ANALOG & DIGITAL MODULATION TECHNIQUES: AN OVERVIEW

TECHNIA International Journal of Computing Science and Communication Technologies, VOL. 3, NO. 1, July 2010. (ISSN 0974-3375)

551

ANALOG & DIGITAL MODULATIONTECHNIQUES: AN OVERVIEW

D.K.Sharma1, A. Mishra2 & Rajiv Saxena3

1Ujjain Engineering College, Ujjain, MP2Madhav Institute of Technology & Science, Gwalior, MP3Jaypee Institute of Engineering & Technology, Guna, MP

[email protected]; [email protected]; [email protected],

Abstract:A tremendous technological transformation during the last twodecades has provided a potential growth in the area of digitalcommunication and lot of newer applications and technologiesare coming up everyday due to these reasons. Restrictingoverself to the domain of modulation techniques a briefoverview over different analog and digital modulationtechniques has been provided in this article through extensiveliterature survey in a tabular manner enabling to analyze andestablish the superiority at a glance of a specific modulationtechnique for a particular application.

1.0 INTRODUCTION:Living in the era of communication every thing

may be video, audio or any information in the form ofelectrical signal is termed as data and there is an enormousrequirement of data transfer between two or more pointthrough the world wide web, every moment of the clock,which is a big threaten to the existing communicationsystems because of the problems like spectral congestion,severe adjacent & co-channel interference problems andnoise corrupted data reception etc. This has resulted inserious need for the research work all around the world forthe development of the communication systems which canhandle the above said problems, where each aspect of thecommunication systems is dealt with the development ofnew encoding techniques, modulation techniques,possibilities for newer transmission channels and off coursethe demodulation and decoding techniques [1, 2].

The design of a communication system is applicationoriented and is dependent on the type of the signal. Thechoice of digital communication technique over its analogcounter part becomes more evident of the fact that it providelarger immunity to noise for even at the price of largebandwidth requirements, where as the requirement of video,Audio and data over the computer network or the mobiletelephony network termed as the third generation (3G)mobile communication poses a serious problem for thebandwidth so The existing modulation techniques need to bemodified for the purpose where it can handle both thesituations of noise and bandwidth efficiency [3, 4].

The major advantage of using digital modulationtechnique is that the use of digital signals reduces hardware,noise and interference problems as compared to theanalogue signal where large number of waveforms will berequired resulting in a larger bandwidth for the symbol to betransmitted [5].

Over the past years various modulation techniqueshave been designed and extensively used for variousapplications but the modern communication system requiresdata transmission at a higher rate, larger bandwidth in order

to have multimedia transmission, hence the existing modulationtechniques are not able to provide a complete solution keepingthis in the view the authors of this article have tried to draw asketch within the existing modulation techniques to derive outexactly what modifications or the alterations in the presenttechniques may sort out the problem or there is still a need fordesigning a new modulation technique for the purpose of thepresent communication system requirements [6, 7].

2.0 Classification of Modulation Techniques.Modulation is the process of varying some parameter of

a periodic waveform in order to use that signal to convey amessage. Normally a high-frequency sinusoidal waveform isused as carrier signal. For this purpose ,if the variation in theparameter of the carrier is continuous in accordance to the inputanalog signal the modulation technique is termed as analogmodulation scheme if the variation is discrete then it is termed asDigital Modulation Technique [8].

Table-1: Type of Modulation TechniquesSr.No

.ModulationTechniques

Type Notation

01 AnalogModulationTechniques

(i) AmplitudeModulation

(ii) FrequencyModulation(iii) Phase

Modulation

A.M.

F.M.

P.M.

02 DigitalModulationTechniques

(i) AmplitudeShift Keying

(ii) FrequencyShift Keying

(iii)Phase ShiftKeying

A.S.K.

F.S.K.

P.S.K.

2.1 Analog Modulation Techniques:-There are basically three type of analog modulation

schemes the amplitude modulation , the Frequency modulationand the phase modulation schemes which have in turn lot ofclass, subclass or derivatives as listed in Table-2 [9, 10]. In caseof the Amplitude Modulation there are several derivatives and itis evident from the comparative table -3 that the Single SideBand Suppressed Carrier (SSS-SC) has smaller bandwidth andpower requirements in contrast with Double Side BandSuppressed Carrier (DSB SC) and Double Side Band FullCarrier (DSB FC) and Single Side Band Full Carrier (SSB FC)but for detection of this signal, we require sharp cutoff Low PassFilter (LPF) which is not practically viable. Using the VestigialSide Band (VSB) technique in place of (SSB SC), we can


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achieve a low pass filter with a gradual cut off but it requiresmore BW and power than SSB-SC and less then the DSB-SC and DSB-FC and hence ideally SSB-SC is proves to bebetter than other AM schemes but practically, VSB provesto be a much better candidate then the other amplitudemodulation techniques [11, 12].

