Articles
GSM Interference Analyzer ROGER (TS9958)
Full-coverage, mobile and automatic measurement
of GSM interference
When you compare the load carried by the channels of todays GSM networks with
road traffic, GSM is constantly on the brink of the rush hour. New transmission
methods such as GPRS and EDGE will soon have completed the pilot phase all
over the world and fill remaining channel resources with packet data. Then, at
the latest, high network quality will be a decisive factor in order to be competitive,
and this means no or only insignificant interference. Plus, the higher the channel
load, the less effective is frequency hopping, which is today successfully used
against interference.
Photo 43386/1
FIG 1 Interference measurement system ROGER
compact and lightweight for mobile use
Interference must be identified
locally
Apart from high network loading,
major causes of interference are
poor or incorrect antenna installation and inadequate setting of RF
power. But whatever the cause, interference has to be identified locally.
And since mobiles reveal neither their
location nor the source of interference, a suitable measurement technique is needed. ROGER (TS 9958)
is a system developed and optimized
by Rohde & Schwarz especially for
detecting and identifying interference
(FIG 1).
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How ROGER works
Signal display
Just a short configuration of ROGER,
and the test tour can start. The test
run is automatically controlled by up
to four mobile phones, doing away
with any manual control. High vehicle
speeds are no problem for ROGER
either. Interference measurement is performed in three steps [1]:
detection of interference,
measurement of interfered/
interfering signals,
assignment of these signals to
base stations.
There are signal displays for two test
modes: for C0 (BCCH) and Cx (TCH)
measurements. From the disturbed composite signal, ROGER filters out frequency-correction bursts (FCCHs) for
the identification of C0 carriers and
displays them. The time axis is structured in lines comparable to a TV
frame, arranged such that neighbouring FCCHs of an M51 frame (51
TDMA frames) come vertically one
below the other. Because of the idle
burst at the end of each M51 frame, a
staircase pattern is obtained for each
detected C0 carrier. So the graphical presentation of the C0 channel
of the serving cell (SC) reveals a staircase with the FCCHs of the SC itself
The signals found can be assigned
to the emitting base stations already
during the test tour or afterwards on
a conventional PC.
Number 168 (2000/III)
Articles
and further patterns in the case of C0
interference. In the analysis window
for adjacent channels or TCH channels of the SC, each staircase pattern
indicates the presence of C0 interference (FIG 2).
In Cx measurement, the composite
signal is analyzed in greater detail. Synchronization as well as dummy-burst
and training sequences are filtered
and visualized grouped according to
timeslots. The measured sequences of
different base stations are shown in
time grids corresponding to two vertical stripes in the Cx display. Different base stations are represented by
stripes at different positions along the
x axis. Interference can be identified
immediately: from any further stripes
displayed next to the two SC stripes
(FIG 3).
In mobile measurements, the selected
signals fluctuate due to fading, reflection and other external influences, resulting in a variety of signal patterns.
ROGER therefore processes interference signals for graphical representation, as the human eye can analyze
complex patterns with high reliability.
FIG 2 Display of FCCH bursts of composite signal on C0 channel of SC. Two FCCH patterns
are clearly discernible, which indicates strong C0 interference. These signals can be assigned to
a base station using a time template that represents the expected arrival time of the signal of a
base station. The BTS can be identified as the signal source if the measured signal comes within
the template
Fast identification of base
stations
To trace interference back to the emitting base station, a feature characteristic of each BTS is used: the expected
arrival times of specific signals at the
measuring instrument. The times are
determined from the measurement position, the expected sending time and the
site of the BTS. When a BTS is selected,
the expected arrival time is superimposed on the displayed sequences in
the form of a template. If this matches
a signal measured, the latter can be
assigned to the selected BTS. In the
case of Cx measurements, the number
of possible base stations is reduced
by a factor of eight to those whose
base-station colour code and trainingsequence code are identical (FIGs 3
and 4). The selected BTS is additionally
FIG 3 Display of filtered composite signal for analysis of Cx interference, grouped according to
timeslots of TDMA frames. Next to two vertical stripes, other signals are visible that indicate Cx
interference. These signals can be assigned to base stations analogously to C0 interference
News from Rohde & Schwarz Number 168 (2000/III)
Articles
FIG 4 Power measurement of interferer at place of interference (power
domain). The frequency distribution of the individual power values of
the serving cell (violet) and an interferer (red) are shown. In the upper
frequency chart SC power can be modified, in the lower one that of
the interferer. The two charts come with appropriate default settings and
are needed for a very detailed analysis of power statistics. Up to four
interferers can in this way be analyzed. In the time-domain chart above,
the mobile data in the interference pocket are displayed. The test distance
to be evaluated can be selected
shown on a map, allowing comparison
of the propagation conditions of server
and interferer [1].
Power measurement
In the interference charts above, the
power values are colour-coded, allowing a basic evaluation of interference.
For purposes of optimization, the measurement system provides the dynamic
C/I value for each base station after
the SC and interference signals have
been selected. The measured and averaged power values can be visualized
and if necessary modified. Modification enables evaluation of the range
of interference obtained with mobile
measurements (FIG 4). The results of
power analysis are stored in a file, and
a test report of the analyzed interference signals is generated (FIG 5). The
latter may serve as a basis for network
modifications.
FIG 5 Test report of detected interference. Values are automatically generated after identification of interference
Hardware
ROGER consists of:
Test Receiver TS 55-RX,
up to four test mobiles of different
make,
a GPS receiver,
a process controller equipped
with A/D converter card and
signal-processing card.
Test Receiver TS 55-RX is accommodated in the controller, making ROGER
a highly compact, lightweight unit.
The system uses Coverage Measurement Software ROMES3 from
Rohde & Schwarz [2], affording a stateof-the-art operating concept and the
repeated use of position data sources
and mobile-phone linkups. Using an
indoor module, the software even
allows interference detection inside
buildings [2].
ROGER can optionally be fitted with
a position trigger so that it can carry
out classic measurement of coverage
News from Rohde & Schwarz Number 168 (2000/III)
in addition to interference. In particular the option of extending the system
by up to eight additional mobiles of
different standards (GSM900/1800,
CDMA, GPRS) allows space- and costsaving performance of different tasks
with a single unit.
Dr Jrgen E. Schlienz;
Otmar A. Wanierke
REFERENCES
[1] Wanierke, Otmar A.: Mobile interference
measurements in GSM networks. News
from Rohde & Schwarz (1998), No. 160,
pp 24 25
[2] Maier, Johann; Spachtholz, Andreas: Coverage Measurement Software ROMES3
Acquisition, analysis and visualization of
data in coverage measurements. News
from Rohde & Schwarz (2000) No. 166,
pp 29 32
Reader service card 168/01
