Radical innovations in technology and exponential increase in demand have been the key enablers for the evolution of mobile networks from the first-generation Total Access Communication Systems (TACS) to the fifth-generation communication systems (5G) which are now being deployed commercially. The Cisco VNI predicts that by 2022, 4G Long Term Evolution (LTE) traffic will constitute 71% of the total mobile connections, with 5G accounting for about 12% ¹ . One of the greatest developments in mobile communications was the transition from analogue (1G) to digital (2G) mobile telephony with the release of Global System for Mobile communications (GSM), giving a taste of data with General Packet Radio Service (GPRS) offering enhanced data rates ranging between 0.4-1 Mbps. Then the significant move towards spread spectrum disrupted 2G leading to what is now known as Universal Mobile Telecommunications Service (UMTS) or 3G, paving the way for mobile broadband with data rates of up to 42 Mbps. 4G was necessitated by the need for both spectral and energy efficiency, proposing scalable bandwidths and data rates of up to 150 Mbps. Innovation in waveform numerology and introduction of several other network functionalities took mobile communications from serving merely mobile phone users to serving everything that connects to the internet. 5G became truly application and user-centric. Interestingly, the switch between consecutive generations from 2G till 5G, lasted roughly 10 years. With 5G now being commercially deployed, we are now starting to think about shaping our strategy for 6G Dang et al. (2020).

Over the past year, a number of researchers have put forward their perspective and vision of the future 6G network. One of the most promising area that most authors have targeted for 6G is the Non-Terrestrial Network (NTN) Giordani and Zorzi (2021); Kota and Giambene (2021). More recently, researcher at Ericsson published their perspective on the use of 6G for UAVs and beyond Mozaffari et al. (2021). All these works present an overview of their vision expanding on how the new and upcoming technologies will form part of the 6G network in supporting UAVs and NTN. Bekkouche et al. (2019) proposed a framework for the management and orchestration of UAVs’ services in a Multi-access Edge Computing (MEC) Network Function Virtualization (NFV) environment by combining the functionalities provided by the management and orchestration framework with the functionalities of a UAV Traffic Management (UTM) system. Shrestha et al. (2021) also presented their perspective on the use of 6G for UAV traffic management. Their work focuses mostly on the UAV operation and its requirements whilst in operation, lacking some technical insights into mobile network technology and the requirements at the Key Performance Indicator (KPI) level.

While the industry remains busy deploying 5G over the terrestrial domain and the academia presents their perspective on the future mobile networks, we focus our research on utilizing 6G to enable the next generation of airborne transport systems. The main contributions of this paper are as follows:

1. A novel use case of Urban Air Mobility (UAM) for the 6th generation of mobile networks.

The rest of the paper is organized as follows: the remaining of Section 1 talks about the evolution of smart mobility in mobile networks leading to the proposition of using mobile networks for Airborne Vehicles (AVs). Section 2 puts forward our vision and architecture for 6G AWN for UAM. In Section 3, we discuss the KPIs for 6G AWN in line with UAM. Then in Section 4, we present some challenges, threats and opportunities for UAM and the next generation of mobile networks. Finally we conclude the paper in Section 5.

In order to achieve true application driven networking for 5G, various use cases and their requirements were first identified, followed by the development of technology. In terms of mobility, connected cars make our roads safer, travels shorter and environment healthier, while addressing the socio-economic benefits of transport. Intelligent mobility addresses end-to-end movement of people and goods while seeking a balance between cost, comfort, speed and convenience against our needs. 6G enabled UAM services will share data behind the scenes to dynamically adjust based on demand and synchronise with other services using network intelligence. We expect 6G will play a leading role in the management of aerial transport, and over the next decade, use cases and applications that will bring more value to our mobility will be defined.

In the following, we discuss some mobile network KPIs and the role they will play in evolving our mobility and subsequently AWNs (Figure 3).

Accessibility: This is a significant network and user side performance indicator that evolves as the network evolves. For 6G networks, where multiple network functionalities at the core will be shared, and user applications will rely on massive network function virtualisation, performance measures that specify whether the user of a service has received successful registration for a specific service or not will be invaluable. The evolution of this KPI will depend on the performance and capabilities of network technologies as they progress towards the next generation. In terms of aerial mobility, evolution and scaling up of current network technologies will be inevitable to accommodate the new set of users. Non-accessibility of the network can have serious consequences, as these aerial vehicles will rely on aerial networks to operate.

Integrity: This specifies the degree to which a service is provided without any unwarranted impairments. Emerging applications for 6G, requiring higher data rates and lower latencies will put much stress on the integrity KPI. In addition to latency and throughput, data security and privacy will become a major part of integrity KPI for 6G. Data integrity is significantly important for applications involving human life on stake. Aerial mobility, including UAVs and PAVs, will make use of AWNs for navigation, taking off and landing manoeuvres. In such circumstances, unwanted delay or a cybersecurity attack can result in catastrophic consequences.

Utilization: This KPI focuses on the consumption of network resources as the application usage grows. This KPI is significant as it evaluates the utilisation of network resources and helps in determining whether any network scale up or scale down operations are required or not. Self-organising networks, as specified in 3GPP TR 22.829 (2019), take into account this KPI to organise virtual resources and configure network architecture accordingly. With a steep increment in network usage by airborne network entities, this KPI will be key for 6G to address the user/application data consumption and give insights to operators/management entities for better decision making.

