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A Low-Complexity LoRa Synchronization Algorithm Robust to Sampling Time Offsets

Orion Afisiadis, Mathieu Pierre Xhonneux

LoRaWAN is nowadays one of the most popular protocols for low-power Internet of Things communications. Although its physical layer, namely LoRa, has been thoroughly studied in the literature, aspects related to the synchronization of LoRa receivers have received little attention so far. The estimation and correction of carrier frequency and sampling time offsets (STOs) are, however, crucial to attain the low sensitivity levels offered by the LoRa spread-spectrum modulation. The goal of this article is to build a low-complexity, yet efficient synchronization algorithm capable of correcting both offsets. To this end, a complete analytical model of a LoRa signal corrupted by these offsets is first derived. Using this model, we propose a new estimator for the STO. We also show that the estimations of the carrier frequency and the STOs cannot be performed independently. Therefore, to avoid a complex joint estimation of both offsets, an iterative low-complexity synchronization algorithm is proposed. To reach a packet error rate of 10(-3), performance evaluations show that the proposed receiver requires only 1 or 2 dB higher signal-to-noise ratio than a theoretical perfectly synchronized receiver, while incurring a very low computational overhead.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

A Maximum-Likelihood-Based Two-User Receiver for LoRa Chirp Spread-Spectrum Modulation

Orion Afisiadis, Alexios Konstantinos Balatsoukas Stimming, Andreas Peter Burg, Joachim Tobias Tapparel, Mathieu Pierre Xhonneux

Long Range (LoRa) is an emerging low-power wide-area network technology offering long-range wireless connectivity to Internet of Things (IoT) devices. For energy efficiency reasons, LoRa end nodes implement a nonslotted ALOHA multiple access scheme to transmit packets to the gateway. Due to the lack of synchronization between end nodes, collisions between uplink packets have been identified as the main obstacle to the scaling of dense LoRa networks. To tackle this issue, we present in this article a LoRa receiver that is capable of decoding colliding packets from two interfering end nodes. The proposed two-user detector is derived from the maximum-likelihood principle using a detailed model of two colliding LoRa packets. As the complexity of the maximum-likelihood sequence estimation is prohibitive, complexity-reduction techniques are introduced to enable practical implementations of the receiver. An in-depth performance analysis highlights that the proposed two-user detector inherently leverages the differences in received power, time offsets, and frequency offsets between the users to separate and demodulate their respective signals. To demonstrate the practicality of the proposed detector, an interference-robust synchronization algorithm is then designed and evaluated. Simulation results indicate that a LoRa receiver combining the proposed synchronization algorithm and two-user detector is capable of detecting and demodulating two interfering users with satisfactorily error rates.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

On the Advantage of Coherent LoRa Detection in the Presence of Interference

Orion Afisiadis, Alexios Konstantinos Balatsoukas Stimming, Andreas Peter Burg, Sitian Li, Joachim Tobias Tapparel

It has been shown that the coherent detection of long range (LoRa) signals only provides marginal gains of around 0.7 dB on the additive white Gaussian noise (AWGN) channel. However, ALOHA-based massive Internet-of-Things systems, including LoRa, often operate in the interference-limited regime. Therefore, in this work, we examine the performance of the LoRa modulation with coherent detection in the presence of interference from another LoRa user with the same spreading factor. We derive rigorous symbol- and frame error rate (FER) expressions as well as bounds and approximations for evaluating the error rates. The error rates predicted by these approximations are compared against error rates found by Monte Carlo simulations and shown to be very accurate. We also compare the performance of LoRa with coherent and noncoherent receivers and we show that the coherent detection of LoRa is significantly more beneficial in interference scenarios than in the presence of only AWGN. For example, we show that coherent detection leads to a 2.5-dB gain over the standard noncoherent detection for a signal-to-interference ratio (SIR) of 3 dB and up to a 10-dB gain for an SIR of 0 dB. Moreover, we show that with coherent detection it is easier to obtain useful and relevant FER values even for negative SIR values.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021