Why did Beidou choose CDMA?

In terms of technical development, there is frequency division multiple access (FDMA) first and then code division multiple access (CDMA), which can be said to be more advanced than the former. Using advanced technology to design navigation constellation and choose code division multiple access to modulate navigation signals is superior to frequency division multiple access in anti-interference performance, compatibility with other navigation systems and receiver design. In addition, under the same index restriction, using frequency division multiple access (FDMA) to modulate navigation signals is technically more complicated for space segments and user machines. In addition, from the point of view of system design, CDMA has higher spectrum utilization. At present, due to the extremely limited spectrum resources in L-band, it is impossible to use frequency resources in a "extravagant" way as when designing GLONASS. So Beidou, like GPS and Galileo, adopts code division multiple access. Therefore, regardless of technology or compatibility, Beidou satellite will choose CDMA.