Introduction to UAV 5G Aerial Networking Technology Based on Millimeter Wave IAB
Compared to traditional fixed base station cellular communication systems, low-altitude UAV communication features low cost, strong adaptability, flexible deployment, and wide coverage. By equipping UAV platforms with lightweight integrated base stations, an aerial mobile communication platform is constructed. Thanks to its mobility and line-of-sight transmission advantages, it can significantly expand network coverage, improve system capacity in hotspot areas, and achieve true accompanying coverage and on-demand coverage. In some special scenarios, such as disaster relief emergency communication and defense early warning in complex military tasks, the aerial mobile communication platform based on UAVs can adapt to various complex communication environments and demanding communication needs by adjusting the UAV’s posture and flight trajectory.In April 2022, the Office of the Under Secretary of Defense for Research and Engineering (OUSD R&E) in the United States released a Request for Solutions (RFS) for the “Innovative Post B5G (IB5G) Program,” aimed at leveraging various novel communication technologies in the post-5G era to enhance operational capabilities, allowing the U.S. military to dominate future network battlefields. One important research direction of this project is the integration of ground and airborne 5G tactical networking, proposing the realization of 5G self-organizing mobile communication networks based on distributed MIMO and IAB (Integrated Access and Backhaul) technologies, integrating D2D (Device-to-Device) or Sidelink capabilities, providing more new architectural options and system design schemes for future tactical network systems.Self-organizing distributed tactical network with MIMO-capable nodesIn UAV mobile communication, the long-standing backhaul bottleneck problem needs to be addressed. The combination of UAV communication and millimeter waves offers unique advantages such as wide bandwidth, short wavelength, spatial sparsity, flexible beamforming, high working altitude, and controllable mobility, applicable in various scenarios. One of these is using millimeter wave communication to solve the wireless backhaul performance bottleneck of UAV base stations. Since both access and backhaul in UAV communication scenarios require wireless transmission, implementing 5G UAV communication based on the millimeter wave IAB architecture has become a potential solution for flexible and large-scale deployment of 5G, and is an important topic for improving network reconfigurability.In June 2022, the 3GPP 5G Rel-17 version standardized IAB enhanced features, and the plan for Rel-18 considers supporting mobile IAB, with typical application scenarios being IAB networking under UAV communication.According to public reports, Huawei successfully completed field tests of 39 GHz 5G millimeter wave integrated access and backhaul (IAB) technology, showing that the integrated access and backhaul technology can significantly improve the coverage and capacity of millimeter waves. In the tests, 5G terminal devices were in motion, accessing the main base station through IAB relay stations, with a throughput exceeding 650 Mbps and end-to-end latency reaching 1.6 milliseconds.Dr. Tong Wen, Chief Technology Officer of Huawei Wireless Networks, stated: “Millimeter wave IAB technology is a key innovative technology of the 5G new air interface, capable of extending service coverage beyond fiber infrastructure. This will be another important tool for operators to optimize network investments based on business needs.”Huawei IAB field test imagesThis article first introduces the network architecture and standardization progress of IAB technology, and then summarizes the key technologies for implementing UAV communication based on millimeter wave IAB.1. Introduction to IAB TechnologyIAB is defined as sharing the same wireless channel for access (base station to mobile device) and backhaul (base station to base station or base station to core network). As shown in the figure below, base station gNB #1 uses optical fiber for wired backhaul, while gNB #2 and gNB #3 backhaul through base station gNB #1. gNB #1 uses the same spectrum or wireless channel to serve mobile devices within its coverage area and provides backhaul connections for the other two base stations (gNB #2 and gNB #3). Wireless channel resources can be reused through time division, frequency division, and spatial division. Since the coverage capability of 5G networks is lower than that of 4G networks, increasing the density of access nodes brings many difficulties and challenges for network coverage, construction costs, deployment, and management. IAB technology can reduce the wired backhaul dependency at each access node location and achieve broader, more effective, and low-cost network coverage through simpler deployments.IAB networking schematicIAB channel sharing schematic1. IAB Network ArchitectureIAB aims to utilize the network element functions and interfaces of existing networks to achieve compatibility with existing 5G networks. Therefore, IAB base stations also support interfaces such as Uu, F1, E1, NG, and X2, and include the functions of gNB base stations and Mobile-Termination (MT) functions.The architecture of the IAB base station is shown in the figure below, which includes one IAB-donor and multiple IAB-nodes. The IAB-donor is a base station with IAB functionality, which includes CU-CP, CU-UP, and DU, connected to the core network via optical fiber, providing access services to UE and wireless backhaul links to