HAPS as the Middle Layer for Making 6G Truly Ubiquitous

One of the recurring themes in 6G discussions is ubiquitous connectivity. The ambition is not simply to make mobile broadband faster, but to extend reliable connectivity to places where today’s terrestrial networks struggle to reach. That includes remote rural areas, mountains, forests, oceans, air corridors, disaster zones and other locations where building and maintaining ground infrastructure may be difficult, expensive or commercially unattractive.

This is where Non-Terrestrial Networks, or NTN, become important. NTN is often associated with satellites, especially Low Earth Orbit satellite constellations, but the 6G NTN story is broader than that. It also includes High Altitude Platform Stations, better known as HAPS, which operate in the stratosphere, typically around 20 km above the Earth.

The HAPS Alliance whitepaper, 6G from the Stratosphere: The Role of HAPS in Key Use Cases, positions HAPS as a middle layer between terrestrial networks and satellites. In this multi-layered architecture, the surface layer provides high-capacity connectivity in areas covered by terrestrial networks, the stratospheric layer adds wide-area and flexible coverage from HAPS, and the space layer extends connectivity to regions beyond the practical reach of both ground networks and HAPS.

That middle position is what makes HAPS interesting for 6G. A HAPS platform is much closer to users than a satellite, so it can offer better link budgets and lower propagation delay. At the same time, it can cover a much larger area than a terrestrial base station. The whitepaper suggests that a single HAPS platform can typically cover a radius of around 50 to 100 km, making it useful for wide-area coverage, temporary capacity, emergency restoration and connectivity in underserved regions.

From a latency perspective, the difference is significant. At a nominal altitude of 20 km, the one-way propagation delay to a user directly below the platform is around 0.067 ms, with a round-trip propagation delay of around 0.133 ms. Even at the edge of a 100 km coverage radius, the round-trip propagation delay is estimated at around 0.68 ms. The overall end-to-end latency will also depend on processing, routing and network architecture, but the whitepaper argues that 1 to 10 ms end-to-end latency is a realistic target for 6G-class HAPS systems with regenerative payloads.

This matters because 6G is expected to support more than broadband. IMT-2030 includes usage scenarios such as immersive communication, massive communication, hyper-reliable and low-latency communication, integrated sensing and communication, AI and communication, and ubiquitous connectivity. HAPS will not be the answer to every requirement in every scenario, but they could provide a useful layer for several of them.

One of the most important HAPS use cases discussed in the whitepaper is Direct-to-Unmodified-Smartphone connectivity, or D2US. The idea is that users should be able to connect directly to HAPS using ordinary smartphones, without needing a special satellite terminal or dedicated antenna. This could make HAPS valuable in rugged terrain, remote islands, maritime environments, suburban coverage gaps and even airspace. Compared with satellite direct-to-device connectivity, the shorter distance between the platform and the user improves the link budget, which can make voice, video and broadband services more practical on standard devices.

Disaster recovery is another strong use case. When earthquakes, floods, hurricanes, wildfires or other emergencies damage terrestrial infrastructure, HAPS can be deployed or activated to provide temporary wide-area connectivity. They can support direct access for users and emergency responders, act as temporary backhaul for surviving terrestrial sites, and work with satellites and ground gateways to restore communications. Since HAPS operate above the affected area, they are less vulnerable to ground-level damage and can provide a resilient layer during the recovery phase.

The whitepaper also highlights Integrated Sensing and Communication, or ISAC. This is one of the key 6G topics because future networks are expected not only to communicate, but also to sense the environment. HAPS could support wide-area environmental monitoring, disaster early warning, public safety, drone control, traffic management and panoramic scanning. The platform’s altitude gives it a broad view of the area below, while its lower altitude compared with satellites can help with latency and resolution. However, there are technical challenges, including efficient waveforms, beamforming, power constraints, payload weight, sensing accuracy and operation in higher frequency bands.

Vehicular connectivity is another area where HAPS could have a role, especially for roads and transport corridors outside dense terrestrial coverage. The whitepaper discusses HAPS support for C-V2X, autonomous vehicles, platooning, traffic optimisation and vulnerable road user safety. Some V2X use cases have latency requirements that may still need terrestrial infrastructure, especially the most demanding safety-critical scenarios. However, HAPS could support many wider-area vehicular applications and provide an additional layer of resilience and coverage, particularly in unserved and underserved areas.

The network sharing and neutral host angle is also worth noting. A HAPS platform could potentially serve multiple operators or tenants across the same footprint, using shared network models such as MOCN or MORAN. This could be useful for large events, private networks, emergency response, transport corridors and rural coverage. In a future 6G environment, where terrestrial, stratospheric and satellite layers are expected to work together, neutral host HAPS platforms could offer Network as a Service for multiple service providers.

Of course, there are still many open challenges. HAPS platforms need suitable spectrum, efficient payloads, advanced antennas, reliable backhaul, integration with terrestrial and satellite networks, mobility management, interference coordination, energy-efficient operation and regulatory support. Direct-to-smartphone services also need careful coordination with existing terrestrial networks, especially where the same or adjacent spectrum may be used.

The most useful way to think about HAPS is not as a replacement for terrestrial networks or satellites, but as a complementary stratospheric layer. Terrestrial networks will continue to provide the highest capacity and lowest latency where infrastructure exists. Satellites will continue to provide global and wide-area reach, especially over oceans, deserts and remote regions. HAPS can sit between these two layers, providing lower-latency wide-area coverage, flexible deployment and targeted capacity where needed.

For 6G, this could be important. The promise of ubiquitous connectivity cannot be fulfilled by terrestrial networks alone. It will require a coordinated system of ground, air and space assets, intelligently managed and dynamically optimised. HAPS may therefore become one of the practical building blocks that helps turn the 6G vision from a set of ambitious capabilities into something that can actually serve people, vehicles, industries and emergency services in difficult-to-reach places.

The HAPS Alliance whitepaper makes a clear case that the stratosphere should be considered a serious part of the 6G network architecture. The real question now is how quickly the industry can solve the remaining technical, commercial and regulatory challenges so that HAPS can move from promising concept to scalable deployment.

The whitepaper can be downloaded from here.

Related Posts: 

Posted by on

Comments

Post a Comment