Today’s wireless communication systems are sensitive to interference, especially with LTE when this occurs at the centre of the channel spectrum, and engineers need to locate and isolate any problems and visualise the cause of the interference in order to deal with it. From the operators’ point of view, interference can create dropped calls, shrink the cell coverage, decrease the data rate (by increasing the bit error rate) and reduce the quality of service between the mobile phones and the network. These problems cost the operators money as they lose subscribers.
A major change occurred in wireless communication systems with the adoption of centralised radio access networks (C-RAN) by the industry as a contribution to cost reduction. With the use of C-RAN, the new architecture of the mobile front-haul connection is configured with centralised baseband units (BBUs) controlling multiple, distributed remote radio head (RRH) units at antenna sites (Figure 1). The RRHs sit on top of the antenna towers, whereas the BBUs are at ground level.
In the past, the BBUs and the RRHs were connected together via coaxial cables that were sensitive to effects such as power losses, aging, corrosion and intermodulation. To prevent most of these effects, a new common standard, CPRI (Common Public Radio Interface), has been adopted by most network infrastructure vendors, using optical fibres rather than coaxial cables. Operators also use optical fibre to reduce the costs of installation and maintenance of each cell site.
This article focuses on the new challenges involved in making CPRI measurements in the field.