HAPS as IMT Base Stations

Description

HIPS (HAPS as IMT Base Stations) is a 3GPP concept that defines the integration of High Altitude Platform Stations (HAPS) into the terrestrial mobile network architecture as aerial base stations. HAPS are quasi-stationary platforms, such as solar-powered unmanned aerial vehicles or airships, operating in the stratosphere at altitudes typically between 17-22 km. In the HIPS framework, these platforms are equipped with 3GPP-defined base station functionalities (gNB for 5G NR), effectively acting as flying cell towers. They connect to the core network via satellite or terrestrial backhaul links and provide radio access to user equipment (UE) on the ground, forming a non-terrestrial network (NTN) layer.

The architecture involves the HAPS platform hosting the radio unit (RU), distributed unit (DU), and potentially part of the centralized unit (CU) as per the 3GPP NG-RAN split. It communicates with UEs using 3GPP NR air interface, but with specific adaptations for the long propagation delays and unique channel conditions of stratospheric links. The HIPS system must manage challenges like significant round-trip delays (on the order of tens of milliseconds), Doppler shifts due to platform motion, and wide beam coverage areas. Key technical aspects include synchronization, timing advance mechanisms, and modified random access procedures to accommodate the high-altitude geometry.

HIPS operates as part of an integrated air-ground network, where it can provide seamless service continuity with terrestrial networks. It supports both transparent payload (bent-pipe) and regenerative payload (on-board base station processing) modes. The system is designed to deliver enhanced mobile broadband (eMBB) and massive IoT services over large geographic areas, complementing terrestrial infrastructure. Its role is pivotal in achieving ubiquitous 5G coverage, disaster recovery, and temporary event capacity, aligning with 3GPP's vision for comprehensive NTN integration in Release 17 and beyond.

Purpose & Motivation

HIPS was introduced to address the challenge of providing cost-effective, wide-area mobile broadband coverage in regions where deploying terrestrial infrastructure is economically or geographically unfeasible, such as remote rural areas, oceans, and disaster zones. Traditional terrestrial networks require dense base station deployment, which is prohibitively expensive for low-population-density areas. HAPS offers a middle-ground solution between terrestrial towers and satellites, with lower latency than geostationary satellites and wider coverage than ground cells.

The motivation stems from the need for global connectivity as part of the 5G evolution and the ITU's IMT-2020 vision. Prior to HIPS, NTN efforts in 3GPP focused mainly on satellite access; HIPS expands this to include stratospheric platforms, which can be more rapidly deployed and reconfigured than satellites. It solves limitations of previous isolated aerial solutions by standardizing HAPS integration within the 3GPP ecosystem, ensuring interoperability, efficient spectrum use, and seamless handovers with terrestrial networks. This standardization in Release 17 enables commercial viability and scalable deployment of HAPS for mobile operators.