UNIT II

5/22/2018 UNIT II

1/131

UNIT II

DIGITAL

COMMUNICATION

IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.01

5/22/2018 UNIT II

2/131

Syllabus

Pulse modulations concepts of sampling and

sampling theorems, PAM, PWM, PPM, PTM,

Quantization and coding: DCM, DM, slope

overload error. ADM, DPCM, OOK systems

ASK, FSK, PSK, BSK, QPSK, QAM, MSK,

GMSK, applications of Data communication.


5/22/2018 UNIT II

3/131

INTRODUCTION

Modulation is the process of frequency

translation in which any one

parameter(Amplitude, frequency or phase)

of high frequency carrier signal is varied in

accordance with instantaneous value of

low frequency modulating signal.

Modulation is either analog or digital.


5/22/2018 UNIT II

4/131

INTRODUCTION

Many signals in modern communication

systems are digital

Additionally, analog signals are transmitted

digitally

Digitizing a signal results in reduced

distortion and improvement in signal-to-

noise ratios


5/22/2018 UNIT II

5/131

INTRODUCTION

A digital signal is superior to an analogsignal because it is more robust to noiseand can easily be recovered, corrected

and amplified. For this reason, thetendency today is to change an analogsignal to digital data.

The process of transmitting signals in theform of pulses (discontinuous signals) byusing special techniques.


5/22/2018 UNIT II

6/131

PULSE MODULATION INCLUDES

Pulse Amplitude Modulation

Pulse Width Modulation

Pulse Position Modulation

Pulse Code Modulation

Delta Modulation


5/22/2018 UNIT II

7/131

PULSE MODULATION

Analog Pulse Modulation Digital Pulse Modulation

Pulse Amplitude (PAM)Pulse Width (PWM)

Pulse Position (PPM)

Pulse Code (PCM)

Delta Modulation(DM)


5/22/2018 UNIT II

8/131

Sampling

The process of transmitting signals in theform of pulses (discontinuous signals) byusing special techniques.

The signal is sampled at regular intervalssuch that each sample is propotional to theamplitude of signal at that instant.Thistechnique is called sampling.

Sampling is common in all pulsemodulation techniques.


5/22/2018 UNIT II

9/131

Concepts of Sampling &

sampling Theorem


5/22/2018 UNIT II

10/131

Sampling

Analog signal is sampled every TSsecs.Tsis referred to as the sampling interval. f

s= 1/T

sis called the sampling rate or

sampling frequency.There are 3 sampling methods:Ideal - an impulse at each sampling

instantNatural - a pulse of short width with

varying amplitudeFlat top - sample and hold, like natural

but with single amplitude value


5/22/2018 UNIT II

11/131

Three different sampling methods for

PCM

4.11IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.011

5/22/2018 UNIT II

12/131

Sampling Rate Nyquist showed that it is possible to

reconstruct a band-limited signal fromperiodic samples, as long as the sampling

rate is at least twice the frequency of the ofhighest frequency component of the signali.e. fs 2fm

where fs is sampling rate

Sampling rates that are too low result inaliasingor foldover


5/22/2018 UNIT II

13/131

Sampling


5/22/2018 UNIT II

14/131

SamplingSampling alone is not a digital techniqueThe immediate result of sampling is a

pulse-amplitude modulation (PAM)signal

PAM is an analog scheme in which theamplitude of the pulse is proportional to theamplitude of the signal at the instant ofsampling

Another analog pulse-forming technique is

known as pulse-duration modulation(PDM). This is also known as pulse-widthmodulation (PWM)

Pulse-position modulation is closelyrelated to PDM


5/22/2018 UNIT II

15/131


In PAM,amplitude of pulses is varied in

accordance with instantaneous value of

modulating signal.


5/22/2018 UNIT II

16/131


Low

PassFilter

Multiplier

Pulse

train

generator

Modulating

Signal PAM

Signal

The carrier is in the form of narrow pulses havingfrequency fs.The uniform sampling takes place in multiplier

to generate PAM signal.Samples are placed Ts sec away

from each other.


16

5/22/2018 UNIT II

17/131


Depending upon the shape and polarity ofthe sampled pulses, PAM is of two types,

Natural PAM sampling occurs when top

portion of the pulses are subjected tofollow the modulating wave.


