A Dual Waveform Differential Chirp Spread Spectrum Transceiver for LEO Satellite Communications

In recent years, chirp spread spectrum (CSS) has been used in the Long Range (LoRa) technology, which has been widely deployed for terrestrial low power and long-range wireless Internet of Things (IoT) communications. In order to ensure the connection of any object whatever its position on earth, constellations of satellites in low Earth orbit (LEO) can be used. However, the sensitivity of the receivers to establish earth-space CSS communications requires modifications. In this work, we propose a modification of the LoRa physical layer that we refer to as differential chirp spread spectrum (DCSS) to cope with the requirements of LEO satellite communications. The latter modulation is naturally insensitive to any carrier frequency offset (CFO) induced by the Doppler effect and existing mismatches between local oscillators at transmission and reception. Hence we propose a time synchronization approach regardless of the CFO using non-chirp-based learning sequences. This processing makes our receiver have no limitation on the maximum CFO estimable as in the current LoRa receivers. In addition, in the presence of a Doppler shift variable in time, DCSS is much more efficient than CSS, making our proposed receiver a good candidate for the LEO satellite communications.