Enhancing Non-Terrestrial Network Performance with Free Space Optical Links and Intelligent Reflecting Surfaces

TL;DR

This work analyzes end-to-end performance of a four-node non-terrestrial network (OGS, HAP, IRS, and users) where the ground-to-HAP link is optical and the HAP–IRS–user RF cascade experiences Shadowed Rician and Nakagami-$m$ fading. It develops a unified analytical framework that yields end-to-end SNR statistics for a fixed-gain AF relay, with the FSO link modeled by Gamma-Gamma turbulence and generalized pointing errors, and the RF cascade approximated by a mixture-Gamma distribution; all performance metrics (OP, BER, ergodic capacity) are expressed in terms of the bivariate Fox-$H$ function, with simple high-SNR asymptotics provided. The paper also provides a thorough numerical analysis to validate the model and investigate how parameters such as zenith angle, HAP altitude, and RF shadowing affect reliability, while demonstrating the advantages of heterodyne detection over IM/DD. The findings offer practical design insights for deploying HAP–IRS–FSO NTN systems, including phase optimization at the IRS and the impact of pointing errors and atmospheric conditions on end-to-end performance.

Abstract

The integration of non-terrestrial networks (NTNs), which include high altitude platform (HAP) stations and intelligent reflecting surfaces (IRS) into communication infrastructures has become a crucial area of research to address the increasing requirements for connectivity and performance in the post-5G era. This paper presents a comprehensive performance study of a new NTN architecture, which enables communication from the optical ground station (OGS) to end users through the utilization of HAP and terrestrial IRS nodes. In this configuration, the HAP acts as an amplify-and-forward (AF) relay terminal between the free-space optical (FSO) link and the RF links. Specifically, the RF links are modeled using the Shadowed Rician and the generalized Nakagami-$m$ models, where the FSO link is characterized by the Gamma-Gamma distribution with generalized pointing errors. The FSO system operates under either intensity modulation with direct detection or heterodyne detection. Using the mixture Gamma model, we approximate the non-centered chi-square distribution that describes the total fading of the RF link, and we assess the performance of the end-to-end system by analyzing the ergodic capacity, the average bit-error rate (BER), and the outage probability, calculated using the bivariate Fox-H function. We also provide simple asymptotic expressions for the average BER and the outage probability at high signal-to-noise ratio (SNR). Finally, the proposed analysis is validated with numerical and Monte-Carlo simulation results, showing an exact match.

Figures (9)

• Figure 1: Integrated HAP-ground communication system with IRS.
• Figure 2: Outage probability of the RF channel under different shadowed conditions using different values of $N_x$ when $q_H=1$.
• Figure 3: Outage probability for different $H_H$ values under IM/DD and heterodyne detection techniques, considering LS conditions with $q_H = 1$ and $\zeta = 40^\circ$.
• Figure 4: Outage probability under IM/DD and heterodyne detection techniques for different zenith angles under HS condition with $q_H=1$.
• Figure 5: Outage probability for different shadowed conditions of the RF link under IM/DD and heterodyne techniques with $q_H=1$.

Theorems & Definitions (12)

• Lemma 1
• proof
• Lemma 2
• proof
• Lemma 3
• proof
• Lemma 4
• proof
• proof
• Lemma 5