Energy harvested end nodes and performance improvement of LoRa networks

Abstract

LoRa technology is derived from chirp spread spectrum (CSS) having embedded forward error correction (FEC). A wide band is used for transmissions to counter interference and to handle frequency offsets. The paper investigates low power wide area networks (LPWAN) transmissions in the uplink, where the end nodes are powered by using energy harvested from the surroundings. Long-range (LoRa) networks demonstrate their capability to support Internet of Things (IoT) applications, where the end nodes utilize the harvested energy for transmission to gateways using different spreading factor (SF) codes. The work fairly improves the throughput of the LoRa nodes while keeping the other parameters, like time duration of the energy harvesting (EH), SF, and transmit power, optimally. Initially, a mathematical expression is derived for collisions between packets of the end nodes; keeping this as an important factor, an algorithm is proposed that fairly assigns SFs to the nodes. Simulation results confirm the improvement in packet error rate and time on air when fewer LoRa nodes are used for lower SFs, as compared to higher SFs. The number of LoRa nodes that can communicate using SF=7 is almost four times as compared to using SF=11, while maintaining a low packet error rate. Also, for SF=7, changing the coding rate from 1 to 4 increases time on air by around 20ms, while time on air increases by 1,200ms for SF=12. The energy efficiency is also compared for different SFs and different transmission powers. A lower SF and lower transmission powers are more suitable for smaller distance and provides better energy efficiency.