Bridging Connectivity Gaps: HAP-Based Network With Parallel FSO-RF Transmission for Ubiquitous Internet Access

High-altitude platform (HAP)-based networks can be a flexible and cost-effective alternative to satellite systems for future connectivity. This paper proposes a reliable HAP-based solution for providing ubiquitous Internet coverage over wide areas. We combine free-space optical (FSO) and radio frequency (RF) links to exploit their complementary properties. The proposed network uses a primary HAP (P-HAP) for dedicated FSO and RF transmissions and a backup HAP (B-HAP) to provide extra access when the primary FSO suffers outages. The FSO links support high-data-rate transmissions to access points in densely populated regions. Meanwhile, the RF link ensures ubiquitous coverage, enabling connectivity for dispersed users who would otherwise be challenging and costly to reach. Due to the low altitude of HAPs, these users can access the Internet using conventional mobile phones without requiring additional terrestrial infrastructure. We develop an analytical model for packet transmission over the FSO and RF links via P-HAP and B-HAP. Important performance metrics are derived, such as throughput, loss probability, resource utilization, packet error rate, and goodput. Extensive Monte Carlo simulations are presented to verify the analytical results.