Doppler frequency code division multiple access method for LEO mega-constellation navigation systems

Low Earth orbit (LEO) satellites have the potential to provide positioning, navigation and timing services in the future and are attracting widespread attention from researchers. However, due to rapid movements and a large number of LEO satellites, directly adopting the traditional code division multiple access (CDMA) method in the LEO mega-constellation navigation system will introduce a high computational complexity to the signal acquisition process of receivers and make the acquisition time much longer if receivers are in case of a cold start without assistance data. To decrease the acquisition complexity, we propose a new Doppler frequency code division multiple access (DFCDMA) method for LEO mega-constellation navigation systems. In this method, navigation signals broadcast by satellites in the same orbital plane are modulated with the same pseudorandom noise (PRN) code, while navigation signals broadcast by satellites in different orbital planes are modulated with distinct PRN codes. Through theoretical analysis and simulation, it is shown that satellites in the same orbital plane can achieve multiple access only by Doppler frequency at any position on the ground if the receiver velocity is less than 500 m/s. Therefore, low-speed receivers can distinguish LEO navigation signals by multiple combinations of different Doppler frequencies and PRN codes acquired. The results show that DFCDMA can achieve a fast acquisition and significantly shorten the acquisition time with similar acquisition sensitivity compared to CDMA in case of a cold start. Compared with the Doppler frequency code phase division multiple access (DFCP-DMA), DFCDMA has higher availability in the case of LEO mega-constellations and multipath environments.