Quasi-Earth Fixed Cell

Description

A quasi-earth fixed cell is a Non-Terrestrial Network (NTN) cell concept introduced in 3GPP Release 17 to address the unique mobility challenges of satellite-based radio access. Unlike terrestrial cells that are permanently fixed to a geographic location, or fully mobile satellite cells that continuously sweep across the Earth's surface, a quasi-earth fixed cell remains stationary relative to a specific geographic area for a defined time duration before potentially being reassigned to cover a different area.

This concept is particularly relevant for NGSO (Non-Geostationary Orbit) satellite constellations, such as LEO (Low Earth Orbit) and MEO (Medium Earth Orbit) systems. These satellites orbit the Earth at high speed — a LEO satellite completes an orbit in approximately 90 minutes — which means their beams would normally sweep across the ground rapidly. However, by using steerable beam technology, the satellite can dynamically adjust its beam pointing direction to keep the beam footprint fixed over a specific geographic area for an extended period.

The quasi-earth fixed cell approach differs from the earth-fixed cell concept in an important way. An earth-fixed cell continuously maintains its coverage over the same geographic area by constantly adjusting beam pointing as the satellite moves. A quasi-earth fixed cell maintains coverage over one area for a limited duration, then switches to cover a different area. This switching may occur when the satellite's elevation angle or beam steering capability reaches its limits, or when the satellite hands over coverage responsibility to another satellite.

From the UE perspective, a quasi-earth fixed cell behaves similarly to a terrestrial cell during the period it covers a given area. The UE performs normal cell selection, reselection, and measurement procedures as defined in TS 36.304 and TS 38.304. However, the network must manage the transition when the cell moves to a new area, potentially requiring the UE to hand over to a different cell or satellite.

The concept is specified in TS 36.304 (E-UTRA idle mode procedures), TS 36.331 (E-UTRA RRC protocol), and TS 38.304 (NR idle mode procedures). These specifications define how UEs handle mobility in NTN scenarios, including the timing and measurement requirements specific to quasi-earth fixed cells. The network broadcasts assistance information to help UEs predict when a cell will stop covering their location, enabling proactive handover decisions.

Quasi-earth fixed cells represent a middle ground between fully earth-fixed and fully earth-moving cell architectures, offering flexibility to satellite operators while maintaining manageable mobility procedures for UEs. This approach is expected to be particularly valuable for providing coverage in rural, maritime, and aviation scenarios where terrestrial infrastructure is unavailable.

Purpose & Motivation

The quasi-earth fixed cell concept was created to solve a fundamental challenge in NTN design: how to provide stable, predictable cell coverage from satellites that are constantly moving at high orbital velocities. Traditional earth-moving cells, where the beam footprint sweeps across the ground with the satellite, create rapid cell changes that stress UE mobility procedures and make idle mode camping difficult. Fully earth-fixed cells require continuous beam steering that may exceed satellite antenna capabilities over extended periods.

The quasi-earth fixed approach addresses this by allowing a satellite beam to remain pointed at a fixed geographic area for a practical duration — long enough for UEs to camp, establish connections, and complete transactions — before the beam is redirected. This reduces the frequency of cell reselection and handover events compared to earth-moving cells, while being more feasible than indefinite earth-fixed coverage.

This concept was motivated by the commercial deployment of LEO satellite constellations for broadband internet access. Operators needed a standardized framework that could accommodate the physical constraints of satellite beam steering while providing a user experience comparable to terrestrial networks. The quasi-earth fixed cell model, standardized in Release 17, provides this framework and has been refined in subsequent releases as NTN deployments have matured.