Hardware doppler shift emulation and compensation for LoRa LEO satellite communication

Abstract

LoRa is the de facto wireless platform of the Internet of Things (IoT). It is a smart and globally accepted low power ultra-long range, low-data rate communication protocol widely used for low frequent sensor telemetry and actuation. The simplicity, license-free spectrum, and long range enable the end user to transmit data even to Low Earth Orbit (LEO) satellites. The relative velocity of the LEO satellite from a stationary transmitter on the earth's surface is very high and time-varying, because of its short orbital period and high velocity, due to which the doppler shift and doppler rate are more prominent. LEO satellite signal reception is highest at the mid-pass duration, but unfortunately, the doppler rate is also highest during the same duration. In LoRa communication protocol, a higher spreading factor (SF12) results in better signal reception and communication range. But the LoRa communication at a higher spreading factor (SF12) results in packet error mainly due to the doppler rate, due to which a transmitter is forced to choose a lower spreading factor (SF9). This paper looks at precisely emulating the doppler shift and doppler rate for the entire pass duration of a LEO satellite using dynamic external hardware clock manipulation on an SX1262 LoRa chipset. A low-cost transmitter-side hardware doppler compensation technique is also explored, which encourages the transmitter to use a higher spreading factor (SF12) for better-guaranteed packet reception by the satellite.