September 24, 2018
Increased bandwidths, more complex signal transmissions, and the ever-growing use of broadband services have changed how base stations are designed and how field engineers and site technicians maintain towers. Two by-products of LTE networks that have become prevalent over the past few years are the Common Public Radio Interface (CPRI) standard being integrated into base stations and passive intermodulation (PIM), which causes interference that can make networks fall short of achieving established KPIs.
We wrote a detailed post on CPRI on our sister blog, Base Station Transmits. It is a good resource to learn about the standard and how it is used in today’s LTE networks. In this post, however, we will focus on how to leverage CPRI to conduct PIM measurements.
PIM Test Options
PIM has become more problematic as network speeds have increased, bandwidths have widened, and signals have become more complex. Anritsu has developed a new technique – PIM Over CPRI – that takes a different approach to solving PIM problems. It is superior to the conventional method of using a spectrum analyzer to conduct noise floor measurements. This latter procedure is faulty, as it will not detect the cause of PIM, only if it is present.
The PIM Over CPRI capability is available in the BTS Master™ handheld analyzer. With this technique, field technicians can make “true” PIM measurements using CPRI links for a more time- and cost-efficient procedure for cell site maintenance.
Four Benefits of PIM Over CPRI
PIM over CPRI utilizes many of the same principles of the CPRI RF measurement capability Anritsu introduced in 2016. It builds off the field-proven process and expands it so field technicians can determine if passive intermodulation is present at the cell site more efficiently.
There are four main advantages associated with the PIM over CPRI method.
- “True” PIM Over CPRI – Since PIM Over CPRI uses live traffic, there are some inherent benefits relative to PIM testing with an RF PIM analyzer. One is that the LTE subcarriers and resource blocks (RBs) are used instead of calibrated 43 dBm or 46 dBm two-tone continuous wave (CW) signals. This allows a PIM desensitization value to be calculated rather than measuring a dBc result from a CW stimulus.
PIM desensitization can be thought of as the amount of noise in the uplink signal caused by PIM. This measurement is calculated by synchronizing and comparing all available downlink signals of a sector (up to 2x MIMO) against each individual uplink signal. The PIM over CPRI measurement supports SISO, 2x2 MIMO, and 2x4 MIMO.
The technique also determines if the PIM found is internal or external to the MIMO antenna system. This unique ability is essential when ascertaining if a tower crew needs to be called on-site to conduct repairs to correct a problem.
- Ground Level Testing – Traditional PIM measurements require crews to drive to the site and scale the tower to conduct measurements. With PIM Over CPRI, field technicians have both feet firmly on the ground when conducting PIM measurements. The result is considerable time and cost savings.
A tower crew is only called out to the site if the measurements reveal PIM inside the antenna system, which can be caused by faulty connectors and cables or metal shavings. If the PIM source is outside the antenna system, however, no crew is required. The field technician simply takes out his Anritsu’s PIM Hunter™ to find the PIM cause in a much faster and economical manner.
- Live Traffic – As mentioned earlier, the method allows tests to be conducted using live traffic. This helps better determine the PIM source and the means to correct it. Mobile operators will also realize operating expense (OpEx) and capital expense (CapEx) savings by conducting tests without having to take the site down.
The PIM Over CPRI process makes PIM measurements on a live system at ground level by monitoring regular LTE traffic carried by an optical fiber connection. By tapping into the CPRI data on the downlink and uplink between the remote radio head (RRH) and baseband unit (BBU), the BTS Master MT8220T base station analyzer can calculate the PIM desensitization of the LTE uplink, as shown in figure 1.
Figure 1 - Band Agnostic – Co-located base stations are the rule, not the exception, in today’s wireless networks, which means that PIM solutions need to test multiple frequencies. The BTS Master with PIM Over CPRI is band agnostic, one reason why it can measure live traffic in both the uplink and downlink in real-world scenarios, so field technicians can discover self-generated PIM.
A common scenario of why this is important is when multiple 15 kHz subcarriers in a single LTE signal mix and create PIM in their own uplink band, as well as 2nd and 3rd order harmonics. Figure 2 shows a typical set-up that measures harmonic PIM to locate an 850 MHz downlink generating PIM in a 1,700 MHz uplink and solve this problem.
Figure 2
How to Conduct PIM Over CPRI Measurement
PIM Over CPRI measurements with the Anritsu BTS Master can be conducted easily. Technicians just connect the test solution to two SFPs and an optical tap to measure the live traffic at the site. A step-by-step process to perform the test will be provided in our next post. Until then, learn more about PIM and how to test it by visiting our Passive Intermodulation technology page.
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