Thursday, October 30, 2008

Lecture 9

Phase Shift Keying


  • Baseband transmission – the transmission of digital information over a low pass / low frequency channel.
  • Use nyquist for low pass channel
    • Represent binary information with a polar NRZ
    • Multiply X1(t) with sine wave generator
    • Which transforms


  • To demodulate
    • Multiply PSK signal with sine wave generator
    • Output can be expressed as

  • Filter out the high frequency component
  • Ak is retrieved
  • Trig identities
    • 2cos2 (x)= 1 + cos(2x)
    • 2sin2(x)= 1 – sin(2x)
    • 2cos(x)sin(x) = 0 + sin(2x)
  • After demodulation of this example we still only end up with W
  • Our goal was 2W (nyquist)
  • To fix this we put ASK and PSK together
  • Send two signals simultaneously on same carrier
  • F1 = f
  • F2 = f + π/2 (shifted 90 degrees)
  • To do this
    • Take the bitstream, split into two sequences f odd and even numbered bits
    • Determine the bipolar NRZ Bk for even and Ak for odd
    • Multiply Ak with cosine wave and Bk with sin wave: modulate
  • To demodulate see slides, involved multiplying whole signal by two trig identities which will give you Ak and Bk independently

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