The Amplitude modulated signals require nonlinearamplifiers which generate spurious out-of-band spectralcomponents which are filtered out with a great difficulty.Frequency Modulation proves to be better in comparison toamplitude modulation and phase modulation, and thederivative of frequency modulation, narrow band FM(NBFM) is usually employed to overcome above mentionedproblems in the communication system [13, 14].Table-3 provides representation, bandwidth requirement andpower requirement properties of various analog modulationtechniques. The great merit of FM over AM is that FMallows us to suppress the effects of noise at the expense ofbandwidth. The major limitation of the analog modulationsystems for communicating over long channels is that oncenoise has been introduced at any place along the channel,then it is carried out till the end. Because the analogmodulation system ( AM, FM and PM ) are extremelysensitive to the noise present at the receiver end in contrastto this if a digital signal is modulated and transmitted thereceived signal is far less sensitive to receiver .

2.2 Digital Modulation Techniques:-After the conversion of an Analog signal to digital

by sampling different type of digital modulation schemescan be achieved by the variation of different parameter ofthe carrier signal for example the Amplitude variation givesBASK, Frequency variation gives BFSK and the phasevariation gives BPSK. Also sometimes a combinationalvariation of this parameter is done to generate the hybridmodulation technique viz. a combinational variation ofAmplitude and Phase Shift Keying (APSK). Many moredigital modulation techniques are available and can also bedesigned depending upon the type of signal and theapplication [17].

Thus a better digital modulation technique is to bethought over by the designer which has an ability ofexploiting the available transmitted power and thebandwidth to its full extent [18, 19].

In order to achieve a discrete signal it is essential tohave the modulating signal of the form of a NRZ rectangularpulse thus yielding the modulated parameter as a discretesignal switching or keying between two discrete values [20].However, ASK, FSK, and PSK with Nyquiste pulse shapingat the base band form the basic technique of digitalmodulation, but other methods are also possible withhybridization of two or more basic digital modulationschemes with or without pulse shaping [21, 23].

3. Classification of Digital Modulation.These digital modulation techniques can be

classified basically either on the basis of their detectioncharacteristics or in terms of their bandwidth compactioncharacteristics [24]. Various types of digital modulationtechniques are listed in Table-4 and few of them have beencomprehensively emphasized here in details providing acomparative analysis.

3.1 Binary Amplitude Shift Keying [BASK]The BASK is obtained by the alteration of the

amplitude of the carrier wave [1, 11]. It is a coherent modulationtechnique hence the concept of the co-relation between thesignal, number of basis functions, the I and Q components andthe symbol shaping are not applicable here. It has very poorbandwidth efficiency. The basic merit of this technique is itssimple implementations but is highly prone to noise and theperformance is well established only in the linear region whichdoes not make it a viable digital modulation technique forwireless or mobile application in the present scenario. Thecombination with PSK [20] yields derivatives like QAM and M-Ary ASK, which have much important application withimproved parameters.

3.2 Binary Frequency Shift Keying [BFSK]When two different frequencies are used to represent

two different symbols, then the modulation technique is termedas BFSK.BFSK can be a wideband or a narrow band digitalmodulation technique depending upon the separation betweenthe two carrier frequencies, though cost effective and providessimple implementations but is not a bandwidth efficienttechnique and is normally ruled out because of the receiverdesign complexities [1-3, 12].

3.3 Binary Phase Shift Keying [BPSK]When the phase of the carrier wave is altered with

reference of the modulating signal then the resultant modulationscheme is termed as Phase Shift Keying. The digital modulationtechnique can be said to be the simplest form of phasemodulation and is known as binary because the carrier phaserepresents only two phase states [13]. It is normally used for highspeed data transfer application, provides a 3dB power advantageover the BASK modulation technique and is robust and simple inimplementation but proves to be an inefficient user of theprovided bandwidth and is normally termed as a non-linearmodulation scheme. It provides small error rates than any othersystems. The modulation techniques provide a number ofderivatives [20].

3.4 Differential Phase Shift Keying [DPSK]For the perfect detection of a phase modulated signal, it

become evident that the receiver needs a coherent referencesignal but if differential encoding and phase shift keying areincorporated together at the transmitter then the digitalmodulation technique evolved is termed as Differential PhaseShift Keying [1, 14]. For the transmission of a symbol 1, thephase is unchanged whereas for transmission of symbol 0, thephase of the signal is advanced by . The track of the phasechange information which becomes essential in determining therelative phase change between the symbols transmitted. Thewhole process is based on the assumption that the change ofphase is very slow to an extent that it can be considered to bealmost constant over two bit intervals (7).

