introduction to gsm&gsm mobile rf transceiver

12 www.rfdesign.com June 2003

The GSM system was specified by the EuropeanTelecommunications Standards Institute

(ETSI at www.etsi.org). GSM has evolved intoGSM900, DCS1800 (also known as PCN) andPCS1900 (in the USA). GSM is now a global sys-tem for mobile communications, spanning Europe,Asia, Africa and much of South America.

The GSM standards define a radio communica-tions system that works properly only if each com-ponent part operates within precise limits.Essentially, mobiles and base stations must trans-mit enough power with sufficient fidelity to main-tain a call of acceptable quality, without transmit-ting excessive power into the frequency channelsand timeslots allocated to others. Similarly,receivers must have adequate sensitivity and selec-tivity to acquire and demodulate a low level signal.

This document provides an overview of the keymeasurements required for testing GSM trans-ceivers. It also discusses GSM mobile performancederivation. It is intended to help an RF designerwith no GSM RF system knowledge get up tospeed with the GSM system.

GSM modulationGSM900 is the original GSM system. It uses fre-

quencies in the 900 MHz band (numbered one to124), and is designed for wide area cellular opera-tion with maximum output powers of 1 W to 8 Wallowed for mobile applications.

GSM uses a digital modulation format called 0.3Gaussian minimum shift keying, or 0.3 GMSK.The 0.3 describes the bandwidth of the Gaussianfilter in relation to the bit rate.

GMSK is a special type of digital FM modulation.Ones and zeroes are represented by shifting the RFcarrier by plus or minus 67.708 kHz. Modulationtechniques that use two frequencies to represent onesand zeroes are called frequency shift keying (FSK). Inthe case of GSM, the data rate of 270.833 kbps is cho-sen to be exactly four times the RF frequency shift.This has the effect of minimizing the modulation spec-trum and improving channel efficiency. FSK modula-tion where the bit rate is exactly four times the fre-quency shift is called minimum shift keying (MSK). InGSM, the modulation spectrum is further reduced byapplying a gaussian pre-modulation filter. This slowsdown the rapid frequency transitions, which wouldotherwise spread energy into adjacent channels.

0.3 GMSK is not phase modulation. It is the fre-quency shift, or change of phase state, that conveysinformation. GMSK can be visualised from an in-phase and quadrature (I/Q) diagram. Without theGaussian filter, if a constant stream of ones is beingtransmitted, MSK will effectively stay 67.708 kHzabove the carrier center frequency. If the carriercenter frequency is taken as a stationary phase ref-erence, the 67.708 kHz signal will cause a steadyincrease in phase. The phase will role 360 degreesat a rate of 67,708 revolutions per second. In one bitperiod (1/270.833 kHz), the phase will get a quarterof the way round the I/Q diagram, or 90 degrees.Ones are seen as a phase increase of 90 degrees.Two ones cause a phase increase of 180 degrees,three ones 270 degrees and so on. Zeroes cause thesame phase change in the opposite direction.

The exact phase trajectory is tightly controlled.GSM radios need to use digital filters and I/Q ordigital FM modulators to accurately generate thecorrect trajectory. The GSM specification allows nomore than 5 degrees RMS and 20 degrees peakdeviation from the ideal trajectory.

GSM transceiver measurementsGSM mobile transmitter and receiver measure-

ments originate from section 05.05.V8.12.0 (“RadioAccess Network: Radio Transmission and Reception,”Release 1999) of the ETSI 3GPP standards.

Performance is critical in three areas; in-chan-nel, out-of-channel, and out-of-band. An example ofa spectrum of the three areas is shown in figure 1.

In-channel measurements determine the linkquality seen by the user in question. Measurementsinclude phase error and mean frequency error,mean transmitted RF carrier power, and transmit-ted RF carrier power versus time.Figure 1. Visualization of the three areas of concern in the spectrum

Introduction to GSMand GSM mobile RF

transceiver derivationThe GSM system works properly

only when its component partsoperate within precise limits.Learn the key measurements

required for testing GSM transceivers.

By Paul Kimuli

embedded systems

Continued on page 16


Out-of-channel measurements deter-mine how much interference the usercauses other GSM users. These mea-surements include spectrum due tomodulation and wideband noise, spec-trum due to switching, and Tx and Rxband spurious.

Out-of-band measurements deter-mine how much interference the usercauses other non GSM users of theradio spectrum, such as the military,police, and aviation. All other spurious(such as harmonics and wideband) areincluded here.

