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7/27/2019 GSM RF Equipment Testing - Final Conference Paper
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GSM RF Equipment Testing and Performance Analysis
Ahmad H. Fares, Ali M. Khachan, and Ahmad M. Bakri Kasbah
Department of Electrical and Computer Engineering
American University of Beirut
Beirut - Lebanon
{ahf05, amk18, amb07}@aub.edu.lb
Abstract: In this paper, we present four Radio
Frequency (RF) measurements used to evaluate
fundamental performance parameters of Global System forMobile Communications (GSM) equipment. We describe
the relevant theory for each measurement, and then we
proceed to explain the algorithms associated with it. These
algorithms are implemented using LabVIEW in a GSMMeasurement Toolkit (GMT). GMT is developed for
National Instruments (NI) to be used with PXI-5660 RF
Signal Analyzer (RFSA) to test RF equipment. Using thistoolkit, a Base Transceiver Station (BTS) downlink signal
and a Mobile Station (MS) uplink signal are tested to ensuretheir conformance to 3
rdGeneration Partnership Project
(3GPP) standards.
1. Introduction
GSM is the most widely deployed mobile system with morethan half a billion users spanning the globe. Measurements
are indispensable for both GSM manufacturers and
operators who are very concerned about the cost of testequipment. These measurements are used in quality control,
calibration, and maintenance of both mobile and base
stations [1] [2].
GSM is a trunked radio system in which the number of
available channels is less than the number of possible users.This process of sharing channels among users is feasible
because the probability of everyone demanding a channel atthe same instance is very low. Multiple users of the system
are granted access through the division of the system into
frequency and time. GSM utilizes a combination of
Frequency-Division Multiple Access (FDMA) and Time-Division Multiple Access (TDMA), in addition to
frequency hopping. The GSM frequency band is divided
into 124 uplink/downlink carriers. Each carrier is divided intime into 8 time slots to allow at least 7 users to access the
network using the same carrier.
The process of testing consumes a lot of resources in termsof time and budget. NIs virtual instrumentation technology
promises a great reduction in test costs and enables thecustomer to administer the test setup and apply customized
configurations. The four measurements, described
throughout this work and included in the GMT, are adjacentchannel power, modulation accuracy, mean transmitted RF
carrier power, and transmitted RF carrier power versus
time. For each measurement, we introduce its theory andillustrate its implementation in LabVIEW. The NI PXI-
5660 RFSA and GMT are used to test a BTS downlink
signal and an MS uplink signal [3].
2. Adjacent Channel Power
According to 3GPP standard, GSM adjacent channel power
measurement is divided into two sub-measurements:spectrum due to modulation and wideband noise and
spectrum due to switching. These two measurements are
frequently referred to as output RF spectrum (ORFS) [5].
2.1. Spectrum due to Modulation and WidebandNoise
Spectrum due to modulation and wideband noise
measurement checks whether the modulation process iscausing excessive spectral spread. In this test, one timeslot
(except for time slot 0) shall be set up to transmit full powerwhile all other time slots shall be turned off. In case of slow
frequency hopping (SFH), any carrier may be used, else
only the carrier transmitted shall be used [5]. First, thepower is measured on the carrier frequency using a video
filter with bandwidth of 30 KHz. The measurement shall be
gated over 50 - 90 % of the useful part of a single time slot,
and the measured value over this part of the burst shall beaveraged. The averaging shall be over at least 200 time
slots and only the active burst shall be included in the
averaging process. The same above procedure is repeated atdifferent offsets below and above the carrier frequency.
These offsets are 100 KHz, 200 KHz, 250 KHz, and 400KHz, in addition to offsets from 600 KHz to 1800 KHz insteps of 200 KHz. The test limits are expressed in relative
terms (dBc), relative to the carrier power. As for themeasurements performed on the different offsets, and
according to 3GPP Technical Specification (TS) 05.05, the
results should not exceed the limits shown in Table 1 forthe BTS. The algorithm for this measurement is
summarized in Figure 1.
