Fractional Chirp Rate Based CSS Multiple Access for LEO Satellite: Design and Implementation

Satellite Internet of Things (IoT) provides the possibility for global IoT service. The chirp spread spectrum (CSS) modulation is expected to be applied in the low earth orbit (LEO) satellite link for its resistance to the Doppler shift and time delay. However, the CSS modulation can hardly support the need for LEO satellite to serve a large number of users simultaneously. Hence, we propose a new CSS based modulation, named fractional chirp rate CSS (FCR-CSS) based modulation, which not only retains the advantages of CSS modulation but also can be used for the non-orthogonal multiple access in LEO satellite link. We design an access scheme where different terrestrial users transmit FCR-CSS signal with distinct chirp rates but the same spreading factor. We implement this scheme in a hardware system with the field-programmable gate array (FPGA) devices. The feasibility of our scheme is tested by simulation and experiments. The results validate that our scheme can achieve the multiple access with a satisfactory bit error rate performance.