A Hybrid Analytical Framework for NB-IoT NTN Performance Under Discontinuous LEO Coverage

The integration of Narrowband Internet of Things (NB-IoT) with Non-Terrestrial Networks (NTN) represents a paradigm shift for achieving ubiquitous massive Machine-Type Communications (mMTC). However, the sparse deployment of current Low Earth Orbit (LEO) satellite constellations results in discontinuous coverage for User Equipment (UEs), which creates severe system-level challenges, most notably bursty traffic behavior where a large number of UEs attempt to transmit simultaneously during brief connectivity windows. This contention-heavy access pattern threatens to compromise the performance and reliability of the system. This paper presents a hybrid analytical framework, integrating link-level simulations into a two-dimensional Markov chain model, to characterize system-level performance under these conditions. The model is utilized to derive key performance metrics, including mean message delay and message loss probability, as a function of constellation size, orbital characteristics, number of UEs and traffic intensity. Our results demonstrate that system performance is robust against varying radio link quality but is highly sensitive to the duration of coverage gaps and the resultant collisions on the Narrowband Physical Random Access Channel (NPRACH). Quantitatively, we reveal that the ITU-R M.2410 requirement of 99% message delivery is not met for the standard mMTC traffic intensity of one message per two hours, even with a 24-satellite near-polar LEO constellation at either 600 km or 1,200 km altitudes considered in our study as a reference scenario. While LEO 1,200 km constellations offer better performance due to longer connectivity windows, the 99% requirement is only satisfied at a much lower traffic intensity of one message per day. The results highlight that vertical industries must carefully evaluate their communication requirements to ensure compatibility with these limitations. For the service providers, significant constellation densification is essential for supporting robust, large-scale NB-IoT NTN deployments.