unit ii
DESCRIPTION
communication engineeringsyllabus anna university chennaiV Semester / EEE 2008 regulation Electrical and electronics engineeringTRANSCRIPT
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UNIT II
DIGITAL
COMMUNICATION
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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.
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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.
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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
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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.
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PULSE MODULATION INCLUDES
Pulse Amplitude Modulation
Pulse Width Modulation
Pulse Position Modulation
Pulse Code Modulation
Delta Modulation
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PULSE MODULATION
Analog Pulse Modulation Digital Pulse Modulation
Pulse Amplitude (PAM)Pulse Width (PWM)
Pulse Position (PPM)
Pulse Code (PCM)
Delta Modulation(DM)
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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.
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Concepts of Sampling &
sampling Theorem
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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
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Three different sampling methods for
PCM
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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
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Sampling
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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
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Pulse Amplitude Modulation
In PAM,amplitude of pulses is varied in
accordance with instantaneous value of
modulating signal.
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Pulse Amplitude Modulation
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.
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Pulse Amplitude Modulation
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.
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Pulse Amplitude Modulation
Flat topped PAMsampling is often usedbecause of the ease of generating the
modulated wave. In this pulses have flat
tops after modulation.
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P l A li d
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Pulse Amplitude
Modulation The PAM signal can be detected by
passing it through a low pass filter.
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Pulse Width Modulation
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.
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Pulse Width Modulation
Fig:
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Pulse Width Modulation
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.
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Pulse Width Modulation
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Pulse Width Modulation
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).
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Pulse Width Modulation
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.
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Pulse Width Modulation
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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.
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Pulse Position Modulation
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Pulse Position Modulation
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.
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Pulse Position Modulation
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.
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Pulse Position Modulation
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.
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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.
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Digital Pulse Modulation
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.
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Digital Pulse Modulation
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.
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Digital Pulse Modulation
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)
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Pulse Code Modulation(PCM)
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
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Pulse Code Modulation(PCM)
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.
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Pulse Code Modulation(PCM)
The basic elements of a PCM system.IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.038
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Pulse Code Modulation(PCM)
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.
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Pulse Code Modulation(PCM)
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.
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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
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Pulse Code Modulation(PCM)
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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
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Quantization
Two types of quantization: (a) midtread and (b)
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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
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Quantization
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Quantization
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Quant izat ion and encod ing of a
sampled signal
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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
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Quantization Error (cont.)
Round-off error
Overload error
Overload
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Quantization Noise
Illustration of the quantization process
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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.
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DPCM
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DPCM
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Quantization error feedback
in the DPCM coder
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Signal distortions due to
intraframe DPCM coding
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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
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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.
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Delta Modulation
The process of delta modulationIFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0
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Delta Modulation
Components of Delta Modulation
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Delta Modulation
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Delta Modulation
DM system. (a) Transmitter. (b) Receiver.IFETCE/EEE /M.SUJITH /III YEAR/V SEM/EC 2311/CE/PPT/VER1.0
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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.
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Delta Modulation
Two types of quantization errors :
Slope overload distortionand granular noise
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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.
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Simple Implementation of a DM
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Simple Implementation of a DM
system
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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
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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
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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.
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Adaptive Delta Modulation
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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.
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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
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DPSK Transmitter
dK
dK-1
bK
AcCos(2fct)
S(t)=AcCos(2fct)
Encoder
Delay Tb
Product
Modulator
What Should We Do to make Encoder?
