Haps for 6G Networks: Survey Bridges Terrestrial and Non-Terrestrial Infrastructure for Wide-Area Coverage

High-altitude platform stations, or HAPS, represent a crucial step towards realising the full potential of future 6G networks, seamlessly connecting terrestrial and space-based infrastructure, and researchers are now comprehensively surveying their capabilities. G. Svistunov, A. Akhtarshenas, and D. López-Pérez, alongside colleagues M. Giordani, G. Geraci, and H. Yanikomeroglu, present a detailed overview of how these stratospheric platforms can deliver wide-area, low-latency, and energy-efficient broadband communications. This work highlights the significant role HAPS play in extending connectivity to remote areas, supporting flexible network backhauling, and enabling massive internet of things deployments, as well as providing reliable communications for emerging applications like autonomous systems and immersive experiences. By examining current architectures, field trials, and key enabling technologies such as artificial intelligence-driven resource allocation, this survey establishes HAPS as a foundational element in building globally integrated, resilient, and sustainable 6G networks.

Scientists are exploring diverse applications including enhanced mobile broadband, massive machine-type communications, ultra-reliable low-latency communications, disaster recovery, and connectivity for remote areas. Key research areas involve developing optimal network architectures, efficiently allocating resources like spectrum and power, extending coverage to underserved areas, mitigating interference, and managing user mobility. Studies demonstrate that HAPS, operating in the stratosphere, significantly improve network stability and reduce signaling overhead by acting as intermediaries between satellites and user equipment, or as standalone communication managers. Experiments employing frequency selective optical (FSO) links between HAPS and low Earth orbit (LEO) satellites demonstrate high-speed, low-latency handovers even with high satellite mobility.

When functioning as quasi-stationary communication nodes, HAPS reduce handover frequency for terrestrial users, providing seamless service in high-mobility scenarios like vehicles and aircraft. A system combining beamforming for HAPS-to-ground communication and FSO backhaul to LEO/medium Earth orbit satellites demonstrates efficient handover management in dynamic environments. Optical links between HAPS and satellites further enhance reliability by ensuring robust backhaul connectivity. Researchers are also exploring HAPS for enhanced Internet of Things (IoT) sensing and monitoring, providing wide, stable coverage and integrating edge computing for low-latency data processing.

Operating at altitudes of 17-35 kilometers, HAPS achieve coverage up to 600 kilometers, surpassing terrestrial solutions in scalability and redundancy. A joint HAPS-unmanned aerial vehicle (UAV) framework for disaster scenarios ensures continuous service with sub-millisecond latency, while narrowband IoT (NB-IoT) and long range (LoRa) access schemes minimize collisions under dense deployments. Complementary LoRa-based monitoring optimizes parameters such as altitude, transmission power, and device density, confirming the viability of HAPS for large-scale environmental IoT. Research demonstrates that integrating HAPS into heterogeneous networks, alongside terrestrial base stations and autonomous vehicles, is essential for ultra-reliable low-latency communication (URLLC) services, sustaining data rates exceeding 700 kbps with high reliability even under dynamic conditions. Scientists optimized multi-connectivity path selection to satisfy quality of service constraints while minimizing link usage, revealing that line-of-sight interference is a key performance limiter, which was subsequently addressed through refined multi-connectivity strategies. For unmanned aerial vehicle (UAV) control, experiments show HAPS reduce handover frequency and ensure continuous connectivity for high-mobility UAVs. Addressing energy efficiency, the work highlights that offloading traffic from terrestrial base stations to HAPS-supported hypercells can reduce network energy consumption. A heuristic algorithm, prioritizing least-utilized base stations for offloading and leveraging HAPS’s self-sufficient energy capabilities, achieved up to 29% reduction in hourly energy consumption over a week, with savings reaching 41% during nighttime hours. The research defines three main network components within the non-terrestrial network architecture: platforms carrying communication equipment, terminals interacting with these platforms, and gateways connecting non-terrestrial networks to core networks. Academics demonstrate how HAPS, operating in the stratosphere, can bridge terrestrial and satellite networks to provide wide-area, low-latency connectivity, particularly benefiting underserved regions and enabling applications like autonomous systems and immersive experiences. The study details how HAPS function not simply as relays, but as intelligent nodes within standardized network frameworks, integrating multiple frequency bands and optical communication options. Researchers identified key enabling technologies, including advanced channel modelling, efficient resource allocation, and artificial intelligence-driven control mechanisms, all vital for optimizing HAPS performance and ensuring reliable, energy-efficient operation. While acknowledging the significant potential, the study also highlights existing challenges, notably the high initial costs associated with HAPS deployment and the need for supportive business models and regulatory frameworks. Future research priorities include developing efficient multi-band communication strategies, ensuring interoperability with evolving 3GPP standards, and designing sustainable platforms to facilitate widespread adoption.