A baseband unit (BBU) is a telecommunications network device used to process baseband signals. Baseband is the term used to describe the original frequency that is transmitted before modulation. A traditional radio access network (RAN) consists of a BBU connected to one or more remote radio units (RRUs) located near the antenna.

The baseband unit is responsible for communicating with the core network through the physical interface, while the remote radio unit performs the transmit and receive radio frequency functions. These two components are usually connected together by optical fibers.

Centralized functionality and fixed positioning at base station tower feet are two aspects of what 5G is redefining the meaning of a traditional BBU. Next-generation RAN architectures divide the BBU functionality into distributed units (DU) for real-time functions and centralized units (CU) for non-real-time functions such as radio resource control (RRC). Virtualization and disaggregation of 5G baseband units increases capacity and reduces latency.

What Does a Baseband Unit Do?

RAN is composed of baseband processing unit and radio frequency processing unit. The baseband unit acts as a centralized "hub" for the base station, handling uplink and downlink data traffic and controlling RRU functions. A traditional BBU contains a digital signal processor (DSP) to convert signals from analog to digital and vice versa.

Additional processing done by the baseband unit includes alarm monitoring and system clock manipulation for synchronization. The BBU also controls the transfer of user data, session management and mobility functions that define communication links between users.

Base station architecture initially placed the BBU and RRU on digital and radio racks within a cabinet or enclosure at the base of the cell tower. Base stations are connected to antennas by radio frequency cables, which are prone to loss and interference. The Common Public Radio Interface (CPRI) protocol, published in 2003, defines the transport and connectivity between the RRU and the BBU, allowing greater physical separation of these elements over the fronthaul link.

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