Wireless Communications

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Wireless Communications
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Wireless Communications

• Wireless is more and more widely deployed
  cellphone, wireless LAN,
• The fundamental fact is that if the sender sends
  a sine wave, the receiver will receive a sine
  wave at the same frequency. But with
• A different phase
• A new amplitude
• How do you design communication schemes based on
  that?

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Wireless Communications

• AM stronger signal when 1, weaker signal when
  0.
• FM faster waveform when 1, slower signal when
  0.
• BPSK 0 degree when 0, 180 degree when 1.

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Basic Wireless System

• The very basic wireless communication system
• Sender given the bit stream, convert it to the
  baseband waveform by a Low Pass Filter, then
  multiply with the carrier waveform (2.4GHz if
  802.11g, 1.8GHz if some cellphones), and send.
• Receiver given the signal received from the
  antenna, multiply it with a locally generated
  carrier, send it to the low pass filter,
  regenerate the baseband waveform.

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Basic Wireless System

• The sender cannot send the square waveform but
  has to send I(t) which is bandwidth limited from
  0Hz to some cutoff frequency BHz. The signal in
  the air in this simple system occupies frequency
  range f-B,fB.
• Because cos(2pf1t)cos(2pft) cos2p(f1tf)t
  cos2p(f1-f)t (constant dropped)
• The f-B,fB must be within the frequency band
  allocated for this system (around 20MHz in
  802.11g, around 25MHz in GSM) because the medium
  is shared

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Basic Wireless System

• How can the receiver regenerate the baseband
  waveform? A simplified explanation
• The sender sends I(t)cos(2pft).
• Assume there is no phase difference, the receiver
  multiplies I(t)cos(2pft) with cos(2pft), and gets
  
• I(t)cos2(2pft) I(t)1/2 1/2cos(4pft).
• Then, after the low pass filter, what is left is
  the low frequency component I(t).
• http//math2.org/math/trig/identities.htm

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Two Orthogonal Channels

• The sender sends I(t)cos(2pft) Q(t)sin(2pft).
• The receiver multiplies the received signal with
  cos(2pft) , and will get
• I(t)cos(2pft) Q(t)sin(2pft) cos(2pft)
  I(t)1cos(4pft) Q(t) sin(4pft)
• (constant dropped), and after the LPF, will have
  I(t).
• At the same time, the receiver also multiplies
  the received signal with sin(2pft) , and will get
  
• I(t)cos(2pft) Q(t)sin(2pft) sin(2pft) I(t)
  sin(4pft) Q(t) 1-cos(4pft)
• (constant dropped), and after the LPF, will have
  Q(t).
• So, the sender can send TWO baseband waveforms at
  the same time. We often use a complex number to
  represent the symbol, where I(t) is the real part
  and Q(t) is the imaginary part.

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BPSK, QPSK, QAM

• BPSK is using only one channel. And in this
  channel, only two possible voltages.
• Quadrature phase-shift keying (QPSK) is using
  both channels. In each channel, only two
  voltages.
• Quadrature amplitude modulation (QAM) is using
  both channels. In each channel, multiple
  voltages. If 4 levels of voltage, it is 16QAM. If
  it 8 levels of voltage, 64QAM.

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Modulation/Demodulation

• Modulation is the process of turning the bits
  into the baseband waveforms.
• Demodulation is the reverse.