Interference investigation
on UMTS base stations
Thomas Hasenpusch
Federal Network Agency
Germany
www.bundesnetzagentur.de
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
31.05.2016
2
UMTS: CDMA Principle

Transmitter side:
0
A
0
1
0
1
0
1
+
0110
1
A
0110100101101001
„slow“ user
data
narrow
spectrum
Thomas Hasenpusch, Bundesnetzagentur
f
Coding with 4-bit key
-> fast data
wide
spectrum
f
31.05.2016
3
UMTS: Immunity Against Interference

Receiver side
A
A
wideband
interferer
narrowband
interferer
f
f
0110100101101001
+
0110
A
A
system
gain
0000111100001111
0
wide
spectrum
f
decoding with
4-bit key
high pass flt.
(integration)
1
0
„slow“ user
data
1
narrow
spectrum
f
This is what we see with the
spectrum analyzer
Thomas Hasenpusch, Bundesnetzagentur
31.05.2016
4
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
31.05.2016
5
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
31.05.2016
6
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
31.05.2016
7
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
31.05.2016
8
Measurement at base station antenna (1)

If interference cannot be pciked up in meas. vehicle:

Measurement point at UMTS base station is antenna port:
Tx/Rx
Tx/Rx
Band pass
filter
Thomas Hasenpusch, Bundesnetzagentur
31.05.2016
9
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
31.05.2016
10
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
Ref
72 dBµV
Ref -20
dBm
70
-20
* Att
Att
* RBW 1 MHz
* VBW 3 MHz
* SWT 20.5
100 ms
ms
0
10dB
dB
-30
60
11 PK
PK**
VIEW
CLRWR
2 PK
VIEW
1 [T1 ]
Marker 2
9.74 dBµV
-52.85
dBm
1.975000000
GHz
20.008000 ms
Marker 1 [T1 ]
-52.81 dBm
10.004000 ms A
TRG
-40
50
1
-50
40
-60
2
PA
PS
TRG -58.2 dBm
30
PRN
-70
20
-80
10
3DB
6DB
1
-90
0
-100
-10
-110
-20
-120
1.9675GHz
GHz
Center 1.975
Thomas Hasenpusch, Bundesnetzagentur
2.05
ms/
10
MHz/
Span 100 MHz
31.05.2016
11
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
31.05.2016
12
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
* Att
Ref -35 dBm
RBW 300 kHz
VBW 1 MHz
SWT 165 ms
0 dB
-40
1 AP
VIEW
-50
-60
2
1
-70
3
Delta 4 [T1 ]
1.30
120.120000
Marker 1 [T1 ]
-67.69
0.000000
Delta 2 [T1 ]
1.22
39.930000
Delta 3 [T1 ]
4.03
4
80.190000
dB
ms
dBm
A
s
dB TRG
ms
dB
ms
TRG -71.6 dBm
PA
PS
-80
PRN
-90
3DB
-100
-110
-120
-130
Thomas Hasenpusch, Bundesnetzagentur
Center 1976 MHz
16.5 ms/
31.05.2016
13
UMTS interference Investigation
Thank you for your attention!
www.bundesnetzagentur.de