  Recent years have witnessed the proliferation of Low-power Wide Area Networks
(LPWANs) in the unlicensed band for various Internet-of-Things (IoT)
applications. Due to the ultra-low transmission power and long transmission
duration, LPWAN devices inevitably suffer from high power Cross Technology
Interference (CTI), such as interference from Wi-Fi, coexisting in the same
spectrum. To alleviate this issue, this paper introduces the Partial Symbol
Recovery (PSR) scheme for improving the CTI resilience of LPWAN. We verify our
idea on LoRa, a widely adopted LPWAN technique, as a proof of concept. At the
PHY layer, although CTI has much higher power, its duration is relatively
shorter compared with LoRa symbols, leaving part of a LoRa symbol uncorrupted.
Moreover, due to its high redundancy, LoRa chips within a symbol are highly
correlated. This opens the possibility of detecting a LoRa symbol with only
part of the chips. By examining the unique frequency patterns in LoRa symbols
with time-frequency analysis, our design effectively detects the clean LoRa
chips that are free of CTI. This enables PSR to only rely on clean LoRa chips
for successfully recovering from communication failures. We evaluate our PSR
design with real-world testbeds, including SX1280 LoRa chips and USRP B210,
under Wi-Fi interference in various scenarios. Extensive experiments
demonstrate that our design offers reliable packet recovery performance,
successfully boosting the LoRa packet reception ratio from 45.2% to 82.2% with
a performance gain of 1.8 times.
