March 7, 2016
To answer the need for more throughput at lower cost, wireless network providers have moved to using a remote radio head (RRH) where the radio equipment is connected to the baseband unit (BBU) by fiber optic cable. This creates a new level of flexibility in cell site deployment, including siting the RRH at the masthead to reduce RF losses or locating the BBU at a remote location for improved operational efficiencies.
This flexible architecture needs an equally flexible interface between the BBU and RRH. That’s where Common Public Radio Interface (CPRI) technology fits. CPRI pushes the complexity into the higher layers of the system so a connection between the BBU and the RRH can be established with minimal configuration.
While the RRH-based configuration has alleviated common problems with the traditional cell site, it creates a new requirement for testing these sites. Testing is particularly important for two reasons. The first is the increased number and density of wireless networks. Secondly, the congested spectrum makes networks much more vulnerable to RF interference and Passive Intermodulation (PIM). The final interface exists at RF, but testing requires costly tower climbing crews. Operators need a more cost-effective ground-based test method.
Fortunately, new test solutions are available that make it possible to conduct a wide range of performance tests on the site from the ground using the CPRI link to troubleshoot interference issues that are the most common cause of reduced cell site performance. Being able to perform RF measurements through the CPRI link provides a powerful tool for hunting down interference and PIM without tower climbs and eliminate visiting the cell site where the BBU is sited remotely.
How CPRI Works
A CPRI link transports digitized RF signals in a complex baseband format. Each sample has an in-phase (I) and quadrature (Q) component. The CPRI specification was created to handle the signal data in this format. The I and Q samples are interleaved together to create a single word. Those words are put together in a pattern to satisfy the target sampling rate and bit width of the signal.
CPRI allows different bit widths to be used, depending on the air interface requirements and whether the signal is in the uplink or downlink. In the downlink, the number of bits can range from 8-20. To allow for higher sampling rates in the uplink, 4-20 bits are used. Also, smaller width IQ samples can be sign-extended or padded with reserve bits to fill the space of larger bit widths. This is useful if the IQ sample processing in the downlink and uplink have different widths, but the user prefers to have the same size for both when mapping the samples onto the CPRI link.
Analyzing RF Interference over CPRI
Field technicians can use handheld instruments such as the one in figure 1 to extract IQ data from the link to perform signal and interference analysis using the digital data in a manner similar to that of measuring RF data. Modulation testing can be performed in the downlink to verify correct BBU operation, CPRI levels and compliance of the digital signal with the RRH capability – with both feet on the ground.
Figure 1
CPRI-based RF testing is especially powerful in the uplink. Since user equipment (UE) has much lower transmit power compared to an RRH, interference has the biggest system impact in the uplink; figure 2 shows actual Spectrum of the Uplink via the CPRI link. The uplink is also susceptible to interference from PIM. As such, field technicians and engineers need handheld analyzers that can conduct fast sweeps, as well as the ability to tune and pan on a displayed signal and zoom into a particular area of interest. Faster sweep speeds and more detailed signal analysis allow for more efficient location of PIM and interference sources.
Figure 2
Optical Taps
Interference hunting using CPRI has also been aided by installing optical taps at cell sites. An optical tap couples a percentage of the optical signal so an external device can monitor both the uplink and downlink CPRI link with no disruption. RF interference can then be analyzed on a live system without locking or disabling the network. In chain or ring configurations, multiple RRHs can be investigated simultaneously. PIM detection can be performed by connecting one fiber containing the affected uplink data and another to potential PIM sources (downlink data).
Using a handheld field analyzer, such as the Anritsu BTS Master™, capable of conducting CPRI RF measurements to be made at ground level eliminates the need to call a tower climbing crew. Many common causes of poor KPI indicators can be diagnosed, especially those resulting from accidental or illegal transmitters interfering with the uplink.
Combined with fewer tower climbs, reduced maintenance time, and lower operating expenses, uplink testing over CPRI becomes an even more powerful tool. To learn more, you can download a new white paper entitled Improving Wireless Network Flexibility Using CPRI Technology.
