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1
Communication Systems
Lecture 4
Dong In Kim
School of Info/Comm Engineering
Sungkyunkwan University
2
Outline
Overview of Modulation
What is modulation?
Why modulation?
Classifications of modulations
History of AM & FM Radio Broadcast
Linear Modulation:
Double sideband modulation
3
Overview of Modulation
What is modulation?
The process of varying a carrier signal in order to use that
signal to convey information.
Why modulation?
1. Efficient transmission of signals using antennas
of practical size:
The optimal antenna size is related to wavelength:
Voice signal: 3 kHz
Wavelength: λ = c / f =
If modulated by a 100 MHz carrier:
Wavelength λ =
4
Overview of Modulation
Why modulation?
2. Utilizing the channel for transmission of more
than one signal (multiplexing)
3. Modulation allows us to get better trade-off
between bandwidth and signal-to-noise ratio
Chapter 6
5
Overview: Types of Modulation Analog modulation
The message is continuous in time and value
Continuous-wave modulation (focus of this course)
A parameter of a high-freq carrier is varied in accordance with the
message signal
( ) ( )cos[ ( )]c cx t A t t t
If a sinusoidal carrier is used, the modulated carrier is:
Linear modulation: A(t) is linearly related to the message.
AM, DSB, SSB
Angle modulation:
Phase modulation: Φ(t) is linearly related to the message.
Freq. modulation: dΦ(t)/dt is linearly related to the message.
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Overview: Types of Modulation
Angle modulation:
Message
Carrier
Phase modulation
Freq modulation
Linear modulation
(Amplitude modulation)
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Overview: Types of Modulation
Analog pulse modulation
Message value is continuous. Transmission happens at discrete times.
Transmitted signal is a sequence of pulses
The amplitude, width, or position of the pulse can be varied over a
continuous range according to the message value at the sampling
instant.
PAM: Pulse amplitude modulation
PWM: Pulse width modulation
PPM: Pulse position modulation
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Overview: Types of Modulation
Digital modulation (also called shift keying)
The message has discrete values:
Binary: 0, 1
M-ary: M different values
Sinusoidal carrier: uses a finite number of distinct signals to represent digital data:
ASK (Amplitude shift keying): uses a finite number of amplitudes.
PSK (Phase shift keying): uses a finite number of phases.
FSK (Freq shift keying): uses a finite number of freqs.
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Overview: Types of Modulation
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Overview: Types of Modulation
Digital pulse modulation
Use sampling and quantization to convert analog data
into digital.
Transmit a coded pulse sequence.
Examples:
DM (Delta Modulation)
PCM (Pulse Coded Modulation)
A standard way to digitize audio, image and video.
Applications: public switched telephone network (PSTN)
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Overview: Types of Modulation
Figure 3-65 Generation of PCM. (a) PCM modulator. (b)
Quantization and encoding. (c) Transmitted output.
PCM:
12
Outline
Overview of Modulation
What is modulation?
Why modulation?
Types of modulations
History of AM & FM Radio Broadcast (chapter 1)
Linear Modulation:
Double sideband modulation
Amplitude modulation
Single side band modulation
Vestigial side band modulation
13
History of Radio
Spark AM FM L-band Digital
1895 by Marconi 1906 by Fessenden 1931 by Armstrong 2000 by Sirius
(Canadian) (500–1500MHz)
Marconi in Newfoundland.
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Early History of Radio
1887: Heinrich Hertz first detected radio waves.
1894: Guglielmo Marconi invented spark transmitter with antenna in Bologna, Italy.
1897: Marconi formed his company in Britain at age 23, awarded patent for “wireless telegraph”.
1905-06: Reginald Fessenden (A Canadian) invented a continuous-wave voice transmitter, first voice broadcast in Christmas Eve 1906.
1906: Lee de Forest patented his audion tube, had visited the Fessenden lab in 1903 and stole the design for a "spade detector" (de Forest sued Armstrong over the basic regenerative patent from 1915 to 1930, and was finally awarded the basic radio patent, causing him to become known as the "father of radio."
1912-1933: Edwin Armstrong invented the Regenerative Circuit (1912), the Superheterodyne Circuit (1918), the Superregenerative Circuit (1922) and the complete FM System (1933). He spent almost his entire adult life in litigation over his patents and ultimately committed suicide by jumping to his death from a high-rise in New York City in 1954.
1912: Due to Titanic disaster, all ships were required to have radios with 2 operators and auxiliary power and all transmitters must be licensed.
1920: The first licensed commercial AM radio services.
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AM and FM Radio AM radio ranges from 535 to 1605 kHz
The bandwidth of each station is 10 kHz.
