How is information sent and received without wires

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How is information sent and received without wires
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A Wireless Communication System
Antenna
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The Transmitter

• Lets start from voice. How can we send ones
  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
Amplifier
Modulator
Carrier Freq Generator
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The Receiver

• At the receiver end, the antenna receives the
  rocketthe 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
Amplifier
Demodulator
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Antenna

• Antenna size is closely related to the wavelength
  ?, which is equal to the speed of light (a
  constant value) divided by the radio frequency
  being used
• ?speed of light (3x108 m/s)/frequency
• 300 kHz (AM radio), ? 3x108 / 300,000 1,000 m
• 3 GHz (3x109/s, Wireless LAN), ?0.1m10 cm
• Quarter-wave antenna ¼ ?
• Half-wave dipole ½ ?
• 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)?
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Some examples
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Parabolic Antenna Gain

• ? Antenna efficiency, 45-75 for parabolic
• D diameter
• ? wave length

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Example antenna gain

• Assume ?50, D0.6m, frequency12GHz.
• Therefore, ?3x108/12x1090.025m

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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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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), see pp.
  97.
• 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), see pp
  100.
• QPSK (pp. 101)

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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 (its called sampling).
• Convert the measurements into binary form. For
  example, 2-gt010, 3-gt011, 5-gt101, 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 signals 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.)
• 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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How can many people simultaneously use their
phones?

• A home telephone has a line. Different homes use
  different lines. They dont interfere with each
  other.
• Wireless phones share the same mediumthe 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, voice signal is modulated onto a
  specific carrier frequency for a mobile phone.
  Carrier frequencies of phones in a small area (a
  cell, as we will explain later) are different and
  sufficiently separated.
• In fact, each phone needs not just a single
  frequency but a small frequency band. For voice
  signals, that band (called a channel) is about 4
  kHz wide. Therefore a 48 kHz wide frequency band
  can accommodate 12 channels of voice.
• 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

• Different time slots are assigned to different
  MSs (mobile stations, which can be phones, PDAs,
  or computers). They may use the same carrier
  frequency but because they use different time
  slots, they dont communicate at the same time.
  Therefore they dont 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, .
• Time division and frequency division can be used
  simultaneously.
• Time Division Duplexing (TDD) downlink and
  uplink use different time slots so they dont
  interfere with each other

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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
  dont want to get cut off in the middle of
  conversation.
• Data transmission often doesnt have to be
  continuous. You can send data several times.