What is RRH (Remote Radio Head), How it is connected to BBU ?
Remote Radio Head (RRH) is installed on the tower near the antenna. RRH is also know and Remote Radio Unit (RRU). The RRH contains the base station’s RF circuitry plus analog-to-digital or digital-to-analog converters and up/down converters.
How RRH Works
It is sitting on top of cell tower that mainly performs following functions:
- Convert optical signal to electrical signal and vice versa using CPRI
- In transmitter section of RRH, it convert digital signal to RF and amplifies that signal to the desire power level and Antenna connected to it, radiates the RF signal in air
- In Receiver section of RRH, it receives the desired band of signal from antenna and amplify it.
- And convert RF signal back to digital signal in the receiver chain.
RRH Block Diagram
RRH has been made of RF transmitter chain and RF receiver chain which includes the ADC (Analog to digital converter), DAC (Digital to analog converter), Mixer, PA (Power amplifier), LNA (Low Noise Amplifier) etc. Basic Block diagram of RRH is as shown below:
RRH is connected to the Base band unit (BBU) via optical cable. Optical cable is used because it has less loss and it is cheaper as compared to RF Coaxial cable. One base-band unit is connected to multiple RRHs depending upon the capability of base-band unit.
Internal parts of RRH is shown in following picture.
Types of RRH
There are following type of RRH or RRU available.
- Single Band RRH: Support single frequency band e.g. 1800 MHz
- Multi-band RRH: A multi band RRH support more than one bands. e.g. 900/1800/2100MHz.
- Massive MIMO RRH: These RRH comes with multi antenna ports support massive MIMO feature
- Integrated RRH: Integrated RRH have RRH and Antenna into a single enclosure. This are also know as active antenna system
Common RRH Issues
When RRH is operation in the field, its health can be monitored with following alarms and issue can be identified.
- VSWR Alarm: High VSWR alarm is reported when there is connection problem between RRH and the antenna system. It could because of loose antenna connectors, rusted connectors and cable fault
- Over Temperature: A RRH has desired operating temperature range. When Temperature beyond this range an over temperature is raised by RRH
- Optical Alarm: RRH connects to BBU or DU system using CPRI cable which is an optical cable. If cable in not properly plugged or damaged an optical alarm is raised.
- Power Alarm: Most of the RRH works on DC power and have certain defined operating voltage range. If voltage is outside working range this alarm is raise.
- Synchronization Alarm: RRH used clock over CPRI from BBU/DU or it can use a dedicated GPS
