The radio equipment control (located in a conveniently accessible site) contains the radio functions of the digital baseband domain, whereas the radio equipment itself (close to the antenna) contains the analogue RF functions.
The basic principle of a CPRI handheld analyser is that it captures a small amount of the optical signal power while the RRH is still communicating with the BBU and mobiles are still connected to the live network. To achieve this, it is necessary to use an optical tap (coupler) to insert the CPRI tester between the RRH and the BBU. For example, the Anritsu MT8220T BTS MasterTM can embed a CPRI board as an option to allow the field technicians to carry out diagnostics of the kind of spectrum the BBU is getting from the antenna via the RRH and optical fibre.
Most radio technicians worldwide will be familiar with RF spectrum measurements using the widely used MT8220T BTS Master, and because the man-machine interface for CPRI spectrum measurement is very similar to that for standard RF spectrum measurements they can intuitively understand what is happening on the optical link.
The user plan for the CPRI standard defines one other important parameter, which is the antenna carrier (normally designated AxC). This parameter contains the IQ data necessary for either reception or transmission of only one carrier at one independent antenna element, and has to be known by the users in order to select the right antenna to analyse the CPRI signal from the ground level.
As previously mentioned, looking at the spectrum shape of an RF signal often allows the user to quickly diagnose if everything is correct. This is illustrated in Figure 3, which sows two screen shots showing one “normal” RF CPRI spectrum plot and another one with an interferer located inside the channel itself.