Main cause for downlink interference in LTE

Hello Experts,
I have a question on downlink interference in LTE:
What can be the main cause for that and how do you resolve the issue?
I went out to test for downlink interference with spectrum analyzer which am not familiar with, I understand uplink interference better.
In TDD both downlnk and uplink are using same frequency.

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In LTE all cells have the “same frequence”, so coverage ovelarpping must be reduced for just allow handover continuity. So, please check if main server area is well defined and if coverage overalap is not too much. You can verify it by DT or even better verifying the Time advance (TA) distribution. A possible way to solve it is by optimizing electrical tilt and also verifying antenna vertical and horizontal beam. For urban areas, for example horizontal beam should be kept up to 60 or 65degress. Verify also PCI conflict and antennas VSWR alarms.

Another possible issue is the external interference, mainlym but not exclusive, with TDD networks. For case like that the spectrum analyzer can be used for interence hunting in order to identify external sources at the frequence range of LTE.

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Regarding Spectrum Analyzer:

With a directional antenna pointing to the sector antenna, for example. So using this antenna in the area reported as interfered one it is possible by swepping the area to get the external interference source.

Here for example you can see Indication of seven repeatedly transmitted beams plus control data in the time domain:

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Do conduct a physical audit at the target site and nearest neighbor sites also, if you are working at field.

Well it is a Pandora box. There are many reasons for interference and you have to hunt one by one.

  1. Internal Causes (Overlapping/Overshooting, PCI clashes, Hardware issues, Installation snags, Inter-System Interference at BTS, etc.)
  2. External Causes (repeaters, Satellite linking centers, TV boosters, Military installations, VIP Convoys, drones, etc)
    So you first run an offline scan to confirm if the cause is internal or external. If it is external, go on hunting for the source in service area. In our experience it is mostly a repeater installed to boost WiFi/LTE services or military installations. If its internal, check for PCI clashes, installation quality, equipment replacements, underlying legacy system switch-offs, etc

Finally what makes this mantra more frightening is PIM. So far PIM hunting there is slightly a targeted regime to explore the source of it. Again a few question like are you sharing Radio Far End, DAS, Colocation and Co-existence scenarios, etc. Mostly its a rusty connector, a bill board, a pole or any metal body acting as a passive modulator of different frequencies in air and generating products in your band.

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But mostly it leaves us hailing the Murphy’s Law, rest is in this story.