ECE 4710: Lecture #28 1

OQPSK & /4 DQPSK

Offset Quadrature Phase Shift Keying OQPSK /4 Differential QPSK /4 DQPSK Both are :

Variations of QPSK Seek to minimize amplitude modulation of QPSK

» Envelope is more constant when pulse shaping filters are used and this improves spectral efficiency when non-linear amps are utilized

Allow non-linear Class C amplifiers to be used to preserve DC battery supplies in mobile units (cell phone)

Very important modulation methods for wireless mobile radio applications

ECE 4710: Lecture #28 2

QPSK

QPSK if baseband m(t) is rectangular pulse then envelope of RF signal is constant

QPSK signal constellation

Signal points located on

circle of constant radius = Ac Instantaneous () change

from one signal point to next“00”“11”

I

Q)(tg

cAcA

“01”

“10”

cjA

cjA

ECE 4710: Lecture #28 3

AM QPSK

Pulse shaping creates time-varying QPSK amplitude Amplitude goes to zero for 180° bit transitions

causing signal to pass thru origin of

constellation diagram

90° transitions cause amplitude

to stay constant

Necessary to minimize

signal BW“00”“11”

I

Q)(tg

“01”

“10”

AM!!

ECE 4710: Lecture #28 4

nn

nn DD

nthytyDnthxtx

21

)( & )( 11

OQPSK

Bandpass OQPSK signal

I and Q time domain waveforms

)()()()()( where

)2sin()()2cos()()(tj

cc

etRtyjtxtg

tftytftxts

amplitudes IQ permitted,

rate symbol

shape pulse symbol)( where 1

nn yx

RD

thNote that Q waveform is

shifted by Ts / 2

relative to I waveform

“Offset” QPSK

ECE 4710: Lecture #28 5

QPSK vs. OQPSK

“00”“11”I

Q)(tg

“01”

“10”

No 180° Phase

Transitions

Input : 100110110100 I : 101100Q : 010110

I

Q

Q OFFSET

ECE 4710: Lecture #28 6

QPSK vs. OQPSK

“00”“11”I

Q)(tg

“01”

“10”

AM!!

“00”“11”I

Q)(tg

“01”

“10”

Pulse-Shaped QPSK Pulse-Shaped OQPSK

No 180° Phase

Transitions

ECE 4710: Lecture #28 7

OQPSK

Offset Q waveform by Ts / 2 Only 90° phase transitions can occur in signal constellation 180° phase transitions eliminated Offset QPSK also called “Staggered QPSK” SQPSK

AM on OQPSK is greatly reduced compared to pulse-shaped QPSK Non-linear Class C amplifiers used on pulse-shaped OQPSK without causing

significant regeneration of spectral sidelobes

Pulse-shaped OQPSK advantages: M = 4 multi-level signaling reduced signal BW Pulse-shaping reduced signal BW Small amount of AM Class C amps preserve battery life of mobile units

(cell phones) in wireless applications

ECE 4710: Lecture #28 8

/4 DQPSK

/4 Differential QPSK /4 DQPSK Created by alternating between two QPSK signal

constellations rotated by /4 = 45° wrt each other Given a point on one constellation next two bits in data

stream determine next signal state on other constellation Two new data bits cause phase shift of ±45° or ±135° Example: Data

Differential encoding since data represented

by phase change & not absolute value of

signal phase

ECE 4710: Lecture #28 9

/4 DQPSK Constellations

I

Q)(tg

Start

I

Q)(tg

“10” = -45°

Constellation #1 Constellation #2

“00” = +135°

“11” = +45°

ECE 4710: Lecture #28 10

/4 DQPSK Constellation

Combined Constellations

I

Q

Start“10” = 45°

“00” = +135°

“11” = +45°

#2 Constellation

#1 Constellation

4

ECE 4710: Lecture #28 11

/4 DQPSK

AM on pulse-shaped /4 DQPSK is also reduced compared to pulse-shaped QPSK ±45° and ±135° phase transitions have less amplitude

modulation compared to ±180° phase transitions on QPSK AM is larger on /4 DQPSK compared to OQPSK since

OQPSK has only ±90° transitions Use of non-coherent Rx is advantage of /4 DQPSK

compared to OQPSK» Simple & cheap Rx implemented» Good for manufacturing mobile units at low cost

ECE 4710: Lecture #28 12

/4 DQPSK Constellation

Combined Constellations

I

Q

#2 Constellation

#1 Constellation

Possible Transitions

No Phase Transitions thru Origin AM is

minimized

±135° Phase Transitions have more

AM than ±90° OQPSK Phase

Transitions

ECE 4710: Lecture #28 13

QPSK and OQPSK require absolute measure of Rx signal phase for data detection Product detector (mixer) required for coherent detection

to measure absolute signal phase states /4 DQPSK requires measure of phase shift

between sequential symbols Non-coherent detection possible FM detector + integrator with bit synchronization

Coherent detection also possible if desired» 3 dB S/N performance increase over non-coherent detection

Detection

/4 DQPSK FM Detector dtd

Decode10 01 11dt

df

ECE 4710: Lecture #28 14

Wireless Communications

OQPSK & /4 DQPSK widely used in wireless communications applications which require: Good spectral efficiency

» Wireless spectrums are expensive ($$)» Available BW must be used efficiently to support large number of

users» Multi-level signaling + pulse-shaping is needed

Long battery life in mobile units» Class C amplifiers with 80-90% DC to RF efficiency» AM minimized on pulse-shaped OQPSK & /4 DQPSK

Non-linear amps used without causing regeneration of spectral sidelobes which would reduce spectral efficiency

ECE 4710: Lecture #28 15

Wireless Communications

OQPSK was used by IS-95 CDMA cellular standard Verizon Wireless Sprint PCS Reverse link (mobile unit to base) modulation only

» Conserve battery life in mobile unit

QPSK used on forward link (base to mobile) /4 DQPSK was used by IS-136 TDMA cellular

standard until 2002 ATT Wireless Cingular Wireless Both forward and reverse links

ECE 4710: Lecture #28 16

MPSK & QAM PSD’s

For rectangular pulse shapes the PSD of baseband complex envelope, g(t), for BPSK, DBPSK, MPSK, QAM, QPSK, OQPSK, & /4 DQPSK all have the same functional (sin x / x)2 form

2sin

)(

b

bg Tf

TfKf

P

levels # 2 M

in Wattspower Tx is where2 PTPK b

Rate Data 1 bT

R

ECE 4710: Lecture #28 17

PSD for Rectangular Pulse

Spectral Sidelobes

MPSK & QAM PSD

Baseband FNBW

R

RF Null-to-NullTransmission BW

R2TB

Spectral Efficiency

2

TBR

Note: This spectral efficiency is for N-to-N BW only