5/22/2018 UNIT II

18/131


Flat topped PAMsampling is often usedbecause of the ease of generating the

modulated wave. In this pulses have flat

tops after modulation.


P l A li d

5/22/2018 UNIT II

19/131

Pulse Amplitude

Modulation The PAM signal can be detected by

passing it through a low pass filter.

FigIFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0

19

5/22/2018 UNIT II

20/131


In this type, the amplitude is maintained

constant but the width of each pulse is

varied in accordance with instantaneous

value of the analog signal.


5/22/2018 UNIT II

21/131


Fig:


5/22/2018 UNIT II

22/131


That is why the information is contained in

width variation. This is similar to FM.

In pulse width modulation (PWM), the

width of each pulse is made directly

proportional to the amplitude of the

information signal.


5/22/2018 UNIT II

23/131



5/22/2018 UNIT II

24/131


A simple method to generate the PWM

pulse train corresponding to a given signal

is the intersective PWM: the signal (here

the green sinewave) is compared with asawtooth waveform (blue). When the latter

is less than the former, the PWM signal

(magenta) is in high state (1). Otherwise itis in the low state (0).


5/22/2018 UNIT II

25/131


The block diagram of next slide can beused for generation of PWM as well asPPM.In this case a sawtooth signal of

frequency fs is a sampling signal. It is applied to inverting terminal of a

comparator with modulating signal at non

inverting terminal.O/P remains high as long as modulating

signal is higher than that of ramp signal.


5/22/2018 UNIT II

26/131



5/22/2018 UNIT II

27/131

Pulse Position Modulation In this type, the sampled waveform has

fixed amplitude and width whereas the

position of each pulse is varied as per

instantaneous value of the analog signal.

PPM signal is further modification of a

PWM signal.


5/22/2018 UNIT II

28/131



5/22/2018 UNIT II

29/131


The vertical dotted lines shown in last slide

treated as reference lines.

The PPM pulses marked 1,2 and 3 go

away from their respective reference

lines.This corresponds to increase in

modulating signal amplitude.

Then as modulating signal decreases the

PPM pulses 4,5,6,7 come closer to their

respective reference lines.


5/22/2018 UNIT II

30/131


The PPM signal can be generated fromPWM signal.

The PWM pulses obtained at the

comparator output are applied to amonostable multivibrator which isve edge

triggered.

Hence for each trailing edge of PWMsignal, the monostable output goes high.Itremains high for a fixed time decided by itsown RC components.


5/22/2018 UNIT II

31/131


Thus as the trailing edges of the PWM

signal keeps shifting in propotion with the

modulating signal,the PPM pulses also

keep shifting.

Therefore all the PPM pulses have the

same amplitude and width.The information

is conveyed via changing position ofpulses.


5/22/2018 UNIT II

32/131

Digital Pulse Modulation

Merits of Digital Communication:

1.Digital signals are very easy to receive.The receiver has to just detect whetherthe pulse is low or high.

2.AM & FM signals become corrupted overmuch short distances as compared to

digital signals. In digital signals, theoriginal signal can be reproducedaccurately.


5/22/2018 UNIT II

33/131


Merits of Digital Communication

3.The signals lose power as they travel,which is called attenuation. When AM

and FM signals are amplified, the noisealso get amplified. But the digital signalscan be cleaned up to restore the quality

and amplified by the regenerators.4.The noise may change the shape of the

pulses but not the pattern of the pulses.


5/22/2018 UNIT II

34/131


Merits of Digital Communication:5.AM and FM signals can be received by

any one by suitable receiver. But digitalsignals can be coded so that only the

person, who is intended for, can receivethem.

6.AM and FM transmitters are real time

systems. i.e. they can be received only atthe time of transmission. But digitalsignals can be stored at the receivingend.

7.The digital signals can be stored.IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0 34

5/22/2018 UNIT II

35/131


The process of Sampling which we have

already discussed in initial slides is alsoadopted in Digital pulse modulation.

