Will High Altitude Platforms Become an Integral Part of 6G?

The sixth generation of the in the evolution of wireless evolution promises to be disruptive in terms of the extreme solutions envisioned to meet the projected targets in terms of data rates, latencies, coverage, use cases, etc.

The drive to ubiquitous connectivity and ever-increasing capacity requirements have pushed designers to foresee a vertical expansion of the network into what is known as the Space Radio Access Network (RAN). Non-terrestrial networks are expected to be a key enabler of 6G systems, given the difficulty for terrestrial ones to guarantee coverage (such as rural areas) and capacity requirements at all times. The Earth topology provides significant restrictions in terms of the installation of transmission stations and the implementation of backhauling solutions for existing terrestrial networks. Natural and man-made disasters are yet another reason to provide a backup plan to solve the resulting disconnection of millions of persons from the communications grid. A core component in this vertical three-dimensional hierarchy is related to high altitude platforms (HAPs). So, what makes HAPs such an attractive option for 6G networks? And what are the challenges that are delaying such deployments?

What are Non-Terrestrial Networks and High-Altitude Platforms?

Using satellite communications is not something new as it has been widely used for worldwide telephony systems, weather monitoring, geographical information systems, and strategic communications in general. This relatively old technology has recently come back into the limelight with the endeavors of Elon Musk in SpaceX and Starlink.

New technology has emerged to extend the satellite ecosystem into terrestrial networks. HAPs and unmanned aerial vehicles (UAV) came to the fore as intermediate layers to complete the hierarchical Space RAN network that connects to the terrestrial one, thus providing coverage that far exceeds the Earth range and unlocking yet-to-be-seen network design and customization opportunities. Imagine a subscriber being able to maintain a connection, whether it is on the ground, in an airplane, or even on a boat trip, without the slightest disruption or drop in performance. This is a potential byproduct of the new network model and the seamless connection handover between the different layers.

Satellites constitute the farthest point of the forthcoming network orbiting at distances ranging from several hundreds of kilometers with Low-Earth Orbit (LEO) satellites to around 36000 kilometers with Geosynchronous Equatorial Orbit (GEO) satellites. On the other side of the Space, RAN spectrum are UAVs which are aircrafts flying at several hundred meters. UAVs are extremely popular as they are relatively easy to deploy and manage while providing a temporary extension of the wireless network.

The middle layer is governed by high-altitude platforms. The International Telecommunications Union (ITU) defines HAPs as “radio stations located on an object at an altitude of 20-50 kilometers and at a specified, nominal, fixed point relative to the Earth”. There are various types of these platforms that float in the stratosphere, such as balloons or aircrafts powered by renewable energy sources. HAPs also provide the best trade-off between the high coverage of satellites and the relative low latency of UAVs.

What are Some Promising Use Cases in HAPs?

The inherent properties of high-altitude platforms in terms of coverage, latency and cost have paved the way for several use cases that would justify the significant investments in that area. The HAPSs Alliance, which is the main consortium of companies working on developing HAPs solutions include the likes of Ericsson, Nokia, T-Mobile, NTT Docomo, and Airbus. A recent article by the GSMA highlights the main use cases as follows:

• Coverage extension: HAPs provide an excellent and viable alternative to provide greenfield coverage, that is, for areas with no cellular connectivity, and white spot reduction, which involves dealing with coverage holes. The altitude at which HAPs operate provides a possible line of sight connection regardless of the terrain below.
• Emergency response solutions: The fragility of terrestrial infrastructure to disasters makes HAPs an excellent means for sustaining communication services for a period of time. This was notably the case after the Hurricane that hit Puerto Rico in 2017. In that case, Google’s “Project Loon” balloons were trialed to restore connectivity in the area.
• Capacity enhancement: HAPs can be employed to boost the capacity of the terrestrial network for temporary events such as festivals.
• Large scale IoT: HAPs can be employed when a widescale internet of things network is required as they resolve the coverage constraint hindering such deployments.
• Fixed wireless access: Telco operators around the world have been implementing fixed wireless access solutions based on the current 5G technology. HAPs could provide an alternative to the terrestrial solutions, especially if a higher frequency of operation is used (that is, with higher bandwidth).
• Backhauling solutions: Perhaps the best use case of HAPs and non-terrestrial networks, in general, is the flexibility it offers in connecting isolated terrestrial networks through efficient backhauling solutions.

Why aren’t HAPs Commercially Ready Yet?

The discontinuation of Project Loon, which was at the forefront of HAP development, highlights the sad reality of a promising project. The discontinuation of the Facebook Aquila solar-powered plane is another event that raises additional questions regarding the HAPs concept as a whole. HAPs are undoubtedly promising solutions for future networks, but the cost and complexity of optimizing the operational process are too much to result in a commercially viable solution. Investments are there indeed, but risks and challenges are far bigger. The work done on Project Loon will feed into other projects with the numerous patents and breakthroughs achieved. Project Taara holds all the hopes of finishing the work started by Google in Project Loon.

The deployment of such solutions also has other challenges inherent to traditional communication systems, including spectrum allocation, regulatory affairs, and integration with operators’ networks. Other challenges relate to the technology itself, mainly in terms of energy generation, heat management, operability, and reliability.

Summary

The path to the 6G evolution is certainly marked by bold changes in the design of the network architecture. Non-terrestrial networks in general and high-altitude platforms, in particular, constitute the most promising candidates in the projected revamp. The use cases they unlock are just too tempting to be overlooked. This said we are still far from seeing a sustainable HAPs-based solution in action. However, with the investments pouring in, the technology is getting more and more mature, but will it be ready in time when 6G is actually deployed?

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