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Testing, Refining, and Improving Stratospheric Connectivity: NTT Docomo’s HAPS Trials

At MWC 2025, NTT Docomo highlighted its latest initiatives under the NTT Group's "NTT C89" space-business strategy, such as mobile-connectivity services using unmanned vehicles, or high-altitude platform stations (HAPS), that fly in the stratosphere for days or months, using relays to provide mobile connectivity in mountainous and remote areas, including at sea and in the sky. A presentation on NTT C89 Aerospace Business Strategy is available here while a presentation on NTT DOCOMO's Non-Terrestrial Network (NTN) for Extreme Coverage Extension is available here.

Stratospheric connectivity, enabled by High Altitude Platform Stations (HAPS), is emerging as a key solution for extending mobile coverage to remote and underserved areas. However, ensuring that these airborne platforms can provide stable, high-quality connectivity requires extensive testing and refinement. At MWC 2025, NTT Docomo showcased its progress in this domain, highlighting multiple real-world trials designed to validate and improve HAPS-based communication.

The Challenge of Stratospheric Connectivity

Unlike terrestrial networks, stratospheric platforms operate at an altitude of around 20 km, where atmospheric conditions, signal propagation, and handover processes present unique challenges. HAPS must maintain stable connectivity despite varying wind speeds, temperature fluctuations, and the need to integrate seamlessly with existing terrestrial and satellite networks.

To tackle these complexities, NTT Docomo has been actively testing different aspects of HAPS connectivity, focusing on signal propagation, latency, and practical use cases for mobile communication.

Real-World Testing: NTT Docomo’s Key HAPS Trials

Kenya LTE Trial (February 2025)

NTT Docomo partnered with Airbus to conduct an LTE connectivity trial in Kenya using the Zephyr S, a solar-powered HAPS aircraft. The goal was to establish a stable mobile link from the stratosphere to standard LTE smartphones on the ground. The test successfully demonstrated:

  • A persistent LTE connection over a wide coverage area.
  • Effective signal transmission at an altitude of 20 km.
  • Feasibility of direct-to-device communication using standard LTE protocols.

Zephyr S Stratospheric Test: Measuring UHF and 2 GHz Bands

Another critical trial involved testing long-distance communication in the ultra-high frequency (UHF) and 2 GHz bands. Conducted from the stratosphere, the test provided valuable insights into:

  • Signal stability over long distances.
  • Potential for spectrum optimization in different frequency bands.
  • Adaptation of existing terrestrial communication technologies for HAPS.

Q-Band Propagation Measurement in Austria

NTT Docomo also conducted a world-first test measuring Q-band (38 GHz) propagation from the stratosphere. This test is significant because:

  • Higher frequencies like Q-band enable high-capacity backhaul links.
  • The results contribute to future standardization efforts for 5G and 6G NTN.
  • Understanding atmospheric attenuation effects at such high frequencies is critical for real-world deployment.

Refining HAPS Connectivity: Lessons Learned

These trials provide critical data to refine and improve HAPS-based mobile networks. Some key takeaways include:

  • Signal Stability: Atmospheric effects at 20 km altitude require adaptive beamforming and advanced interference management.
  • Latency & Handover: Ensuring seamless transition between terrestrial, satellite, and HAPS networks is essential for maintaining user experience.
  • Commercial Viability: By validating different frequency bands and technologies, NTT Docomo is laying the groundwork for commercial HAPS deployment in the near future.

The Future of Stratospheric Connectivity

The insights gained from these trials will help accelerate the commercialization of HAPS networks, making them a viable alternative for global connectivity challenges. As 5G and 6G networks evolve, integrating HAPS with terrestrial and satellite solutions will be key to achieving truly ubiquitous coverage.

With companies like NTT Docomo at the forefront of these advancements, stratospheric connectivity is no longer just a theoretical concept but a rapidly evolving reality that could revolutionize how we connect the world.

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