3.5 Quadrature Phase Shift Keying (QPSK)Another extension of the PSK digital modulation technique

is the division of the phase of the carrier signal designed byallotting four equally spaced values for the phase angle [1-3] as

BPSK by having the information capacity double to it, i.e. the


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QPSK has four message points in the constellation diagramand so it becomes a highly bandwidth efficient digitalmodulation technique. But the exact phase retrieval becomesa very important factor for the receiver designconsiderations, failing which can give rise to erroneousdetection of the signal. This factor increases the receiverdesign complexities. To compensate for these problems,normally the idea of pulse shaping the carrier modulatedsignal is employed with the Root Raised Cosine Pulseshaping for achieving better performances which in turnprovides a demerits that the constant envelope property ofthe signal is lost but then there is a lost but there is aremarkable improvement in the ISI performance of thisdigital modulation technique [15-18].

3.6 Minimum Shift Keying [MSK]Minimum Shift Keying (MSK) is a modified form of

continuous phase FSK. Here, in this case, the spacingbetween the two carrier frequencies is equal to half of the bitrate which is the minimum spacing that allows the twofrequencies states to be orthogonal [1-3].

An MSK signal can e said to be derived from either anOffset Quadrature Phase Shift Keying (OQPSK) signal byreplacing a square pulse by ½ cosinusoidal pulse oralternatively from an FSK signal. The information capacityof an MSK signal is equal to that of QPSK signal but due tothe ½ cosine pulse shaping the bandwidth requirement islesser than that required by QPSK. It achieved smoothphase transitions thus providing a constant envelope. It haslower out of band power and can be said to be morespectrally efficient than the QPSK modulation technique[19-25].

The major demerits which this digital modulationscheme suffer s is that it is in the class of linear modulation.The spectrum is not enough compact to realize [27] data rateapproximating RF channel bandwidth. Table-2 [26, 27]summarizes representation and different properties of thistechnique.

3.7 Gaussian Minimum Shift Keying [GMSK]An MSK signal is generated by applying a half

sinusoidal pulse in place of a square pulse. If a Gaussianpulse shape is used instead then the resultant digitalmodulation technique is an improved version of the MSKdigital modulation technique in the sense of bandwidth andspectral efficiency and is termed as GMSK digitalmodulation technique (Gaussian Minimum Shift Keying).Moreover, the major advantage in this technique is thesufficiently lower side lobe levels and the narrower mainlobe as compared to a QPSK and MSK pulse [18].

GMSK can be viewed as either a frequency orphase modulation scheme, although the rate of change ofphase is limited by the Gaussian response but he phasecarrier can still advance or retard up to 90o over the courseof the bit period. The severity in pulse shaping lies on thebandwidth time product (BT) because of the reason that theachieved phase change over a bit period may fall short by

act on bit error rate [28] butit still provides improved bandwidth efficiency over MSK.

The bandwidth of a GMSK system is defined bythe relationship between the premodulation filter bandwidthB and the bit period TB. Thus the decision of value of BT

and data rate is crucial in the sense that there has to be a trade ofbetween the BER and out of band interference [29, 30] as thenarrow filter will result in provocation of Inter SymbolInterference (ISI) which on the other hand will reduce the signalpower enormously [30].

The generation of a GMSK signal can be done by anyone of the two methods as in the case of MSK signals, theFrequency Shift Keying modulation method. Only differencewhich comes in here than the generation of MSK signal is thatthe pulse shaping by half root raised cosine pulse is replaced by aGaussian pulse shape.

3.8 Orthogonal Frequency Division Multiplexing (OFDM):-The OFDM is a modulation scheme having multicarrier

transmission techniques here the available spectrum is dividedinto many carriers each one being modulated at a low rate datastream. The spacing between the carriers is closer and thecarriers are orthogonal to one another preventing interferencesbetween the closely spaced carriers hence OFDM can be thoughtof as a combination of modulation and multiplexing techniques,each carrier in a OFDM signal has very narrow bandwidth so theresulting symbol rate is low which means that the signal has hightolerance to multipath delay spread reducing the possibility ofinter symbol interferences (ISI) which is the requirement for