It is mainly of two types:

Pulse Code Modulation(PCM)

Delta Modulation(DM)


5/22/2018 UNIT II

36/131


Pulse-Code Modulation (PCM) is the mostcommonly used digital modulation scheme

In PCM, the available range of signal

voltages is divided into levels and each isassigned a binary number

Each sample is represented by a binarynumber and transmitted serially

The number of levels available dependsupon the number of bits used to expressthe sample value

The number of levels is given by: N = 2m


5/22/2018 UNIT II

37/131


PCM consists of three steps to digitizean analog signal:

1. Sampling

2. Quantization3. Binary encoding

Before we sample, we have to filter thesignal to limit the maximum frequency of

the signal .Filtering should ensure thatwe do not distort the signal, ie removehigh frequency components that affectthe signal shape.


5/22/2018 UNIT II

38/131


The basic elements of a PCM system.IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.038

5/22/2018 UNIT II

39/131


Analog to digital converter employs twotechniques:

1. Sampling: The process of generating pulses of

zero width and of amplitude equal to theinstantaneous amplitude of the analog signal.

The no. of pulses per second is called sampling

rate.


5/22/2018 UNIT II

40/131


2. Quantization: The process of dividing

the maximum value of the analog signal

into a fixed no. of levels in order to

convert the PAM into a Binary Code.

The levels obtained are called

quanization levels.


5/22/2018 UNIT II

41/131

Time

V

o

l

ta

g

e

7

6

5

4

3

2

1

0

111

110

101

100

011

010

001

000

L

ev

e

l

s

B

in

a

r

y

Co

d

e

sTime

Time

V

o

l

t

a

g

e

0 1 0 1 0 1 1 1 0 1 1 1 1 1 0 1 0 1 0 1 0

Sampling,

Quantizationand

Coding


5/22/2018 UNIT II

42/131



5/22/2018 UNIT II

43/131

Quantization By quantizing the PAM pulse, original

signal is only approximated

The process of converting analog signals

to PCM is called quantizing Since the original signal can have an

infinite number of signal levels, the

quantizing process will produce errorscalled quantizing errors or quantizingnoise


5/22/2018 UNIT II

44/131

Quantization

Two types of quantization: (a) midtread and (b)

midriseIFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.044

5/22/2018 UNIT II

45/131

Quantization

Coding and Decoding

The process of converting an analog signal

into PCM is called coding, the inverse

operation is called decoding

Both procedures are accomplished in a CODEC


5/22/2018 UNIT II

46/131

Quantization


5/22/2018 UNIT II

47/131

Quantization


5/22/2018 UNIT II

48/131

Quant izat ion and encod ing of a

sampled signal


5/22/2018 UNIT II

49/131

Quantization Error

When a signal is quantized, we introducean error - the coded signal is anapproximation of the actual amplitudevalue.

The difference between actual and codedvalue (midpoint) is referred to as thequantization error.

The more zones, the smaller which

results in smaller errors.BUT, the more zones the more bits

required to encode the samples -> higherbit rate


5/22/2018 UNIT II

50/131

Quantization Error (cont.)

Round-off error

Overload error

Overload


5/22/2018 UNIT II

51/131

Quantization Noise

Illustration of the quantization process


5/22/2018 UNIT II

52/131

Pulse Code Modulation

In PCM system,N number of binary digits

are transmitted per sample.Hence the

signaling rate and channel bandwidth of

PCM are very large.

Also encodind,decoding and quantizing

circuitary of PCM is complex.


DPCM

5/22/2018 UNIT II

53/131

DPCM


5/22/2018 UNIT II

54/131

Quantization error feedback

in the DPCM coder


5/22/2018 UNIT II

55/131

Signal distortions due to

intraframe DPCM coding


5/22/2018 UNIT II

56/131

Delta Modulation

In Delta Modulation, only one bit istransmitted per sample

That bit is a one if the current sample is

more positive than the previous sample,and a zero if it is more negative

Since so little information is transmitted,delta modulation requires higher samplingrates than PCM for equal quality ofreproduction


5/22/2018 UNIT II

57/131

Delta Modulation

This scheme sends only the differencebetween pulses, if the pulse at time tn+1ishigher in amplitude value than the pulse attime tn, then a single bit, say a 1, is used

to indicate the positive value. If the pulse is lower in value, resulting in a

negative value, a 0 is used.

