Broadband RFI due

to faulty switching
power supplies

GSM 900 MHz uplink band with broadband RFI due to faulty switching Occupied spectrum for GSM900 mobile phone in normal mode, dual
power supplies in LC plasma displays used for advertisements. screen display. Lower RF-PAN displays the full GSM900 uplink band and
the upper part displays a single zoomed mobile TCH uplink channel with
GMSK spectrum.
J Different marker, delta and line settings
J For a more detailed analysis, the RMS detector of the demodulation path
should be used

Defective TX filter in a GSM mobile phone, GSM 1800 MHz band. The Interference from a broadband digital signal on the same GSM 900 MHz
occupied spectrum exceeds the speciied spectrum mask ▷ adjacent channel.
channels are blocked.

Use the DIFF mode in the PSCAN display mode to suppress the user
signal, leaving only the short-time RFI signal.

Rohde & Schwarz R&S®PR100 Radio Interference Detection and Analysis 15

4.2.4 Uplink RFI screenshots

Lower curve: DIFF mode activated to display a wanted signal ▷ waiting

for interferer signal to appear.

Upper curve: DIFF mode activated, GSM uplink mobile TX signal and
interferer signal. Lower curve: DIFF mode activated, short-time interferer
signal is displayed in PSCAN mode.

Difference mode activated, interferer

available in PSCAN mode

Difference mode activated, GSM

signal eliminated, no RFI present

Difference mode inactive, only GSM

user signal present, no interferer

DIFF mode with spectrum and waterfall screen.

16
Detecting RFI in the 5.1.3 General tips for detecting RFI in the
downlink band
RFI in the downlink band reduces the mobile phone

downlink band receiver’s ability to demodulate and decode the physical

channels, which leads to lower voice quality and
decreased data rates for packet-switched services. Key
performance indicators (KPIs) such as call setup times,
5.1 Preparation, preliminary checks, tips latency times and handover times will be impacted.
Make sure there are no base station hardware alarms relat-
ed to power amplifiers (VSWR problems), TMA, duplexers, J Error correction in the baseband component of the
combiners and microwave, fiber-optic or DSL links. Check receiver is effective up to a certain BER, beyond which
the GPS module if the base station has one (clocking refer- calls can drop or handovers can fail
ence). An unstable clock reference can lead to an incorrect J An RFI signal with –80 dBm/10 kHz bandwidth is a
base station transmit or receive center frequency. relative power value. The impact of the RFI depends on
the signal strength of the base station downlink. Mobile
From the RAN management system, retrieve a list of inter- phones in close proximity to the base station or at the
fered channels that have been identified with counters for outer edge of the cell range calculate different C/I values
events such as dropped calls, blocked calls and dropped because the signal strength of the base station varies
handovers J Locating the cause of service reduction (RFI) could also
be accomplished with coverage and QoS measurement
5.1.1 FOSiG method systems such as R&S®ROMES. These systems can
The FOSiG method in the downlink band means that no perform a full uplink and downlink analysis, including
mobile TX signal is available, which isolates the interfering all KPI measurements and GPS data logging, which
signal for detection. together provide the optimal tools for detecting RFI in the
affected area
This is accomplished by blocking the specific channel via
an O & M command for the controlling unit (no adjacent 5.1.4 RFI sources in the downlink band
channels) The base station can also be a source of RFI, e.g. when
faulty TX filters create spectrum interference in adjacent
J If the entire channel is blocked by RFI, switch off the channels. QPSK modulation for WCDMA is characterized
base station downlink signal so that no broadcast by steep filter slopes, eliminating the need for complete
information is available for mobile phones (no uplink user channel separation. The UARFCN of the adjacent
trac) operator is 200 kHz outside of the 5 MHz bandwidth of the
other operator ▷ analyze BTS with the R&S®FSH4.
5.1.2 Non-FOSiG method
The non-FOSiG method does not permit the suppression J Antenna blades, wires and soldered components can
of base station signals. A composite base station and in- cause PIM due to faulty connections or metal corrosion
terferer signal is provided instead. ▷ analyze the cabling and antenna components with the
R&S®FSH4 handheld spectrum analyzer
J If the base station cannot be switched off due to high J Incorrect channel or scrambling code planning, crosstalk
trac levels, reduce the TX power rom other network operators ▷ use the R&S®ROMES
J If the downlink signal remains in service, the noise level software plus the R&S®TSMx to scan the network
will increase, especially in CDMA systems. Depending J The most challenging sources of RFI are not related to
on the distance between the mobile phone and the base commercial operator base station systems as mentioned
station, the uplink signal may be clear in urban areas previously. Other sources that should be considered
J Determine what a normal downlink signal looks like in include:
the spectrum and waterfall W
Nonharmonic RF within switching power supplies close
J Technologies such as GSM and TETRA are TDMA-based to shopping windows, railway stations, street lamps, fat
systems in contrast to C2K, UMTS, TD-SCDMA, which lighting, LCD or plasma fat screens, computers, kitchen
are code-domain-accessed systems that work on the appliances
same frequency with different codes

Rohde & Schwarz R&S®PR100 Radio Interference Detection and Analysis 17

 W
Audio and video transmitters using AM, FM FSK and
AFSK modulation in various ISM or professional bands
typically found in schools, universities, concert halls
and sports arenas. They are sometimes used to monitor
building and garage entrances as an example. Even
when outside of the desired band, they can produce
harmonics caused by deective TX lters or intermodu-
late with other signals
W
Illegal (non-licensed) GSM, UMTS, CDMA2000®, TETRA
and WiMAX™ repeaters used to extend coverage in
hotels, ports and on private property
W
Short-range wireless communications: Bluetooth®,
RF USB, ZigBee, WLAN, RFID, DECT
W
Hobby RC electronics: 35 MHz, 40 MHz, 72 MHz,
433 MHz, 2.4 GHz
W
Amateur radio signals, which can intermodulate with
commercial services
W
Various wireless consumer electronics products from
other countries that use different frequencies (e.g. USA
versus Europe)
W
Computer cables, power line communications that have GSM1800 downlink band, markers active.
powerful spike spectra
W
RFI ading caused by multipath propagation ▷ perorm
fading analysis with R&S®TSMx scanners

RFI in the downlink channel is a common cause of mobile

phone service reduction. Relying on the R&S®PR100 to-
gether with network quality analysis tools is therefore
highly recommended. A decrease in service quality can be
detected immediately by using the R&S®ROMES software
and a mobile phone.

The two figures show a mobile phone operating at Broadband RFI emitted by switching
1858.2 MHz with RFI from a direct adjacent channel at power supplies: supermarket
1858.4 MHz. With normal 400 kHz channel spacing, the
next usable channel should be at 1858.8 MHz.

5.2 R&S®PR100 base station downlink spectral WCDMA band 1 downlink spectrum and waterfall analysis, six channels
analysis on air.
J Connect an omnidirectional antenna and set the
frequency range of interest. Choose PSCAN mode with
RBW of 100 kHz, set the downlink band with start and
stop frequency or select a stored user preset with GSM,
CDMA2000®, WCDMA, TETRA band values
J Spectrum and waterall display is the rst choice
J If needed, run the marker menu with marker, delta
marker and lines on the screen
J For GSM systems, a frequency reuse factor is used
to separate channels due to the special envelope of a
GMSK-modulated carrier, which occupies much more
Broadband RFI
than 200 kHz of bandwidth emitted by switching
J Rohde & Schwarz FFT technology in the R&S®PR100 power supplies close
to railway stations
permits the detection of leakage transmit power

TETRA downlink band 4, four channels on air.

18