Downlink Coverage and Bit Rate Performance of LEO-Based Non-Terrestrial Networks for NB-IoT

Abstract

The integration of Narrowband Internet of Things (NB-IoT) with Non-Terrestrial Networks (NTNs) has emerged as a prominent enabler for global IoT connectivity in regions where terrestrial infrastructures are unavailable or impractical to deploy. Although IoT traffic is inherently uplink-oriented, performance of the downlink transmissions is equally vital for maintaining reliable system operations, yet it has so far received considerably less attention. This paper presents a comprehensive study of downlink coverage and bit rate performance in LEO-based NB-IoT NTN systems. Using simulations with realistic satellite constellation models at 600km and 1,200 km altitudes, we evaluate minimum link availability, downlink Signal-to-Noise Ratio (SNR) distributions, and achievable physical-layer bit rates under various antenna configurations and constellation sizes. Our results demonstrate that 1,200 km constellations consistently provide stronger downlink performance than their 600km counterparts. Notably, for LEO 1200, with 60 satellites equipped with 0.4 m aperture antennas, end devices can maintain valid connections with the satellites for approximately 30% of the time-almost four times higher than for LEO 600, and more than 65% of devices achieve bit rates above 3.5 kbps, satisfying the 3GPP TS 22.261 requirement of 2 kbps experienced data rate. Furthermore, smaller aperture antennas, despite offering lower peak gain, are shown to improve robustness by producing wider beams, which yield more stable SNR distributions and superior minimum link availability compared to larger apertures.