Fractional Chirp Rate Based CSS Division Multiple Access over LEO Satellite Internet-of-Things

Low earth orbit (LEO) satellites are bringing new opportunities for the integration between terrestrial Internet-of-Things (IoT) and satellite IoT. Due to its high robustness against large time delays and Doppler shifts, chirp spread spectrum (CSS) modulation, i.e., the key technology of the Long-Range (LoRa), is expected to empower the satellite link. However, the ALOHA protocol employed by LoRa will inevitably lead to collisions over the satellite channels. In this paper, we focus on the concurrent uplink transmission over the LEO satellite IoT, which is based on CSS. We carefully analyze the relationship between the chirp rate and its spreading factor (SF). Then, we propose the fractional chirp rate based CSS modulation, and support terrestrial users to achieve the non-orthogonal multiple access with the same SF, which ensures that the users possess the same noise immunity. We derive the bit error rate (BER) for both the synchronous and asynchronous scenarios. The performance of our scheme is tested by simulation. Results show that our scheme can achieve the multiple access while maintaining a satisfactory BER performance and is robust over the asynchronous scenario. Furthermore, we build a hardware system using the field-programmable gate array (FPGA) devices to validate the feasibility of this system.