Green UAV communications for 6G: A survey

Xu Jiang, Min Sheng, Nan Zhao, Chengwen Xing, Weidang Lu, Xianbin Wang
2021 Chinese Journal of Aeronautics  
Unmanned Aerial Vehicles (UAVs) have received a wide range of attention for military and commercial applications. Enhanced with communication capability, UAVs are considered to play important roles in the Sixth Generation (6G) networks due to their low cost and flexible deployment. 6G is supposed to be an all-coverage network to provide ubiquitous connections for space, air, ground and underwater. UAVs are able to provide air-borne wireless coverage flexibly, serving as aerial base stations for
more » ... ground users, as relays to connect isolated nodes, or as mobile users in cellular networks. However, the onboard energy of small UAVs is extremely limited. Thus, UAVs can be only deployed to establish wireless links temporarily. Prolonging the lifetime and developing green UAV communication with low power consumption becomes a critical challenge. In this article, a comprehensive survey on green UAV communications for 6G is carried out. Specifically, the typical UAVs and their energy consumption models are introduced. Then, the typical trends of green UAV communications are provided. In addition, the typical applications of UAVs and their green designs are discussed. Finally, several promising techniques and open research issues are also pointed out. ·2 · Chinese Journal of Aeronautics security 1 . To achieve ubiquitous coverage, satellite communication, Unmanned Aerial Vehicle (UAV) communication, and maritime communication will be integrated into terrestrial cellular networks to provide seamless network access 2 . Due to the characteristics of low cost, Line-of-Sight (LoS) air-ground channel and flexible deployment, UAV communication network is deemed as a promising solution to extend the coverage of existing networks in the harsh environment and improve the Quality-of-Service (QoS) in hotspot areas. UAVs, also known as drones, have been employed in various applications for almost one hundred years. Since UAVs can be controlled remotely and do not need pilots onboard, they are extensively used in military applications such as remote surveillance and armed attack to save lives of pilots. In recent years, commercial UAVs are extensively used due to the advancement of manufacturing technology and reduced cost. Nowadays, UAVs have been used for reconnaissance and surveillance, public safety, transport management, search and rescue, data collection, and so on. Benefiting from the miniaturization of communication devices, as well as the flexible deployment and low cost of UAVs, UAV-assisted communication has become a promising technique for the future 6G networks. As a new member of the 6G networks, UAV is significantly different from the conventional terrestrial communication nodes. Compared to the terrestrial systems, the new characteristics of the UAV-assisted communication systems are summarized as follows: (1) High altitude. UAVs usually have a higher altitude than the conventional base stations and mobile users. Usually, the wireless link between the ground node and the UAV has no obstacles. Thus, the air-ground channel experiences less scattering and has less path loss than the conventional terrestrial channel. The high possibility of LoS channels has both pros and cons on the air-ground communications. On one hand, the LoS-dominant channels bring more reliability and less path loss in air-ground transmissions than the Non-Line-of-Sight (NLoS) terrestrial communications. On the other hand, the LoS channels produce severe interference to other coexisting nodes in a wireless network. Therefore, the position of UAV in the Three-Dimensional (3D) space needs to be investigated to exploit the LoS-dominant air-ground channels. (2) High mobility. In the conventional terrestrial communications, the nodes are usually in fixed locations. For UAV communications, UAVs can be remotely controlled to fly with high speed in the 3D space. As a result, the UAV can be flexibly deployed to establish wireless links. This characteristic is more useful for quick response in emergency situations, such as disaster rescue and military action. Besides, the UAV's mobility can also be exploited to move close to target user to achieve better channel gain and avoid obstacles. Thus, the trajectory of the UAV can be designed to achieve better communication performance. (3) Limited energy. Different from the terrestrial communication infrastructures that have sufficient power supplies, small UAVs only have limited onboard energy due to their size and weight constraints. In addition, UAVs need to provide energy for communication and propulsion simultaneously. The propulsion energy consumption for maintaining the UAV aloft and supporting its mobility is much higher than the communication-related energy consumption. Therefore, the endurance time for a small UAV is limited. As a result, although the UAV is suitable for quick deployment in harsh environment, energy-efficient design is crucial to prolong its lifespan. As pointed out in Ref. 1 , the future 6G wireless network is supposed to be a space-air-ground-underwater network which provides ubiquitous coverage. On one hand, UAVs can work as communication platforms in the air to serve the ground users due to its airborne nature and flexible deployment. On the other hand, UAVs can also work as relays to connect the ground users and the spacecrafts with LoS channels. As a result, UAV-based communication is supposed to play an important role in the 6G networks. However, energy consumption is a critical issue in wireless networks, especially for the long-range communication which has high path loss. Besides, small-size UAVs are known to have limited on-board energy. Therefore, green UAV communication is an important issue and has aroused plenty of research attentions recently. Since UAVs have the advantages of low cost, high mobility, flexible deployment and clear LoS air-ground communication links, UAV communications have aroused extensively attentions in both industry and academia recently. In 2017, a study was conducted by the Third-Generation Partnership Project (3GPP) to investigate the support for UAVs in the Long-Term Evolution (LTE) cellular networks 3 . In 2018, the Civil Aviation Authority of China (CAAC) released a test report for the flight safety of cellular-connected low-altitude UAVs 4 . In 2020, a field test of UAV-enabled 5G aerial base station was released by China Mobile. It is reported in Ref. 5 that the tethered UAV base station can cover a radius of 6.5 km at the altitude of 200 m. Due to the great potential of integrating UAVs into 6G networks, various applications of UAVs have been investigated in literature, including UAVs as aerial users 6-10 , UAVs as aerial base stations 11-15 , UAVs as relays 16-21 , UAV-enabled data collecting 22-25 , UAV-assisted data offloading 26-29 , etc. Although UAVs have a wide range of applications in future wireless networks, its energy budget is limited due to the reliance on onboard battery 30 . It is shown that there exist throughput-energy trade-off and delay-energy trade-off in UAV-enabled wireless networks. To prolong the lifetime of UAVs in wireless networks, various energy-saving Chinese Journal of Aeronautics · 15 · UAV-assisted backscatter communication Backscatter communication is based on reflecting the induced wireless signals, which requires in principle only a switch. Energy consuming signal processing devices such as mixers, amplifiers or filters are not needed 119 . Thus, backscatter communication has ultralow power consumption and low device complexity. Although backscatter communication was initially proposed for short range communication, recent advances have shown the potential of achieving communication range for hundreds of meters or even kilometers. Thus, UAV-assisted backscatter communication is becoming a promising technique for ultralow power communication recently. In Ref. 120 , the UAV was employed as a carrier transmitter to assist the backscatter communication. If there is no direct link between the backscatter device and the receiver, the UAV can also work as a data relay 120 . The UAV can work as a receiver to collect data in back scatter wireless sensor networks, where the UAV trajectory and sensor activation decisions can be jointly optimized to enhance the reliability of backscatter communication 121 . The energy efficiency of UAV-assisted backscatter communication for data collection was investigated in Ref. 122 and Ref. 123 , which show that by employing the mobility of UAVs, the energy efficiency of UAV-assisted backscatter communication can be significantly improved. The above-mentioned research results show that employing UAVs is a promising solution to achieve green backscatter communication.
doi:10.1016/j.cja.2021.04.025 fatcat:t5yhxsloefc23b77larlmbxjm4