dct fundamentals 1,2,3 2012
TRANSCRIPT
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Digital Communication
Fundamentals
Dr. M.Meenakshi
Department of ECECollege of Engineering, Guindy
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Analog versus Digital
Propagation distance
Original
pulse
Regenerated
pulse
Data VoiceMedia
A bit is a bit!
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In Analog communications we want,
In Digital communication systems,
Data rate (R bps) Limited by Channel CapacityProbability of error (BER)
Performance Metrics
)()( tmtm
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Low
Pass
Filter
Sampler QuantizerChannel
Encoder
Line
Encoder
Pulse
Shaping
Filters
Source
Encoder
Modulator
Multiplexer
Input
Signal
Analog/
Digital
To Channel
DetectorReceiver
Filter
De-
ModulatorFrom Channel
Channel
Decoder
Digital-to-Analog
Converter
De-
Multiplexer
Signal
at the
user end
Carrier
Analog Digital
Carrier
Recovery
Symbol timing
Recovery
DigitalAnalog
Twisted Pair
Co-axial Cable
Optical Fiber
Wireless
Space
Digital communication system
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Analog to Digital Conversion
Low
PassFilter
Sampler
Quantizer
Analog
Digital
Source
Encoder
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Sampling Process
)()()( txtxtxs )(tx
)(txs
)(tx
)()()( fXfXfXs
|)(| fX
|)(| fX
|)(| fXs
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LP filter
Nyquist rate
Aliasing effect
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Aliasing effect
aliasing
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Aliasing effect
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Quantized
values
Amplitude quantizing: Mapping samples of a
continuous amplitude waveform to a finite set of
amplitudes.
Quantization
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Quantization
Average
quantization noise
power
Signal peak power
Signal power to
average quantization
noise power
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Quantization example
111 3.1867
110 2.2762
101 1.3657
100 0.4552
011 -0.4552
010 -1.3657
001 -2.2762
000 -3.1867
amplitude
x(t)
110 110 111 110 100 010 011 100 100 011
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Non-uniform quantization
compression+expansion companding
)(ty)(tx )( ty )( tx
x
)(xCy
x
yCompress Quantize
Channel
Expand
Transmitter Receiver
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Source Coding
Pulse Code Modulation
Delta Modulation
Eliminate redundancy
Lossless coding Huffman Coding,Shannon-Fano Coding, etc.
Lossy compression coding LPC, JPEG, MPEG, etc.
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Source Coding
Redundancy elimination maximizes
information rate in a transmission
What is Information Rate ?
Entropy Average Information per bit
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Baseband Signal Processing
Channel
Encoder
LineEncoder
PulseShaping
Filters
Multiplexer
Digital
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Channel Coding(Error Control Coding)
To reduce the impact of channel
errors by controlled introduction ofredundancy
Decrease in effective data rate
Increased coding gain
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Channel Coding(Error Control Coding)
Forward Error Correcting Codes
Linear Block CodesConvolutional Codes
Turbo Codes
Feedback based Error Correcting Codes
ARQ methods: Stop and wait,
Go-back N, Selective repeat
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Multiplexing
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Line Coding Formats
Converting Data into time varying Signals
Must consume least bandwidth
Reduce dc content
Clock extraction for symbol timing
Error correction capability if possible
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Line Coding Formats
(Converting Data into Signals)
Numerous techniques
NRZ-L
NRZ-I
Manchester
Differential Manchester
Bipolar AMI
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Line Coding Formats
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Line Coding Formats
0 1 0 0 1 1 0 0 0 1 1
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Line Coding Formats
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Pulse Shaping Filters
Time Domain Frequency Domain
Bandlimiting signals in frequency domain
spreads signal in time domain
Inter-Symbol Interference ISI
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Inter Symbol Interference
Time Domain Frequency Domain
fT1 T2 t
f1 f2 f
f
T1 T
2 t
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Nyquist Criterion
Time Domain Frequency Domain
T1 T
2 t
f1 f2 f
f
T1 T
2 t
f
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Application of Nyquist Criterion
Time Domain
0 T 2T 3T 4T 5T 6T 7T
Bit 1
Bit 0
At 4thbit interval
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Application of Nyquist Criterion
Time Domain
1 1 0 1 0 0 1 0
1 1 -1 1 -1 -1 1 -1
0 T 2T 3T 4T 5T 6T 7T
t
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Pulse Shaping Filters
Nyquist Criterian to overcome ISI
Pulse Shaping Filters
Raised Cosine Filters
Gaussian shaping filters
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Baseband RC Filter
-4T -3T -2T -T 0 T 2T 3T 4T
hRC(t)
1/Ts
-4T -3T -2T -T 0 T 2T 3T 4T
hRC(t)
1/Ts
-4T -3T -2T -T 0 T 2T 3T 4T
hRC(t)
1/Ts
= 0= 0.5
= 1
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Baseband Gaussian Filter
t
hG(t)
2sT
2sT
2
3 sT
2
3 sT t
hG(t)
2sT
2sT
2
3 sT
2
3 sT
2sT
2sT
2
3 sT
2
3 sT
= 0.5
= 0.75= 1.0
= 2.0
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Bandpass Processing
ModulatorTo Channel
Carrier
Modulationis the process of
encoding information from a messagesource in a manner suitable fortransmission through the channel.
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Passband Modulation
Analog
AM AmplitudeModulation
FM FrequencyModulation
PM PhaseModulation
Digital
ASK AmplitudeShift Keying
FSK FrequencyShift Keying
PSK PhaseShift Keying
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ASK, FSK, PSK
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Bandpass Signal Reception
DetectorReceiver
Filter
De-
ModulatorCarrier Ref.
Carrier
Recovery
Timing Ref.
Symbol
TimingRecovery
Analog Analog Digital
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Signal Decoding and Reception
Channel
Decoder
Digital-to-Analog
ConverterDe-
Multiplexer
Digital Analog
Signalat the
user
end
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Goal of Advanced Modulation &
Coding Techniques
Modulation / coding is difficult task in hostile
channels like the mobile radio channels
Small-scale fading and multipath conditions.
The goal of a modulation / coding scheme is:
To transport the message signal through the radiochannel with best possible quality.
To occupy least amount of (eg. radio ,RF) spectrum.
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Low
Pass
Filter
Sampler QuantizerChannel
Encoder
Line
Encoder
Pulse
Shaping
Filters
Source
Encoder
Modulator
Multiplexer
Input
Signal
Analog/
Digital
To Channel
DetectorReceiver
Filter
De-
Modulator
From Channel
Channel
Decoder
Digital-to-Analog
Converter
De-
Multiplexer
Signal
at the
user end
Carrier
Analog Digital
Carrier
Recovery
Symbol timing
Recovery
DigitalAnalog
Twisted Pair
Co-axial Cable
Optical Fiber
Wireless
Space
Digital communication system
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Digital Communication Techniques
Line coding, pulse shaping techniques
Carrier and Symbol timing estimation
Information theory, Source coding
Channel coding techniques
Advanced coding / modulation techniques
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ThankYou