communication systems lecture 4class.icc.skku.ac.kr/~dikim/teaching/3032/notes/eee3032p_04.pdf ·...

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Communication Systems

Lecture 4

Dong In Kim

School of Info/Comm Engineering

Sungkyunkwan University

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Outline

Overview of Modulation

What is modulation?

Why modulation?

Classifications of modulations

History of AM & FM Radio Broadcast

Linear Modulation:

Double sideband modulation

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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 =

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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

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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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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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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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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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Figure 3-65 Generation of PCM. (a) PCM modulator. (b)

Quantization and encoding. (c) Transmitted output.

PCM:

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Outline


What is modulation?

Why modulation?

Types of modulations

History of AM & FM Radio Broadcast (chapter 1)

Linear Modulation:


Amplitude modulation

Single side band modulation

Vestigial side band modulation

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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)

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Outline


What is modulation?

Why modulation?

Types of modulations

History of AM & FM Radio Broadcast

Linear Modulation:





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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.

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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.

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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:





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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.

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( ) [ ( ) ( )].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

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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

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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

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In the time domain

(Note that 1 cos(2 ) 0)ct

( ) ( )[1 cos(2 )]c cd t A m t t

2cos( )ct

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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 !

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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.

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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.

communication systems lecture 4class.icc.skku.ac.kr/~dikim/teaching/3032/notes/eee3032p_04.pdf ·...

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