Modulation and Multiplexing
ICS 620
Overview
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Frequency Spectrum
Modulation techniques
Multiplexing--TDM vs FDM
Multiple Access
Signal formats
The Bands
ELF VLF
LF
MF
HF VHF UHF SHF EHF
Radio
Submillimeter
Range
3KHz 30KHz 300KHz 3MHz 30MHz300MHz 3GHz 30GHz 300GHz 3THz
Far
InfraRed
Optical
300mm 1500nm
1PetaHz
Near
InfraRed
R
e
d
700nm
1ExaHz
O
r
a
n
g
e
Y
e
l
l
o
w
600nm
G
r
e
e
n
B
l
u
e
500nm
I
n
d
i
g
o
V
i
o
l
e
t
Ultraviolet
400nm
X-Ray
Frequency Spectrum
• Limited resource
• Managed
• WARC
• FCC
• Bands
Analog Modulation
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The purpose of a communication system is to transmit information signals
(baseband signals) through a communication channel
The term baseband is used to designate the band of frequencies
representing the original signal as delivered by the input transducer
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For example, the voice signal from a microphone is a baseband signal,
and contains frequencies in the range of 0-3000 Hz
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The “hello” wave is a baseband signal:
Analog Modulation
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Since this baseband signal must be transmitted through a
communication channel such as air using electromagnetic waves, an
appropriate procedure is needed to shift the range of baseband
frequencies to other frequency ranges suitable for transmission, and a
corresponding shift back to the original frequency range after reception.
This is called the process of modulation and demodulation
Remember the radio spectrum:
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AM
FM radio/TV
radio
For example, an AM radio system transmits electromagnetic waves
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with frequencies of around a few hundred kHz (MF band)
The FM radio system must operate with frequencies in the range of 88108 MHz (VHF band)
Analog Modulation
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Since the baseband signal contains frequencies in the audio frequency range (3
kHz), some form of frequency-band shifting must be employed for the radio
system to operate satisfactorily
This process is accomplished by a device called a modulator
The transmitter block in any communications system contains the modulator
device
The receiver block in any communications system contains the demodulator
device
The modulator modulates a carrier wave (the electromagnetic wave) which has
a frequency that is selected from an appropriate band in the radio spectrum
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For example, the frequency of a carrier wave for FM can be chosen from
the VHF band of the radio spectrum
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For AM, the frequency of the carrier wave may be chosen to be around a
few hundred kHz (from the MF band of the radio spectrum)
The demodulator extracts the original baseband signal from the received
modulated signal
To Summarize:
Modulation is the process of impressing a low-frequency information signal
(baseband signal )onto a higher frequency carrier signal
Modulation is done to bring information signals up to the Radio Frequency (or
higher) signal
Types of Analog Modulation
Amplitude Modulation (AM)
 Amplitude modulation is the process of varying the amplitude of
a carrier wave in proportion to the amplitude of a baseband
signal. The frequency of the carrier remains constant
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Frequency Modulation (FM)
 Frequency modulation is the process of varying the frequency of
a carrier wave in proportion to the amplitude of a baseband
signal. The amplitude of the carrier remains constant
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Phase Modulation (PM)
 Another form of analog modulation technique which we will not
discuss
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Amplitude Modulation
Carrier wave
Baseband signal
Modulated wave
Amplitude varyingfrequency constant
Frequency Modulation
Carrier wave
Baseband signal
Small amplitude:
low frequency
Large amplitude:
high frequency
Modulated wave
Frequency varyingamplitude constant
AM vs. FM
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AM requires a simple circuit, and is very easy to generate.
It is simple to tune, and is used in almost all short wave broadcasting.
The area of coverage of AM is greater than FM (longer wavelengths
(lower frequencies) are utilized-remember property of HF waves?)
However, it is quite inefficient, and is susceptible to static and other
forms of electrical noise.
The main advantage of FM is its audio quality and immunity to noise.
Most forms of static and electrical noise are naturally AM, and an FM
receiver will not respond to AM signals.