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DPSK Transmitter..Modified
dK
dK-1
bK
AcCos(2
fct)
S(t)=AcCos(2fct)
Delay Tb
Product
ModulatorEx- NOR
Gate
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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
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DPSK Receiver
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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
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Quadrature Phase Shift Keying (QPSK)
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
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QPSK Waveform
00 11 00 11 10 10
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QPSK Signal Phase
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Constellation Diagram
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Quadrature Phase Shift Keying (QPSK)
( ) 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
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QPSK Transmitter
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QPSK R i
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QPSK Receiver
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S h i i Ci i
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Synchronization Circuit
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Mi i Shift K i (MSK)
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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
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Mi i Shift K i (MSK)
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Minimum Shift Keying (MSK)
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
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Mi i Shift K i (MSK)
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Minimum Shift Keying (MSK)
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
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C i Ph C di
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Carrier Phase Coding
For dibit 00
(t)
tTb 2Tb
-/2
-
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Carrier Phase Coding
For dibit 10
Tb 2Tb
/2
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Carrier Phase Coding
Tb 2Tb
/2
For dibit 11
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Carrier Phase Coding
For dibit 01
(t)
tTb 2Tb
-/2
-
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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
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Basic Encoding Techniques
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A lit d Shift K i
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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
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A lit d Shift K i
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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
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Phase Shift Keying (PSK)
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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
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Differential PSK (DPSK)
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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
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Four-level PSK: Quadrature PSK (QPSK)
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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
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QPSK and Offset QPSK (OQPSK)Modulators
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Modulators
)2sin()(2
1)2cos()(
2
1)(:
)2sin()(2
1)2cos()(
2
1)(:
tfTtQtftItsOQPSK
tftQtftItsQPSK
cbc
cc
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Example of QPSK and OQPSK Waveforms
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41101
4
31100
4
31110
41111
:
QPSKf or
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Performance of ASK, FSK, MFSK, PSK andMPSK
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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
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Performance of MFSK and MPSK
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MFSK: increasingMdecreases BER and decreases bandwidth Efficiency
MPSK: IncreasingMincreases BER and increases bandwidth efficiency
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Binary Frequency-Shift Keying
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(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
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Binary Frequency Shift Keying (BFSK)
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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
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Multiple Frequency-Shift Keying
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(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
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Multiple Frequency-Shift Keying
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(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
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Multiple Frequency-Shift Keying
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(MFSK)
Total bandwidth required2Mfd
Minimum frequency separation required
2fd=1/Ts
Therefore, modulator requires a bandwidth of
Wd=2L/LT=M/Ts
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Multiple Frequency-Shift Keying
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(MFSK)
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Phase-Shift Keying (PSK)
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Phase-Shift Keying (PSK)
Two-level PSK (BPSK) Uses two phases to represent binary digits
ts tfA
c2cos
tfA c2cos1binary
0binary
tfA c2cos
tfA c2cos
1binary
0binary
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Phase-Shift Keying (PSK)
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Phase-Shift Keying (PSK)
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
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Phase-Shift Keying (PSK)
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Phase Shift Keying (PSK)
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
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Phase-Shift Keying (PSK)
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Phase Shift Keying (PSK)
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
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Performance
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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
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Performance
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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
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Quadrature Amplitude Modulation
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Quadrature Amplitude Modulation
QAM is a combination of ASK and PSK Two different signals sent simultaneously on the
same carrier frequency
tftdtftdts cc 2sin2cos 21
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Quadrature Amplitude Modulation
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Quadrature Amplitude Modulation
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Reasons for Analog Modulation
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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
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GMSK
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GMSK as implemented by quadrature signal processing at baseband
followed by a quadrature modulator
Generating a GMSK Waveform
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Architecture of a GMSK ModulatorGMSK modulator using a VCO
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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
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Pulse Shaping
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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
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Summing and Integration
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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)
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I&Q Signals
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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
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GMSK Properties
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p
Improved spectral efficiency Power Spectral Density
Reduced main lobe over MSK
Requires more power to transmit data than manycomparable modulation schemes
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Applications to Digital Data
communications
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Telegraph
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g p
Morse Code Dots and dashes
Slow
No error correction
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Message Switching Systems
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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
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Computers
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Benefits Inquiry
File updating
Timesharing Other applications (TPS, MIS, DSS, EX, EC)
Types
Centralized Distributed