FSO-based Multi-Layer Airborne Backhaul Networks

The deployment of non-terrestrial networks (NTNs) is envisioned to realize a truly global coverage for 6G and beyond. Advances in autonomous avionics and lightweight composite materials have positioned high-altitude platform stations (HAPSs) as viable NTN nodes for future networks in addition to rotary-wing unmanned aerial vehicles (UAVs). In this paper, we address the system-level design of a multi-layer airborne backhaul network where HAPSs and rotary-wing UAVs provide free space optical (FSO) backhaul links to the ground base stations. While HAPS fleets operate in circular tracks at stratospheric altitudes and provide wide coverage, rotary-wing UAVs operate at low and medium altitudes complementing the HAPSs. The aerial backhaul architecture should be designed to provide a seamless connection with base stations without any coverage gap. We present a step-by-step system design methodology for FSO-based airborne backhaul systems. For a given coverage area, we discuss how to select the number of required layers, the number of HAPS tracks, the number of HAPS per track, the number of UAVs in the lower altitudes, the operation altitude of the middle-layer UAVs, and the number of laser sources per airborne node. We present several numerical results to highlight our findings for typical rural and urban areas.