A Low-Power Demodulator for LoRa Backscatter Systems With Frequency-Amplitude Transformation

The radio range of backscatter systems continues growing as new wireless communication primitives are continuously invented. Nevertheless, both the bit error rate and the packet loss rate of backscatter signals increase rapidly with the radio range, thereby necessitating the cooperation between the access point and the backscatter tags through a feedback loop. Unfortunately, the low-power nature of backscatter tags limits their ability to demodulate feedback signals from a remote access point and scales down to such circumstances. This paper presents Saiyan, an ultra-low-power demodulator for long-range LoRa backscatter systems. Saiyan is based on an observation that a frequency-modulated chirp signal can be transformed into an amplitude-modulated signal using a differential circuit. Moreover, we redesign a LoRa backscatter tag which integrates Saiyan and a ring oscillator-based modulator. The LoRa backscatter tag enables re-transmission, rate adaption and channel hopping - three PHY-layer operations that are important to channel efficiency yet unavailable on existing long-range backscatter systems. We prototype Saiyan and the LoRa backscatter tag on two PCB boards and evaluate their performance in different environments. Results show that Saiyan achieves 3.5- 5\times gain on the demodulation range, compared with state-of-the-art systems. Our ASIC simulation shows that the power consumption of Saiyan and the LoRa backscatter tag are around 93.2~\mu W and 94.7~\mu W . Code and hardware schematics can be found at: https://github.com/ZangJac/Saiyan .