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CELLULAR
COMMUNICATIONS
4. Modulation
Modulation
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Radio signals can be used to carry information
 Audio,
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data, video
Information is used to modify (modulate) a single
frequency known as carrier
Modified(modulated) signal is transmitted to
receiver
At the receiver the information is removed from the
radio signal
Information is reconstructed into original format
through in a process of demodulation
Some key points
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Spectrum is scarce
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Spectrum is scarce natural resource.
There is only limited range of wavelength that can be used for
communications
Regulated by government (FCC)
Modulation techniques should make effective use of spectrum, i.e.
transmit as much as possible information using given amount of spectrum
Efficient use of energy
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Mobile devices has limited battery
Transmitting unnecessary energy on a radio carrier may interfere with
other transmitters
Reliably Transmit information with minimal possible amount of energy
Radio Carrier
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Single alternated waveform.
If carries no information appears at receiver:
Amplitude Modulation(AM)
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Change amplitude of the signal according to
information
Simplest digital form is “on-off keying”(telegraph
Morse code)
Amplitude Modulation
Fully modulated signal
AM efficiency
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Carrier:
w=2f
Message: m(t), Signal y(t)=m(t)*c(t)
Let consider highest frequency in a message wc and
its maximum/minimum amplitude M
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Modulated Signal:
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After some trigonometry:
AM Energy usage
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Fully modulated A=2M
Energy at carrier and one of sideband is wasted
33% of the transmitted energy carries information
Audio AM
Frequency Modulation
FM efficiency
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Modulation index (max
change in carrier
frequency due to
modulation): M
Bandwidth of FM signal
is BW = 2 (M + 1 ) fm
fm maximum modulating
frequency used
Energy efficiency
increased by increasing
bandwidth
AM vs FM
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FM is more resilient to noise
FM: signal level variation does not affect quality
provided the signal is strong enough to recover its
frequency
Used for 1G analogue mobile phone systems
Digital Version of FM
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Frequency Shift Keying (FSK)
Phase Modulation
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Another form of FM
Binary Phase Shift Keying (BPSK)
Quadrature Phase Shift Keying(QPSK)
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BPSK, 180% change in phase represent change in
bit
QPSK 90% change in phase represent change in 2
bit sequence
Quadrature Amplitude Modulation
16-QAM
Circular 16-QAM
Other QAMs
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HSPA+ (aka high speed GSM+) is 64QAM
HDTV is 256QAM
ADSL 16/64 QAM
Spread Spectrum Techniques
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Conserve spectrum by keeping transmission as narrow
as possible
Sometimes it’s beneficial to spread transmission over
wide frequency range (spread spectrum)
Fading and noise might be different for different
frequencies
 Spreading over wide range of frequencies will help to
reduce errors/signal noise
 Spreading power over many frequencies result in very low
power transmission at each frequency
 Reduce interference to other transmitter , single frequency
transmission appears as a noise
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Spread Spectrum
F
Normal Signal
F
Signal with Spread Spectrum
Frequency Hopping
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Transmitter sends a signal at each frequency during
very short period of time
Transmit next piece of data on other frequency
Hop hundreds of time per second between different
frequencies
To receive the signal, receiver must be able to
follow the hop sequence of the transmitter
Both receiver and transmitter must know hop
sequence and be synchronized in time
Frequency Hopping
Adaptive Frequency Hopping
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Don’t transmit on a bad frequencies/channels
Measure error rate on each channel
Spread Spectrum Illustration
Transmit 3 Pictures to 3 Destinations
XOR each image with a mask
Result
Add Them Up and send to all dest.
Each recipient decodes using his mask
Direct Sequence Spread Spectrum
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AM/FM transmit around single carrier
Frequency Hopping transmit at wide range of carriers
but one carrier at the time
DSSS transmit at wide range of carriers simultaneously
Very low power at each carrier
 Appears as a noise at each carrier
 Transmission across carriers is “synchronized” so signal can
be recovered
 Several transmissions on the same set of carriers(spectrum)
as looks as noise for each other
 Different transmissions use different “synchronization”
methods/codes
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White Noise
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Completely random signal, alternates widely
Spectrum of white noise
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Same average power at each frequency
Filtered (Bandlimited) Noise
How to make a carrier to look like
band limited noise?
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Make it look randomly alternating
Modulate it with randomly alternating signal
(analog) or bits (digital)
Represent data that we want to transmit with a
longer sequence of bits that “looks like random”
(pseudo-random)
Use less time to modulate each bit (e.g. BPSK)
Transmit modulate rapidly alternating signal
 Same
total energy
 Speeded over wide ranges of frequencies
Example :DSSS with PN
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Transmitter/Receiver should be able to generate same
synchronized Pseudo Random Noise sequences
DSSS-PN Receiver/Transmitter
Spreading
PN Sequences
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PN generator produces periodic sequence that
appears to be random
PN Sequences
Generated by an algorithm using initial seed
 Sequence isn’t statistically random but will pass many test
of randomness
 Sequences referred to as pseudorandom numbers or
pseudonoise sequences
 Unless algorithm and seed are known, the sequence is
impractical to predict
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Some Properties of PN sequences
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Balance property
 The
number of "1"s in the sequence is one greater than
the number of "0"s.
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Run property: Of all the "runs" in the sequence of
each type (i.e. runs consisting of "1"s and runs
consisting of "0"s):
 One
half of the runs are of length 1.
 One quarter of the runs are of length 2.
 One eighth of the runs are of length 3.
 ... etc. ...
Autocorrelation property
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Autocorrelation is large when signal/mask perfectly synchronized
Synchronization between rx/tx
Hopefully does not give a large peak when there is no signal
Orthogonal Sequences
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Cross correlation: same as autocorrelation but
among different sequences
Several different sequences with zero crosscorrelation between them allow several
transmissions at the same channel (“range of
carriers”)
 Base
for Code Division Multiple Access method (CDMA)
 3G/UMTS use version of CDMA(WCDMA)
 Will talk about it later
Orthogonal Frequency Division Multiplex(OFDM)
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OFDM/COFDM Used in
 WiFi
(802.11)
 ADSL
 WiMax
 4G
 More
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Provide very high data rates (e.g. up to 150Mbps
802.11n)
Multichannel Communications
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Transmit bits in parallel using several carriers (frequencies)
Transmission over each carriers take certain amount of
bandwidth around this carrier
Carriers need to be separated from each other to avoid
interference
Relatively small amounts of parallel transmissions can be
fitted in a given spectrum
OFDM
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Select orthogonal carriers
Reach maximum at different times
Can pack close without much interference
More carriers within the same bandwidth
More on OFDM