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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
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ECE 4710: Lecture #26 2 BPSK Signal Constellation BPSK “1” “0” Real (In-Phase) Imaginary (Quadrature)
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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
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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
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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
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ECE 4710: Lecture #26 6 QPSK Constellation QPSK “00” “11” I Q “01” “10” Signal points located on circle of radius A c
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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
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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 )
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ECE 4710: Lecture #26 9 QPSK Generation QPSK
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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
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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
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ECE 4710: Lecture #26 12 MPSK PSD MPSK PSD for Rectangular Pulse Modulation Spectral Sidelobes
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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
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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!!
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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%
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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
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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
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