bandpass modulation03

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BandpassBandpass ModulationModulationPrachi Raut



BPSK Modulation and ReceptionBPSK Modulation and Reception



y In general,

)cos(2)()(0t st bt

S [ !



Generation of carrier waveGeneration of carrier wave

)cos(2)()( 0

U[ ! t Pst bt V BPSK

)(2cos0

U[ t

)(2cos

2

1

2

1)(cos

00

2 U[ U[ ! t t

)cos(0

U[ t

Received Signal :

Signal after squaring:

Signal after filtering:

Signal after frequency division:



Reception of BPSK signalReception of BPSK signal

¼½»

¬-«

! )cos(2

1

2

12)()(cos2)(

00

2 U[ U[ t Pst bt Pst b

)(2cos

2

)(

2

)(0

U[ ! t Ps

t b Ps

t b

2)(

Ps

t b

Output of Synchronous Demodulator :

Output of Integrator and Dump Circuit :



Constellation DiagramConstellation Diagram



9

BPSKBPSK Constellation DiagramConstellation Diagram

-PsTb PsTb I



Spectrum of BPSK signalSpectrum of BPSK signal



Differential Phase Shift KeyingDifferential Phase Shift Keying(DPSK)(DPSK)



DPSK WaveformDPSK Waveform



)()()(b

T t bt d t b !

)()(b

T t bt b !

)()(b

T t bt b !

When d(t) = 1

When d(t) = 0

)cos(2)()(0

t st bt D S

[ !



DPSK DemodulationDPSK Demodulation



DPSK DemodulationDPSK Demodulation

y Output contains a term

yWhen

)()(b

T t bt b

d(t)=0 b(t)= b(t-Tb) Output = 1V

d(t) = 1 b(t)=- b(t-Tb) Output = -1 V



Differentially Encoded Phase ShiftDifferentially Encoded Phase ShiftKeying (DEPSK)Keying (DEPSK)



QuadratureQuadrature Phase Shift KeyingPhase Shift Keying(QPSK)(QPSK)



QPSK WaveformsQPSK Waveforms



Offset Vs NonOffset Vs Non--Offset QPSKOffset QPSK

y Off set QPSK is also called as StaggeredQPSK

y In OQPSK, one bit changes state at a time

y In QPSK, both bits change state at a time

y Amplitude changes are more pronouncedin QPSK than OQPSK



QPSK RECEIVER QPSK RECEIVER



Problem«««??Problem«««??

y In QPSK, abrupt phase changes occur dueto which spectral components at highfrequencies may arise

y Sharp transitions between 1 and 0 statesand vice versa create signals that havesidebands extending out a long way from

the carrier.y This creates problems for communication

systems, as any sidebands outside theallowed bandwidth cause interf erence to

adjacent channels.



The Solution !!!The Solution !!!

y Phase transitions can be made zero,if rectangular pulse shapes used in QPSK are

replaced by sinusoidal pulse shapes.y This is known as Minimum Shift Keying

(MSK)

y MSK has a baseband waveform which is

much smoother than QPSK basebandwaveform

y Waveform of MSK has phase continuity



t Tb

t t b Pst

Tb

t t be Pst V

o MSK 00sin

42cos)(2cos

42sin)(2)( [ T [ T ¼½

»¬-

«¹ º ¸

©ª¨

¼½

»¬-

«¹ º ¸

©ª¨

!



MSK TransmitterMSK Transmitter



MSK ReceiverMSK Receiver



Gaussian MSK (GMSK)Gaussian MSK (GMSK)

y Another way of generating continuous

phase signal is filteringy If a Gaussian filter is used, the

modulation technique is called GMSK



MM--aryary PSK ModulatorPSK Modulator



MM--aryary PSK DemodulatorPSK Demodulator



Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)



Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)

yWhen d(t)=1

yWhen d(t) = 0

t st

FS ¡ )cos(2)(

0;! [

t st ¢

FS £

)cos(2)(0;! [



FSK DemodulatorFSK Demodulator



Constellation Diagram for FSKConstellation Diagram for FSK

t mf t ub

b

T 2cos2

)(1

!

t nf t ub

b

T 2cos2

)(2

!

u1 and u2 are mth and nth harmonic of f b respectively



Orthogonal FSK Non-orthogonal FSK



MM--aryary FSKFSK



Spectrum of MSpectrum of M--aryary FSKFSK



QuadratureQuadrature Amplitude ShiftAmplitude Shift

Keying (QASK) orKeying (QASK) orQuadratureQuadrature Amplitude ModulationAmplitude Modulation(QAM)(QAM)

y In BPSK,QPSK and M-ary PSK all signalshave same amplitude

y Noise immunity can be improved bymaking symbols of diff erent phase as wellas diff erent amplitude

y

This type of modulation is called QASK orQAM.



16 QASK constellation Diagram16 QASK constellation Diagram



t sk t sk t vQAS

¤

0201sin2.0cos2.0)( [ [ !

Where k1 and k2 are each equal to + 1 or +3



QASK ModulatorQASK Modulator



QASK DemodulatorQASK Demodulator



ReferencesReferences

y Herert Taub, Donald L. Schilling,Gautam Saha ³Taub¶s Principles of Communication Systems´ , ThirdEdition, Tata McGraw Hill

y Bernard Skalar, ³ Digital

Communications´, Pearson Education,2001, 2nd edition

bandpass modulation03

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