ECE 4710: Lecture #28 1 OQPSK & /4 DQPSK Offset Quadrature Phase Shift Keying OQPSK /4...
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Transcript of ECE 4710: Lecture #28 1 OQPSK & /4 DQPSK Offset Quadrature Phase Shift Keying OQPSK /4...
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