Enhanced LoRa-Like Transmission for Multiuser Interference Environments Using Variable-Slope Chirp Spread Spectrum Signals

Abstract

We consider the setting of Long Range (LoRa) transmission in the uplink of a multi-user system, where users operate with the same spreading factor. LoRa is a popular low-power wide area network wireless protocol that is designed for IoT applications. Traditionally, LoRa employs chirp spread spectrum (CSS). In this paper, we propose a novel chirp-based transmission scheme for LoRa-like systems, namely, variable-slope CSS (VS-CSS). While not adhering to the official LoRa standard, this scheme employs a modulation approach based on chirp spread spectrum (CSS), utilizing the same bandwidth and spreading factor ranges defined by LoRa. In this transmission scheme, the transmitted chirp signal consists of two sub-chirps with different slopes (i.e., different chirp rates), while keeping the initial and final frequencies the same for all symbols. The motivation for adopting this tailored LoRa-like design is to improve the performance when the system is subjected to interference from transmissions utilizing the same spreading factor. We analyze the cross-correlation properties of the VS-CSS signals. Furthermore, we derive a tight approximation of the bit error rate (BER) for VS-CSS transmission in an AWGN channel for single-user and two-user LoRa systems experiencing the same spreading factor interference with a basic correlator receiver. Our approximation takes into account the spreading factor (SF), partial cross-correlation properties, in addition to the signal-to-interference ratio (SIR). Our theoretical and numerical results show that VS-CSS outperforms traditional CSS in terms of BER under the same-SF interference. Moreover, VS-CSS is more robust against same-SF interference for lower SIR operation thresholds in comparison to traditional CSS without the need to use complex receivers.