Reducing Passive Inter-Modulation
Passive Inter-Modulation, also known as PIM is a problem that can affect FDD (Frequency Division Duplexing) LTE systems.
Explaining the PIM Problem:
Passive Inter-Modulation (PIM) products are caused by passive components such as duplexers, connectors, cables, and antennas. These components are nonlinear and create intermodulation products that can leak into the receive path of a FDD system. Also, duplexers used in FDD systems have a limited isolation between transmitters and receivers. These intermodulation products will appear as distortion and noise in the receiver.
External sources of PIM interference
In addition, there are external sources of Passive Inter-Modulation Interference. A base-station located near any structure with rusty components can create intermodulation products as the rusty elements act as nonlinear transmitters. This can create unwanted signals in the base -station receiver as can be seen in the figure below.
How to solve Passive Inter-modulation issues:
When installing LTE base stations, ensure you are using low-PIM cables and connectors. Use the cables provided or recommended by the base station vendor. Ensure that the antennas are specified to be low-PIM as well.
Some Linearization algorithms can remove all these causes of Passive Inter-Modulation by characterizing the non-linearities and removing the products in the receive path. Models of the internal causes of PIM are created and “subtracted” from the receiver signal. In addition, a receiver can be used to sample the signal and estimate the causes of PIM from external components and subtract them from the receive path.
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