This scheme works well for small changes

in signal values between samples. If changes in amplitude are large, this will

result in large errors.


5/22/2018 UNIT II

58/131

Delta Modulation

The process of delta modulationIFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0

58

5/22/2018 UNIT II

59/131

Delta Modulation

Components of Delta Modulation


Delta Modulation

5/22/2018 UNIT II

60/131

Delta Modulation

DM system. (a) Transmitter. (b) Receiver.IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0

60

5/22/2018 UNIT II

61/131

Delta Modulation

Distortions in DM system1. If the slope of analog signal is much

higher than that of approximated digital

signal over long duration,than thisdifference is called Slope overload

distortion.

2. The difference between quantized signaland original signal is called as Granular

noise. It is similar to quantisation noise.


5/22/2018 UNIT II

62/131

Delta Modulation

Two types of quantization errors :

Slope overload distortionand granular noise


5/22/2018 UNIT II

63/131

Delta Modulation

Distortions in DM systemGranular noise occurs when step sizeis

large relative to local slope m(t).

There is a further modification in thissystem,in which step size is not fixed.

That scheme is known as Adaptive Delta

Modulation.


Simple Implementation of a DM

5/22/2018 UNIT II

64/131

Simple Implementation of a DM

system


5/22/2018 UNIT II

65/131

Slope Overload Error

Slope overload

When the analog signal has a high rate of

change, the DM can fall behind and a

distorted output occurs


5/22/2018 UNIT II

66/131

Tradeoffs

Simplicity versus Quality

In order to obtain the high quality DM

requires very high sampling rates, typically

20 the highest frequency of interest, asopposed to Nyquist rate of 2


5/22/2018 UNIT II

67/131

Adaptive Delta Modulation

A better performance can be achieved ifthe value of is not fixed.

The value of changes according to the

amplitude of the analog signal. It has wide dynamic range due to variable

step size.

Also better utilisation of bandwidth ascompared to delta modulation.

Improvement in signal to noise ratio.


5/22/2018 UNIT II

68/131

Adaptive Delta Modulation


5/22/2018 UNIT II

69/131

Conclusion for Pulse Modulation

The main advantage of these pulse

modulation schemes are better noise

immunity and possibility of use of

repeaters which makes communicationmore reliable and error free.


5/22/2018 UNIT II

70/131

Differential Phase Shift Keying (DPSK)

Why We Require? To Have Non-coherent Detection

That Makes Receiver Design

How can we do? 0 may be used represent transition

1 indicate No Transition


5/22/2018 UNIT II

71/131

DPSK Transmitter

dK

dK-1

bK

AcCos(2fct)

S(t)=AcCos(2fct)

Encoder

Delay Tb

Product

Modulator

What Should We Do to make Encoder?


5/22/2018 UNIT II

72/131

DPSK Transmitter..Modified

dK

dK-1

bK

AcCos(2

fct)

S(t)=AcCos(2fct)

Delay Tb

Product

ModulatorEx- NOR

Gate


5/22/2018 UNIT II

73/131

Differentially Encoded Sequence

Binary Data 0 0 1 0 0 1 0 0 1 1

Differentially

Encoded Data

1 0 1 1 0 1 1 0 1 1 1

Phase of DPSK 0 0 0 0 0 0 0 0

ShiftedDifferentially

encoded Data

dk-1

1 0 1 1 0 1 1 0 1 1

Phase of

shifted Data

0 0 0 0 0 0 0

PhaseComparision

Output

- - + - - + - - + +

Detected

Binary Seq.

0 0 1 0 0 1 0 0 1 1


73

5/22/2018 UNIT II

74/131

DPSK Receiver


5/22/2018 UNIT II

75/131

Quadrature Phase Shift Keying (QPSK)

Extension of Binary-PSK

Spectrum Efficient Technique

In M-ary Transmission it is Possible to Transmit M Possible Signal

M = 2nwhere,

n= no of Bits that we Combine

signaling Interval T= nTb

In QPSK n=2 === > So M =4

and

signaling Interval T= 2Tb


5/22/2018 UNIT II

76/131


M=4 so we have possible signal are 00,01,10,11

Or In Natural Coded Form 00,10,11,01

3( ) cos(2 )

