RU (Radio Unit) or RRU (Remote Radio Unit) in wireless networks is a radio transceiver that connects to the Baseband Unit (BBU) of a base station.
The RU contains the base station's RF circuitry plus analog-to-digital/digital-to-analog converters and up/down converters. RUs also have operation and management processing capabilities and a standardized optical interface to connect to the rest of the base station.
When deployed as part of a DAS infrastructure, RUs are usually connected with a DAS Head-End via fiber optic cable.