Retainability: This KPI measures the preservation of Quality of Service (QoS) flow associated with each user application. QoS is vital for users to achieve the quality of experience and network providers to avoid customer churn. Radio and network resources can be scarce; hence monitoring of QoS is vital. For airborne network applications, the transmission requirements for safety and traffic management applications will be extremely strict. Hence, stringent QoS flows will be required to address emerging aerial applications. Nevertheless, this KPI will be vital in determining the success of 5G networks and subsequent 6G airborne networks.

Mobility: Until 5G, mobility involved successful handovers between gNBs and Next Generation Radio Access Networks (NG-RANs) while maintaining the end-to-end link to the network slice. With aNBs, the mobility for 6G will need to cater for handovers in the 3D airspace for instance between the ground and aerial base stations. The success of these handovers depends on many factors including radio network planning, network orchestration and availability of resources. Efficient mobility is important for fast-moving vehicles, and more significantly for AVs. 3GPP TR 36.777 specifies some enhancements in LTE for aerial vehicles where the mobility KPIs further include handover rate, handover failure rate, radio link failure rate, handoff times and ping pong rates. To enhance this KPI, AI will play its role in making handovers in airborne networks predictive and seamless.

Nevertheless, poor radio planning can cause substantial deterioration in all the above KPIs, leading to inadequate network performance and substandard user experience.

There are many challenges on our way to 6G AWN. The enabling technologies for AWN will address these challenges and pave way for airborne networks and astute mobility. In this section, we discuss some of the main challenges and threats followed by a brief summary of some future research directions.

The United Kingdom Civil Aviation Authority (CAA) has developed an elaborate system to control the airspace. The aircraft generally follows the predefined routes called airways that are implemented to connect heavy traffic regions while ensuring adequate radar coverage for controlling traffic. Furthermore, the existing radio network that assists large aircraft instruments and pilots in all operations of a flight is a complex harness of electromagnetic spectrum. Considerable development and testing efforts are required in revising and planning radio networks for massive aerial mobility. The challenge will be to extend this airspace control system to include AWN entities and subsequent aerial vehicles in an official and elaborate manner to ensure safety, resilience and efficiency.

Cybersecurity is also one of the most important issues in the air traffic control system since many functionalities are heavily dependent on communication between the network agents. Potential mechanisms to protect airborne communications from cyber-attacks are required. GPS spoofing, false data injection and similar cyber-attacks are a major threat to the safety of aerial mobility. There are still many unanswered questions and concerns in terms of data and control integrity in addition to regulatory and compliance issues that will be major obstacles in the deployment and adoption of aerial mobility. Significant research is needed to address security and privacy concerns.

As evident from 3GPP studies and field trials, problems like interference and energy efficiency are also the most challenging aspects of aerial UEs. At the same time, operators and service providers will ponder upon RF planning in the 3D airspace. Proper harnessing of the electromagnetic spectrum will be important in planning the airborne networks. In light of the above challenges, we require techniques and mechanisms that address these concerns and evolve our mobile networks in the right direction.

We list below some of the future research directions and opportunities that we envisage will enable the 6th wave of mobile technologies for mobility use cases as presented in this article.

Network and Application Intelligence: AI can make our networks smarter and proactive to the environment, similarly, developing features like predictive handovers and AI-assisted channel sensing can address aerial network problems like interference and energy efficiency.

THz Transceivers: Development of transceivers and antenna arrays utilizing the THz band will be challenging. For applications like internet of nano things and connected micro-mobility, such bands will be instrumental. Beam-tracking of these waves specially in moist air environment will require investigation to foresee their practicality. At the same time, integrating beyond mmWave bands and IRS with 6G airborne networks is an open challenge that requires substantial research efforts.

6G Satellite: High-throughput satellites are ideal for providing unprecedented coverage and capacity, and given that UAM requires ubiquitous coverage, we shed some light on challenges of LEO satellite constellations for 6G mobile communications. Challenges include securing highly reliable, flexible and scalable 6G links at a viable cost while technical challenges include managing the frequent handovers between high velocity LEO satellites, seamless integration of robust and ultra-low latent inter-satellite routing algorithms. With all these challenges, there are several research opportunities in the field of LEO satellites for 6G and more particularly AWNs.

Intelligent Reflecting Surfaces: Given the need for ubiquitous coverage for UAM, programmable, software defined and passive IRS metasurfaces can be used as inter-satellite radio switches presenting numerous technical solutions to coverage and capacity problems. These smart surfaces mounted along or either in part of the solar panels can also play an enabling role in reducing energy consumption for satellite communication. Major implementation challenge include tracking high mobility satellites in order to maintain links in real-time with minimum power consumption.

Security and Privacy: Due to the nature of mobile communications, research on cybersecurity will always remain ongoing. Efforts are required to secure our mobile networks specifically for applications that involve human life at risk. Vulnerability in communication links for AWNs can simply result in hijack of controls or data breaches, hence demanding much research efforts in addressing the security and privacy challenges.

In this paper, we have presented our vision of 6G AWNs enabling urban air mobility. A 6G architecture for airborne networks is presented and enabling technologies specific to aerial use cases are discussed in detail. We envisage airborne networks utilizing aerial vehicles as base stations (aNBs) to enable a multitude of applications for future astute mobility. We also examine the required evolution of KPIs for 6G airborne networks and UAM applications. Finally, we discussed the challenges and threats in pursuing AWNs and subsequent future research opportunities in the space of UAMs and 6G. Our work in this paper highlights the need of rigorous research efforts in bringing the vision to life, however, these insights and past developments of mobile networks and their respective applications make a solid case for the realisation of Astute Urban Air Mobility.