Transcript Document

How is information sent and
received without wires
A Wireless Communication System
Antenna
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Analog and digital signals
Voice signal from a microphone is an
analog signal, which changes
continuously.
A digital signal only has two states, say,
low voltage, and high voltage,
representing zero and one.
An analog signal can be converted into a
digital signal, or vice versa.
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How to convert an analog signal to a
digital one?
Measure the amplitude of the signal at
regular intervals (it’s called sampling).
Convert the measurements into binary
form. For example, 2->010, 3->011, 5>101, and so forth.
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Two representations of a signal
 A signal can be viewed in time domain or in frequency
domain. The two views are the two representations of
the same signal.
 Time domain: a signal’s amplitude (strength) changes
over time, therefore a time series graph can be used to
characterize a signal
 Frequency domain: a signal occupies a frequency band
 The faster a signal changes (higher data rate) the wider
its frequency band. The magnitudes of the two are
similar. For example, if the data speed is several Mbps
(Mega-Bits Per Second), then it will occupy a frequency
band of several MHz (Mega-Hertz) wide.
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View from the Frequency domain
 As an example, human voice occupies a
frequency band roughly from 300 hertz to 3400
hertz. (A music piece has much wider frequency
band.)
 A TV signal would occupy a lot wider frequency
band, typically several mega hertz.
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The Transmitter
Let’s start from voice. How can we send one’s
voice thousands of miles away? We first need a
transmitter.
 Voice (the “goods”) is transformed into an
electrical signal (the “package”).
 This signal is carried by a high frequency
electrical current (the “rocket”).
 The antenna (the “launcher”) sends the high
frequency current out in the form of radio wave.
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How Does Transmitter work?
The microphone transforms the voice into
an electrical signal.
The modulator “loads” the voice signal
onto a high frequency electrical current.
The amplifier magnifies the high frequency
current and sends it to the antenna.
The antenna, driven by the current, emits
radio wave to the space.
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Diagram of A Transmitter
Amplifier
Modulator
Amplifier
Carrier Freq
Generator
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The Receiver
At the receiver end, the antenna receives
the “rocket”—the high frequency radio
wave.
The receiver separates (“unloads”) the
electrical voice signal from the “rocket.”
The receiver transforms (“unpacks”) the
electrical signal into voice.
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How does the receiver work?
The antenna receives the radio wave sent
by the transmitter.
The demodulator “unload” the voice
signal.
The speaker turns the voice signal back to
voice.
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Diagram of A Receiver
Amplifier
Demodulator
Amplifier
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Antenna
 Antenna size is closely related to the wavelength
λ, which is equal to the speed of light (a constant
value) divided by the radio frequency being
used:
λ=speed of light (3x108 m/s)/frequency
300 kHz (AM radio), λ= 3x108 / 300,000 = 1,000 m
3 GHz (3x109/s, Wireless LAN), λ=0.1m=10 cm
 Quarter-wave antenna: ¼ λ
 Half-wave dipole: ½ λ
 Parabolic reflective antenna
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Antenna Gains
 Omnidirectional (isotropic) antennas and
directional antennas
 Antenna gain is defined as the power output in a
particular direction compared to that produced in
any direction by an isotropic antenna. For
example, antenna gain of 3 dB in a particular
direction means an improvement over an
isotropic antenna by 3 dB, or a factor of 2.
 The increased power radiated in a given
direction is at the expense of other directions.
Antenna gain does not mean obtaining more
output power.
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What is dB (decibel)?
Ppar
Power of parabolic
dB  10 log(
)  10 log
Power of isotropic
Piso
P
dBm  10 log
1m W
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Some examples
1 tim e is 10 log(1)  0 dB
2 tim es is 10 log(2)  3 dB
10 tim es is 10 log(10)  10 dB
100 tim es is 10 log(100)  20 dB
1000tim es is 10 log(1000)  30 dB
1 / 10 is 10 log(1 / 10)  10 dB
1 / 100 is 10 log(1 / 100)  20 dB
P
dBm  10 log
1m W
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Parabolic Antenna Gain
D 2
Gain   ( )

η: Antenna efficiency, 45%-75% for parabolic
D: diameter
λ: wave length
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Example: antenna gain
Assume η=50%, D=0.6m,
frequency=12GHz.
Therefore, λ=3x108/12x109=0.025m
D 2
3.14 * 0.6 2
G   ( )  0.5(
)  2840

