Many cellular networks have begun to deploy a distributed architecture where the traditional radio has been split into a baseband unit (BBU) and remote radio head (RRH). This provides the ability to replace the coaxial RF cable from the radio unit at the base of a tower to the antenna array at the top of a tower with a fiber optic cable. RF cables have limitations in that they suffer power loss and are subject to performance degradation over time due to damage or corrosion.
The BBU and RRH are typically connected with a fiber link conforming to the common public radio interface, or CPRI. When installing a system with fiber connections, it is important to validate that the CPRI interface is performing to specification. By installing an optical Test Access Point (TAP) or splitter into the link, the spectrum of the uplink and downlink can be monitored for interference, Passive Intermodulation effects and noise.
Ferdinand Gerhardes, Anritsu GmbH on behalf of Livingston