The higher is the transmission rate, the large will be thebandwidth of the signal as compared with the coherencebandwidth of the propagation channel, at this stage the differentspectral components present in the signal will experiencedifferent fading characteristics, this frequency selective fadinghas to be characterized using appropriate techniques in order toachieve acceptable error rate at the detection or output in order toachieve characterization in frequency selective fading the basicapproach is to partition the signal into frequency bands, each oneof which is narrow as compared to the coherence bandwidth ofthe channel and subsequently each of this signal component isthen modulated onto a different sub carrier and the signalcomponents are sent parallel over the channel. Hence, eachsignal component will now experience non- frequency-selectivefading because now the high rate serial data sequence isconverted into a number of lower rate parallel sequences andthen each of them is modulated onto a sub carrier, the effectivemethod to achieve this is orthogonal frequency divisionmultiplexing (OFDM). The modulation parameters dependent onthe data rate used shall be set according to (Table-12) RateDependant Parameter.

4.0 ComparisonThe BASK technique is simpler and economic in

implementation and is less prone to errors but provides lessbandwidth efficiency and operates efficiency in the linear regiononly, which does not make it an efficient technique for thewireless communication systems. On the other hand, he BFSKtechnique is still less prone to errors and the bandwidthrequirement is the same as that of BASK (Table-4) but is not abandwidth efficient technique. The error performance parameteris better to BASK (Table-9,10). It requires matched filerdetection and because of this, the receiver design complexitiesincrease and so it is seldom used for wireless or mobileapplication.

The BPSK modulation technique is still better than theabove mentioned two modulation techniques. It is a coherent


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modulation technique and can be used for high speed datatransfer application and has a basic advantage of doubleinformation capacity (Table-7) over BASK and BFSK.Simple implementation and robustness makes it a usefultechnique for satellite communication but on the other handit has proved an inefficient use of the bandwidth and iscategorized under a class of non-linear modulationtechniques (Table-5). The error performance is better and isoptimized to achieve minimum possible error rate (Table-6,7). The detection of phase shift (Table-8) makes thereceiver design complex, so the technique is not of interestfor the wireless or mobile communication applications.

The DPSK technique provides information capacitysimilar to BPSK and is considered to be more viabletechnique than BPSK and is a non coherent orthogonalmodulation (Table-4, 5). But the receiver complexities aremore than BPSK because memory is required in the systemto keep the track of relative phase difference.

The most widely used technique is the QPSK modulationtechnique which has an information capacity double toBPSK (Table-4) over the same bandwidth and requirescoherent detection, so it can be considered to be highly BWefficient. Since the modulation envelope is also constanthence it is said to be spectrally efficient modulationtechnique also. Thus it provides major advantages overBPSK and has also overcome the major drawbacks of theBPSK.

In detection of a QPSK signal, the detection of exactphase shit becomes an important criterion which on theother hand increases receiver design complexities as well.The improvement further in this modulation technique canbe achieved by pulse shaping the modulated carrier. Thepulse shaping by ½ co-sinusoidal pulse shaping provides abetter performance modulation technique, the MinimumShift Keying (MSK), which can also be viewed ascomprising of two CPFSK signals. This has a majoradvantage that the out of band power is significantly lowerthan QPSK (Table-7) and the 99% of total power of MSK is1.2 TB thus spectrally efficient and constant envelopes. Ithas proved to be a better modulation technique than QPSKin the sense that the signal coherence and deviation ratio arelargely unaffected by variation in input rates (Table 11).But the basic demerit (Table 7) of MSK modulationtechnique is that the spectrum is not enough compact for therealization of higher data rates. The GMSK modulationtechnique is a variation of MSK where the co-sinusoidalpulse shaping of the modulated carrier is replaced by theGaussian pulse shaping. This improves the envelope andthe spectral efficiency (Table 6, 7). A BT = 0.3 GMSK hasbeen more popular than its other variants as it is optimizedfor the better bandwidth and error performances at thisvalue. The major disadvantage shown by this modulationtechnique is its high susceptibility to ISI at higher data ratesdue to the narrow symbol shape (Table 7). The technique ishighly used in GSM mobile communication.

The average probability of bit error at the output of ademodulator and decoder is the performance measure of thedemodulator decoder combination. To be more precise theprobability of error is a function of code characteristics,waveforms, the transmitted power, characteristics of thetransmission channel and the demodulation and decodercombination. Hence the reconstructed signal at the receiving

end is an close approximation of the transmitted signal and thedifference or some function of the difference in the original andthe reconstructed signal. This marks a measure of performance interms of distortion in a digital communication system. (Table-10) summarizes the BER equations of digital modulationtechniques.

