Passive Intermodulation (PIM) as a source of 4G/LTE interference
What is PIM?
One cause of self-disturbance in LTE Networks is Passive Intermodulation (PIM). PIM is interference resulting from the nonlinear mixing of two or more frequencies in a passive circuit. If the interference coincides with a network’s base receive frequencies, it can cripple network performance and throughput. Possible PIM sources include poor connections, damaged cables or water infiltration into transmission equipment. PIM can also be caused by objects outside the path, such as light posts, buried conduit, fences or site equipment. There are so many possible external sources, PIM is sometimes known as “the rusty bolt effect.”
Rusty objects can cause PIM: the crystals found in corrosion or rust on an antenna or cable connector can cause PIM. Even corrosion outside the intended radio signal chain can cause interference: a rusty fence, rusty bolts, corroded rooftop air conditioners or even a rusty barn roof in proximity to the base station are a hazard. Of course, it’s also possible that loose connectors in an antenna feed line or poorly configured transmitters can cause PIM.
PIM frequencies are predictable. If you have two signals at frequencies F1 and F2, the third-, fifth- and seventh-order intermodulation
products will be found equally spaced above and below the two signals (see figure 2).
So for instance, if a base station transmits two signals at 925 MHz and 960 MHz (the extremities of the EGSM band), intermodulation products will be found at 890 MHz, 855 MHz and 820 MHz. The problem here is that the 890MHz intermodulation signal is in the middle of the EGSM receiving band (880- 915 MHz), so the intermodulation product will therefore interfere with valid incoming signals that the base station is trying to receive.
PIM in Wideband LTE Channels
For LTE, most European operators have allocated a single, wideband channel (of up to 20 MHz). Even here, though, PIM – which requires two strong signals – can occur. This is because the channel includes sub-carriers in the OFDM modulation scheme that LTE uses. So if the infrastructure is liable to cause Passive Intermodulation, all the sub-carriers will intermodulate with each other and so disturb the receiver (see above figure).
Q: Where does most PIM occur?
A: PIM is a problem in almost any wireless system but is most noticeable in cellular basestation antennas, transmission lines, and related components.
Q: What actually causes Passive Intermodulation?
A: The interaction of mechanical components generally causes the nonlinear elements, especially anywhere that two different metals come together. Junctions of dissimilar materials are a prime cause. PIM occurs in antenna elements, coax connectors, coax cable, and grounds. It is caused by rust, corrosion, loose connections, dirt, oxidation, and any contamination of these factors. Even nearby metal objects such as guy wires and anchors, roof flashings, and pipes can cause PIM. The result is a diode-like nonlinearity that makes an excellent mixer. As nonlinearity increases, so does the amplitude of the PIM signals.
Q: What conditions are necessary to cause Passive Intermodulation?
A: Typically, two relatively strong RF signals relatively close in frequency are required to trigger Passive Intermodulation effects. The outputs from two or more high-power (20 W or so) transmitters are enough to create the PIM effects. The higher the power used, the greater the PIM signals generated.
Q: What other conditions affect Passive Intermodulation?
A: Passive Intermodulation tends to increase as components age. Older systems are particularly susceptible. Environments where there are wide temperature variations, salt air or polluted air, or excessive vibrations exacerbate Passive Intermodulation.
Q: Besides interfering signals, what are the other negative effects on the wireless system?
A: The main effect is on the receiver with its high sensitivity. Interfering signals can raise the noise floor and block desired signals. Interfering signals can also reduce receiver sensitivity. End effects include dropped calls, decreased system capacity, and decreased data rates.
External Interference by Jammers
Interference is not only caused internally: external interference also affects cellular networks. The biggest such source is jammers installed for the specific purpose of blocking mobile phone transmissions. These devices, which can be readily bought online for less than €300, are used by places of worship and cinemas to prevent mobile phones from causing disturbance, and by universities to stop students using mobile phones to find information during examinations.
In fact, usage of jammers is strictly forbidden in Europe: this is mainly to ensure that emergency services (available by dialing 112 in the European Union) can be reached at any time from any place. But a secondary problem is that users of jammers cannot control their coverage, and this means they frequently block mobile phone reception close to, as well as inside, the buildings that host them.
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