Baseband BBU is centralized in the computer room, and RRU can be installed on the floor. Optical fiber transmission is adopted between BBU and RRU, and RRU is connected to antenna through coaxial cable and power divider (coupler), that is, optical fiber for trunk line and coaxial cable for branch line.
For the downlink direction: the optical fiber is directly connected from BBU to RRU, and the baseband digital signal is transmitted between BBU and RRU, so that the base station can control a user's signal to be transmitted from the designated RRU channel, which can greatly reduce the interference to users on other channels in the cell.
For the uplink direction: the user's cell phone signal is received by the nearest channel, and then transmitted to the base station through optical fiber from this channel, which can also greatly reduce the interference between users on different channels.