The basic research work carried out in the field ofcommunication lead to the development of new modulationtechniques, coding techniques, error rate performances analysisbut the ever increasing demand of the faster communicationsystem with large bandwidth requirements has again generated anew hunger towards the development of newer techniques, somany modulation techniques like BPSK, DPSK, MSK, GMSK,M-ary QAM have been developed. The major consideration withany modulation technique developed is that its detectionperformance should show a better bit error rate (BER)performance, several methods have been devised for the exact orimproved BER performances of the modulation techniques.

The main objective of a communication systemdesigner is to transmit message as speedily as possible, with leastprobability of error. Fast communication is possible by: (i)reducing the time of each massage; but this, in turn, increase thebandwidth and (ii) simultaneous transmission of severalmessages over a single physical channel. This process is knownas multiplexing. So OFDM can be a good candidate over otherdigital modulation schemes.

5.0 Conclusions:An analysis of the digital modulation technique carried

out in this article reveals that the selection of a digitalmodulation technique is solely dependent on the type ofapplication. This is because of the fact that some of thetechnique provide lesser complexities in the design of themodulation and demodulation system and prove economic likethe BASK, BFSK, BPSK and DPSK techniques and can bevisualized for the systems which really does not require highamount of precisions or when economy is the major aspect andthe BER performances can be tolerated.

On the other hand when the system designer has a soleconsideration for the techniques like BASK, BFSK, BPSK and

designer has to think in terms of better modulation techniqueslike the QPSK, MSK and GMSK, where GMSK has proved itsperformance over the other two in the area of mobilecommunication because of the spectral efficiency. But the

criterion for higher data rate communication is taking the lead inalmost every area of communication and thus the ISI and BERrealization become very important and crucial aspect for anyfuture digital modulation technique.

Taking the above facts into consideration, the design ofa digital communication system is very trivial and is very muchapplications oriented, as one application may require higherprecision in data reception where as the other may compromiseon this aspect but may be rigid on the aspect of the availablebandwidth or power, thus the parameters like the modulationbandwidth, power, channel noise and the bit error rate becomevery important parameters in the designing of digital/wirelesscommunication system.

Reference:


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Table-2: Classification of Analog Modulation TechniquesSr.No. MODULATION

TECHNIQUESREPRESENT

ATIONTYPE

1Amplitude Modulation

Double-SidebandSuppressed Carrier

AMDSB-SC Linear


Double-Sideband With FullCarrier

AMDSB-FC

Linear


Single-Sideband SuppressedCarrier

AMSSB-SC

Linear


Single-Sideband With FullCarrier

AMSSB-FC

Linear


Vestigial-SidebandAMVSB

Linear

6 Narrow-Band FrequencyModulation

NBFM Non-Linear

7 Wide-Band FrequencyModulation

WBFM Non-Linear

8 Phase Modulation PM Non-Linear

Table-3: Performance Analysis of Analog Modulation Schemes

Sr.No.

TYPE OFANALOG

MODULATION

BANDWIDTH

(B. W.)

%POWERSAVING

POWERREQUIREM

ENT

1 AM-DSB-FCm2 Standard 3/2 Pc

2 AM-DSB-SCm2 66.67% 5/4 Pc

3 AM-SSB-FCm

16.67% 1/2 Pc

4 AM-SSB-SC m 83.33% 1/4 Pc

5 AM-VSB m >SSB-SC Greater thanSSB-SC

6 NBFMm2 Same as

DSB-SCSame asDSB-SC

7 WBFMFm

.

Morethan

NBFM

More thanNBFM

m =

modulatingfrequency

F =modulation index in

FM

P =modulation in PM

Pc = carrierpower

Table 4: Classification & Performance Analysis of Digital ModulationTechniques [1-7]

Sr. Modulation Representation Type BW


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No. Technique requirementBinaryModulationScheme

01BinaryAmplitudeShift Keying

BASK Noncoherent 2RB

02BinaryFrequencyShift Keying

BFSK Noncoherent 2RB

03 Binary PhaseShift Keying BPSK Coherent 2RB

04DifferentialPhase ShiftKeying

DPSK Noncoherent 2RB

QuadratureModulationScheme

01QuadraturePhase ShiftKeying

QPSK Coherent 2RB

02MinimumPhase ShiftKeying

MSK Coherent Less thanQPSK

M-rayModulationScheme

Where

NMN ,2

01 M-ary PhaseShift Keying M-ary PSK Coherent 2 /N

02

M-aryQuadratureAmplitudeShiftModulation

M-ary QAM Coherent2 /N

03M-aryFrequencyShift Keying

M-ary FSK Coherent M 2 /N

OrthogonalFrequencyDivisionMultiplexing

OFDM Coherent

01Binary PhaseShift KeyingOFDM

BPSK-OFDM CoherentLess than

Othertechniques

02

QuadratureAmplitudeModulationOFDM

QAM-OFDM CoherentLess than

Othertechniques

03

16-QuadratureAmplitudeModulationOFDM

16-QAM-OFDM

Coherent

Less thanOther

techniques

04

64-QuadratureAmplitudeModulationOFDM

64-QAM-OFDM Coherent

Less thanOther

techniques

Table-11: Modulation Parameters of Digital Modulation Techniques inmulticarrier Modulation Schemes

Sr. Data Modulati Codin Coded Coded Data

No.

