De-spreading over the air
Shuai Tong, Yangliang He, Yunhao Liu, Jiliang Wang
Abstract
Unlicensed LPWANs on ISM bands share the spectrum with various wireless techniques, such as Wi-Fi, Bluetooth, and ZigBee. The explosion of IoT deployments calls for an increasing need for long-range cross-technology communication (CTC) between LPWANs and other techniques. Yet, existing technologies cannot achieve real long-range CTC for commodity wireless. We propose L2X, which provides long-range CTC to diverse receivers with LoRa transmitters. At the heart of L2X, we design an energy-concentrating demodulation mechanism that de-spreads LoRa chirps over the air. Therefore, L2X enables non-LoRa receivers to detect and demodulate LoRa signals even under extremely low SNR. We address practical challenges in L2X design. We propose a packet detection method to detect low-SNR LoRa transmissions at non-LoRa receivers. To decode LoRa transmissions, we accurately synchronize the demodulation window with incoming packets and propose a cross-domain demodulation approach to enhance the demodulation SNR. We implement L2X, all using commodity devices, and extensively evaluate its performance. The results show that L2X achieves 1.2 km CTC with the signal -9 dB below the noise floor, improving the distance by 30X compared with state-of-the-arts.