The importance of PIM testing

The modern communications system is required to perform at a very efficient level. This quality standard has changed greatly during the past few years and will continue to change as current 3G and 4G systems compete for market share and increase infrastructure and coverage. Line sweep testing is more important than ever and emerging Passive Intermodulation (PIM) testing requirements have lifted the bar even higher. Ultimately, this means contractors, cell technicians and performance engineers must work closely together to achieve the required quality of service.

PIM basics
PIM, often referred to as the diode effect or the rusty bolt effect, is a form of passive intermodulation distortion that occurs in components normally thought of as linear, such as cables, connectors, and antennas. However, when subjected to the high-power RF signals found in cellular systems, these devices can generate spurious signals.

An on-site PIM test is a comprehensive measure of linearity and construction quality. PIM appears as a set of unwanted signals created by the mixing of two or more strong RF signals in a non-linear device, such as in a loose or corroded connector.

This pair of formulas can predict PIM frequencies for two carriers:
nF1 – mF2
nF2 – mF1

F1 and F2 are carrier frequencies and the constants n and m are positive integers. When referring to PIM products, the sum of n + m is called the product order, so if m is 2 and n is 1, the result (2+1=3) is referred to as a 3rd order product. Typically, the 3rd order product is the strongest, causing the most harm, followed by the 5th and 7th order products, which also cause significant harm. An example is shown in Figure 1.

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Because PIM amplitude becomes lower as the order increases, higher order products typically are not strong enough to cause direct frequency problems, but they usually assist in raising the adjacent noise floor. Once this raised noise floor crosses into the Rx band, it then has an open door into the BTS (base transceiver station).

A standard PIM test will almost always test for the 3rd order because it is always the most powerful. The actual problem that exists within a site, however, may be 5th or 7th order PIM products that are causing the degradation in site performance. PIM products can be very wideband, covering wide swaths of frequencies.

As shown in Figure 2, a PIM example for the widely used 900MHz band assumes two GSM carriers, one at 935MHz and the other at 960MHz. In this case, the 910MHz 3rd order product is in the uplink band.
All the calculations outlined have assumed that only two carriers are present. That is not the case in the real world. At the base station, one needs to consider the increase in susceptibility to multi-carrier-specific issues.

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Figure 2: PIM causing receiver de-sense at 910 MHz. Note the frequency of the F1 harmonic falls into the DCS1800 band

When three or more carriers are involved, the calculations quickly become complex. The intermodulations created within a multi-carrier system often appear as a raised wideband noise floor. This is often referred to as a ‘shark’s fin’, as the BTS Rx filtering gives this noise a characteristic shape when monitored on the Rx path coupling ports.

Line sweep testing
Line sweep testing and PIM are very different tests. Both are extremely important and accurately measure a cell site’s ability to provide service and perform optimally. Line sweeping measures the signal losses and reflections of the transmission system. PIM testing is a measure of construction quality, and poor quality will result in self-interference.

PIM testing performance measurements are not relevant unless accompanied by comprehensive line sweep tests. It is important to recognise that PIM test measurements performed on a transmission system that has poor microwave performance are irrelevant indicators of the transmission system performance.

Unfortunately, not understanding how these tests relate to performance has not only resulted in compromised test data, it has also created the need to re-test the systems repeatedly. In turn, connections and components are becoming increasingly overworked, and greatly contribute to line sweep and PIM problems.

PIM requires both low system loss and good return loss (VSWR) to perform to an acceptable standard. If PIM testing is performed prior to line sweep testing, the operator may not be aware of the impedance characteristics of the transmission line. High insertion loss attenuates the PIM test signals, preventing the high-power characteristics from reaching the very components that require this stringent testing. Poor return loss reflects a percentage of the PIM test signals back into the test set, causing some signal cancellation that can report a false positive. It is becoming increasingly common for poor line sweep performance to create a false ‘pass’ of a PIM test.