Rate(Mbits/s)

on gRate( R )

bitsPerSub-carrier

BPSC

bitsPerOFDMSymbol

CBPS

bitsPerOFDMSymbol

DBPS

1 6 BPSK ½ 1 48 24

2 9 BPSK ¾ 1 48 36

3 12 QPSK ½ 2 96 48

4 18 QPSK ¾ 2 96 72

5 24 16-QAM ½ 4 192 96

6 36 16-QAM ¾ 4 192 144

7 48 64-QAM 2/3 6 288 192

8 54 64-QAM ¾ 6 288 216

Table-12: Numerical Values for the OFDM [Multicarrier Modulation Schemes]Parameters

Sr.No.

PARAMETERS VALUE

1 Sampling Rate = 1/T 20Mhz

2 Useful Symbol Part Duration 64*T3.2 microsecond

3 Cyclic Prefix Duration 16*T, 0.8 Sec. (mand.)8*T, 0.4 Sec. (Opti.)

4 Symbol Interval 80*T, 4 Sec, +

72*T +

5 Number of Data Sub-carriers

SD

48

6 Number of Pilot Sub-carriers

SP

4

7 Total Number of Sub-

carriers ST

52 ( SD + SP )

8 Sub-carrier Spacing F 0.3125 Mhz ( 1/ )

9 Spacing Between the twooutmost sub-carriers

16.25 Mhz

( ST F )

10 FFT Size, FFT64

11 Used Sub-carrier index { -26 to -1 , +1 to +26 }

Table-6: Parametric Study of Digital Modulation Techniques [1, 8-30]Sr. Digital No. of Type of No. of Message Information BW BW Efficiency Symbol


557

No. ModulationTechnique

Type

Symbols Envelope points Capacity required Shaping

01 BASK 01 NotConstant 01 Poor 2RB Poor Not required

O2 BFSK 01 Constant 01 Better thanBASK (NC) 2RB Not BW Efficient Not required

03 BPSK 02 Constant 02 Double toBFSK (NC) 2RB

Used for Highspeed datatransfer

Not required

04 DPSK 02 Constant 01 Same asBPSK 2RB

For Mediumspeedcommunication

Not required

05 QPSK 04 Constant04

Expressed interms of SignalEnergy/Symbol

Double ofBPSK 2RB

Highly BWefficient

Required,RectangularPulse

06 MSK 04 Constant04

Expressed interms of Signal

Energy/Bit

Same asQPSK

Less thanQPSK

Out of BandPowerSignificantlylower thanQPSK, 99% oftotal Power ofMSK is 1.2TB

RequiredHalf Co-Sinusoidalpulse

07 GMSK 04 Constant 04 Same asMSK

Narrow BT-0.3 popular Excellent

RequiredGaussianPulse

08

OFDMBPSKOFDMQAM-OFDM

16-QAM64-QAM

02041664

NotConstant

02041664

HighThanabove

Less thanOther

techniques

ExcellentThanabove

Better thanAboveSchemes

Table-7: Merits & Demerits of Digital Modulation Techniques [1, 7-14, 19-30]Sr.No.

Type of DigitalMode Tech Derived From MSLL Merits Demerits

01 BASK ASK - 13 db Simple implementation, low costNot an BW efficient

technique, more noiseprone, operates only in

linear region02 BFSK FSK - 13 db Simple implementation, low cost Received design complex

03 BPSK PSK - 13 dbSimple implementation, robust, used

mostly for satellite communication, 3 dBPower advantage over BASK

Inefficient use of BW, non-linear modulation scheme

04 DPSK PSK - 13 db Reduces complexities of Receiver designfor non coherent case

Efficient less than coherentPSK

05 QPSK PSK - 13 dbTwice the data in same BW, hence BWefficient, more spectrally efficient than

BPSK

Complex receiver design,pulse shaping is required

but then it losses itsconstant envelope property

06 MSKFrom OQPSK byreplacing squarepulse by ½ Co-sinusoidal pulse

- 13 db

Constant envelope, out of band power islower, minimum spacing allows two

frequencies to be orthogonal, spectrallyefficient and easily generated, smoothphase transition as compared to QPSK

