Multi-Layer Airborne FSO Systems: Performance Analysis and Optimization

What is it about?

Future non-terrestrial networks may use high-altitude platforms (HAPSs) together with lower-altitude UAVs to provide high-capacity optical backhaul. Unlike fixed terrestrial links, these airborne laser links are affected by platform motion, changing transmission distances, atmospheric attenuation, geometrical loss, and pointing errors. We develop statistical channel models that capture these airborne-specific effects and use them to derive the end-to-end error performance of both two-hop HAPS networks and three-hop HAPS–UAV networks. Importantly, we then develop a power-allocation strategy that optimizes the transmit power across the different hops, rather than simply assigning the same power to every link.

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Photo by John Adams on Unsplash

Photo by John Adams on Unsplash

Why is it important?

Using equal transmit power on every hop can be highly inefficient because the hops experience very different propagation conditions. In a single-layer system, the long gateway-to-HAPS link typically dominates the overall error performance. In a multi-layer system, the dominant link can change with transmit power: the longer HAPS link dominates at lower SNR, while the UAV link can become the bottleneck at higher SNR because of pointing errors. The proposed optimization allocates power so that the error performance is balanced among the hops and prevents one link from limiting the entire connection. The study also tests the optimization against realistic parameter mismatches and finds that the overall BER remains largely robust to the considered altitude deviations.

Perspectives