Brightlight: Hybrid Testbed for Backhaul-Aware Handover in 6G Satellite–Terrestrial Networks

Abstract

The integration of terrestrial and non-terrestrial networks is a key enabler for seamless global connectivity in 6G systems. Existing simulation tools typically address only one domain, lacking unified architectures for capturing transient protocol-level behavior during satellite mobility events. This paper introduces BrightLight, a hybrid emulation–simulation testbed for space–terrestrial integrated networks (STINs) that combines Linux network namespaces, NS-3 mmWave channel modeling, and an Open5GS core to execute real protocol stacks under configurable satellite mobility and gateway impairments. To demonstrate the platform's ability to capture fine-grained handover dynamics, we evaluate backhaul-aware handover over a Starlink-based constellation topology, comparing conventional satellite switching against inter-satellite link (ISL) assisted rerouting under ground-segment congestion. BrightLight successfully captures transient throughput evolution, TCP buffer drainage effects, and RTT dynamics throughout the handover process, revealing that routing via ISLs to uncongested ground stations substantially reduces latency and eliminates throughput degradation. These results validate BrightLight as an effective platform for studying protocol-level handover behavior in 6G STIN architectures.