Linear modulation, thespectrum is not compact

enough to realize data ratesapproximating rf CHANNEL

bw

07 GMSK

From FSK byreplacing ½ Co-Sinusoidal pulse

by Gaussianpulse

Fast roll of factorwith BT = 0.3,

narrow main lobe,lower side lobe

level

Constant envelope, spectrally efficient,widely used in GSM mobile

communication with BT = 0.3Promotes ISI at higher bit

rate transmission

08

OFDM

BPSK-OFDMQAM-OFDM

16-QAM64-QAM

From multicarriermodulation

scheme__

Robust to ICI & ISI, High SpectralEfficiency, Efficiently implementation by

FFT, Low sensitivity to timesynchronization errors, Tuned sub channelreceiver filter are not required, Facilitates

single frequency network i., Complexequalization.

1. Sensitive to Doppler Shift.2. To frequency

synchronization problem. 3.Inefficient transmitter power

consumption since linearpower amplifier is required.

Table 8: Detection Performance Analysis of Digital Modulation Techniques [1-7, 19-30]

Sr.No.

DigitalModulationTechniques

Demodulation Performance Combination with othertechniques Derivatives

01 BASK Simple demodulation With PSKQAM Quadrature AmplitudeModulation (used extensively indigital microwave links M-ray ASK

02 BFSK Simple demodulation (Matched filter Special case of orthogonal M-ray FSK


558

detection) modulation

03 BPSK Phase shift detection makes the Rxcomplex With ASK-QAM

QAM, incoherent detection, QPSK,OQPSK, BPSK, /4PSK, 16 PSK,MPSK

04 DPSKReceiver requires memory to measurerelative phase difference between waveforms received in successive intervals

Non coherent orthogonalmodulation when consideredover two bit interval

/4 DPSK

05 QPSK Phase shift detection is importantDifferent phase variation.Replacement of a square pulseby ½ sinusoidal pulse to giveMSK

OQPSK-Q channel shifted by ½symbol QPSK to OQPSK to

/4 QPSK when differentiallydecoded referred to as /4 DQPSK

06 MSK Direct injection of NRZ data to frequencymodulator with Modulation Index 0.5

Replacement of ½ co-sinusoidalpulse by Gaussian pulse to giveGMSK

GMSK

07 GMSKBandwidth time product BT is an importantfactor performance is measured by SNRversus BER

Nil GMSK BT = 0.3GMSK BT = 0.5

08 OFDMBandwidth time product BT is an importantfactor performance is measured by SNRversus BER

BPSK,QAM,16-QAM,64-QAM

BPSK,QAM,16-QAM,64-QAM

Table-9: Performance Characteristics of Digital Modulation Techniques (1-7, 12-18)

1 DigitalModulationTechnique

ErrorProbability

ErrorPerformance

ISIStatus

Dimensions

No.ofBasisFunctions

01 BASK

BWAmplitudeNoisepower

for

o

o

,,,

,8

exp21 2

2 Efficient only in linearregion

Nil One One

02 BFSK

,&&

,2

exp21

densityBitNoisebo

Performs Well at high

values as PSK &

FSK for same signalenergy and bit rate.

Nil One One

03 BPSKerfc

21

Small error rate thanany other system butrestriction of AWGN on1 bit decoding .It isoptimum as it achievesminimum possible errorrate.

LessProne

Two Two

04 DPSK

2exp

21

Required is 3

dBless than that ofBFSK for same errorrate.

LessProne

Two Two

05 QPSK 2

Performance better overBPSK & BFSK butmajor draw back is usedof square pulse, can beimproved by shapingwith root raised cosineimproving ISI.

ProneToISI

Two Two

06 MSK 2

The signal coherenceand derivation ratio arelargely unaffected byvariations in input datarate.

LessProneThanQPSK

Two Two

07 GMSK

BTBT

Qe

,85.0,25.0

,68.0,2 The carrier is lag of lead

by090 over bit

period, w.r.t. BTresulting in BER.

MoreProneThanMSK

Two Two


559

08 OFDM

ddd

WWWerfc

S

TP

22

02

2 221

Minimum than otherschemes

Nil 2 inBPSK4 inQPSK16 in16-QAM

2 inBPSK4 inQPSK16 in16-QAM

Table-10: BER Equations of Digital Modulation Techniques

Sr.No.

MOD.

N

M2

B.W.

