Principle of Communication

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Chapter 5

AM, FM, and Digital Modulated Systems

Binary Bandpass Signalling Techniques OOK BPSK FSK

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On–Off keying (OOK) [amplitude shift keying (ASK)] - Consists of keying (switching) a carrier sinusoid on and off with a unipolar binary signal. - Morse code radio transmission is an example of this technique. - OOK was one of the first modulation techniques to be used and precedes analog communication systems.

Binary Phase-Shift Keying (BPSK) - Consists of shifting the phase of a sinusoidal carrier 0o or 180o with a unipolar binary signal.

- BPSK is equivalent to PM signaling with a digital waveform.

Frequency-Shift Keying (FSK) - Consists of shifting the frequency of a sinusoidal carrier from a mark frequency (binary 1) to a space frequency (binary 0), according to the baseband digital signal.

- FSK is identical to modulating an FM carrier with a binary digital signal.

Binary bandpass signaling techniques

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Binary bandpass signaling techniques

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tmAtg c

The complex envelope is

CarrierCos(2fct)

Messagem(t)

OOK outputAcm(t)Cos(2fct)

The OOK signal is represented by

ttmAts cc cos

On-Off Keying (OOK) Also known as Amplitude Shift Keying (ASK)

22 sin

2 b

bb

cg fT

fTTf

Af

2

2cA

The PSD of this complex envelope is given by

2Awhere m(t) has a peak value of

So that s(t) has an average normalized power of

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On-Off Keying (OOK)

Tb – bit period ; R – bit rate

1bT

R

1 0 1 0 1 0 1Message

UnipolarModulation

BipolarModulation

OOK signal

m(t)

m(t)

s(t)

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On-Off Keying (OOK)

cgcgv ffPffPfP 4

1)(

PSD of the bandpass waveform is given by

Null-to-Null bandwidth is and absolute bandwidth isRBT 2

The Transmission bandwidth is

TB

BBT 2

Where B is the baseband bandwidth

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OOK in Binary outputEnvelopeDetector

LPFOOK in Binary output

cos(2 )cf t

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

2( ) cos (2 )c cA m t f t

1( )

2 cA m t

Non-Coherent Detection

Detection of OOK

Coherent Detection with Low-pass filter

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Carrier:Cos(2fct)Message: m(t)

BPSK outputAcCos(2fct+Dpm(t))

-90 Phase shift

Binary Phase Shift Keying (BPSK)

1 0 1 0 1 0 1Message

UnipolarModulation

BipolarModulation

BPSK output

1bT

R

m(t)

m(t)

s(t)

Generation:

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Binary Phase Shift Keying (BPSK)

tmDtAts pcc cos

ttmDAttmDA cpccpc sinsincoscos

ttmDAtDA cpccpc sinsincoscos

2

h

The BPSK signal is represented by

pilot carrier term data term

The level of the pilot carrier term is set by the value of the peak deviation

The digital modulation index ‘h’ is given by

If Dp is small, then there is little power in data term & more in pilot term

To maximize performance (minimum probability of error)

2900 pDOptimum case :

ttmAts cc sinBPSK signal :

2∆θ – maximum peak-to-peak deviation during time Ts

1)( tmlet

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tmjAtg c

b

bbcg fT

fTTAf

sin2

Binary Phase Shift Keying (BPSK) The complex envelope for this BPSK is given by

The PSD for this complex envelope is given by

cgcgv ffPffPfP 4

1)(

PSD of the bandpass waveform is given by

Average normalized power of s(t) : 22

cA

Null-to-Null BW

PSD of optimum BPSK

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fc

2R = 2/Tb

If Dp /2Pilot exists 22

/)(

)/)(sin(

8

Rff

Rff

R

A

c

cc

Binary Phase Shift Keying (BPSK)

Power Spectral Density (PSD) of BPSK:

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Frequency Shift Keying (FSK)

Cos(2f1t)Message: m(t)

FSK outputAcCos(2f1t+1) or

AcCos(2f2t+2)Osc. f2

Osc. f1

Cos(2f2t)

Discontinuous FSK :

sent is '0'binary a when interval timein thefor t ,cos

sent is '1'binary a when interval timein thefor t ,coscos

22

11

tA

tAttAts

c

ccc

tt

ttt

c

c

22

11

The discontinuous-phase FSK signal is represented by

for t during a binary ‘1’ signal

for t during a binary ‘0’ signal

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Frequency Shift Keying (FSK)

The Continuous-phase FSK signal is represented by

FrequencyModulator

fc

Message: m(t) FSK output

Continuous FSK :

t

fcc dmDtAts cos

tj cetgts Re

tjceAtg

t

f dmDt

or

where

for FSK

t

fcc dmDtfA 2cos

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1 0 1 0 1 0 1Message

UnipolarModulation

BipolarModulation

FSK output(Discontinuous)

1bT

R

FSK output(Continuous)

Mark(binary 1) frequency: f1

Space(binary 0) frequency: f2

m(t)

m(t)

s(t)

s(t)

Frequency Shift Keying (FSK)

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Frequency Shift Keying (FSK)

ComputerFSK modem(Originate)

ComputerCenter

FSK modem(Answer)

Digitaldata

PSTN

Dial up phone line

f1 = 1270Hzf2 = 1070Hz

f1 = 2225Hzf2 = 2025Hz

FSK modem with 300bps