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40962098 Phase Shift Keying

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    ECE 513 - Digital and DataCommunications 1Prepared by: Engr. Jan Ray C. Rulida

    Holy Name University. Copyright 2010

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    is a modified form of QPSK wherethe bit waveforms on the I and Q

    channels are offset or shifted inphase from each other by one-half of a bit time

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    O ne-half bit delay

    Balancedmodulator

    + 90

    Balancedmodulator

    Bandpassfilter

    Linearsummer

    Bandpassfilter

    BPF

    Referencecarrier

    oscillator(sin ct)

    sin ct

    cos ct

    I-Channelinput data

    Q-Channelinput data

    O QPSK output

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    Because changes in the I channel occurat the midpoints of the Q channel bits and

    vice versa, there is never more than asingle bit change in the dibit code and,therefore, there is never more than 90shift in the output phase.

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    A n advantage of O QPSK is the limitedphase shift that must be imparted

    during modulation. A disadvantage of O QPSK is thatchanges in the output phase occur at

    twice the data rate in either the I or Qchannels.

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    W ith O QPSK, the baud and minimumbandwidth are twice that of

    conventional QPSK for a giventransmission bit rate.Sometimes called O KQPSK (offset-

    keyed QPSK)

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    three bits are encoded, forming tribits there are eight possible conditions

    (000, 001, 010, 011, 100, 101, 110, and111), therefore producing eightdifferent output phases

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    N=3 and M =8To encode eight different phases, the

    incoming bits are encoded in groupsof three, called tribits (2 3 = 8)

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    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    2-to-4-levelconverter

    Q I C

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    8-PSK Phasor and Constellation diagrams

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    Binary Input O utput

    PhaseQ I C

    0 0 0 -112.5

    0 0 1 -157.5

    0 1 0 -67.5

    0 1 1 -22.5

    1 0 0 112.51 0 1 157.5

    1 1 0 67.5

    1 1 1 22.5

    8-PSK truth table

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    it can be seen that the angularseparation between any two

    adjacent phasors is 45 (half of whatit is with QPSK)an 8-PSK signal can undergo almost

    a 22.5 phase shift duringtransmission and still retain itsintegrity

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    B andwidth Considerations:

    The bitrate in the I, Q, or C channel is equalto one-third of the binary input data rate( f b / 3)

    B andwidth: B = =

    where: f N = double-sided Nyquist bandwidthf b

    = input bit rate

    b f N 3b f

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    M -ary Encoding (for M =8)N = log 2 M

    N = 3 (for M = 8)

    where: N = number of input bitsM = number of output conditions

    possible with N bits

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    the tribit code between any twoadjacent phases changes by only

    one bit; this type of code is calledthe G ray code or, sometimes, the

    maximum distance code .

    G ray code is used to reduce thenumber of transmission errors.

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