Table 1 BTS limits for spectrum due to modulation [5]
Maximum relative level (dB) at specifiedcarrier offsets (kHz)
CarrierPowerlevel
(dBm)100 200 250 400 600 to
< 1200
1200 to
< 1800
43 +0,5 -30 -33 -60* -70 -73
41 +0,5 -30 -33 -60* -68 -71
39 +0,5 -30 -33 -60* -66 -69
37 +0,5 -30 -33 -60* -64 -67
35 +0,5 -30 -33 -60* -62 -65
33 +0,5 -30 -33 -60* -60 -63
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Figure 1 Spectrum due to modulation and wideband
noise measurement procedure
2.2. Spectrum due to Switching
Spectrum due to switching is the second measurement
defined with adjacent channel power. RF power quickly
ramps up in GSM transmitters. This process of rampingshould occur precisely and at a specific speed. Spectrum
due to switching measurement checks for undesirable
spectral components resulting from very quick power ramp.
Quick power ramping causes significant interference inadjacent channels. This measurement usually detects faults
in a transmitters output power amplifier. For the
transmitter under test, all timeslots shall be set up totransmit full power. Similar to spectrum due to modulation,
if SFH is on, any carrier may be used. Otherwise, only the
carrier transmitted shall be used. At 400, 600, 1200, and
1800 kHz offsets, power shall be continuously measuredusing a video bandwidth of 100 kHz. The power measured
shall not exceed the limits shown in Table 2 for a BTS [5]
[6].
Table 2 BTS limits for spectrum due to switching
Offset
(kHz)
GSM 900
Power (dBc)
DCS 1800
Power (dBc)
400 -57 -50
600 -67 -58
1200 -74 -66
1800 -74 -66
3. Modulation Accuracy
Modulation accuracy test is characterized by phase errorand frequency error sub-measurements. It reflects the
performance of the transmitter; a significant phase errorindicates a problem with the I/Q base-band generator, the
Gaussian LPF, the modulator, or the RF amplifier of the
transmitter. On the other hand, a significant frequency errorindicates a problem with the synthesizer (phase-lock loop)
[4].
Figure 2 Modulation accuracy test procedure
Both sub-measurements depend on bits obtained after
demodulation and are performed when all carriers aretransmitting full power in all their time slots. In case of
SFH, the BTS shall be hopping over the maximum number
of carriers or else the test shall be only performed over the
B, M, and T channels [5]. As defined in CCITT
Recommendation O.153, any 148-bit subsequence of the511-bit pseudo random sequence can be used to trace the
trajectory of the expected phase. This expected phase is
subtracted from the actual phase of the measuredwaveform. Phase error is determined using the root mean
square (RMS) and the peak of the variation of thesubtraction result, as shown in Figure 2. RMS and peak
values should not surpass 5o
and 20o
respectively. Finally,the mean gradient of the subtraction result constitutes the
frequency error. Frequency error should not exceed 0.05
ppm [5].
4. Mean Transmitted RF Carrier Power
GSM BTS and MS are classified into classes according to
the maximum power they can emit. Mean transmitted RFcarrier power shall always be less than that maximum level.
Were it not the case, transmitter power sources and
amplifiers might be malfunctioning. Transmitted RF carrier
power shall be measured at the input of the TX combiner. Itis defined to be the mean power of the useful part of a GSM
burst, shown in Figure 3. It keeps changing due to dynamicpower control of the BTS in a predefined power steps
defined by 3GPP TS 11.20 [5].
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Figure 3 Mask limits of a GSM burst for BTS
5. Transmitted RF Carrier Power versus
Time
Transmitted RF carrier power versus time measurementevaluates the envelope of carrier power in the time domain
within a predefined mask. To prevent interference, the
power of GSM transmitters must ramp up then down within
the allocated timeslot. If transmitters ramp up too slowly,
data at the beginning of the burst may be lost. Furthermore,if transmitters ramp down too slowly, the user of the next
timeslot will experience interference. A problem with the
units output amplifier is highly probable if the transmitterfails this measurement, which is performed using an
analyzer in a zero-span mode. An example of the mask used
in transmitted RF carrier power for BTS is shown in Figure3 [4].
6. Implementation
The above four measurements are implemented usingLabVIEW, with Spectral Measurements Toolset and
Modulation Toolkit, and included in the GMT. LabVIEW, a
data-flow programming platform, provides a great graphicaldevelopment environment for signal acquisition,
measurement analysis, and data presentation. It delivers the
flexibility of a programming language and avoids the
complexity of traditional development tools. The basic unitin LabVIEW is the VI. The VI is an instrument driver
divided into a front panel and a block diagram. The front
panel forms the Graphical User Interface (GUI) of thedriver while the block diagram forms graphical code which
is compiled into machine code.
