oqpsk

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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

October 2014

Contents

1. Drawback of QPSK Modulation

2. OQPSK Modulation

Digital Modulation Schemes (OQPSK) 1 / 7

Drawback of QPSK Modulation

Transmitting binary sequence

11000111

11 , 00 , 01 , 11

Digital Modulation Schemes (OQPSK) 2 / 7

split

in-phase (I) component

of the signal

the quadrature (Q)

component of the signal

Signal Duration = 2Tb

Drawback of QPSK Modulation

Digital Modulation Schemes (OQPSK) 3 / 7

In-phase components for QPSKquadrature components for QPSK

11 , 00 , 01 , 11

1 , 0 , 0 , 11 , 0 , 1 , 1

Note that changes can occur only at

even multiples of Tb

both I and Q components

change simultaneouslyresulting in a

change of 180◦in the phase !

Drawback of QPSK Modulation (Application)

180◦ phase changes cause abrupt changes in the signal,

resulting in large spectral side lobes.

Digital Modulation Schemes (OQPSK) 4 / 7

In high amplifiers and for certain satellite applications,

OQPSK offers better performance

Solution (QPSK modified to became OQPSK)

Digital Modulation Schemes (OQPSK) 5 / 7

misaligned by Tb

The OQPSK signal can be written as:

Solution (QPSK modified to became OQPSK)

Company nameDigital Modulation Schemes (OQPSK) 6 / 7

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.

Solution (QPSK modified to became OQPSK)

Company nameDigital Modulation Schemes (OQPSK) 7 / 7

The absence of 180◦ phase jump is, however, offset by more

frequent ±90◦ phase shifts

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Reference:

Digital Communicationm, John G. Proakis , Masoud Salehi

Direct Download link: www.Teleinfra.net/OQPSK.pptx

www.Teleinfra.net/OQPSK.pdf

Email:

M.naslcheraghi@Teleinfra.net

M.naslcheraghi@mail.sbu.ac.ir

Thank You