Phase error and frequency errorPhase error is one of the parameters

used in GSM to characterize modula-tion accuracy. Poor phase error usuallyindicates problems with I/Q basebandgenerators, filters, modulators, oramplifiers in the transmitter circuitry.

Frequency error measurements indi-cate poor synthesizer/phase lock loopperformance (such as if synthesizersmay not settle quickly enough as theyshift frequencies between transmis-sions). In GSM systems, poor frequencyerror can cause target receivers to failto gain lock to transmitted signals. Alsothe transmitter could cause interfer-ence with other users.

To measure phase and frequencyerror, test sets can be used to sampletransmitted output of the devices undertest to capture the actual phase trajecto-ry. This is then demodulated and theideal phase trajectory is derived mathe-matically. Subtracting one from theother gives error signals. The mean gra-dient of these signals gives frequencyerror. The variation of this signal is thephase error and is expressed in terms ofroot mean square (rms) and peak. Figure2 demonstrates this test procedure.

Figure 3 shows a measurement ofthe phase error on one transmittedburst and how it relates to the limitsset by the GSM standard.

Mean transmitted output powerGSM systems use dynamic power

control to ensure that each link is

maintained sufficiently with a mini-mum of power. This allows overall sys-tem interference to be kept to a mini-mum, and in the case of an MS, batterylife is maximized.

Power measurements outside ofspecifications usually indicate a fault inthe power amplifier circuitry, the cali-bration tables or the power supply.

The mean output power is mea-sured during the useful part of theGSM burst. When performing thismeasurement, the GSM test equip-ment derives the correct timing refer-ence by demodulating incoming sig-nals and gating over the useful part ofthe GSM burst.

The transmitters of the GSM sys-tems must ramp up and down withinthe time division multiple access(TDMA) structure to prevent adjacenttimeslot interference. If transmittersturn on too slowly, data at the begin-ning of the burst might be lost, degrad-ing link quality. If the transmittersturn off too slowly, the user of the nexttime slot in the TDMA frame will expe-rience interference.

Therefore, transmitted RF carrierpower versus time measurements areperformed to assess the envelope of car-rier power in the time domain against aprescribed mask. The measurementsalso check that the transmitters’ turnoffs are complete. If a transmitter failsthis measurement, it usually indicatesa problem with the units PA or powercontrol loop.

Adjacent channel powerAs part of the out-of-channel mea-

surements, the adjacent channelpower (ACP) is defined by two mea-surements: spectrum due to modula-tion and wideband noise, and spec-trum due to switching. These two mea-surements are usually grouped togeth-er and referred to as output RF spec-trum (ORFS).

The modulation process in a trans-mitter causes continuous wave carriersto spread spectrally. The spectrum dueto modulation and wideband noise

measurement is used to ensure that themodulation process does not causeexcessive spread. This would causeinterference to adjacent channel users.

To perform these measurements,analyzers are tuned to spot frequen-cies and time gated across part of themodulated burst. Using this mode, thepower is measured. The analyzer isthen retuned to the next frequency oranother offset of interest. This processcontinues until all offsets are mea-sured and verified against permissiblelimits. The result of these measure-ments is a set of frequency versuspower points that define the spectrumof the signals. However, spectral com-ponents that result from the effect ofbursting do not appear because theramps are gated out.

The test limits for these measure-ments are expressed in dBc (powerbelow carrier). It follows that the firststep of the measurement is to take areading of the center frequency towhich the transmitter is tuned.

Spectrum due to switchingGSM transmitters ramp RF power

rapidly. The transmitted RF carrierpower versus time measurementsdescribed earlier ensure that thisprocess happens at the correct timesand is fast enough. However, if RFpower is ramped too quickly, undesir-able spectral components exist in thetransmission. This measurement alsoensures that these components staybelow the acceptable level.

To perform spectrum due to switch-ing measurements, the analyzers aretuned to and measure multiple offsetfrequencies in zero span mode with notime gating.

Spurious measurementsThe out-of-channel measurements

are necessary to ensure GSM transmit-ters do not place energy into the incor-rect parts of the spectrum. This wouldcause interference to other users of thespectrum. These anomalies are referredto as spurious transmissions.

