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Why is PIM such a big issue?
The expanded capabilities of the next generation of RRUs brings with it a high risk of PIM. The following features make PIM within the RF system much more likely:
- Multiple allocations within a band.
- Very broad band carriers.
- Operator infrastructure sharing.
- Dual band RRU.
Manufacturing and installing the RF system (cables, antennas, filters etc.) to the highest PIM standards clearly will help alleviate the problem but it cannot solve it, as demonstrated in this example.
A worked example for an operator allocation in Band 3 with 2x10MHz carriers:
- Typical PIM specification for an integrated site is -140dBc (with 2x20W).
- Carrier power = 20W → Total PIM = -97dBm.
- Looking at the figure opposite, only a portion of the broadband PIM power lies in the receive band. In this case, –3.4dB of the total PIM power.
- However, PIM is dominated by the peaks in the RF power, so an OFDM signal causes ~3dB higher total PIM than the test signal.
- PIM in Rx = -97.0+3.0-3.4 = -97.4dBm.
- Typical noise floor in Rx bandwidth = -102.7dBm.
- Noise rise due to PIM = 6.4dB.
- Approximates to 37% uplink capacity loss.