Download - Basic Concepts

Basic Concepts

„Modulation & Detection” Zdzisław Papir

• Definition of modulation• Spectra of information source signals• Purposes of using modulation• Does the free lunch exist?• Definition of demodulation• Definition of detection• The whole picture – transmission system• Classification of modulations

Definition of modulation


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


ttxt

ftAtc

0

0000

cos2,cos

MODULATOR

tAtc 00 cos

ttxt 0cos tx

Amplitude Modulation


ttxt 0cos

t

Baseband modulation(line code, transmission code)


+

–

p

p1

p

pp1

Transition graph for Miller’s code 0Pr1,1Pr pp

Baseband modulation(line code)


+

–

p

p1

p

pp1 0Pr1

1Pr

p

p

1 0 001 1 00 00 1 1 1

Spectra of informationsource signals


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 of usingmodulation


Access to different parts of the electromagnetic spectrum

Microwave - MHzs

Copper plant – baseband up to 10 MHz

Fiber – narrow frequency window at THzs

Satellite - GHzs


Better noise immunity of transmission

TRANSMITTER RECEIVERTRANSMISSIONCHANNEL

noisemodulation detection

improvment of the Signal to Noise Ratio (SNR)

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

(modulation gain)

SNR

Purposes ofusing modulation

http://en.wikipedia.org/wiki/Image:White-noise.png


Frequency

Frequency (FDM)

Time (TDM)

interleaved signal spectra

interleaved data blocks

Purposes of usingmodulation

Division Multiplex

Tim

e

The free lunch does not exist


#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

fBW

fBW

spectrum widening

coefficient

Definition of Demodulation


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 modulated carrier.

Definition of Detection


DETECTION

φ(t) + noisex(t) + noise

c(t)

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

The Whole Picture


TRANSMITTER RECEIVERTRANSMISSIONCHANNEL

noise

modulation detection

Transmitter


LPF MOD BPF tx t

Informationsignal

Modulatedsignal

LPF – Lowpass FilterBPF – Bandpass FilterMOD – Modulator

Carrier tc

Receiver


BPF DEMOD LPF tx t

Informationsignal

Modulatedsignal

LPF – Lowpass FilterBPF – Bandpass FilterDEMOD – Demodulator

Carrier tc

Transmission Channel


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

AWGN – Additive White Gaussian Noise


LPF MOD BPF tx t

Transmission System

tc

BPF DEMOD LPF

noisetx noiset tc

00 SNRII SNR

[dB] power noisepower signal NPSNR

noise t


LPF MOD BPF tx t

Transmission System Quality

tc

BPF DEMOD LPF

noisetx noiset tc

I

0

I

0gain modulationSNRSNRg

II SNR 00 SNR


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)


Classification of Modulationscarrier

informationsignal

D

D

A

A

Samplingtheorem

PAM PFM

PDM

Keying

ASK PSKFSK MSK

GMSK

Line codes

NRZ Miller

HDB PRS

AM

FM SSB

QAM

Classics

DSB