1. OQPSK & p/4 DQPSK Offset Quadrature Phase Shift Keying  OQPSK
/4 Differential QPSK  p/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
“01”
“10”
ECE 4710: Lecture #28

3. AM QPSK Q I Pulse shaping creates time-varying QPSK amplitude AM!!
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
“01”
“10”
AM!!
ECE 4710: Lecture #28

4. OQPSK Bandpass OQPSK signal I and Q time domain waveforms
Note that Q waveform is shifted by Ts / 2 relative to I waveform  “Offset” QPSK
ECE 4710: Lecture #28

5. QPSK vs. OQPSK Q I ECE 4710: Lecture #28 “01” “00” “11” “10”
Input :
I :
Q : Q
“01”
No 180° Phase Transitions I I Q
“11”
“00”
Q OFFSET
“10”
ECE 4710: Lecture #28

6. QPSK vs. OQPSK Q Q I I AM!! ECE 4710: Lecture #28 Pulse-Shaped QPSK
Pulse-Shaped OQPSK Q Q
“01”
AM!!
“01”
No 180° Phase Transitions I I
“11”
“00”
“11”
“00”
“10”
“10”
ECE 4710: Lecture #28

7. OQPSK ECE 4710: Lecture #28 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. p/4 DQPSK /4 Differential QPSK  p/4 DQPSK ECE 4710: Lecture #28
Created by alternating between two QPSK signal constellations rotated by p/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 Dq ° ° ° °
Differential encoding since data represented by phase change & not absolute value of signal phase
ECE 4710: Lecture #28

9. p/4 DQPSK Constellations
“10” = -45°
Start
“00” = +135° I I
“11” = +45°
ECE 4710: Lecture #28

10. p/4 DQPSK Constellation
Combined Constellations I Q
Start
“10” = - 45°
“00” = +135°
“11” = +45°
#2 Constellation
#1 Constellation
ECE 4710: Lecture #28

11. p/4 DQPSK
AM on pulse-shaped p/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 p/4 DQPSK compared to OQPSK since OQPSK has only ±90° transitions
Use of non-coherent Rx is advantage of p/4 DQPSK compared to OQPSK
Simple & cheap Rx implemented
Good for manufacturing mobile units at low cost
ECE 4710: Lecture #28

12. p/4 DQPSK Constellation
Combined Constellations Q
#1 Constellation
#2 Constellation
±135° Phase Transitions have more AM than ±90° OQPSK Phase Transitions
Possible Transitions I
No Phase Transitions thru Origin  AM is minimized
ECE 4710: Lecture #28

13. Detection
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
p/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
p/4 DQPSK
FM Detector
Decode
ECE 4710: Lecture #28

14. Wireless Communications
OQPSK & p/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 & p/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)
p/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, & p/4 DQPSK all have the same functional (sin x / x)2 form
ECE 4710: Lecture #28

17. MPSK & QAM PSD ECE 4710: Lecture #28 Baseband FNBW
PSD for Rectangular Pulse
Spectral Sidelobes
RF Null-to-Null
Transmission BW
Spectral Efficiency
Note: This spectral efficiency is for N-to-N BW only
ECE 4710: Lecture #28