Basic Concepts

„Modulation & Detection” Zdzisław Papir

• Modulation• Purposes of using modulation• Demodulation• Demodulation vs. Detection• The whole picture – transmission system• Classification of modulations

Definition of Modulation

„Modulation & Detection” Zdzisław Papir

Modulation involves three signals:• information carrying signal = modulating signal x(t)• carrier c(t)• modulated signal φ(t)

• Modulation is mapping of the information signal into one of the parameters of the carrier.• The output signal is called the modulated signal.

MODULATORφ(t)x(t)

c(t)

Amplitude Modulation

„Modulation & Detection” Zdzisław Papir

ttxt

ftAtc

0

0000

cos

2,cos

MODULATOR

tAtc 00 cos

ttxt 0cos tx

Amplitude Modulation

„Modulation & Detection” Zdzisław Papir

ttxt 0cos

t

Baseband Modulation(Line Code)

„Modulation & Detection” Zdzisław Papir

+

–

p

p1

p

pp1

Transition graph for Miller’s code

0Pr1,1Pr pp

Baseband Modulation(Line Code)

„Modulation & Detection” Zdzisław Papir

+

–

p

p1

p

pp1 0Pr1

1Pr

p

p

1 0 001 1 00 00 1 1 1

Baseband Modulation(Line Code)

„Modulation & Detection” Zdzisław Papir

+

–

p

p1

p

pp1

0Pr1

1Pr

p

p

S()

Power density spectrum

Bipolar NRZ code

Miller’s code

Biphase code

Spectra of information signals

„Modulation & Detection” Zdzisław Papir

Voice - baseband – up to several kHzs

Sound - baseband – up to tens of kHzs

Video – baseband up to several MHzs

Raw data – baseband up to ...?

Purposes

„Modulation & Detection” Zdzisław Papir

#1 Access to different parts of the electromagnetic spectrum

Microwave - MHzs

Copper plant – baseband up to 10 MHz

Fiber – narrow frequency window at THzs

Sattelite - GHzs

Purposes

„Modulation & Detection” Zdzisław Papir

#2 Better noise immunity of transmission

TRANSMITTER RECEIVERTRANSMISSIONCHANNEL

noise

modulation detection

improvment of the Signal to Noise Ratio (SNR)

SNR (modulation & detection) >> SNR (baseband system)

SNR

Purposes

„Modulation & Detection” Zdzisław Papir

#3 Frequency or Time Division Multiplexof transmission channels

Frequency (FDM)

Time (TDM)

The free lunch does not exist

„Modulation & Detection” Zdzisław Papir

#1 Access to different parts of the electromagnetic spectrum

#2 Better noise immunity of transmission

#3 Frequency or Time Division Multiplex of transmission channels

#4 SPECTRUM WIDENING

• B [Hz] or W [rad/s] – spectrum of the modulated signal• fm [Hz] or ωm [rad/s] – spectrum of the modulating signal

mm

mm

f

BW

fBW

spectrum widening

coefficient

Definition of Demodulation

„Modulation & Detection” Zdzisław Papir

Modulation is MAPPING the information signal onto the carrier.

MODULATORφ(t)x(t)

c(t)

DEMODULATORφ(t) x(t)

c(t)

Demodulation is STRIPPING the information signal from the carrier.

Definition of Detection

„Modulation & Detection” Zdzisław Papir

DETECTIONφ(t)x(t) + noise

c(t)

Detection is STRIPPING the information signal from the carrier in a noisy environment.

The Whole Picture

„Modulation & Detection” Zdzisław Papir

TRANSMITTER RECEIVERTRANSMISSIONCHANNEL

noise

modulation detection

Transmitter

„Modulation & Detection” Zdzisław Papir

LPF MOD BPF tx t

Informationsignal

Modulatedsignal

LPF – Lowpass FilterBPF – Bandpass FilterMOD – Modulator

Carrier tc

Receiver

„Modulation & Detection” Zdzisław Papir

BPF DEMOD LPF

tx t

Informationsignal

Modulatedsignal

LPF – Lowpass FilterBPF – Bandpass FilterMOD – Modulator

Carrier tc

Transmission Channel

„Modulation & Detection” Zdzisław Papir

The common assumption is thatthe transmission channel disturbesthe signal being transmitted withthe Additive White Gaussian Noise.

AWGN – Additive White Gaussian Noise

„Modulation & Detection” Zdzisław Papir

LPF MOD BPF tx t

Transmission System

tc

BPF DEMOD LPF

noisetx noiset tc

0O SNRII SNR

[dB] power noisepower signal NSSNR

„Modulation & Detection” Zdzisław Papir

LPF MOD BPF tx t

Transmission System Quality

tc

BPF DEMOD LPF

noisetx noiset tc

I

O

I

Ogain modulationSNR

SNRg

II SNR 0O SNR

„Modulation & Detection” Zdzisław Papir

Classification of Modulations

MODULATORφ(t)x(t)

c(t)

Digital - Analog

Digital - Analog

• Analog waveform – sin or cos waveform (A)

• Digital waveform – sequence of pulses (D)

„Modulation & Detection” Zdzisław Papir

Classification of Modulationscarrier

informationsignal

D

D

A

A

Samplingtheorem

PAM PFM

PDM

Keying

ASK PSK

FSK MSK

GMSK

Line codes

NRZ Miller

HDB PRS

AM

FM SSB

QAM

Classics