Impact of LoRa Imperfect Orthogonality: Analysis of Link-Level Performance

TLDR

LoRa uses chirp spread‑spectrum modulation with quasi‑orthogonal spreading factors to provide wide coverage, low power consumption, and robustness to interference. This letter evaluates link‑level performance of LoRa in a dense network with a central gateway, focusing on collisions at a single receiver. We numerically analyze LoRa modulation to show that inter‑SF collisions can cause packet loss when interference power is high, and we validate these results experimentally with commercial devices and software‑defined radios. Our results contradict the assumption of SF orthogonality, demonstrating that inter‑SF collisions degrade performance and that assigning higher SFs to distant users does not necessarily increase link capacity in congested networks.

Abstract

In this letter, we focus on the evaluation of link-level performance of LoRa technology, in the usual network scenario with a central gateway and high-density deployment of end-devices. LoRa technology achieves wide coverage areas, low power consumption and robustness to interference thanks to a chirp spread-spectrum modulation, in which chirps modulated with different spreading factors (SFs) are quasi-orthogonal. We focus on the performance analysis of a single receiver in presence of collisions. First, we analyze LoRa modulation numerically and show that collisions between packets modulated with different SFs can indeed cause packet loss if the interference power received is strong enough. Second, we validate such findings in experiments based on commercial devices and software-defined radios. Contradicting the common belief that SFs can be considered orthogonal, our results demonstrate that inter-SF collisions are indeed an issue in LoRa networks and, thus, allocating higher SFs to users far from the gateway might not necessarily improve their link capacity, in case of congested networks.