The audio quality of a FM signal increases as the frequency deviation
increases (deviation from the center frequency), which is why FM
broadcast stations use such large deviation.
The main disadvantage of FM is the larger bandwidth it requires
Amplitude Modulation
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Uses a higher frequency
carrier
Most efficient use of
frequency
Time and Frequency Domain
Susceptible to Noise
Most Efficient Use of
Frequency
• Maximum frequency required
is:
• Twice baseband
• Just baseband (special
conditions)
Higher Frequency Carrier
Signal
time
Carrier
time
Higher Frequency Carrier
Power Spectral Density
watts
Carrier
Signal
frequency
Baseband
Time Domain
Signal
time
Carrier
time
Time Domain Continued
time
Detection of Signal
time
time
Frequency Domain
Unmodulated
watts
Carrier
Signal
frequency
Baseband
watts
Modulated
Carrier
Signal
frequency
Baseband
Baseband
Spectrum of AM signal
Susceptible to Noise
time
Antenna
Carrier Signal
Oscillator
Low-Power
Amplifier
Information
Signal
Modulation
Device
Final Amplifier
Simple block diagram of
AM modulation
Single Sideband (SSB)
 Variant of AM is single sideband (SSB)
 Sends only one sideband
 Eliminates other sideband and carrier
 Advantages
 Only half the bandwidth is required
 Less power is required
 Disadvantages
 Suppressed carrier can’t be used for
synchronization purposes
Frequency Modulation
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Uses a higher frequency
carrier
Usually more bandwidth
Time and Frequency Domain
Resistant to some Noise
Time Domain
Signal
time
Carrier
time
Time Domain
time
Frequency Domain
Unmodulated
watts
Carrier
Signal
frequency
Baseband
watts
Modulated
Carrier
Signal
frequency
Resistant to Some Noise
time
Phase Modulation
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Uses a higher frequency
carrier
Fairly efficient use of
frequency
Time and Frequency Domain
Used mainly for data
Time Domain
(Instantaneous View)
Unmodulated Carrier
Modulated Carrier
Phase
Input
Phase Shift Keying
0O
90O
180O
Time Plot
270O
Pulse Modulation
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Uses the sampling rate
PAM
PDM, PWM
PPM
PCM
Starts with Sampling
Volts
time
PAM
Pulse Amplitude Modulation
Volts
time
PDM (a.k.a. PWM)
Pulse Duration Modulation (Pulse Width Modulation)
Volts
time
time
max = largest Positive
min = largest Negative
PPM
Pulse Position Modulation
Volts
time
time
max = largest Positive
min = largest Negative
Pulse Code Modulation
By quantizing the PAM pulse, original
signal is only approximated
 Leads to quantizing noise
 Signal-to-noise ratio for quantizing noise
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SNR dB  20 log 2  1.76 dB  6.02n  1.76 dB
n
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Thus, each additional bit increases SNR by
6 dB, or a factor of 4
PCM
Pulse Code Modulation
Volts
8
8
5
-1
-2
-9
time
-7
01000 01000 10010 11001 10001 00101 10111
time
Multiplexing
 Frequency
Division
Multiplexing
–Separate each baseband
signal into a discrete band
–Uses AM-SSB/SC to position
each baseband
Frequency Division Multiplexing
1
frequency
2
frequency
3
frequency
4
frequency
1
2
3
4
frequency
Multiplexing
• Time Division Multiplexing
• Separate each digital
baseband into discrete time
slots
• Cyclical in nature
Time Division Multiplexing
1
2
3
4
Rotation Analogy
FDMA
Frequency Division Multiple Access
Frequency
Chan D
Chan C
Chan B
Chan A
Time
TDMA
Frequency
Time Division Multiple Access
Chan B
Chan A
Time
CDMA
Code Division Multiple Access
Code
Summary
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Frequency Spectrum
Modulation techniques--AM,
FM, Phase, and Pulse
Modulation
Multiplexing--TDM vs FDM