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
Top Related