19 Sep 2011
Demystifying PIM – Passive Intermodulation Products
Peter Jackson, Director, EMEA Director for CCI gives a guide to PIMs, passive intermodulation products, their effects and how they can be avoided
Passive Intermodulation (PIM) has become new benchmark in determining the health of a cell site. Today’s mobile handset users expect consistent high throughput from their devices and, consequently, push current networks to their limit.
Forthcoming fourth generation (4G) networks feature an increased mobile data rate of 100 Mb/s and this higher transmission rate will expose PIM vulnerabilities in today’s networks like never before. Fourth generation FDD networks require superior network transmission fidelity, higher than previous generations.
Network operators also face the challenge of maintaining customer loyalty in an unforgiving competitive arena. As such, good network PIM performance is now imperative.
This article attempts to clarify PIM to all who have interest in this subject, but should be of particular interest to field technical personnel, as well as anyone responsible for performance engineering, some of whom may be required to perform PIM measurements as part of their daily work routines. This article does not focus on PIM sources such as loose connectors, contaminants, dirt, etc…. Although important, such PIM sources can easily be resolved with regular cell site transmission line maintenance and work quality awareness of installers and site technicians.
What is PIM?
Passive Inter-Modulation is an undesired, non-linear, signal energy generated as a bi-product of two or more carriers sharing the same down-link path in wireless networks. Due to network hardware configurations, this multi-carrier interaction can cause significant interference in the up-link receive band, which can lead to reduced receiver sensitivity. To the mobile phone user, this often translates to a loss in audio fidelity in conversations, decreased data speeds, or, in extreme circumstances, dropped calls or an inability to make or receive calls or utilize data services. Since there is a mathematical correlation between the known carrier frequencies and the resultant interference signal in the receive band, accurate measurements of PIM signals can be achieved consistently. For practical PIM testing applications, we will only concern ourselves with those PIM signals which interfere directly with a network’s receive band. Typically these PIM signals are:
3rd order PIM, = 2 x F1 – F2
5th order PIM, = 3 x F1 – 2 x F2
To illustrate this point, CCI’s PIM-Pro 850 analyzer has a default set-up with two transmit frequencies at 869 and 894MHz, producing a 3rd-order IM at 844MHz and a 5th-order IM at 819 MHz. In this example, the focus would be on the 3rd-order IM at 844 MHz, since it falls within the receiver range of 824 to 849MHz.
The 5th-order IM at 819 MHz is outside the receiver range and, as such, can be ignored for the purposes of PIM testing. It is important to observe that the actual IM frequency is determined by the two transmit frequencies and the spacing between them. A 25MHz frequency spacing between the transmitters also results in a 25MHz spacing between the IM signals.
Typically, the 3rd and 5th-order PIM signals are the most likely to fall within the receive band with enough PIM energy to cause disturbances, while 7th and 9th-order PIM signals are usually very low in power. CCI’s PimPro Passive Intermod Analyzer allows a user to select which order PIM is to be measured and highlights those which fall in the receive band for simplicity.
It should be noted that PIM signals exist as a result of the combined transmission of multiple carrier frequencies within a transmission line path. The objective is to ensure that these levels, by design and in practice, should occur at an amplitude which is below the Base Station’s receiver sensitivity. The amplitude of these undesired signals is directly influenced by the fidelity of the transmission line path, including all components and junctions that can introduce a non-linear effect to the signals passing through them.
What causes PIM?
Ferromagnetic materials, when in the current path, exhibit a non-linear voltage-to-current ratio. This non-linear effect is accentuated at higher power levels because of increased current density. Looking at Ohm’s law from the perspective of ‘power’ helps clarify the fact that the squaring effect of current results in a higher magnetic flux, which makes metals with high bulk resistivity, such as, iron, steel and nickel exhibit a magnet-like memory effect. This effect is better known as ‘magnetic hysteresis’. Metals that exhibit this asymmetrical magnetic flux are often the main contributor of PIM energy.
Poor metal-to-metal contact junctions can create additional non-linearities resulting in PIM. Such non-linearities can come from under-torqued male-to-female DIN 7-16 mates, as well as irregular contact surfaces, such as poorly manufactured connectors and surface metal oxidation. Oxidation (corrosion) creates tiny air gaps, which promote voltage potential barriers, in turn resulting in a non-linear voltage-to- current ratio, sometimes referred to as the ‘diode effect’.
PIM: dBc or dBm?
Although PIM measurements can be presented using both (dBc and dBm) engineering units, it is more meaningful and consistent to keep measurements in dBm. This is particularly true when trying to compare PIM measurements at different carrier power levels, where measurements in dBc may be misleading. Using dBc simply means that the value is relative to the transmitter power. For example, a -100dBm PIM level generated from two 43dBm tones (20W) equates to a PIM of -143dBc.
Figure 1 - BTS Receiver Sensitivity (dBm) Requirement. Figure shows an example of an operator needing to keep PIM signals below -106 dBm since the Base Station (BTS) Rx sensitivity is at -105dBm.
Table 1 - BTS Receiver Sensitivity Input Power and Measurement Requirements (dBm vs. dBc).
Table 1 demonstrates that the power of the transmitter dramatically affects the PIM dBc value, where the desired PIM dBm level is the same for all three. Testing at 40W is shown to be a more stringent network test with the combination of higher power and the need for a more sensitive receiver. As can be seen in table 1, a device tested at 40W actually performs 13dB better than a device tested at 2W even though both devices meet the desired -106dBm PIM performance level.
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About the author
Peter Jackson is EMEA Director for CCI. He has over 25 years’ RF broadcast and telecoms experience, providing innovative solutions through OEM, System Integrator and Product Supplier businesses for the International RF industry.
CCI provides innovative, cost-effective, revenue-increasing RF solutions for cellular infrastructure, including equipment for 2G, 3G and LTE co-location, coverage enhancement, capacity improvement plus portable PIM test equipment.
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