1 BPSK 2,1

2 QPSK 4,2 /

23 SQPSK 4,2 /

24 MSK 4,2 /

25 M-PSK M,N /

N6 16-

QAM16,4 /

47 M-

QAMM,N /

N8 QPR L

levels

/

49 LQPR L

levels

]/})1{

/6()(log4/[]/11[2

2/12

2/122

ob NELL

erfcL/

L

10

M-FSK M,N M

/

L11

OFDM-BPSK 2

.1cos

cossin

21 1

0

21

022

fmkfmkcwhere

dcEecE

mP

Imk

N

kmk

Imk

o

I

bMinimumB.W.

isrequir

edthanotherschemes

12

OFDM-QPSK

4

Qmk

kmk

mk

Qb

ccewhere

dccm

coscos

.cossin21

1

0

21

000

22

MinimumB.W.

isrequir

edthanotherschemes

13

OFDM-16QA

M

1600

0001QQ

iMinimumB.W.isrequir


560

1

0

21

000

002

.cos2coscos2cos

.cos2cossin2sin2sin221

22

kmk

Qmk

Qmkmkmk

QQi

dcdcdcdce

ddcdcdcdcdmedthanotherschemes

Table 5: Mathematical Representation of Digital Modulation Techniques [1-8, 17, 18]

Mod. Mathematical Representation Type Correlationbetweensignals

Component Q component

1 BASK X(t) = Am (t) cos2 fct for 0 < 1 < TB, m(t) = 1 for Bit 1 & 0 for Bit 0Signal energy representation

X(t) = tfTE

cB 2cos2

NonCoherent

Noncorrelation

Nil Nil

2 BFSK

Xi(t) =B cos(2 fct) for 0 < t t < TB, i = 1,2 fc =

B

c

X1 (t) = 1, (f1) & X2 (t) = 0, (f2) and

NonCoherent

Noncorrelation

Nil Nil

3 BPSK

X1(t) =B

B

TE2

cos(2 fct) & X2(t) =B

B

TE2

cos(2 fct+ )

X1(t) for Bit 1 & X2(t) for Bit 0

Coherent Noncorrelation

Nil Nil

4 DPSK Over dual bit interval

X1(t) =

BBcB

B

BcB

B

X2(t) =

BBcB

B

BcB

B

TtTfortfTE

TtfortfTE

2)2cos(2

0)2cos(2

It is a special case of Non coherent Orthogonal Modulation for TB = 2TB & EB= 2EB X1(t) for Bit 1 and X2(t) for Bit 0

Noncoherent

Correlationexists

Nil Nil

5 QPSK(Phasedivision)Phaseangles45,135,225 and315degree

Xo(t) =

)2sin(4

)12sin(2

)2cos(4

)12cos(2

tfnTE

tfnTE

cB

B

cB

B

for 0 < t < TB, Where n = 1`,2,3,4 and forBit 10 00 01 11Phase /4 3 /4 5 /4 7 /4

Coherent Correlationexist

Xt(t) =

B

n = 1, 2, 3, 4

XQ(t) = -

4

)12(sinn

EB

n = 1, 2, 3, 46 MSK

X1(t) = cos 02B

c TtABtf

Where the value of = 0 for A = 1 and the value of = for A = -1. Thusthe above expression can be of the form

(1) X1(t) = cosB

c for A = 1 & B = + 1

Coherent Correlationexist

Xt(t) = +

tT

TE

B

B

B

2cos

2

Tb < 1 < Tb

XQ(t) = +

tT

TE

B

B

B

2

sin

2

Q < 1 < Tb


561

(2) X1(t) = cosB

c Tttf2 for A = -1 & B = + 1

7 GPSK

G(t) =

2log5.0

2

2log5.0

2

21

e

Bban

e

Bban

TtBQ

TtBQ

T

Q(t) =t

2

, Bban is the bandwidth of the filter

Coherent Correlationexists

I(t) = cos[C(t)] For C to be aconstant such thatT

T

dttCG2

)(

Q(t) = sin[C(t)]

8 OFDM.0;/2exp1 1

0

ttjt

T = Signal Duration, N = N-Point IDFT,1

=Scale factor,

Signal

samples .1,,.........2,1,0;nn

Sub carrier frequency = =

Sub-carriers. EB = Energy of the Bit, TB = Time duration of the Bit, fc =Carrier Frequency, m(t) = Modulation Index, A = Amplitude, nc = Noise

NonCoherent

Noncorrelation

Nil Nil

9 OFDMWith(BPSK,QAM,16-QAM,64-QAM)

1

0

2k

kk

tfj c

is complex data symbol, T = OFDM block duration, tg = pulse

shape , = carrier frequency.

NonCoherent

Noncorrelation

Nil Nil

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ANALOG & DIGITAL MODULATION TECHNIQUES: AN OVERVIEW