Spectral-Efficient Network Design for High-Altitude Platform Station Networks with Mixed RF/FSO Systems

Integrating terrestrial networks with burgeoning high-altitude platform stations (HAPSs) will be a disruptive challenge for beyond-5G systems provisioning large-scale three-dimensional connectivity. Here, we study the problem of forwarding packets between terrestrial terminals and backhaul through multi-HAPS relaying. Considering the limited wireless backhaul, which is the practical constraint of HAPS relaying, dual-hop mixed radiofrequency/free-space optical (RF/FSO) networks are investigated, where backhaul-to-relay and relay-to-user communications employ FSO and RF links, respectively. To maximize the end-to-end network throughput, including downlink and uplink rate, we formulate the optimization problem for variables; the association between aerial and terrestrial terminals, transmit power, and deployment of multiple HAPSs, respectively. We tackle this problem using an iterative algorithm with proposed surrogate functions to efficiently obtain the locally optimal solution. Simulation results corroborate that our proposed optimal method achieves up to 11.3% spectral efficiency compared to the conventional heuristic method. Furthermore, we answer three questions; 1) what the wireless backhaul requirements are, 2) how the number of HAPSs and terrestrial terminals impact the network, and 3) what if a certain user terminal has a particular demand on data rate.