Figure 2. Test procedure to derive phase errorFigure 3. Example of an absolute phase error in one burst in relation to GSMstandards


The spurious transmissions are mea-sured by connecting test sets directly tothe antenna connectors of the MS. Dueto the antennas direct connection to thetest sets, these measurements arereferred to as the conducted spuriousmeasurements. Measurements of thisparameter include Tx/Rx band spuri-ous, cross band spurious, and out-of-band spurious.

The spurious measurements can becategorized as Tx or Rx depending onthe band they inhabit. The Tx bandspurious measurements relate to spuri-ous that fall within the 925 MHz to 960MHz GSM Tx band. The Rx band spu-rious measurements, however, aremeasures of how much energy thetransmitters put in the Rx band (880MHz to 915 MHz). This test ensuresthat Tx spurious don’t “jam” or desensi-tize adjacent receivers. The specifica-tions of the measurements are based on1 m average distance between mobiles.

For the purpose of attenuating the Txband signals during these measurements,the test setups usually include Rx bandpass filters in front of the analyzer inputs.

In some countries GSM900 andDCS1800 systems co-exist. For thisreason the ETSI 3GPP standardsrequire specific cross-band performancecapability. This is to ensure the GSMtransmitters place the minimum ener-gy required into both the DCS1800 andGSM9000 bands.

The out-of-band spurious is a seriesof spectrum analyzer measurementsover a large frequency range — from100 kHz through 12.75 GHz. The 3GPPstandards were written to includewideband spurious limits to which anMS must conform.

ReceiversSensitivity is the fundamental mea-

sure of receiver performance. It speci-fies the minimum signal level for aspecified percentage of errors in thedemodulated information. The reportedvalue for all receiver measurements isbit error rate (BER) or one of the fol-lowing a variations:

• Frame erasure rate (FER) — Thepercentage of erased frames com-pared to the total number of framessent during an observation period.

• Residual bit error rate (RBER) —When frames are erased, the BERof the remaining frames is mea-sured. The RBER parameterdefines this measurement.

BER is a ratio of bits received erro-neously versus total number of bitsreceived. It is measured as follows: Thetest systems output signals carryingknown bit patterns (usually pseudorandom bit sequences or PRBS). PRBSsignals are usually labeled PNx, wherex is the number of bits being permutat-ed in the sequence (such as PN9 = 29 - 1or 511 bits).

During the measurements, thereceivers under test attempt to demodu-late and decode these patterns. Byreturn paths (using a method known asloop back), the receivers send the resul-tant bits back to the test systems forcomparison. The test systems then calcu-late the required metrics. GSM handsetsare tested using this loop-back method.

Most receivers are required to main-tain a specified BER in the presence ofinterfering signals within the channel.For GSM, the performance is measuredas follows: Digitally modulated signalspower levels are set 20 dBs abovereceiver sensitivity at the center of thereceivers’ passband. These signals arecombined with GMSK modulated inter-ferers. Combined signals are theninjected into the antenna ports of thereceivers. The power levels of theGMSK interfering signals are then setto nominal levels at which the receiverBERs must not exceed the receiver sen-sitivity specifications. The difference inpower levels between the two signals isthe interference ratio.

Receiver blocking constitutes one ofthe out-of-channel receiver tests.Blocking tests verify correct receiveroperation in the presence of out-of-channel signals and monitor thereceivers’ susceptibility to internallygenerated spurious responses. There

are three key tests that define thereceivers blocking performance: spuri-ous immunity, intermodulation immu-nity, and adjacent channel selectivity.

Spurious immunity is the ability ofthe receivers to prevent single, out-of-channel interference signals from caus-ing undesired in-channel responses atthe output of the receivers. Spuriousmay be generated within the receiversfrom power supply harmonics, systemclock harmonics or LO spurious.

Intermodulation immunity is ameasure of the receivers’ performancein the presence of distortion productsdue to intermodulation products.These intermodulation products aregenerated when more than one tone ispresent at the input of the receivers.The tones non-linearly mix to formthird-order intermodulation products.The products of concern lie within thereceivers’ passband.

Adjacent channel selectivity is ameasure of the receivers’ ability toprocess the desired modulated signalsin the presence of strong signals in theadjacent channels. Alternate channelselectivity is a similar test in which theinterfering signals are two RF channelsaway from the receivers’ passband.

GSM mobile RF transceiverderivation

Receiver sensitivity is related toreceiver noise figure according to:

Where the receivers’ bandwidth (180kHz for GSM), is the baseband signal-to-noise-ratio and the RF and broad-band implementation gain.