4c cs t A f t

cos(2 )4

c cA f t

cos(2 )4

c cA f t

3cos(2 )

4c cA f t

-135

-45

45

135

Binary Dibit 00

Binary Dibit 10

Binary Dibit 11

Binary Dibit 01


76

5/22/2018 UNIT II

77/131

QPSK Waveform

00 11 00 11 10 10


5/22/2018 UNIT II

78/131

QPSK Signal Phase


ll

5/22/2018 UNIT II

79/131

Constellation Diagram


5/22/2018 UNIT II

80/131


( ) cos(2 ( ))c cs t A f t t

The QPSK Formula

Where, (t)=135,45,-45,-135

( ) cos ( ).cos(2 ) sin ( )sin(2 )c c c cS t A t f t A t f t

(1)

Simplifying Equation 1

This Gives the Idea about Transmitter design


i

5/22/2018 UNIT II

81/131

QPSK Transmitter


QPSK R i

5/22/2018 UNIT II

82/131

QPSK Receiver

IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0 82

S h i i Ci i

5/22/2018 UNIT II

83/131

Synchronization Circuit


Mi i Shift K i (MSK)

5/22/2018 UNIT II

84/131

Minimum Shift Keying (MSK)

In Binary FSK the Phase Continuity is

maintained at the transition Point. This type of

Modulated wave is referred as Continuous

Phase Frequency Shift Keying (CPFSK)

In MSK there is phase change equals to one

half Bit Rate when the bit Changes 0 to 1 and 1

to 0.

1

2 bf

T


84


5/22/2018 UNIT II

85/131


1 2 1 21

2 2

c c c cc

f f f ff

2c

ff

1 2

1 2

2

c c

c c

f ffc

f f f

1 2 1 22

2 2

c c c cc

f f f ff

2c ff

Lets take fc1 and fc2 represents binary 1 and 0 Respectively

Where

Similarly


85


5/22/2018 UNIT II

86/131


The MSK Equation

where

( ) cos(2 ( ))s t Ac fct t

( )t ft

For Symbol 1

( )t ft

2 b

t

T

For Symbol 0

( )t ft

2 b

t

T


86

C i Ph C di

5/22/2018 UNIT II

87/131

Carrier Phase Coding

For dibit 00

(t)

tTb 2Tb

-/2

-


C i Ph C di

5/22/2018 UNIT II

88/131


For dibit 10

Tb 2Tb

/2


C i Ph C di

5/22/2018 UNIT II

89/131


Tb 2Tb

/2

For dibit 11


C i Ph C di

5/22/2018 UNIT II

90/131


For dibit 01

(t)

tTb 2Tb

-/2

-


B i E di T h i

5/22/2018 UNIT II

91/131

Basic Encoding Techniques

Digital data to analog signal

Amplitude-shift keying (ASK)

Amplitude difference of carrier frequency

Frequency-shift keying (FSK) Frequency difference near carrier frequency

Phase-shift keying (PSK)

Phase of carrier signal shifted


B i E di T h i

5/22/2018 UNIT II

92/131

Basic Encoding Techniques


A lit d Shift K i

5/22/2018 UNIT II

93/131

Amplitude-Shift Keying

One binary digit represented by presence of carrier, atconstant amplitude

Other binary digit represented by absence of carrier

where the carrier signal isAcos(2fc

t)

ts tfA c2cos0

1binary0binary


A lit d Shift K i

5/22/2018 UNIT II

94/131

Amplitude-Shift Keying

Susceptible to sudden gain changes

Inefficient modulation technique

On voice-grade lines, used up to 1200 bps

Used to transmit digital data over optical fiber


Phase Shift Keying (PSK)

5/22/2018 UNIT II

95/131

36/45

y g ( )

Phase of carrier signal is shifted to represent data

Binary PSK (BPSK): two phases represent two binary digits

0 0 1 1 0 1 0 0 0 1 0 0 0 0 0

1)(),2cos()(

0),2cos(1),2cos(

0),2cos(

1),2cos()(

tdtftAd

binarytfAbinarytfA

binarytfA

binarytfAts

c

c

c

c

c


Differential PSK (DPSK)

