-reliable links, but cellular-connected UAV communication introduces characteristics and requirements unlike terrestrial-only cellular systems. The paper surveys this paradigm, its benefits and design challenges, proposes enabling technologies for 3D heterogeneous networks, and uses simulations to corroborate the discussion. It highlights 3D beamforming and cellular support as promising directions for coexistence of aerial and ground users.

  • Problem

    Large-scale UAV use requires high-rate, low-latency, and ultra-reliable wireless communications, while aerial users create distinct communication and spectrum requirements compared with terrestrial users.

  • Method

    The article provides an overview of cellular-connected UAV communication, discusses requirements and design considerations, introduces promising technologies, and presents numerical results.

  • Results

    The paper identifies 3D beamforming and other cellular technologies as promising for future heterogeneous networks with coexisting aerial and terrestrial users, supported by simulation results.

  • Takeaways & Limitations

    Cellular-connected UAV communication is presented as a promising basis for integrating UAVs into future 3D heterogeneous wireless networks.

Abstract

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Enabling high-rate, low-latency and ultra-reliable wireless communications between unmanned aerial vehicles (UAVs) and their associated ground pilots/users is of paramount importance to realize their large-scale usage in the future. To achieve this goal, cellular-connected UAV, whereby UAVs for various applications are integrated into the cellular network as new aerial users, is a promising technology that has drawn significant attention recently. Compared to the conventional cellular communication with terrestrial users, cellular-connected UAV communication possesses substantially different characteristics that bring in new research challenges as well as opportunities. In this article, we provide an overview of this emerging technology, by firstly discussing its potential benefits, unique communication and spectrum requirements, as well as new design considerations. We then introduce promising technologies to enable the future generation of three-dimensional (3D) heterogeneous wireless networks with coexisting aerial and ground users. Last, we present simulation results to corroborate our discussions and highlight key directions for future research.

I. INTRODUCTION

Cellular-connected UAVs integrate aerial vehicles into cellular networks to address the limited, unreliable, and insecure links of conventional point-to-point UAV communications. The article surveys their benefits, requirements, design considerations, enabling technologies, simulations, and research directions.

  • Cellular-connected UAV paradigm: Cellular-connected UAVs integrate application-specific UAVs into existing and future cellular networks as aerial user equipments.The paper contrasts this paradigm with traditional direct ground-to-UAV point-to-point links.
  • Potential benefits: Ubiquitous cellular access can support remote UAV command and control with essentially unlimited operation range and connectivity to distant stakeholders.Examples include end users and air traffic controllers, with live video sent to audiences worldwide.
  • Potential benefits: Advanced cellular technologies and authentication mechanisms may improve UAV communication reliability, security, and performance over simple direct links.The passage frames these improvements as potential advantages rather than established guarantees.
  • Potential benefits: Cellular-connected UAVs can support large-scale air-traffic monitoring and management, including authorized remote takeover to avoid foreseen safety threats.The takeover capability is conditioned on appropriate regulations and legislation.
  • Potential benefits: Cellular signals can complement GPS navigation when satellite signals are disrupted by blockage or adverse weather conditions.The paper also mentions differential GPS as another approach to robust navigation.
  • Potential benefits: Reusing millions of deployed cellular base stations can reduce infrastructure deployment costs and bundle UAV control with payload communications.The paper presents this reuse as potentially beneficial to both cellular and UAV operators.
  • Article scope: The article surveys unique communication and spectrum requirements, new design considerations, promising technologies, numerical results, and future research directions for 3D heterogeneous networks.The envisioned networks contain coexisting terrestrial and aerial users.

II. UNIQUE COMMUNICATION AND SPECTRUM REQUIREMENT

With UAVs treated as aerial cellular user equipments, cellular-connected UAV systems have communication and spectrum requirements that differ substantially from terrestrial-only cellular communication.

  • System distinction: Cellular-connected UAVs introduce substantially different communication and spectrum requirements because UAVs operate as new aerial user equipments.The comparison is with conventional cellular communication serving terrestrial users only.

A. Basic Communication Requirement

UAV communication requirements divide into control and non-payload communication for safe flight operation, and payload communication for mission-related data transmission. These categories differ sharply in rate and reliability needs.