0.025
10log(2840)  34.5dB
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The Modulator
The modulator “loads” the voice signal onto
the high frequency current.
There are several ways to load the voice
signal:
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
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Why modulation?
 While voice signal can be sent out through wire
in a wired communication system, its frequency
(300 Hz to 4kHz) is too low to be sent out by
antennas in a wireless communication system.
 Only certain frequencies assigned by FCC, say,
a frequency band around 1.8 GHz, can be used.
Therefore you must use modulation to bring the
frequency inside that frequency band.
 Often some sort of frequency division has to be
used to separate users (phones). Thus each
phone may use a specific carrier frequency in its
modulator.
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Amplitude Modulation
Voice signal controls the amplitude of the
high frequency current (called the
carrier)—the amplitude of the carrier
changes proportionally to the strength of
the voice signal.
As a result, the voice signal becomes the
“envelope” of the carrier.
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View from the Frequency domain
 As an example, human voice occupies a
frequency band roughly from 300 hertz to 3400
hertz. (A music piece has much wider frequency
band.)
 The carrier frequency is always much higher,
say, 100kHz.
 After modulation, the carrier carrying the voice
signal may (depending on the modulation
method) occupy a band of 100,000 Hz to
103,400 Hz.
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Frequency Modulation
The voice signal controls the frequency of
the carrier. That is, the frequency of the
latter changes proportionally to the
strength of the voice signal.
The amplitude is always constant.
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Example of frequency modulation
 Digital FM (FSK, frequency shift keying).
 The difference between the two frequencies
used is an important parameter. If it is too small,
it will be difficult to differentiate them. If it is too
large, the bandwidth will be too wide.
 The minimum is 1/2T where T is the duration of
the transmitted data symbols. This is called the
Minimum Shift Keying (MSK).
 The most popular one is Gaussian MSK
(GMSK).
 The bandwidth efficiency
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Phase Modulation
The voice signal controls the phase of the
carrier. That is, its phase changes as the
voice signal varies (often proportionally).
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Example of phase modulation
Binary digital phase modulation (BPSK).
QPSK
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How can many people simultaneously
use their phones?
A home telephone has a line. Different
homes use different lines. They don’t
interfere with each other.
Wireless phones share the same
medium—the air. A phone can receive all
the signals send to other phones which
are located close enough. Therefore there
has to be a way to separate the signals.
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Ways to separate signals
Frequency division
Time division
Code division
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Frequency Division
 Each phone uses a specific frequency that is
different from the frequencies used by other
phones.
 The transmitter of a base station sends signals
to mobile phones using different frequencies.
Each phone has a pre-assigned frequency. Only
the signal sent at that particular frequency can
be received by that phone.
 A phone also is assigned a unique frequency for
sending signal back to the base station.
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Frequency Division (contd.)
 Specifically, a signal (voice, data, ...) is modulated onto a
specific carrier frequency. Carrier frequencies of mobile
devices in a small area (e.g. a cell, as we will explain later)
are different and sufficiently separated.
 In fact, each mobile device needs not just a single frequency
but a small frequency band. For voice signals, that band
(called a channel) can be 4 kHz wide. Therefore a 48 kHz
wide frequency band can accommodate 12 channels of voice.
(We will see later that bandwidths are different for different
systems.)
 Frequency Division Duplexing (FDD): forward (from a base
station to phones) and backward (from phones to a base
station) links in cell phone system use different frequencies.
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Time Division used in 2G cellular
network
 Different time slots are assigned to different MSs (mobile stations,
which can be phones or computers). They may use the same carrier
frequency but because they use different time slots, they don’t
communicate at the same time. Therefore they don’t interfere with
each other.
 Typically 3 to 8 mobile stations will be given different time slots but
the same carrier frequency.
 An example: If three mobile stations share the same frequency, then
each will be given a time slot and they take turns to transmit or
receive signals. The sequence will be 1, 2, 3, 1, 2, 3, 1, …. This is
the case for the 2G cell phones.
 Time division and frequency division can be used simultaneously.
 Time Division Duplexing (TDD): downlink and uplink use different
time slots so they don’t interfere with each other
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Time Division used in 4G cellular
systems
Time slots can be dynamically assigned to
MSs based on their needs, e.g. a device is
viewing a real time video may be given
more time slots.
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Time Division used in WiFi
Mobile stations needing data transmission
wait in a queue. At a given time only one
MS can transmit.
The management of the queue is
decentralized based on CSMA/CA, which
will be explained in detail when we discuss
WiFi.
It is similar to the CSMA/CD mechanism
used in an Ethernet network.
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Code Division
Each mobile station is given a unique
code.
Signals sent by a transmitter are coded.
An MS can only receive the signal that is
coded with its unique code.
An MS sends back signal using its
assigned code.
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Difference between voice and data
Voice (conversation) has to be continuous.
You don’t want to get cut off in the middle
of conversation.
Data transmission often doesn’t have to
be continuous. You can send data several
times.
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