AM signals can travel quite long distance due to ionospheric refraction
http://www.cybercollege.com/frtv/frtv017.htm
The FM radio band goes from 88 to 108 MHz
FM stations are 200 kHz apart
FM has much better quality than AM
Ionospheric refraction doesn't affect FM or TV signals too much (line-of-sight
propagation, need tall antenna)
16
Outline
Overview of Modulation
What is modulation?
Why modulation?
Types of modulations
History of AM & FM Radio Broadcast
Linear Modulation:
Double sideband modulation
Amplitude modulation
Single side band modulation
Vestigial side band modulation
17
Definition of Bandwidth
A measure of the extent of significant spectral content
of the signal for positive frequencies.
Band-limited signal:
|X(f)|
f
W -W
Bandwidth is W.
|X(f)|
f fc+W fc-W
Bandwidth is 2W.
fc 0
When the signal is not band-limited:
f
|X(f)| Different definitions exist:
- Main lobe, 3 dB, etc.
18
Definition of Bandwidth
Radio spectrum is a scarce and expensive
resource:
US license fee: ~ $77 billions / year
Communications systems should provide the
desired quality of service with the minimum
bandwidth.
19
Linear Modulation
( ) ( )cos[ ( )]c cx t A t t t
If a sinusoidal carrier is used, the modulated carrier is:
Linear modulation:
A(t) is linearly related to the message.
Φ(t) = 0
Type of linear modulation:
Double sideband modulation
Amplitude modulation
Single side band modulation
Vestigial side band modulation
20
Double Sideband Modulation (DSB)
Time domain expression:
A(t) is proportional to the message m(t)
A special case of linear modulation
Input: DSB Output:
( ) ( )cos( )c c cx t A m t t
Envelope should be dashed line.
21
Double Sideband Modulation (DSB)
( ) [ ( ) ( )].2
cc c c
AX f M f f M f f
Effect in frequency domain:
|M(f)|
f
W -W
|Xc(f)|
f fc -fc
Proof:
( ) ( )cos( )c c cx t A m t t
22
Double Sideband Modulation (DSB)
Upper Sideband and Lower Sideband Upper Sideband (USB): [fc, fc + W], [- fc – W, -fc].
Lower Sideband (LSB): [fc – W, fc], [- fc, -fc + W].
f fc -fc
Bandwidth is doubled: 2W (a drawback of DSB)
2W
DSB is also called suppressed carrier DSB (DSB-SC)
No spectral component at carrier frequency, unless m(t) has a DC.
USB LSB LSB
USB
23
Double Sideband Modulation (DSB)
Modulator Implementation:
( )cos( ) 2cos( ) ( )[1 cos(2 )]c c c c cA m t t t A m t t
m(t) can be obtained by filtering out the high freq component.
The lowpass filter is called postdetection filter.
Demodulator:
Ignore channel noise: xr(t) = xc(t)
Use trigonometric identity:
|D(f)|
f
2fc -2fc
LPF
W
24
Double Sideband Modulation (DSB)
In the time domain
(Note that 1 cos(2 ) 0)ct
( ) ( )[1 cos(2 )]c cd t A m t t
2cos( )ct
25
DSB: Phase Error
Coherent Demodulation: So far we assume the demodulator has a carrier that has the
same frequency and phase as the transmitter
Synchronous or coherent demodulation
If a time-varying phase error θ(t) exists:
( )cos( ) 2cos( ( )) ( )[cos(2 ( )) cos( ( ))]c c c c cA m t t t t A m t t t t
After LPF: ( ) ( )cos( ( ))Dy t m t t
2cos( ( ))ct t
LPF ( )Dy t
Serious distortion can be resulted !
26
DSB: Generation of Coherent Carrier
How to generate a phase coherent carrier?
1. Costas phase-locked loop (covered later)
2. Square the received signal!
( ) ( )cos( )r c cx t A m t t
2 2 2 2 2 21( ) ( )cos ( ) ( )[1 cos(2 )]
2r c c c cx t A m t t A m t t
Trigonometric identity again!
A coherent carrier can be obtained by:
1. Extract the high-frequency component 2ωc by a bandpass filter
2. Using frequency divider to divide the frequency of this component by 2 (can be implemented by Phase-Locked Loop (PLL): see pp.197).
Condition: m(t) is a power signal. 21
lim ( ) 02
T
T TP m t dt
Tor m2(t) has a nonzero DC component.
27
DSB: Generation of Coherent Carrier
3. Transmit a carrier component
|Xc(f)|
f fc -fc
Use narrowband bandpass filter to extract the carrier
component.
If the carrier component is sufficient large, no need to
generate a demodulation carrier
Amplitude modulation.