This document discusses offset quadrature phase-shift keying (OQPSK) modulation. It begins by explaining the drawback of traditional quadrature phase-shift keying (QPSK), which is that both the in-phase and quadrature components change simultaneously, resulting in abrupt 180-degree phase changes and large spectral side lobes. It then introduces OQPSK modulation as a modification of QPSK where the quadrature component is offset by one bit period, preventing simultaneous changes in both components and replacing the 180-degree phase jumps with multiple 90-degree phase shifts. This results in better performance for applications using high power amplifiers or satellites. Diagrams are provided to illustrate the difference between the phase transitions of QPSK and O

1. Digital Modulation schemes
OQPSK
Offset Quadrature Phase-shift keying
Shahid Beheshti Univ. of Tehran
Faculty of electrical and computer engineering
Department of Electrical Engineering
Instructor: M. Naslcheraghi
Assistant Professor: Dr. Sadough
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October 2014

2. Contents
1. Drawback of QPSK Modulation
2. OQPSK Modulation
Digital Modulation Schemes (OQPSK) 1 / 7

3. Drawback of QPSK Modulation
Transmitting binary sequence
11000111
split
11 , 00 , 01 , 11
in-phase (I) component
of the signal
the quadrature (Q)
component of the signal
Signal Duration = 2Tb
Digital Modulation Schemes (OQPSK) 2 / 7

4. Drawback of QPSK Modulation
1 , 0 , 0 , 1
In-phase components for QPSK
1 , 0 , 1 , 1
quadrature components for QPSK
11 , 00 , 01 , 11
Note that changes
can occur only at
even multiples of Tb
both I and Q
components
change
simultaneously
resulting in a
change of 180◦
in the phase !
Digital Modulation Schemes (OQPSK) 3 / 7

5. Drawback of QPSK Modulation (Application)
180◦ phase changes cause abrupt changes in the signal,
resulting in large spectral side lobes.
In high amplifiers and for certain satellite applications,
OQPSK offers better performance
Digital Modulation Schemes (OQPSK) 4 / 7

6. Solution (QPSK modified to became OQPSK)
misaligned by Tb
The OQPSK signal can be written as:
Digital Modulation Schemes (OQPSK) 5 / 7

7. Solution (QPSK modified to became OQPSK)
A possible mapping for QPSK signal.
Possible phase transitions in QPSK signaling.
Phase transition diagram for OQPSK signaling.
Results:
1. prevents both components changing at the
same time
2. In constellation-speak, if the signal wave in the
right upper quadrant, the next signal can only
go to either the lower right quadrant or to upper left
quadrant but not across.
But in QPSK all transitions can occur.
Digital Modulation Schemes (OQPSK) Company nam6e / 7

8. Solution (QPSK modified to became OQPSK)
The absence of 180◦ phase jump is, however, offset by more
frequent ±90◦ phase shifts
Digital Modulation Schemes (OQPSK) Company nam7e / 7

9. Thank You
Reference:
Digital Communicationm, John G. Proakis , Masoud Salehi
Direct Download link: www.Teleinfra.net/OQPSK.pptx
www.Teleinfra.net/OQPSK.pdf
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Email:
M.naslcheraghi@Teleinfra.net
M.naslcheraghi@mail.sbu.ac.ir
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