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Interference testing on CPRI links at wireless cell sites

Interference testing on CPRI links at wireless cell sites

Technology News |
By Jean-Pierre Joosting



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.

Figure 1: Evolution of cell site architecture from conventional design with coaxial cable to distributed antenna system with optical fibre.

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.


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.

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.

 

Spectrogram display

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.

Figure 3: Representation of a LTE 10 MHz bandwidth transmission without and with an interferer in the UL band.

In addition, having the ability to observe the behaviour of the interferer over time using the spectrogram display allows the technician to better emphasise the frequency and amplitude stability of the interfering signal. By colour coding the amplitude level, it becomes easy to see at a glance what occurs in the chosen frequency carrier and channel band (Figure 4).

Figure 4: Spectrogram of a LTE 10 MHz channel without and with a zooming capability highlighting an interferer’s behaviour.

Once the interferer has been seen from the ground level and the frequency identified by using markers on the plot, the technician can then use a conventional RF spectrum analyser to hunt for the interferer’s location on the basis that its amplitude will become higher closer to the source. However, as interferers may occur randomly, it is not always easy or fast to locate them in dense environments. In this case, it may sometimes be useful to drive to a neighbouring site to determine if this new site gets more disturbed by the same interferer or not.

 

Conclusion

Mobile phone subscribers all expect good Quality of Service from their cellular network operator including the ability to connect anytime, anywhere. Having deployed CPRI technology in the field, network operators need to work hard to prevent any network communication drops or failures caused by interference within the system. CPRI testing is therefore becoming more and more important, and the latest handheld instruments offer new ways of investigating interference problems and showing the test results.

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