1, the mobile phone sends a digital signal to the antenna;
2, RRU is the signal source, the upper level is BBU, BBU is connected with GPS, GPS receives the signal and transmits it to BBU, BBU goes to RRU at the next level, and RRU goes to antenna, and then the mobile phone terminal receives the signal transmitted by antenna. Now there are devices that integrate RRU and antenna.
If I remember correctly, RRU's filter is a high-pass filter.