Wireless Communications with Unmanned Aerial Vehicles: Opportunities and Challenges
The paper authored by Yong Zeng, Rui Zhang, and Teng Joon Lim provides an in-depth analysis of the utilization of unmanned aerial vehicles (UAVs) for wireless communications. This overview covers various facets including the basic networking architecture, channel characteristics, critical design considerations, and key performance enhancing techniques enabled by UAV controlled mobility.
Introduction
UAVs, also known as drones or remotely piloted aircrafts, have gained traction in both military and civilian applications. The continuous reduction in costs and miniaturization of devices has extended UAV applications into civilian domains, such as weather monitoring, forest fire detection, traffic control, and communication relaying. The focus of this paper is UAV-aided wireless communications, which provides on-demand, flexible, and swift deployment for wireless connectivity, largely benefitted by low-altitude line-of-sight (LoS) links.
Communication Architectures and Channels
The paper delineates two primary communication links in UAV-aided systems: Control and Non-Payload Communications (CNPC) and Data Links.
Control and Non-Payload Communications Link
CNPC links are crucial for UAV operational safety, requiring highly reliable, low-latency, and secure two-way communications to support real-time control, collision avoidance, and aircraft status reporting. Operating typically in the protected L-band (960-977MHz) and C-band (5030-5091MHz), CNPC links must withstand stringent security protocols to prevent unauthorized control—termed "ghost control."
Data Link
Data links facilitate mission-related communications for ground terminals, with bandwidth ranging from several kbps to dozens of Gbps, depending on the application. UAVs may reuse existing spectrum bands or employ dedicated spectrum such as mmWave bands for high-capacity UAV-UAV wireless backhaul.
Channel Characteristics
The UAV-aided communication framework involves UAV-ground and UAV-UAV channels with distinct characteristics.
UAV-Ground Channel
UAV-ground channels resemble air-ground channels but with added complexity due to varied operational environments. LoS links are predominant, but blockages from obstacles cause multi-path components and airframe shadowing. The stochastic Rician fading model is often employed to characterize these channels, with Rician factors influenced by surroundings and frequency.
UAV-UAV Channel
Dominated by LoS components, UAV-UAV channels also face high Doppler shifts due to relative velocities. Potential utilization of mmWave communications for high-capacity UAV backhaul may be impeded by Doppler-induced challenges, necessitating further validation.
Design Considerations
The paper presents three main design considerations for UAV-aided communication systems: path planning, energy-aware deployment and operation, and MIMO technology.
UAV Deployment and Path Planning
Effective UAV path planning tailors the UAV trajectory to optimize communication distance and minimize energy consumption, typically modeled using mixed integer linear programming (MILP). The choice of rotary-wing or fixed-wing UAVs and their specific deployment strategies must align with application scenarios.
Energy-Aware Deployment and Operation
Energy constraints of UAVs necessitate energy-aware deployment strategies for timely replenishment and energy-efficient operation mechanisms. Energy-efficient UAV mobility and communication techniques are critical, demanding further investigation into optimizing energy usage concerning UAV speed, altitude, and transmission strategies.
MIMO for UAV-Aided Communications
While MIMO technology offers substantial gains in terrestrial systems, its application in UAVs is limited by channel characteristics and SWAP constraints. Promising results indicate high spatial multiplexing gain may be achievable in LoS conditions with precise antenna arrangements. Multi-user MIMO and mmWave communications are potential enablers, though practical challenges like beam alignment remain.
UAV-Controlled Mobility for Enhanced Performance
Two key techniques exploit UAV-controlled mobility: mobile relaying and D2D-enhanced information dissemination.
UAV-Enabled Mobile Relaying
Mobile relaying outperforms static relaying by dynamically adjusting UAV positions to optimize link distances, enhancing spectrum efficiency and throughput for delay-tolerant applications. Numerical results demonstrate significant gains under varying UAV velocities and tolerable delays.
D2D-Enhanced UAV Information Dissemination
Utilizing D2D communications for information sharing among ground nodes reduces UAV retransmissions and flying time. A two-phase protocol involves initial UAV broadcast followed by ground node D2D exchanges, improving dissemination efficiency.
Conclusion
The paper provides a comprehensive overview of UAV-aided wireless communications, addressing its basic architecture, channel characteristics, and design considerations. Techniques leveraging UAV-controlled mobility, such as mobile relaying and D2D-enhanced dissemination, present promising avenues for performance enhancement. The challenges and opportunities discussed form a foundation for future research in integrating UAVs into advanced wireless communication systems.