IAB-nodes. Each IAB-node carries two NR functions: one MT, for wireless backhaul connection with upstream IAB-node or IAB-donor; and one DU, for downstream MT access to UE or other IAB-nodes. The DU connects to the CU in the IAB-donor and operates the NR F1 interface via wireless backhaul links, thus the access stacks of IAB-node and IAB-donor serve both F1 and Uu interfaces simultaneously.IAB architectureThis design achieves the functional split of the radio protocol stack, with the control layer and upper layers located in the IAB-donor CU, and the lower layers located in the IAB-node DU, with the split occurring at the RLC (Radio Link Control) layer. Therefore, RRC (Radio Resource Control), SDAP (Service Data Adaptation Protocol), and PDCP (Packet Data Convergence Protocol) layers are located in the CU, while RLC, MAC (Medium Access Control), and PHY (Physical) are located in the DU. Another adaptation layer manages routing above the RLC, thus achieving end-to-end connectivity between DU and CU.IAB supports both in-band (access and backhaul sharing the same frequency band) and out-of-band (access and backhaul using separate frequency bands) methods, with the in-band method being the focus. This makes network design and management more challenging but maximizes spectrum utilization. IAB supports Sub-6GHz and above 6GHz frequencies, and supports both Standalone (SA) and Non-Standalone (NSA) deployments.In terms of topology, the 3GPP Rel.16 standard defines two structures: Spanning Tree (ST) and Directed Acyclic Graph (DAG), as shown in the figure below. For the ST structure, each IAB-node has only one parent node, which can be either an IAB-node or IAB-donor; for the DAG structure, IAB-nodes can connect to multiple upper nodes.IAB network topology structure2. Evolution of IAB StandardsIn fact, the IAB technology was not newly proposed for 5G; the LTE specifications already included Relay, but Relay only supports single hops, and the relay node is associated with a fixed parent base station, with rigid distinctions between access and backhaul resources, resulting in low overall flexibility and thus not widely deployed. In March 2017, the 3GPP RAN working group approved the research project on IAB for NR proposed jointly by AT&T, Qualcomm, Samsung, and KDDI at the 75th RAN plenary meeting, initiating the evaluation and standardization of IAB technology.The 3GPP Rel-16 version standardized the 5G wireless backhaul solution by defining architecture, topology, in-band & out-of-band relay methods to ensure basic networking issues such as network stability, robustness, and flexible coverage expansion. It addressed the duplex limitations and co-frequency interference issues of IAB relay nodes by defining the spectrum reuse methods for access-backhaul links, and resolved the short coverage and short-term blockage issues of millimeter waves by defining the wireless access layer handover mechanism between relay nodes, providing a foundation for the application of IAB technology.The 3GPP Rel-17 version further standardized the enhanced features of IAB, mainly including: allowing frequent topological changes in network deployment, enhancing the robustness of migration between IAB master nodes, providing more stable backhauls, achieving finer topological management and load balancing; enhancing scheduling, congestion, and flow control functions, improving end-to-end transmission performance of the network; duplex enhancement based on spatial division multiplexing and frequency division multiplexing, IAB node synchronized transmission and reception, improving spectrum efficiency; reducing service interruption incidents caused by backhaul link beam failure and IAB node handover failure recovery, thus improving network performance.The 3GPP Rel-18 plan supports mobile IAB, with typical application scenarios being IAB networking under UAV communication, focusing on standardizing and enhancing topological management mechanisms under the mobility of IAB nodes, and the dual dynamics mobility management of IAB and mobile terminals, while also researching interference issues caused by the mobility of IAB nodes. Future standardized versions may further consider the combination of IAB nodes and intelligent super surfaces, utilizing artificial intelligence, machine learning, and other methods for topological management optimization and transmission performance optimization, introducing network coding to improve overall system spectrum efficiency and throughput.2. Key Technologies for UAV Communication Based on Millimeter Wave IAB1. Topological ManagementThe following figure is a typical schematic of a multilayer UAV cellular network based on the millimeter wave IAB architecture. The first layer includes the ground gNB (IAB-donor), responsible for providing access links to ground UE while also providing backhaul links to UAVs; the second layer includes multiple UAVs, which can serve as IAB-nodes to provide access services to both ground and aerial UEs, or act as aerial UEs themselves. The ground and aerial devices form an IAB ST or DAG network topology, creating a multilayer heterogeneous network. The UAV communication networking based on IAB can extend network coverage and enhance network capacity on top of the ground NR infrastructure, where optimizing complex topological establishment and management processes is fundamental to efficient network operation.UAV IAB communication networking architectureThe end-to-end performance of the entire network in the IAB architecture largely depends on the number of hops between the IAB-donor and the terminal relay, the number of relays supported by