The GSM standard specifies a mini-mum -102 dBm sensitivity requirement.Given a worse-case baseband ratio of 9dB and a 2 dB implementation margin,we calculate the noise figure as:

Given this worse-case NF, thereceivers’ designers can then investi-

Figure 4. Single LNA with active mixer Figure 5. Dual LNA with active mixer


gate various front end gain and NF par-tition options according to the equation:

Where Fi is the noise factor of thei’th block in the partition (i = 1, 2, 3 ...).

Although the second equation showsthat the higher the gain of the firstactive stage, the lower the NF of thesystem would be, the receivers’ design-ers need to ensure that the first activestage does not compress the subsequentstages. This would degrade receiver lin-earity. This shows that system sensitiv-ity is a compromise between receiverNF (dominated by choice of front endcomponents) and receiver linearity.

The receivers’ front end low noiseamplifier (LNA) options typically inves-tigated are shown in figures 4 and 5.

The main benefit of the dual LNAoption — when compared with thefirst option — is that the individualLNA noise figure and gain require-ments are significantly relaxed. Withthe single LNA option, the front endLNA would need to be tightly specified

to achieve the same system NF as thedual LNA option.

The main disadvantage of the dualLNA option typically is increased costand (potentially) the extra supply cur-rent required due to addition of a sec-ond LNA.

Rx blocking analysisTable 1 shows the blocking signal

levels with which a GSM mobile isexpected to perform without drop-ping a call. GSM receiver designersspecify the receive strips’ compres-sion points based on the above listedin-band blocking signal level specifi-cations, and uses the out-of-bandblocking signal levels to define thefilter rejection specifications to avoidsignal path compression.

For example, in-band blocking at 3MHz offset (such as -23 dBm) sets thecompression point required for thefront end. Assuming a 1 dB loss switchand a 2.5 dB loss filter prior to a LNAstage in the receive strip, it places atotal 3.5 dB loss prior to the LNA stage.This means the LNA compression pointmust, in the worst case, be -26.5 dBm(such as -23 dBm to 3.5 dBm).

Frequency band MS Blocking Descriptionsignal level

600 kHz ≤ |f-fo| < 800 kHz -43 dBm In band blocking

800 kHz ≤ |f-fo| < 1.6 MHz -43 dBm

1.6 MHz ≤ |f-fo| < 3 MHz -33 dBm

3 MHz ≤ |f-fo| -23 dBm

900 MHz to 915 MHz -5 dBm Out of band blocking

980 MHz to 12750 MHz 0 dBm

Table 1: Blocking signal levels where GSM is expected to perform

Offset frequency dBc/BW Derived Phase noise

±200 kHz -30 kHz/30 kHz -75 dBc/Hz



±600 kHz - 1200 kHz -60 kHz/30 kHz -105 dBc/Hz




> ±6000 kHz -71 kHz/100 kHz -121 dBc/Hz

Table 2: ETSI 05.05 specifications for spectrum due to modulation

RF Design www.rfdesign.com 21

Rx intermodulationGSM receiver intermodulation per-

formance is predominantly affected bythe front end circuitry. The intermodu-lation performance is affected if the IFfilter chosen has good enough attenua-tion at +/-800 kHz and +/-1600 kHz (theoffset frequencies for which this para-meter is tested as specified by the GSM05.05 standard).

The equation typically used to deter-mine system IP3 requirement is:

where

is the interference signal level (-49dBm from GSM 05.05 spec),

is the useful signal and

is the carrier-to-interference ratio forwhich the receiver is designed. Theuseful signal level can be determinedfrom the GSM sensitivity level byadding 3 dB. So, for an 8 dB carrierinterference ratio, the minimum GSMreceiver input intercept is -20 dBm.

ConclusionThis article is meant to only be an

overview of the key measurementsrequired for testing GSM transceivers,and GSM mobile performance deriva-tion. It is intended to help an RFdesigner with no GSM RF systemknowledge get a better appreciation ofGSM module specifications and howthey can affect system performance.

About the AuthorPaul Kimuli is a senior RF field appli-cations engineer for Maxim IntegratedProducts Inc. (www.maxim-ic.com)based in the United Kingdom. He hasa first class degree with Honoursin electronic and electrical engi-neering, from the University ofLeeds. Kimuli can be reached [email protected].

introduction to gsm&gsm mobile rf transceiver

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