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Digital communication- Digital communication- student versionstudent version

Dr. Uri Mahlab

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General overviewGeneral overview

Digital Radio Theoryand Implementation

-How a Digital Radio Works

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Digital Radio Block DiagramDigital Radio Block Diagram

CO

DER MOD

UPCONVERTER

DOWNCONVERTER

DEMOD

DE C

OD

E R

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Analog vs. Digital ModulationAnalog vs. Digital Modulation

AM

FM

PM

DIGITAL

With digital modulation information is in the phase and

amplitude of the signal

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The IQ DiagramThe IQ Diagram

Vq

Vi

magnitude

phase

Q

I

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Analog Modulation on the IQ diagramAnalog Modulation on the IQ diagram

Q

I

B

A

C

D

FM

PM

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BPSK Timing and State DiagramBPSK Timing and State Diagram

Reference

State 1 State 0

= 0 deg. = 180 deg.

0 State 1 State

BPSK

Constellation

Diagram

Q

I

t

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QPSK ModulationQPSK Modulation

Q

I

00

1011

01

Vi

Vq

4 Possible States

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16 QAM State Diagram16 QAM State Diagram

Q

I

0000 00110001

01010100 0111

1100

0010

1101

0110

10011000 1010

1110

1011

1111

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Symbol Rate:Symbol Rate:

“The rate at which the carrier

moves between points in the

constellation”

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Example:Example:

16 Mb/s4 Bits

A 16 QAM radio has 4 bit per state (or symbol).

If the radio operates at 16 Mb/s, then the

carrier must change states

or

4 million times per second (4 MBaud)

SYMBOL RATE = 4MHz

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Some Typical Modulation FormatsSome Typical Modulation Formats

BPSK QPSK 8PSK

16QAM 64QAM

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QPSK ModulatorQPSK Modulator

SERIAL TOPARALLEL

CONVERTERCARRIER PHASE

SHIFT BPFCOMBINER

00

11 10

01

COMBINED VECTOR

STATE DIAGRAM

BALANCED

MODULATION

BALANCED

MODULATION

QUADRATURE DATA STREAM

IN-PHASE DATA STREAM

fs = fb/2

BINARY

NRZ

INPUT

SIGNAL

fb

SYMBOL RATE:fs = fb/2

I

Q

I.F0°

90°

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I, Q, Eye diagram and ConstellationI, Q, Eye diagram and Constellation

EYE

I

Q

+1

-1

+1

-1

CONSTELLATION :

I

Q

1,4

5 3

2

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QPSK DemodulatorQPSK Demodulator

BPF PowerSplitter

CarRec.

PhaseSplitter

Symboltiming

rec.(STR)

Parallelto serial

converter

LPF.

LPF.

ThreshComp.

ThreshComp.

IF

Input

Phase

Demodulation

Phase

Demodulation

Binary

NRZfb

fb/2

fb/2

I

Q

I

Q

0°

90°

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16 QAM Modulator16 QAM Modulator

Data

2-to-4level

convert

2-to-4level

convert

Premod.LPF

Premod.LPF

Phasesplit BPFLO

L.F.

16 QAM

Output

0°

90°

Q

I

4bf

4bfI

Q

2bf

2bf

Binary

NRZ

Data

bf

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16 QAM Demodulator16 QAM Demodulator

BPF CR STR

X2datacon

blner

LPF

LOGIC

1thV

2thV

3thV

4-to-2 level converterof Q channel. Samedesign as I channel.

IF Input

I

Q

0°

90°

4-LevelSignal

4-LevelSignal

4bf

4bf

Regeneration

DataOutfb

2bf

2bf

LPF

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Which waveform requires more bandwidth?Which waveform requires more bandwidth?

A

B

time

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Bandwidth ConsiderationsBandwidth Considerations

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Two random data sequenceTwo random data sequence

timefrequency

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Unfiltered Digital Radio SpectrumUnfiltered Digital Radio Spectrum

0f SFf 0 SFf 20 SFf 30 SFf 40 SFf 0SFf 20 SFf 30 SFf 30 SFf 50

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An UNFILTERD RadioAn UNFILTERD Radio

CO

DER MOD U/C

D/C DEMOD

DEC

OD

ERtime

time

frequency

Data is easier to

recover but signal

requires a lot of

bandwidth

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A FILTERED RadioA FILTERED Radio

CO

DER MOD U/C

D/C DEMOD

time

time

frequency

Signal requires less

bandwidth but data

is filtered

DEC

OD

E R

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Intersymbol InterferenceIntersymbol Interference

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Nyquist FilteringNyquist Filtering

Raised Cosine

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Filter Coefficient & Determines Required B.W.Filter Coefficient & Determines Required B.W.

15.0

3.00

SF2SFRateSymbolFS _

Amplitude

Response

Linear Phase

(Flat Group Delay)

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The Filtering is Distributed in the RadioThe Filtering is Distributed in the Radio

CO

DER MOD

UPCONVERTER

DOWNCONVERTER

DEMOD

DE C

OD

E R

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SUMMARYSUMMARY

As the modulation complexity increases,the radio becomes more spectrally efficient.However, it also become more susceptibleto errors caused by noise and distortions.

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TOTAL PROBABILITYOF NOISE AMPLITUDE

EXCEEDING THISTHRESHOLD

THRESHOLDP(x)

X10 20 30-10-20-30

0.1

0.2

0.3

0.4

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How Error OccurHow Error OccurVOLTAGE

PROBReceived signal withsuperimposed noise

1 ERROR

0 ERROR0

1

THRESHOLDDECISION

NORMAL1 VALUE

NORMAL0 VALUE

BINARY SIGNAL + AMPLTUDENOISE FDP

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Gaussian DistributionGaussian Distribution

0.1

0.2

0.3

0.4

P(x)

X10 20 30-10-20-30

NEVER RECHSZEBO

PROBABILITYDENSITY

FUNCTION

0=RMS VALUE AFTERSUBTRACTING

DC COMPONENT

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Meaning of Eye diagramMeaning of Eye diagram

Threshold