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

Abstract

In this letter we focus on the evaluation of linklevel 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 softwaredefined 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.

On the LoRa Modulation for IoT: Waveform Properties and Spectral Analysis

“…Therefore, the DFT of the twisted signal (15) has only one non-zero element in the position of the modulating symbol a. This means that a possible way to implement a demodulator is to compute the dechirped vector (15), and decide based on its DFT. Remark 1.…”

A Thorough Study of LoRaWAN Performance Under Different Parameter Settings

“…For each value of the SF parameter, Tab. I shows the A key feature of the LoRa modulation is that packets employing different SFs are almost orthogonal: transmissions overlapping in time and frequency can still be correctly decoded by the receiver, provided that the power of the target signal is sufficiently larger than that of interferers [5], [6].…”

Optimum LoRaWAN Configuration Under Wi-SUN Interference

“…Contrasting with [11] and [9], the work in [10] takes into account the effect of interference from the imperfect orthogonality of LORA signals using different SFs. In this sense, Croce et al [12] show that LORA SF separation is not perfect, so LORA nodes are subject to some interference generated by nodes using other SFs.…”

“…Moreover, a LORA gateway can process up to nine channels in parallel, combining different sub-bands and SFs [1]. Besides, LORA features the capture effect, making it possible to recover a LORA signal when two or more signals are received simultaneously, in the same frequency and SF, provided that the desired signal is at least 1 dB above interference [12].…”

“…Provided the branch-and-bound technique guesses T H1 in line 6, the range limits for all SF are computed using (17) in line 7. In the following, the algorithm uses the newly computed vector L to obtain vector B and matrix Y (lines [11][12][13][14][15][16], allowing for the computation of the PPPs densities in A (line 17), using (20). Following that, the number of nodes fitting the generated configuration is computed by first obtaining the area of each SF ring in lines 18-19 as V i = π(l 2 i − l 2 i−1 ).…”