gsm rf equipment testing - final conference paper

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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gsm rf equipment testing - final conference paper

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