5/22/2018 UNIT II

96/131

37/45

In DPSK, the phase shift is with reference to the previous bit

transmitted rather than to some constant reference signal Binary 0:signal burst with the same phase as the previous one

Binary 1:signal burst of opposite phase to the preceding one


Four-level PSK: Quadrature PSK (QPSK)

5/22/2018 UNIT II

97/131

38/45

10)4

2cos(

00)4

32cos(

01)4

32cos(

11)

4

2cos(

)(

tfA

tfA

tfA

tfA

ts

c

c

c

c

More efficient use of bandwidth if each signal element representsmore than one bit

eg. shifts of /2 (90o)

each signal element represents two bits

split input data stream in two & modulate onto the phase of the carrier

can use 8 phase angles & more than one amplitude

9600bps modem uses 12 phase angles, four of which have two amplitudes: thisgives a total of 16 different signal elements


QPSK and Offset QPSK (OQPSK)Modulators

5/22/2018 UNIT II

98/131

39/45

Modulators

)2sin()(2

1)2cos()(

2

1)(:

)2sin()(2

1)2cos()(

2

1)(:

tfTtQtftItsOQPSK

tftQtftItsQPSK

cbc

cc


Example of QPSK and OQPSK Waveforms

5/22/2018 UNIT II

99/131

40/45

41101

4

31100

4

31110

41111

:

QPSKf or


Performance of ASK, FSK, MFSK, PSK andMPSK

5/22/2018 UNIT II

100/131

41/45

MPSKBandwidth Efficiency

ASK/PSK:

MPSK:

MFSK:

10,1

1

r

rB

R

bandwidthontransmissi

ratedata

T

elementssignaldifferentofnumberM

r

M

B

R

T

:,

1

log 2

Mr

M

B

R

T )1(

log 2

Bit Error Rate (BER)

bit error rate of PSK and QPSK are about 3dB superior to ASK

and FSK (see Fig. 5.4)

for MFSK & MPSK have tradeoff between bandwidth efficiency

and error performanceIFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0

100

Performance of MFSK and MPSK

5/22/2018 UNIT II

101/131

42/45

MFSK: increasingMdecreases BER and decreases bandwidth Efficiency

MPSK: IncreasingMincreases BER and increases bandwidth efficiency


Binary Frequency-Shift Keying

5/22/2018 UNIT II

102/131

(BFSK)

Two binary digits represented by two differentfrequencies near the carrier frequency

wheref1andf2are offset from carrier frequencyfcby equal but

opposite amounts

ts tfA 12cos

tfA 22cos 1binary0binary


Binary Frequency Shift Keying (BFSK)

5/22/2018 UNIT II

103/131

Binary Frequency-Shift Keying (BFSK)

Less susceptible to error than ASK

On voice-grade lines, used up to 1200bps

Used for high-frequency (3 to 30 MHz) radio

transmission

Can be used at higher frequencies on LANs

that use coaxial cable


5/22/2018 UNIT II

104/131


Multiple Frequency-Shift Keying

5/22/2018 UNIT II

105/131

(MFSK)

More than two frequencies are used More bandwidth efficient but more susceptible to

error

f i=f c+ (2i 1 M)f d f c= the carrier frequency

f d= the difference frequency M = number of different signal elements = 2 L

L = number of bits per signal element

tfAts ii 2cos Mi1



5/22/2018 UNIT II

106/131

(MFSK)

To match data rate of input bit stream, eachoutput signal element is held for:

Ts=LTseconds

where T is the bit period (data rate = 1/T)

So, one signal element encodesLbits



5/22/2018 UNIT II

107/131

(MFSK)

Total bandwidth required2Mfd

Minimum frequency separation required

2fd=1/Ts

Therefore, modulator requires a bandwidth of

Wd=2L/LT=M/Ts



5/22/2018 UNIT II

108/131

(MFSK)


Phase-Shift Keying (PSK)

