Remote Radio Unit
Remote Radio Unit (RRU) - an enclosed radio transceiver with the baseband and control interfaces exposed to allow for remote control. Usually, these devices are used extensively by telecoms, but remain unknown to the amateur radio world.
Repeaters
Repeaters can be analog, digital or mixed-mode, but the underlying principle remains the same.
Classic approach
To build a simple, half-duplex, analog repeater, a number of components is required:
- receiver,
- transmitter,
- controller board - used to trigger the transmitter when the receiver detects a valid signal,
- duplexer
Of course some additional hardware is required, such as power supply, antenna or feed line. Most of these parts have to be placed in a room or enclosure of some kind, protecting it from external conditions like sun and rain. They need to be connected to the antenna, sometimes tens of meters away.
RRU approach
With the RRU located close to the antenna, the feed line loss and interference are minimized. The receiver, transmitter, duplexer, and power supply are all embedded within the RRU block. If the device supports complex baseband streams (IQ), an additional GNU Radio block applying digital predistortion can be added to greatly improve Adjacent Channel Power Ratio (ACPR).
Interfacing
The difference between the classic and RRU approach is the split between the radio part and the control part. The RRU device does not do anything on its own, other than "pushing" baseband data to and from the device.
Control protocol
The control interface uses Common Amateur Radio Interface (CARI). The protocol is supposed to mimic professionally used Common Public Radio Interface (CPRI) and offer a part of its functionality. CARI allows for basic data, control and supervision plane access, letting the user directly control radio devices over a given medium. ZeroMQ is used for all data transfers.
Hardware
- rru-rf-hw - the RF board
- rru-rf-chassis - milled aluminum case for the RF board
- rru-cm4-hw - RRU Interface Board using a RaspberryPi CM4 module
- rru-enclosure - customized 50C-4 telecom chassis
Software
- cari-host - CARI host for the RRU Interface Board module
Firmware
- rru-rf-fw - firmware for the RF board (STM32 MCU)