Experimental Study of LoRa Modulation Immunity to Doppler Effect in CubeSat Radio Communications

Currently, LoRa technology is one of the most promising technologies in satellite Internet of Things. Particularly those based on satellite constellations in low Earth orbit, including the CubeSat nanosatellite constellations. However, the LoRa specification does not contain clear criteria for the applicability of the LoRa modulation under strong Doppler effect conditions caused by the very high speed of satellites. This is especially true in the case of the dynamic Doppler effect when the Doppler frequency shift changes rapidly with time. This paper presents the results of laboratory testing and outdoor experiments conducted to determine the feasibility of the LoRa modulation in CubeSat radio communication systems. Additionally, possible restrictions associated with the Doppler effect were explored. The experiments showed that the LoRa modulation has very high immunity to the Doppler effect. This immunity allows for the use of LoRa modulation in satellite radio communications in orbits above 550 km without any restrictions associated with the Doppler effect. In lower orbits, the dynamic Doppler effect leads to the destruction of the satellite-to-Earth radio channel when using the LoRa modulation mode with a maximum spreading factor of SF = 12. This destruction occurs when the satellite is flying directly above the ground station, resulting in reduced duration of the radio communication session. The reduction in the duration of a communication session increases with decreasing orbit altitude and reaches about one minute in an ultra-low orbit 200 km high.