By performing the line sweep test prior to PIM testing, the operator can be confident the insertion loss and return loss data are at acceptable levels. This data ensures that the PIM test signals actually reach the components at the highest possible signal level, offering the most accurate indicator of true PIM performance. By constructing a system using modern low PIM practices, the need to break the transmission system back open will be minimised. If the lines are disassembled again to replace or clean a connector, the line sweep data will need to be repeated.

One point that should be conveyed is that poor PIM may affect the site performance by lowering capacity, but poor return loss is a catastrophic failure and will take the site off air.

DCS1800 PIM issues
Throughout Europe, the 900MHz, 1800MHz and 2100MHz communications bands are commonly used and are currently being upgraded for LTE deployment. With real estate at a premium, most cellular networks will share sites and infrastructure.

The European frequency plans present some issues. DTV banding is very close to the cellular systems and several countries are expecting major interference challenges. Another common issue is that any faulty cable TV distribution amplifiers or lines that may be un-terminated can cause interference with a BTS receiver.

Another major problem is illustrated in Figure 3. The 1800MHz band produces 3rd order PIM in its own receive band. This is a very common occurrence for most cellular bands. It is also possible to create PIM that falls into the 2100MHz receive band when a 3rd order PIM on the high side of the 1800MHz band is present. In a Distributed Antenna System (DAS), any PIM generated by the DCS1800 band is certain to affect both receive bands, resulting in poor system performance. Where spread spectrum technologies are utilised, the PIM will be broadband in nature. These multi-banded systems will need to be constructed very well in order to perform under heavy subscriber traffic conditions.

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Figure 3: The 1800MHz band produces 3rd order PIM in its own receiver band.

Airports, sports venues and major tunnels may use a DAS where all operators’ BTS signals are combined and share a common antenna and distribution system. The PIM performance of these systems is critical, as any isolation enjoyed between carriers in the outside network will be lost. Also worth noting is that even high-performance filtering will not fix linearity problems, as the PIM is generated on the common areas of the distribution system. When a PIM signal crosses into the Rx band, any filtering simply allows these signals to pass through to the receivers. The good news is that PIM testing can control these construction issues.

Common performance indicators
BTS alarm conditions are designed to provide insight into the current performance issues. Many network operators are assuming that unfixable performance faults are due to poor PIM levels. It needs to be carefully considered if PIM can actually be a possible problem within the cell site. Unless the site has two or more transmitters on one feed line, self-generated PIM is unlikely. The BTS will offer indications of PIM performance, which is usually reported as an ‘Rx/Main diversity noise imbalance’ alarm. This data indicates that the noise floor between the main line and the diversity line are different in level. All major BTS system manufacturers offer these alarm conditions but different terms are used.

If the feed line that carries the transmit signals has the raised noise floor, then PIM is highly suspected. External interference will be present on both the main and diversity feed lines. This interference or raised noise floor could be externally generated PIM and can be quickly isolated and identified by wilting (shutting down) one or more transmitters on the affected sector.

A cellular site that reports elevated Rx noise floor on both Rx lines (Duplex/Simplex) would indicate that the source is external. This is often a cable TV amplifier that is leaking RF power. The BTS receiver is very sensitive and can easily suffer from external noise that happens to cross into the receive band of the site. Another good example of this type of fault is problems caused by cellular jammers. Often this technology is employed without realising the possible problems it can cause, especially if emergency calls are restricted.

As subscribers utilise more high-speed mobile devices, the performance issues of the cell system will grow at an even greater rate. This degradation in capacity and overall performance needs to be considered as network upgrades are made. PIM testing is fast becoming a commonly accepted test, much like impedance measurements; as technology advances, these performance measurements are becoming more critical.

As new technologies are deployed and old technologies are phased out, the unique opportunity exists to ensure that critical areas of the infrastructure are capable of the performance required by these advanced communication formats. The performance engineering team will be responsible for multiple spread spectrum technologies. Having the right resources will be paramount to maximising site and system efficiency as subscriber traffic increases.

Author profile: Nicholas Cannon is a Product Marketing Manager for Anritsu