Spectral-Efficient LoRa with Low Complexity Detection

TL;DR

The paper tackles the need for higher spectral efficiency in LoRa without resorting to computationally expensive detectors. It introduces SE-LoRa, a modulation that transmits multiple time-shifted LoRa chirps within a single window, and derives the optimal joint ML detector, whose complexity is prohibitive in practice. A low-complexity SIC-based detector is then developed, exploiting the frequency-domain structure of the dechirped SE-LoRa signal to cancel interference from neighboring chirps, achieving complexity on the order of conventional LoRa. Simulations across AWGN, Rayleigh, and Rician channels show substantial spectral-efficiency gains up to $445.45\%$, $1011.11\%$, and $1071.88\%$ for SFs $7$, $9$, and $11$, with SER within $<3$ dB of conventional LoRa in rural channels. Overall, SE-LoRa with SIC-based detection offers a practical pathway to higher data-rate LoRaWAN IoT networks with manageable complexity.

Abstract

In this paper, we propose a spectral-efficient LoRa (SE-LoRa) modulation scheme with a low complexity successive interference cancellation (SIC)-based detector. The proposed communication scheme significantly improves the spectral efficiency of LoRa modulation, while achieving an acceptable error performance compared to conventional LoRa modulation, especially in higher spreading factor (SF) settings. We derive the joint maximum likelihood (ML) detection rule for the SE-LoRa transmission scheme that turns out to be of high computational complexity. To overcome this issue, and by exploiting the frequency-domain characteristics of the dechirped SE-LoRa signal, we propose a low complexity SIC-based detector with a computation complexity at the order of conventional LoRa detection. By computer simulations, we show that the proposed SE-LoRa with low complexity SIC-based detector can improve the spectral efficiency of LoRa modulation up to $445.45\%$, $1011.11\%$, and $1071.88\%$ for SF values of $7$, $9$, and $11$, respectively, while maintaining the error performance within less than $3$ dB of conventional LoRa at symbol error rate (SER) of $10^{-3}$ in Rician channel conditions.

Figures (14)

• Figure 1: SE-LoRa transmission scheme for $\tau=T/4$ (Note that for illustration purposes, we set $s=0$ for all of the presented chirps)
• Figure 2: LoRa frame structure
• Figure 3: DFT of the dechirped LoRa signal with ${\rm SF}=7$ and $s=56$.
• Figure 4: Real part of DFT of the dechirped SE-LoRa signal with $K=3$, $\rm SF=7$, and arbitrary LoRa symbols of $s_{-2}=10$, $s_{-1}=30$, $s_{0}=50$, $s_{1}=70$, and $s_{2}=90$.
• Figure 5: Real part of DFT of the dechirped SE-LoRa signal with $K=4$, $\rm SF=7$, and arbitrary LoRa symbols of $s_{-3}=10$, $s_{-2}=30$, $s_{-1}=50$, $s_{0}=70$, $s_{1}=90$, $s_{2}=100$, and $s_{3}=120$.