Interference testing on CPRI links at wireless cell sites: Page 2 of 4

July 04, 2016 // By Cyril Noger, Anritsu
A key economic aim for wireless network operators is to reduce the amount of money spent in deploying the network in the field while maximising the cell coverage and ensuring better quality of service despite the impact of interference on the quality of reception.

Measurement tools

With CPRI links, it now becomes possible, with appropriate handheld measurement tools, to perform accurate and fast analysis for troubleshooting the radio network by decoding the CPRI link between the RRH and the BBU without the need to climb the tower.

Essentially, CPRI technology converts radio frequency signals from the electrical to the optical domain. This is what the RRH does on top of the tower when it receives the RF signals (for instance W-CDMA for 3G or LTE for 4G technologies) and converts them to optical signals (using CPRI protocol) before they are sent down to the BBU at ground level. The BBU then converts the information from optical to electrical signals to deal with the network at the backhaul stage.

Unfortunately, even though optical fibres are less sensitive to external spurious distortion or interference effects, they do not eliminate the problems that occur if the data coming from the antenna are corrupted by added noise on top of the signal, or if there are any extra unwanted frequencies inside the signal band itself. External interference still arrives from the antenna to the BBU via the RRH and the optical link.

Analysing the CPRI link in the field with a handheld instrument requires the RF IQ data transmitted from the RRH to the BBU (uplink channel) and from the BBU to the RRH (downlink channel) to be decoded. This then gives access to the representation of the RF channel spectrum being carried in the optical fibre. Looking mainly at the uplink, technicians can more easily understand the kind of disturbance affecting the network that prevents mobile phones from connecting normally to the base stations in the field.

As CPRI is a well-defined standard adopted by almost all the big infrastructure vendors, implementing a CPRI board into a handheld analyser allows users to decode and analyse the two main layers of the standard to troubleshoot alarms and errors in addition to looking at the physical transport of the data.

Figure 2 illustrates the link between the radio equipment and the radio equipment control. The “network interface” is connected to the backhaul and the “air interface” is connected to the antennas, as specified in the CPRI specification v6.0 (2013-08-30).

Figure 2: The CPRI standard defines the connection of the BBU and the RRH with protocol layers between them.

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