What is Remote Radio Head ? 

 
RRH, Remote Radio Head

It is a special component located on top of cell towers, base stations, which mainly perform the following functions :

i) converting optical signals into electrical signals;

ii) converting electrical signals into optical signals;

iii) Up/Down frequency conversion;

iv) amplifying radio frequency signals;

The most important features of RRH are items i) and ii), which means that the eNB/base station can send and receive data directly to the top of the tower via optical fiber, compared to the traditional method of using thick RF coaxial cable, because the cable is very short, so Energy loss is almost negligible

The main components inside the RRU and its place in the network

RRH consists of base station RF unit, filter, analog-to-digital converter, digital-to-analog converter and up/down converter. 

The connection between the RRH and the rest of the base station (BBU) is mainly achieved through fiber optic links.

The communication between the RRH and the rest of the base station (BBU), ie control and management, synchronization and transmission of I/Q data, etc., is based on the CPRI protocol. 

Therefore, test systems designed to test RRHs must support both RF and CPRI interfaces to fully characterize the DUT.

CPRI interface between RRH and BBU and the flow of upstream and downstream signals

Why should we use RRH?

As I briefly mentioned above, the biggest motivation for using an RRH is to reduce energy loss while transmitting the signal back and forth between the antenna (the top of the base station's transmit tower) and the processing unit at the bottom of the tower.

In regular base stations, they usually put the amplifier on the bottom and transmit the signal over thick coax. But the path loss of the coaxial cable is relatively large, so the base station RF module has to amplify more signals to compensate for the energy loss of the coaxial cable.

To reduce this problem, it is possible to place the amplifier unit at the top of the tower (this unit is called a TMA (Tower Mount Amplifier)), but I don't think this idea is widely adopted because the installation becomes complicated.

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