5/22/2018 UNIT II

109/131


Two-level PSK (BPSK) Uses two phases to represent binary digits

ts tfA

c2cos

tfA c2cos1binary

0binary

tfA c2cos

tfA c2cos

1binary

0binary



5/22/2018 UNIT II

110/131


Differential PSK (DPSK) Phase shift with reference to previous bit

Binary 0 signal burst of same phase as previous signal

burst Binary 1 signal burst of opposite phase to previous

signal burst


5/22/2018 UNIT II

111/131



5/22/2018 UNIT II

112/131


Four-level PSK (QPSK) Each element represents more than one bit

ts

4

2cos

tfA c 11

4

32cos

tfA c

4

32cos

tfA c

42cos

tfA c

01

00

10


5/22/2018 UNIT II

113/131



5/22/2018 UNIT II

114/131


Multilevel PSK Using multiple phase angles with each anglehaving more than one amplitude, multiple signalselements can be achieved

D= modulation rate, baud

R= data rate, bps

M= number of different signal elements = 2L

L= number of bits per signal element

MR

LRD

2log


5/22/2018 UNIT II

115/131


Performance

5/22/2018 UNIT II

116/131

Performance

Bandwidth of modulated signal (BT) ASK, PSK BT=(1+r)R

FSK BT=2DF+(1+r)R

R= bit rate

0 < r < 1; related to how signal is filtered

DF = f2-fc=fc-f1


Performance

5/22/2018 UNIT II

117/131

Performance

Bandwidth of modulated signal (BT)

MPSK

MFSK

L= number of bits encoded per signal element M= number of different signal elements

RM

rR

L

rB

T

2log

11

R

M

MrB

T

2log

1


5/22/2018 UNIT II

118/131


Quadrature Amplitude Modulation

5/22/2018 UNIT II

119/131


QAM is a combination of ASK and PSK Two different signals sent simultaneously on the

same carrier frequency

tftdtftdts cc 2sin2cos 21



5/22/2018 UNIT II

120/131



Reasons for Analog Modulation

5/22/2018 UNIT II

121/131

Reasons for Analog Modulation

Modulation of digital signals When only analog transmission facilities are

available, digital to analog conversion required

Modulation of analog signals A higher frequency may be needed for effective

transmission

Modulation permits frequency divisionmultiplexing


GMSK

5/22/2018 UNIT II

122/131

GMSK as implemented by quadrature signal processing at baseband

followed by a quadrature modulator

Generating a GMSK Waveform


122

Architecture of a GMSK ModulatorGMSK modulator using a VCO

5/22/2018 UNIT II

123/131

Coder

Bits ar t( ) VCO

h

t( )h t( )

Gaussian filter

g

kk

a s t kT kk

a t kT

Rectangular filter

t( )CoderBits ak s

t

(

)

2 h

t ( )t

cos()

sin()

+

-

st rt ht() ()* ()

GMSK modulator without VCO

kk

a t kT

cos 2 cf t

sin 2 cf t


Pulse Shaping

5/22/2018 UNIT II

124/131

p g

Input: Binary pulse train (+1/-1)

Each binary pulse goes through a LPF with a Gaussian impulse response

The filter smoothes the binary pulses

The filter output is truncated and scaled This process results in a train of Gaussian shaped pulses


124

Summing and Integration

5/22/2018 UNIT II

125/131

g g

The pulses are summed together (left)

The signal is integrated over time to obtain a continuous

waveform which captures the bit transition information (right)


125

I&Q Signals

5/22/2018 UNIT II

126/131

Q g

The resulting waveform is divided into In-Phase andQuadrature components

In-phase: Left

Quadrature: Right

The two signal components are then up-converted to thecarrier frequency


126

GMSK Properties

5/22/2018 UNIT II

127/131

p

Improved spectral efficiency Power Spectral Density

Reduced main lobe over MSK

Requires more power to transmit data than manycomparable modulation schemes


127

5/22/2018 UNIT II

128/131

Applications to Digital Data

communications

IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0

128

Telegraph

5/22/2018 UNIT II

129/131

g p

Morse Code Dots and dashes

Slow

No error correction


129

Message Switching Systems

5/22/2018 UNIT II

130/131

Message Switching Systems

Equipment: teletypewriters

Types: torn tape message system

Point-to-point

Multipoint line

Collision, polling, address, and protocol

Control or master station and subordinate or

slave station


130

Computers

5/22/2018 UNIT II

131/131

Benefits Inquiry

File updating

Timesharing Other applications (TPS, MIS, DSS, EX, EC)

Types

Centralized Distributed

Documents

UNIT II