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8/3/2019 DonadioOQPSK

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OQPSK MODULATION

(Offset Quadrature Phase-shift keying)

MARCO DONADIORF COMMUNICARION SYSTEM ON CHIP

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INDEX

Introduction to digital modulation

Review of PSK modulation

QPSK modulation

Variation on the QPSK : Offset QPSK


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In digital RF system, the carrer is modulated by a digital

baseband signal. Digital modulation offers many advantages

over modulation and is widely used in wireless systems.

Basic concepts of digitalmodulation

Carrier signal: Ac cos (2fct + )

Modulation: m(t)

Modulated signal: Ac (t) cos (2fc(t) t + (t))

Vary Vary Vary

amplitude frequency phase

m(t); discrete

Variations are discrete!!!!!

Binary

OOK BPSK

DPSK

FSK

Multilevel QPSK

MPSK

QAM

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Principle

1 0 1 01 1

0 0

Analog

message

Digital

code

Digital message

Sinusoidal carrier

modulate

AM

FM

PM

AM &

PM

1 0 1 0


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Digital data to analog signal --- Digital Modulation

Amplitude-shift keying (ASK)

Amplitude difference of carrier frequency

Frequency-shift keying (FSK)

Frequency difference near carrier frequency Phase-shift keying (PSK)

Phase of carrier signal shifted

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Signal Vector Representation

PhaseM

agnit

ude

S

0 degreesI

Q

I-Q

Plane

s(t) = Ac(t) cos (2fct + (t))

fixed!!!fixed!!!

Modulated signal


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Signal Changes:

Representation in the IQ Plane

S1

S2I

Q

Magnitude

Change

S1

S2

I

Q

Phase

Change

S1

S2

I

Q

Magnitude

& Phase

Change

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This is a linear modulation scheme in wich the source symbols

shift the phase of a carriersignal.

Binary PSK = the binary baseband data selects oneof two opposite phases of the carrier.

Phase Shift Keying (PSK)

( )

=ts( )tfA c2cos

( ) +tfA c2cos

1binary

0binary

= ( )tfA c2cos

( )tfA c2cos


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EXAMPLE of a BPSK modulation

We need to transmit the sequence 0111 0101 0010 1011 using BPSK

modulation, we will have the following modulated signal:

At every bit transition the signal does a 180 degree phase shift!!!

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Phase-Shift Keying (PSK)

Four-level PSK (QPSK) = each elementrepresents more than one bit.

( )

=ts

+ 42cos

tfA c 11

+4

32cos

tfA c

4

32cos

tfA c

4

2cos

tfA c

01

00

10


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QPSK Modulator

0 1 1 1 0 0 1 0

0 1 0 1

1 1 0 0

Serial-to-parallel converter separates odd en even bits, each group of two

bits (one in each arm) constitutes a symbol; then the symbol rate is half thebit rate indicating that the required bandwidth is half that of BPSK

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Drawback of QPSK

In QPSK the carrier phase changes only once every 2Ts seconds;

when only one of the two quadrature components, in-phase (I) or in

quadrature (Q), changes its sign, a phase shift of90 occurs. A

change in both components generate a phase shift of180. These

phase jump in the ideal case is instantaneous but in the real case

produce a shifting in a nonzero time and cause the envelope toapproach zero (due to filtering effect).


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The brutal change of phase in QPSK result a variation of

envelope due also to the filter respons that limit the bandwidth.

Such variation require linear PA to avoid spectral regrowth.

Linear PA are less efficient than non-linear PA (for 40%

efficiency it consumes 2.5W).

Solution = OQPSK

Offset QPSK (OQPSK)

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OQPSK modulationThe data stream are offset in time by half the symbol period after

S/P conversion, avoiding simultaneous transitions in waveforms at

node A and B. The phase can change only for 90 every Ts.


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QPSK vs OQPSK

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OQPSK Offset Quadrature Phase ShiftKeying

Q channel signal offset from I channel signal

Delayed in time by Ts Eliminates 180 phase transitions

No transitions thru origin !!

Amplitude variations less than QPSK & much less than

BPSK

Non-linear Tx amplifiers can be used to achieve

better utilization of battery power in mobile unit

Retains same BW efficiency as QPSK

Very popular modulation method forreverse-link in

wireless systems