the IAB-donor, and the procedures for network formation, routing selection, and resource allocation. Besides the signaling for initial setup, the CU needs to understand the traffic load and backhaul link quality related to transmission and dynamically update the associations between IAB nodes based on this information, ensuring service continuity (preventing backhaul link loss) and load balancing (avoiding congestion). In the DAG networking architecture, the CU also needs to update and manage redundant routing.In the UAV IAB networking scenario, to ensure backhaul associations continuously point to the designated gNB, UAVs can continuously adjust their positions, and as users can dynamically change network association points, the complexity of topological management is exacerbated. Furthermore, the entire network exists in 3D space, requiring consideration of the heterogeneous 3D layout of UAV-ground base stations, relying on precise air-to-ground NR propagation models, realistic UE deployment, and mobility models, designing suitable adaptive, flexible, and effective optimization algorithms to calculate the deployment positions of UAV nodes and manage the overall network topology, ensuring consistently high performance.2. Resource ManagementIn 5G wireless networks, user traffic comprises various types, including text, voice, video, etc., with different service quality requirements for each type, coupled with the scarcity of spectrum resources, making wireless resource allocation and management a focal point in wireless communication research. The IAB architecture shares wireless channels for access and backhaul, and to avoid the capacity of wireless backhaul becoming a bottleneck for the entire network, the wireless resource allocation scheme for both access and backhaul links needs to be considered holistically.In UAV communication scenarios, it is essential to achieve reasonable allocation and sharing of different nodes in space, time, frequency, and other limited resources. Given the 3D networking characteristics, reasonable allocation of spatial beams can be a focal point. The design of the MAC layer is a crucial foundation for implementing wireless resource allocation and management. Currently, IEEE has provided detailed MAC layer designs supporting millimeter wave bands above 45 GHz, but they are more suited for low mobility scenarios. Current research supporting millimeter wave UAV networks is still focused on the PHY layer, requiring studies on MAC layer design schemes suitable for mobility networks, considering characteristics such as dynamic link fluctuations, beam alignment time consumption, and directional transmission modes.Moreover, considering the topological characteristics of UAV 3D scenarios, it is necessary to account for IAB nodes sharing channel measurements and topological routing information, synchronizing the design of hop-by-hop and end-to-end resource allocation mechanisms, alleviating potential congestion that may arise from intermediate nodes with poor transmission conditions, maximizing network throughput.3. Interference ManagementUAV communication based on millimeter wave IAB technology is influenced by factors such as millimeter wave characteristics, UAV mobility, complex air-to-ground transmission environments, and the IAB architecture, making interference management an important issue to resolve. First, UAVs themselves exist in complex electromagnetic environments, requiring communication devices to overcome electromagnetic interference from the UAVs; second, millimeter wave signals are very sensitive to small-scale changes in the environment, with UAV aerodynamic interference and body sway significantly affecting channel characteristics. Due to the higher frequency, the Doppler shift caused by movement will be more pronounced compared to frequencies below 6 GHz, leading to severe carrier interference and signal fading; third, shared channels for access and backhaul, combined with the presence of wireless backhaul hops, result in a far greater number of transmission links in the network compared to past wireless networks, thus introducing serious interference issues.Specifically, in the time domain, reasonable frame designs are needed to ensure efficient transmission and avoid conflicts during multipath and multi-hop routing generation, employing flexible scheduling algorithms to double spectrum efficiency and reduce network latency to handle interference. In the frequency domain, global spectrum management and real-time allocation can be optimized based on the 3D network topology and time-varying interference, enhancing network throughput while avoiding interference. In the spatial domain, although millimeter wave beamforming technology has brought high spectrum efficiency and anti-interference capabilities, rapid beam tracking and resource reallocation must be designed to address the rapid changes in topology and the number of adjacent UAVs. Additionally, designing optimal allocation strategies considering the collisions and interference caused by beam multiplexing in the network, along with multi-layer mesh architectures, leveraging node positions, sub-channel alignment, and transmission power control, are also methods to consider.3. ConclusionIAB technology is an emerging technology considered by 3GPP to address the densification of cellular networks and expand coverage. UAV communication based on millimeter wave IAB technology is expected to be a promising solution due to higher data rates on access and backhaul links, lower interference, and more flexible service dynamics. 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