High RTWP Issue (External interference suspected)

[yaserahmed25]

High RTWP issue faced on some cells in same region:

1. High RTWP even at low traffic hours (e.g. 4 AM)
2. RTWP reduces to -105 after using 2nd frequency
3. High RTWP appearing on all cells facing in certain direction
4. RTWP decreased on cell when swapped with cell not facing in the direction of the other cells having high RTWP while it increased on the other cells when it faced in this direction

So, clearly external interference on first frequency, but:

Even after one week of rigorous drive tests trying to find the source of interference using "spectrum analyser", we found nothing.

I am losing sleep over it, any ideas/comments would be welcome.

[BryxKeisha]

Hi,

First, I suggest you get a 5 or 7-day data of RTWP of these cells. Then insert them in a chart in Excel and check whether these cells have the same pattern of graphs. If they are, there's a possibilty of external interference. Also, check whether there are repeaters or other sort of antennas present on that area. Check also if these cells have very high traffic.

Please add reputation if you find this useful. Thanks!

[dremka]

Maybe this is wideband interference - it isn't easy to see it on a spectrum analyser - try to set SA with RBW as narrow as possible, use high gain directional antenna (e.g. multielement yagi-uda) and use a preamplifier. But be aware this configuration is very susceptible to strong outband signals (preamp may tend to be unstable) so passband filter (including your rx band of course) is also recommended.
Wideband signal with channel power eg -80dBm has its "floor" on -120dBm...
It is good to start an investigation on a hill or high building to discover a character of interference and then use a triangulation method to locate it (still using hills or high accesible buildings). Investigation from ground level gives mixing result, I always start from an exposed location.
If you have any further question just ask - I have some experience with finding interference sources...

[jayk]

High RTWP issue faced on some cells in same region:

1. High RTWP even at low traffic hours (e.g. 4 AM)
2. RTWP reduces to -105 after using 2nd frequency
3. High RTWP appearing on all cells facing in certain direction
4. RTWP decreased on cell when swapped with cell not facing in the direction of the other cells having high RTWP while it increased on the other cells when it faced in this direction

So, clearly external interference on first frequency, but:

Even after one week of rigorous drive tests trying to find the source of interference using "spectrum analyser", we found nothing.

I am losing sleep over it, any ideas/comments would be welcome.

Locating source of external interference is very difficult task in RF Optimization. You have mentioned about one week of rigorous drive test; I have seen cases, where external interference source was located after months and in some cases could not be found in years. Many have lost sleep on the same issue. My suggestion is to take expert help from vendor or from someone, who has detected interference source many times.

[shishio]

my suggestions
1. external interference suspected, try to locate and eliminate the source using a spectrum alalyzer, install filters on the affected cells.
2. rtwp is due to cell congestion, using the 2nd freq shared the traffic on the cell so rtwp dropped.
3. very high traffic in that direction, enable faster HO to 2G cells around, expand capacity of 3G cells around, use 2nd carrier, bring up new site (last resort)
4. interference is due to internal source, check filters and tighten feeder connectors.

[shishio]

Hope suggestions will be useful for others, kindly add thanks and rep if you find suggestions helpful.

[chronix_ray]

Hi,

You can check for possible repeaters in the area. Also, you can check your own NodeB. Maybe you have faulty filters/combiners/equipments.

Br//

[jan74]

High RTWP issue faced on some cells in same region:

1. High RTWP even at low traffic hours (e.g. 4 AM)
2. RTWP reduces to -105 after using 2nd frequency
3. High RTWP appearing on all cells facing in certain direction
4. RTWP decreased on cell when swapped with cell not facing in the direction of the other cells having high RTWP while it increased on the other cells when it faced in this direction

So, clearly external interference on first frequency, but:

Even after one week of rigorous drive tests trying to find the source of interference using "spectrum analyser", we found nothing.

I am losing sleep over it, any ideas/comments would be welcome.

We have had a lot of this on a network I worked on. It certainly looks like interference. Check your HSUPA allocations. Presumably they are very low when RTWP is high. Users get R99 UL in these situations, if theyre lucky...

One of the main sources of interference like what you have described are US DECT phones which use the lower part of the European WCDMA band. Can you confirm what frequency is affected? Could it be the 1920-1935MHz band? If so, it could well be a DECT phone in a private house that is causing your problem. Not sure why your spectrum analyser has not identified it. Normally triangulation like you describe identifies the problem area...Another culprit can be Television Mast Head Amplifiers, CCTV systems and 3G repeaters. We have seen all of these go haywire and cause problems. I would check again how the spectrum analyser was setup, the pickup antenna used and the location driven. Check, as somebody mentioned, your own Hardware on-site, to rule out any faults. Failing all of that, you probably need to contact the relevant spectrum authorities to report an illegal interferer. If it is a DECT phone, you are likely to start seeing a lot more of them!

[madeira]

I found some problemns by people who uses DECTS , one strategy used is using 2nd and 3rd carrier that are more clear.

[majortom]

Did you ever find this???
Just some additional comments...
Connect the spectrum analyser tuned to the uplink carrier frequency to a receive sample port of the worst offending site to get a signature of what you are looking for.
Check all receive branches in diversity receive systems. With diversity reception, if both antenna rx paths in the same sector are receiving nearly the same level of interfering signal, then it's likely coming from an external source (assuming you've ruled out high traffic as noted above). If only one rx branch is seeing it then likely some internal hardware/cabling issues. Can also be helpful if ya can determine whether there is any intelligence in an external interfering signal. ie - if it's from a narrow band analog source, can it be demodulated so ya can hear it? Try demodulating it with multiple modulation types and bandwidths from narrow to wide... If it's relatively narrowband, is it wandering in frequency? Or does it seem rock solid in frequency?