Integrating Terrestrial and Non-terrestrial Networks: 3D Opportunities and Challenges

Giovanni Geraci

Universitat Pompeu Fabra, Barcelona

A mobile connection is our window to the world. The current social, economic, and political drive to reach global wireless coverage and digital inclusion acknowledges connectivity as vital for accessing fair education, medical care, and business opportunities in a post-pandemic society. Sadly, more than a third of the population on Earth remains unconnected. Indeed, rolling out optical fibers and radio transmitters to every nook and cranny of the globe is not economically viable, and reaching the billions who live in rural or less privileged areas has remained a chimera for decades. The long-overdue democratization of wireless communications will require a wholly new design paradigm to realize ubiquitous and sustained connectivity in an affordable manner. 

Meanwhile, in more urbanized and populated areas, even 5G may eventually fall short of satiating our appetite for mobile internet and new user experiences. Life in the 2030s and beyond will look quite different from today’s: hordes of network-connected UAVs[1] will navigate 3D aerial highways—be it for public safety or to deliver groceries to our doorstep—and flying taxis will reshape how we commute and, in turn, where we live and work. The bold ambition of reaching for the sky will take the data transfer capacity, latency, and reliability needs for the underpinning network to an extreme, requiring dedicated radio resources and infrastructure for aerial services.

In a quest for anything, anytime, anywhere connectivity— even up in the air—next-generation mobile networks may need to break the boundary of the current ground-focused paradigm and fully embrace aerial and spaceborne communications. To this end, the wireless community has already rolled up its sleeves in (re)search for technology enhancements towards a fully integrated terrestrial plus non-terrestrial network (NTN) able to satisfy both ground and aerial requirements. At first glance, terrestrial networks could be re-engineered and optimized to support aerial users or complemented by NTN infrastructure such as low Earth orbit (LEO) satellite constellations or aerial base stations to further improve performance. Cost-related factors may advocate for a progressive roadmap. 

The opportunities unlocked by integrating terrestrial and NTN capabilities could lead to a vast number of new applications and services, including critical communications, massive IoT, and aerial communications. Indeed, beyond standalone cellular networks, primarily designed for 2D usage, an integrated ground-air-space network could support reliable data and control links to multiple UAVs, electrical vertical take-off and landing vehicles, and aircrafts. These services would be guaranteed in specific 3D areas—aerial corridors or waypoint trajectories—where end-devices will be allowed to fly at different heights. The potential of UAVs may only truly be unleashed once the network capabilities and regulations allow for autonomous operation beyond visual line-of-sight. To this end, either augmenting a ground deployment with co-channel uptilted base stations or complementing it with a LEO constellation are both promising avenues for supporting aerial communications, under the right design choices: the former entails advanced interference mitigation capabilities, the latter hinges on a sufficiently dense constellation—to guarantee near-zenith coverage—and a carefully designed beam reuse.

Standardization work on non-terrestrial communications in 3GPP dates back to 2017. This effort can be classified nowadays into two main areas, namely NTN enhancements and terrestrial network support for UAVs. The former aims at defining a global standard for future spaceborne communications, fostering an explosive growth in the satellite industry. Activities within the latter serve the twofold purpose of ensuring that mobile standards meet the connectivity needs for safe UAV operations, and that other users of the network do not experience a loss of service due to their proximity to UAVs. Looking ahead, 3GPP Rel-18 will enhance 5G NR NTN operation by improving coverage for handheld terminals, studying deployments above 10 GHz, addressing mobility and service continuity between terrestrial-NTN as well as across different NTNs, and investigating regulatory requirements for network-verified user location. As 5G use-cases evolve, Rel-18 will also introduce 5G NR support for devices onboard aerial vehicles, studying additional triggers for conditional handover, base station uptilting, and signaling to indicate UAV beamforming capabilities, among others.

However, the availability of terrestrial plus NTN segments is just a prerequisite for realizing a 3D wireless network. Jointly and optimally designing and operating all platforms and nodes requires further disruptive and interdisciplinary research. One chief challenge in realizing an integrated ground-air-space network arises from its extreme heterogeneity, reflected at different levels, including radio propagation features, node and device capabilities, and ownership and operations. Such heterogeneity makes realizing a 3D network a remarkable endeavor and suggests much-needed work to enhance radio access, mobility and multi-connectivity, and network management and orchestration. We hope this overview article will foster new research and breakthroughs, bringing the wireless community one step closer to the era of ground-air-space communications.

Full article: G. Geraci, D. López-Pérez, M. Benzaghta, and S. Chatzinotas, “Integrating Terrestrial and Non-terrestrial Networks: 3D Opportunities and Challenges”, IEEE Communications Magazine, 2023.

Giovanni Geraci

Universitat Pompeu Fabra, Barcelona

Giovanni Geraci is an Assistant Professor and the Head of Telecommunications Engineering at Univ. Pompeu Fabra in Barcelona. He was previously with Nokia Bell Labs, holds a dozen patents on wireless technologies, and is a co-Editor of the book “UAV Communications for 5G and Beyond” by Wiley–IEEE. He serves as Distinguished Lecturer for the IEEE Communications Society and the IEEE Vehicular Technology Society, and he received the 2018 IEEE ComSoc EMEA Outstanding Young Researcher Award as well as Best Paper Awards at IEEE PIMRC’19 and IEEE Globecom’22. Giovanni likes coffee, surprises, and living by the sea. Born and raised in Sicily, he has lived across six continents for work/love/adventure and considers himself one of the luckiest people in the world. 

[1] Short for uncrewed aerial vehicles, commonly known as drones.

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