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

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

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

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

QPSK vs. OQPSK Q I ECE 4710: Lecture #28 “01” “00” “11” “10” Input : 100110110100 I : 101100 Q : 010110 Q “01” No 180° Phase Transitions I I Q “11” “00” Q OFFSET “10” ECE 4710: Lecture #28

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

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

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 00 +135° 01 -135° 10 -45° 11 +45° Differential encoding since data represented by phase change & not absolute value of signal phase ECE 4710: Lecture #28

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

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

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

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

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 10 01 11 ECE 4710: Lecture #28

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

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

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

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