XCopy: BoostingWeak Links for Reliable LoRa Communication

LoRaWAN suffers dramatic performance degradation over a long communication range due to signal attenuation and blockages. To ensure reliable data transfer, LoRaWAN adopts retransmission mechanism where an unacknowledged packet is retransmitted multiple times in the hope of successfully delivering the packet at least once over harsh wireless channels. This retransmission mechanism is ill-suited for LoRa: 1) unsuccessful retransmissions lead to high power consumption for battery-powered LoRa nodes, and 2) a retransmission at another time does not necessarily improve the signal strength over harsh wireless channels. This paper presents the design and implementation of XCopy, which effectively improves the signal strength by coherently combining retransmitted packets received over weak links that would otherwise be thrown away. XCopy develops and puts together novel algorithms to 1) accurately identify the signal copies of the same packet over multiple retransmissions in the presence of interfering packets, and 2) precisely align the signal copies (in time, frequency, and phase) to ensure constructive combining, which turns out to be very challenging in ultra-low SNRs, but made possible by XCopy. Evaluations show that XCopy can deliver significant SNR gains and yield higher packet reception ratio and longer lifetime of LoRa nodes.