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

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 29.11.2018

Coding with 4-bit key -> fast data UMTS: CDMA Principle Transmitter side: f A 0 1 0 1 f A + 0110 0 1 0 1 0110100101101001 „slow“ user data narrow spectrum Coding with 4-bit key -> fast data wide spectrum Thomas Hasenpusch, Bundesnetzagentur 29.11.2018

UMTS: Immunity Against Interference Receiver side f A wideband interferer f A narrowband interferer 0110100101101001 This is what we see with the spectrum analyzer 0110 + f A f A system gain 0000111100001111 0 1 0 1 wide spectrum decoding with 4-bit key high pass flt. (integration) „slow“ user data narrow spectrum Thomas Hasenpusch, Bundesnetzagentur 29.11.2018

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 29.11.2018

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 29.11.2018

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 29.11.2018

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 29.11.2018

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 29.11.2018

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 29.11.2018

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 µ 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 29.11.2018

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 29.11.2018

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 29.11.2018

UMTS interference Investigation Thank you for your attention!