1. Interference investigation on UMTS base stations
Thomas Hasenpusch
Federal Network Agency
Germany

2. UMTS: System Principles
Each bit is „coded“ with a binary key before RF modulation and transmission
Spectrum widens by the number of bits (chips) in the code
Multiple users can transmit on the same frequency at the same time
Receiver at base station separates users by correlation with known binary codes
Immunity against interference raises by „system gain“
Thomas Hasenpusch, Bundesnetzagentur

3. Coding with 4-bit key -> fast data
UMTS: CDMA Principle
Transmitter side: f A f A +
0110
„slow“ user data
narrow spectrum
Coding with 4-bit key -> fast data
wide spectrum
Thomas Hasenpusch, Bundesnetzagentur

4. UMTS: Immunity Against Interference
Receiver side f A
wideband interferer f A
narrowband interferer
This is what we see with the spectrum analyzer
0110 + f A f A
system gain
wide spectrum
decoding with 4-bit key
high pass flt. (integration)
„slow“ user data
narrow spectrum
Thomas Hasenpusch, Bundesnetzagentur

5. UMTS: Principle Restrictions
Separation of signals from multiple users works only if all are received with (nearly) equal level
Fast organisation channel necessary to tell mobile how to adjust its output power
If interference present: Base station asks all mobiles to increase power until useful signal is above interference
Far away mobiles get disconnected if they cannot increase power further
Base station coverage decreases
All base stations (of one network) operate on same frequency
One interferer may disable whole base station
Thomas Hasenpusch, Bundesnetzagentur

6. Interference Recognition by Operator
Modern UMTS base stations measure the level of „unwanted emissions“ (in the code domain)
Indicator is RTWP value (received total wideband power): Equivalent to RMS power from the antenna in 5 MHz bandwidth
Sector 2 and 3 are interfered (3 less).
Equal interference times indicate one interference source
Thomas Hasenpusch, Bundesnetzagentur

7. Interference Recognition by Monitoring Service
Interfering signal must be in the order of system gain to be really „harmful“
When uplink channel is viewed with a spectrum analyzer:
Wideband interference must be visible to left and right of the used channel
Narrowband interference must „peak out“ of wanted signal
Thomas Hasenpusch, Bundesnetzagentur

8. Investigation: Sequence
Determine rough direction of interferer by sector map:
Base station with sector beams
Estimated angle range of interferer
Try to pick up interferer in measurement vehicle
Thomas Hasenpusch, Bundesnetzagentur

9. Measurement at base station antenna (1)
If interference cannot be pciked up in meas. vehicle:
Measurement point at UMTS base station is antenna port:
Band pass filter
Tx/Rx
Tx/Rx
Thomas Hasenpusch, Bundesnetzagentur

10. Measurement at base station antenna (2)
In case of weak interferers, measurement must be done while UMTS station is running!
Otherwise nearby mobiles may cover interfering signal
Interfered station
Neighbour stations
Thomas Hasenpusch, Bundesnetzagentur

11. Common Interference Sources (1)
DECT telephones:
Imported from US (DECT channels reach into UMTS uplink band)
Faulty DECT fixed parts (main emission jumps into UMTS uplink band)
Identification:
In case of faulty phones spectrum is unlike DECT
Time domain (zero span) with largest RBW shows 10ms pulse repetition and about 100µs pulse length R e f 7 2 d B V A t * P S C n r 1 . 9 5 G H z p a M / W 3 T m s K I E D - 4 6 k [ ] R e f - 2 d B m A t 1 C L W 6 D M H z V 3 T G n r . 9 7 5 s / S * P K N 8 4 a k [ ]
Thomas Hasenpusch, Bundesnetzagentur

12. Common Interference Sources (2)
Antenna amplifiers
High amplification in active antennas may lead to feedback between output and input
Identification:
Mostly unmodularted carriers
AM-demod: sometimes humming noise
Often not stable in frequency
Thomas Hasenpusch, Bundesnetzagentur

13. Common Interference Sources (3)
Sideband emissions from RLAN Routers
Insufficient suppression of sideband emissions when router operates in 2.4 GHz band
Identification:
In time domain (zero span): different pulse lengths up to 1 ms, Pulse repetition about 100 ms
Spectrum indifferent A t d B * P S R e f - 3 5 m T G W k H z V 1 M D C n r 9 76 6 . s / I E N 2 8 7 4 a [ ] l
Thomas Hasenpusch, Bundesnetzagentur

14. UMTS interference Investigation
Thank you for your attention!