Our GMT is designed for use along with NI PXI-5660
RFSA for GSM RF equipment testing. All itsmeasurements are integrated in one VI.
6.1. RFSA Hardware
The RFSA used for acquisition is the NI PXI-5660, which
is a modular signal analyzer optimized for automated RF
tests. PXI-5660 features a wide real-time bandwidth, a
highly stable time base, and flexible software tools that can
solve measurement applications ranging from component
characterization in R&D to the remote monitoring of RFnavigation systems. The main components of PXI-5660 are
the NI PXI-5600 and PXI-5620. The PXI-5600 is a 2.7 GHz
broadband down-converter that employs vector RF
measurements. It has an 80 dB spurious-free dynamic rangewith a 30 dBm full scale input range. Correspondingly, the
PXI-5620 is a high-spectral-purity single-channel digitizer
module with a sampling rate ranging from 1 kS/s to 64MS/s. It is characterized by an outstanding distortion-free
performance due to its deep segmented memory and 14-bit
resolution. A GSM 7 dBi directional antenna is used withthe PXI-5660 via its SMA interface. Subsequently, the PXI-
5660 is connected to a PC via a PCI-PCI bridge (NI PXI-
8335), fiber optical cables, and a PCI card [3].
As illustrated in Figure 4, a GSM signal passes through a
range of stages while processed and analyzed by the PXI-5660/GMT package.
Figure 4 PXI-5660/GMT operational overview
First, the antenna detects required signal and passes it to thePXI-5600 down-converter where it is down-converted to an
Intermediate Frequency (IF) of 15 MHz. Thereon, the
signal is appropriately sampled and digitized by the PXI-
5620 digitizer. The sampling rate is carefully set in theGMT block diagram while the digitization is performedusing 2
14discrete levels. After the RF signal becomes
digital, it is transmitted to the PC where users may carry out
different tests using the GMT.
6.2. GMT Main VI
All GMT measurements share the same block diagram. The
general flow of each algorithm includes tuning to one of the
124 frequency carriers, synchronizing in time to a specific
time slot, and finally isolating a number of GSM bursts.The process of tuning to a certain frequency f1 and
acquiring a GSM signal is done through several VIs. These
VIs are responsible for initializing the acquisition
hardware and then configuring a filter with the requiredbandwidth. The filter bandwidth depends on the type of
measurement undertaken. The ORFS measurement requiresa bandwidth of 3.6MHz to ensure that all the offset
frequencies are present for later processing. The other 3
measurements need a bandwidth of 200 kHz only, which isthe effective bandwidth of a single GSM channel.
Synchronization in time is achieved through the
identification of the Frequency Burst, which has the shape
of an unmodulated carrier in the frequency domain, and
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then the deciphering of the following Synchronization
Burst. The last stage of isolating a burst is simply done bysaving the number of samples constituting the burst. The
number of samples/burst depends on the sampling
frequency used by the acquisition hardware [3].
After employing the above procedures, specialized VIs areused to perform measurement-specific operations and signal
processing, such as filtering, power averaging, powerspectrum calculations and GUI functions.
7. RF Test Results
The developed GSM Measurement Toolkit is used to test
the RF equipment, BTS and MS. For the ORFS
measurement, the BTS failed the test while the MS passed
it as shown in the two figures below.
Figure 5 ORFS due to modulation-BTS (Failed)
Figure 6 ORFS due to modulation-MS (Passed)
The BTS, far from the test location, failed to conform to the
3GPP standard due to the highly attenuated signal. On the
other hand, the MS under test, which was placed near the
antenna, passed the test due to limited effect of signalattenuation.
The figure below also shows an MS that passedsuccessfully the ORFS due to switching test. This indicates
that the MS doesnt emit undesirable spectral components
that cause inference in adjacent channels.
Figure 7 ORFS due to switching-MS (Passed)
Over the useful part of the burst, as shown in Figure 8, the
mean transmitted RF carrier power for a BTS is measured
to be -58.6 dBm.
Figure 8 Mean Transmitted RF Carrier Power-BTS
The transmitted carrier power versus time measurement is
conducted on both the BTS and MS as shown below in
Figures 9 and 10.
Figure 9 Transmitted carrier power versus time-BTS
(Passed)
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