Dct Fundamentals 1,2,3 2012

8/13/2019 Dct Fundamentals 1,2,3 2012

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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


fT1 T2 t

f1 f2 f

f

T1 T

2 t


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Nyquist Criterion


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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Nyquist Criterian to overcome ISI


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

[email protected]

Dct Fundamentals 1,2,3 2012

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