  • CNPC: Control and non-payload communication (CNPC) is two-way communication between the aircraft and ground control station or remote pilot for safe, reliable, effective flight operation.Typical messages include telemetry, flight commands, navigation aids, sense-and-avoid information, and air-traffic-control relaying.
  • CNPC: CNPC usually requires hundreds of Kbps, while demanding ultra-reliability, high security, and low latency.These requirements reflect its role in flight control and safety.
  • Payload communication: Payload communication carries mission-related information such as real-time video, images, and relaying data between UAVs and ground users.Aerial videography requires timely transmission of captured video to end users.
  • Payload communication: Full high-definition video requires several Mbps, whereas 4K video requires more than 30 Mbps.Payload communication generally has a much higher data-rate requirement than CNPC.
  • Airborne communication: UAV-enabled airborne communication can require up to dozens of Gbps for wireless backhauling.This rate exceeds the requirements cited for ordinary UAV payload video transmission.

B. Spectrum for Cellular-Connected UAV

Cellular-connected UAVs can support line-of-sight and beyond-line-of-sight operations without satellite dependence, but spectrum protection, bandwidth, and aerial–ground interference require dedicated design choices.

  • Cellular support: Cellular-connected UAVs have the potential to enable both LoS and BLoS operations without relying on satellites.The proposed cellular approach is linked to the need for reliable UAV command and control.
  • Spectrum requirements: CNPC links must operate over protected aviation spectrum, but most existing UAS used direct UAV-to-ground communication over unlicensed spectrum.The contrast identifies a regulatory and deployment gap for conventional UAS communications.
  • Spectrum requirements: Supporting forecast UAV operations requires a maximum of 34 MHz terrestrial spectrum and 56 MHz satellite spectrum for both LoS and BLoS operations.The cited requirement comes from ITU studies.
  • Spectrum requirements: The C-band spectrum at 5030–5091 MHz was made available for UAV CNPC at WRC-12.This allocation addresses the stated CNPC spectrum requirement.
  • Cellular support: One approach licenses C-band spectrum exclusively to cellular operators for CNPC, while payload communications share LTE or future 5G spectrum with ground UEs.Shared-spectrum operation requires proper control of interference between aerial and ground users.

III. NEW DESIGN CONSIDERATIONS

Cellular-connected UAVs introduce distinct coverage, interference, traffic, and spectrum requirements because aerial users operate at higher altitudes and interact differently with cellular networks. The section identifies design considerations for supporting coexistence between aerial and terrestrial users.

  • Cellular-connected UAVs require a paradigm shift in cellular and UAV communication design.
  • 3D Coverage: UAVs require 3D coverage because their altitude can exceed BS antenna height, whereas existing antennas are generally designed for downward ground coverage.
  • Potential solutions include reshaped BS coverage, high-altitude UAV highways, air-to-air relaying, and separate sub-6 GHz downlink and mmWave uplink bands.
  • Unique Channel Characteristics: Higher UAV altitude creates strong LoS UAV-BS channels, improving desired links but making inter-cell interference more critical.
  • Severe Aerial-Ground Interference: High-altitude UAVs can receive interference from many neighboring BSs, while their uplinks can interfere with multiple non-associated BSs.
  • Asymmetric Uplink/Downlink Traffic Requirement: UAV applications such as video streaming and aerial imaging generally require higher uplink rates than current downlink-oriented cellular networks provide.

IV. PROMISING TECHNOLOGIES

The paper surveys technologies for 3D heterogeneous networks containing aerial and terrestrial users. It emphasizes directional control, coordinated cells, and non-orthogonal access as ways to manage coverage, interference, and spectrum efficiency.

  • Sub-Sector in Elevation Domain: Elevation-domain sub-sectorization partitions each horizontal sector’s 3D coverage volume into regions covered by directional antennas.
  • 3D Beamforming: 3D beamforming adaptively designs antenna beams from UAV location or instantaneous CSI, providing finer azimuth and elevation resolution.
  • 3D Beamforming: Pencil-shaped 3D beams can improve interference mitigation by exploiting elevation-angle separation between aerial and terrestrial users.
  • Multi-Cell Cooperation: Cooperation can provide larger macro-diversity gains for high-altitude UAVs, which may have strong LoS links with more neighboring BSs.
  • Multi-Cell Cooperation: Multi-cell cooperation jointly optimizes resources such as assignment, power, beamforming, and association, or jointly transmits and receives through cooperating BSs.
  • Ground-Aerial NOMA: Ground-aerial NOMA exploits different channel conditions by decoding stronger UAV signals first, subtracting them, and then decoding weaker ground-UE signals.

V. SIMULATION RESULTS AND DISCUSSIONS

The simulations use a 57-cell 3GPP urban-macro deployment to compare fixed-pattern antennas with 3D beamforming. The setup varies array configuration while specifying standardized antenna spacing and beamwidth assumptions.

