1. ECE 4710: Lecture #26 1 BPSK  BPSK   m(t) is binary baseband signal, e.g. m i = ±1 and i = 1, 2  Two possible phase states for carrier »  i = 0°, 180° for m i = ±1  Polar form of complex envelope   Signal Constellation Diagrams  Plot g(t) in polar coordinate system  Visual representation of modulation format

2. ECE 4710: Lecture #26 2 BPSK Signal Constellation BPSK “1” “0” Real (In-Phase) Imaginary (Quadrature)

3. ECE 4710: Lecture #26 3  Digital input information signal, m(t), with more than two levels used as input to Tx modulator  Generate multi-level bandpass signal  “Level” is misleading »Implies signal amplitude »Could be multi-frequency or multi-phase signal  Serial binary input converted to multi-level signal by DAC Multi-Level Signaling

4. ECE 4710: Lecture #26 4 Multi-Level Signaling t T 1 0 0 1 0 0 1 1 T 1 0 0 1 0 0 1 1 t Binary Input M = 4-Level DAC Output

5. ECE 4710: Lecture #26 5 QPSK & MPSK  Multi-level digital input to Phase Modulator (PM)  M -ary Phase Shift Keying  MPSK  For M = 4  Quadrature Phase Shift Keying  QPSK  QPSK  m(t) is multi-level baseband signal, e.g. m i = -3,-1,+1,+3  Four possible phase states for carrier  Quadrature phase states  90° difference »  i = 0°, 90°, 180°, and 270° for m i = -3,-1,+1,+3   /4 QPSK  Quadrature phase states »  i = 45°, 135°, 225°, and 315° »Carrier phase shifted by 45° wrt QPSK  45° =  /4

6. ECE 4710: Lecture #26 6 QPSK Constellation QPSK “00” “11” I Q “01” “10” Signal points located on circle of radius A c

7. ECE 4710: Lecture #26 7  /4 QPSK Constellation  /4 QPSK “00” “11” I Q “01” “10” 45° =  /4 Signal points located on circle of radius A c

8. ECE 4710: Lecture #26 8 QPSK Generation  Use m(t) to drive phase modulator (PM)  Not normally done in high performance systems  Quadrature Tx  Cartesian form of PSK complex envelope  Use two quadrature carriers modulated by x and y components of complex envelope »Quadrature carriers  90° phase difference  sin(2  f c t ) & cos(2  f c t )

9. ECE 4710: Lecture #26 9 QPSK Generation QPSK

10. ECE 4710: Lecture #26 10 MPSK Envelope  For rectangular baseband pulse shapes the envelope of BPSK, QPSK, MPSK signals is approximately constant  A c ( not A c (t) ) Polar Baseband Modulation BPSK Bandpass Signal 0 1 0 1 0 1 180° Phase Change Between 1/0 Bits Constant Envelope

11. ECE 4710: Lecture #26 11 MPSK Envelope  Constant envelope  no amplitude modulation (AM)  During data transitions the envelope is  constant because of nearly instantaneous phase transitions but this requires very large BW signal!  Rectangular pulse shape produces (sin x / x ) 2 PSD  Large undesirable spectral sidelobes for f > 1 / T s »Spectrally inefficient »Signal interference between adjacent frequency users  Adjacent Channel Interference (ACI) in cellular radio  Spectral sidelobes eliminated with RC filter »MPSK signal will have time-varying amplitude because of pulse shaping to minimize signal BW  no longer constant envelope

12. ECE 4710: Lecture #26 12 MPSK PSD MPSK PSD for Rectangular Pulse Modulation Spectral Sidelobes

13. ECE 4710: Lecture #26 13 BPSK with Pulse Shaping Polar Baseband Modulation 1 0 1 0 1 0 BPSK Bandpass Signal Raised Cosine Filter  Minimize Signal BW Time-varying amplitude creates AM modulation for PSK signals Note that signal amplitude gradually goes to ~zero at transition period between bits

14. ECE 4710: Lecture #26 14 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 “01” “10” AM!!

15. ECE 4710: Lecture #26 15 AM QPSK  RC filtering minimizes QPSK signal BW  Primary Advantage  AM modulation of QPSK has one major disadvantage  Class A or B linear amplifiers required to preserve AM on QPSK and therefore preserve spectral efficiency »Poor DC to RF efficiencies  typically 40-65% »Serious problem for mobile communication applications  Increase battery capacity requirements by 40-50%  High efficiency non-linear Class C amplifiers have DC to RF efficiencies of 80-90%

16. ECE 4710: Lecture #26 16 AM QPSK  What happens if non-linear Class C amplifiers are used on pulse-shaped QPSK anyway?  Non-linear amplification significantly distorts AM pulse shaping  Spectral sidelobes regenerated by non-linear amplification  Advantage of pulse-shaped signal BW is lost f PSD 1 / T s = FNBW RC Pulse Shaped RC Pulse Shaped after Class C RF Amplifier Spectral Regeneration

17. ECE 4710: Lecture #26 17 AM QPSK  How can we keep minimal signal BW and still use efficient non-linear Class C amplifiers for mobile applications that want to use PSK signals?  Offset Quadrature Phase Shift Keying  OQPSK   /4 Differential QPSK  Both techniques seek to minimize transitions thru origin of constellation diagram  Limit amplitude modulation  Allow for efficient Class C amplifiers with pulse-shaped PSK signals