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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.