  • Simulation Setup: Each cell compares a fixed-pattern ULA of size (M, N) = (8, 1) with a 3D-beamforming UPA of size (M, N) = (8, 4).
  • Simulation Setup: Adjacent antenna elements are separated by half a wavelength, and the antenna element pattern follows the 3GPP technical specification.
  • Simulation Setup: The antenna half-power beamwidth is 65° in both azimuth and elevation dimensions.
  • Simulation Setup: The study evaluates fixed-pattern and 3D-beamforming configurations using the parameters summarized in the simulation setup.

A. UAV C&C with Dedicated Channel

Dedicated-channel simulations examine UAV cell association and received SNR across altitudes under fixed antenna patterns and 3D beamforming. Higher altitude changes association behavior, while 3D beamforming improves association and SNR performance.

  • Cell Association: At low altitude with fixed BS patterns, UAVs are most likely associated with nearby cells, but at 200 m they more often associate with distant cells through sidelobes.
  • Cell Association: 3D beamforming keeps UAVs almost surely associated with nearby cells even at 200 m by flexibly focusing signals toward them.
  • SNR Performance: Higher UAV altitude reduces SNR variations under both fixed patterns and 3D beamforming because LoS probability increases.
  • SNR Performance: 10 dB higher 5th percentile SNR is observed at 200 m than at 1.5 m with a fixed BS antenna pattern.
  • SNR Performance: 3D beamforming significantly improves SNR at all UAV altitudes, with a larger improvement at 200 m due to more dominant LoS propagation.

B. Shared Channel by UAV and Ground UE

Sharing cellular channels between aerial and ground UEs creates stronger interference for aerial users, degrading system spectral efficiency as UAV numbers increase. 3D beamforming substantially improves achievable sum rates across UAV populations.

  • Simulation setup: The simulation reuses one channel among 20 UEs, with NUAV = 0, 5, or 10 aerial UEs distributed horizontally within a 1000 m-radius disk.Ground UEs are fixed at 1.5 m altitude, while aerial UE altitudes are uniformly distributed from 1.5 m to 300 m.
  • Performance effects: As the number of aerial UEs increases, overall system spectral efficiency degrades for both fixed antenna patterns and 3D beamforming.This trend is reported from the empirical CDF comparison in Fig. 6.
  • Performance effects: Aerial UEs typically achieve poorer downlink rates than ground UEs because stronger non-associated-BS interference outweighs their stronger associated-BS links.The higher line-of-sight probability of aerial UEs increases interference from non-associated base stations.
  • 3D beamforming: 3D beamforming significantly improves system spectral efficiency for all evaluated UAV numbers, even with the low-complexity MRT scheme.The comparison uses fixed BS antenna patterns as the baseline.
  • 3D beamforming: For NUAV = 10, the 5th-percentile UEs’ sum rate rises from about 30 Mbps with a fixed pattern to more than 68 Mbps with 3D beamforming.The result demonstrates the potential of 3D beamforming in systems combining aerial and ground users.

VI. CONCLUSIONS AND FUTURE DIRECTIONS

The paper surveys cellular-connected UAV communication, covering its requirements, challenges, design considerations, and enabling technologies for 3D networks with aerial and ground users. It highlights trajectory optimization, mmWave communication, and swarm connectivity as future directions supported by simulation results.

  • Overview: Cellular-connected UAVs integrate UAV applications into existing and future cellular networks as new aerial user equipments.The article presents this integration as a promising approach for UAV communications.
  • Overview: The article reviews unique communication and spectrum requirements, new design considerations, and promising technologies for 3D heterogeneous wireless networks.These networks contain both aerial and ground users, and simulations are provided to corroborate the discussion.
  • Quality of Service-Aware Trajectory Design: UAV trajectories can be jointly optimized with communication resource allocation for metrics including spectral efficiency and energy efficiency.Trajectory design may avoid cellular coverage holes and account for propulsion energy consumption.
  • Quality of Service-Aware Trajectory Design: Autonomous UAVs may self-optimize their positions using real-time radio measurements.This is identified as a direction for future cellular-connected UAV systems.
  • Millimeter Wave Cellular-Connected UAV: MmWave communication above 28 GHz offers wide bandwidth for high-rate UAV communications, while UAV altitude and mobility require efficient 3D beamforming.LoS-dominating UAV-BS channels offer favorable conditions, but blockage vulnerability remains a challenge.
  • Cellular-Connected UAV Swarm: For UAV swarms, cellular-assisted U2U communications can connect the aerial network to the cellular core network without directly connecting every UAV to cellular base stations.The large number and close separation of swarm UAVs make individual direct connections challenging and inefficient.
  • Cellular-Connected UAV Swarm: Future research should investigate effective aerial-network topologies and seamless integration of aerial and ground networks.The paper identifies these as open issues for cellular-connected UAV swarms.
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