Antenna basics: PIM metrics
Passive Intermodulation (PIM) is a manifestation of intermodulation distortion, commonly found in linear components such as cables, connectors, and antennas. When these devices are exposed to high RF power from cellular systems, they generate intermodulation signals of-80dBm or higher. Under the influence of nonlinear elements (such as semiconductors), two frequencies f1 and f2 can produce harmonics of the original frequencies, such as (2f1, 3f2,etc.). The combination of these harmonics forms intermodulation products, including even and odd orders. However, even-order intermodulation products have less impact on mobile communications because they are farther from the original frequencies, and signals used in mobile communications typically do not contain static components. Therefore, even-order intermodulation products usually do not occur.
The influence of odd-order intermodulation products cannot be ignored
Odd-order intermodulation products, such as 2f2-f1 or 4f1-3f2, pose significant hazards due to their frequencies being close to the original signal. Once these intermodulation products fall into the RX band, they cannot be effectively suppressed by filters, potentially causing interference with operating frequencies and affecting communication quality.
An interesting phenomenon in practice is:Odd-order intermodulation products, due to their frequencies being close to the original signal, often produce unexpected interference effects. When these intermodulation products inadvertently fall into the RX band, conventional filters often struggle to effectively suppress them, potentially causing interference at the operating frequency and subsequently affecting the performance of the entire communication system. This phenomenon requires sufficient attention and proper handling in practical communication engineering.
As the order of the out-of-band products increases, their bandwidth also grows accordingly.In cellular communication systems, PIM can cause a series of interference issues, including reduced receiver sensitivity and even complete communication blockage. This interference not only affects the cell base station that generates it but also impacts nearby receivers.Taking LTE as an example, in the LTE band 2, the downlink frequency range is from 1930MHz to 1990 MHz, and the uplink frequency range is from 1850MHz to 1910 MHz. If two transmission carriers are at 1940MHz and 1980MHz respectively, and signals are transmitted from a base station system with PIM, the intermodulation products will include a component at 1900 MHz.This component falls precisely into the reception frequency band, thus affecting the receiver. At the same time, the intermodulation product at 2020MHz may also interfere with other systems.
GSM:
In the GSM communication system, there is also the issue of passive intermodulation. Due to the overlap between the frequency bands of GSM and LTE, PIM can also interfere with the GSM system. This interference not only affects the base station that generates the interference but may also affect nearby receivers, thereby impacting the performance of the entire communication network.
Passive Intermodulation (PIM)
Passive intermodulation in the GSM communication system should not be overlooked. Due to partial overlap between the frequency bands of GSM and LTE, PIM issues can also adversely affect the GSM system. This interference not only directly impacts the problematic base station but may also affect surrounding receivers, thereby degrading the performance of the entire communication network.
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