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Digital Modulation

Digital Modulation. Two general classifications of digital modulation methods: 1) Linear : amplitude of Tx signal varies linearly with modulating information signal, m ( t ) .

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Digital Modulation

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  1. Digital Modulation • Two general classifications of digital modulation methods: 1)Linear : amplitude of Tx signal varies linearly with modulating information signal, m(t). 2) Constant Envelope : non-linear methods where amplitude of Tx signal held constant regardless of the variation in the modulating information signal, m(t). ECE 4730: Lecture #14

  2. Digital Modulation • Linear Modulation  amplitude of Tx RF signal varies linearly with modulating information signal • Bandwidth efficient  many users supported in limited spectrum • AM is analog example of linear scheme • Inefficient power usage • Must use linear RF power amplifiers • Class A ~ 3040% DC to RF efficiency • Lower battery life for mobile units! ECE 4730: Lecture #14

  3. Digital Modulation • Linear Modulation • Use non-linear amplifiers anyway? • Regeneration of spectral sidelobes • Reduces bandwidth efficiency • Increases ACI • Higher-order modulation schemes like QPSK or OQPSK improve this situation some ECE 4730: Lecture #14

  4. Digital Modulation • Constant Envelope Modulation non-linear methods where amplitude of Tx RF signal held constant • Very DC-RF efficient (not related to p & BER performance!!) • Non-linear RF power amplifiers used with few problems • Class C ~80-90%% DC to RF efficiency • Bandwidth occupancy not affected by non-linear amps like linear schemes • Low out-of-band radiation • 60-70 dB below Tx signal level • Very low ACI ECE 4730: Lecture #14

  5. Digital Modulation • Constant Envelope Modulation • High immunity against amplitude fluctuations like Rayleigh fading in MRC • Constant envelope  no information in signal amplitude !! • FM is analog example of constant envelope scheme • Poor spectral efficiency • Larger occupied BW compared to linear methods ECE 4730: Lecture #14

  6. 1 0 1 Q t 1 0 I Tb 180° Phase Transitions BPSK Modulation • BPSK Binary Phase Shift Keying + = “1” and = “0” f1(t) = basis function (N = 1) ECE 4730: Lecture #14

  7. BPSK Modulation • Binary Phase Shift Keying • Amplitude appears constant ! • How is this linear modulation?? • Time domain representation of BPSK waveform is ideal • 180° phase transitions require infinite BW • Phase transitions force carrier amplitude to pass thru origin of constellation diagram • Amplitude varies in time (goes to zero as it passes thru origin)! • Pulse shaping to reduce BW also causes time-varying amplitude ECE 4730: Lecture #14

  8. BPSK Modulation BPSK RF signal BW Null-to-Null RF BW = 2Rb = 2 /Tb 90% BW = 1.6Rb 100% BW = 1.5Rb for RCF with a = 0.5 ECE 4730: Lecture #14

  9. BPSK Modulation • BPSK Demodulation in Rx • Requires reference of Tx signal in order to properly determine phase of incoming Rx signal • Synchronous or “Coherent” detection • Complex & costly Rx circuitry • Good BER performance for low SNR power efficient ECE 4730: Lecture #14

  10. DPSK Modulation • DPSK Differential Phase Shift Keying • Very popular variation of BPSK • Non-coherent Rx can be used • No need for coherent reference signal from Tx • Easy & cheap to build • Same BW properties as BPSK • Non-coherent detection  power efficiency is 3 dB worse than coherent BPSK (higher BER for same Eb / No) • Partially coherent detection  power efficiency is only 1 dB worse than coherent BPSK • Most widely used for DPSK  simple Rx and good SNR vs. BER ECE 4730: Lecture #14

  11. XOR Truth Table DPSK Modulation • Bit information determined by transition between two phase states • Incoming bit = 1  phase stays same as previous bit • Incoming bit = 0  phase switches state • Table 6.1, pg. 299 Input Binary Data  Previous Encoded Data  DPSK Encoded Data  ECE 4730: Lecture #14

  12. QPSK Modulation • QPSK Quadrature Phase Shift Keying • Four different phase states in one symbol period • Two bits of information in each symbol Phase: 0 p/2 p 3p/2  possible phase values Symbol: 00 01 10 11 Two basis functions N = 2 and Es = 2Eb !! ECE 4730: Lecture #14

  13. Q “01” p/2 “11” p “00” 0 I 3p/2 “10” QPSK Modulation QPSK Signal Constellation Constellation more dense than BPSK A) Reduce RF BW by 2 x of BPSK for same Rd OR B) 2  the data rate in same signal BW !! ECE 4730: Lecture #14

  14. QPSK Modulation QPSK RF signal BW Null-to-Null RF BW = Rb = 1 /Tb= 2 /Ts 2  smaller than BPSK ECE 4730: Lecture #14

  15. QPSK Modulation • How does SNR vs. BER performance compare to BPSK? • # states / N = 2 for both BPSK and QPSK • Same effective density • Same power efficiency!! (Eb / No @ specified BER) • Some (not all) state transitions pass thru origin • Phase shift = p (180°) ; e.g. p/2  3p/2 or 0 p • Linear RF amplifiers still required to preserve spectral efficiency  less efficient DC to RF conversion • Fewer transitions than BPSK ECE 4730: Lecture #14

  16. Q AM!! “01” I “00” “11” “10” QPSK Modulation • QPSK amplitude variations • Amplitude goes to  zero for 180° bit transitions causing signal to pass thru origin of constellation diagram • 90° phase transitions cause amplitude to stay  constant ECE 4730: Lecture #14

  17. OQPSK Modulation • OQPSK Offset Quadrature Phase Shift Keying • Q channel signal “offset” from I channel signal • Delayed in time by ½Ts • Eliminates 180° phase transitions • No transitions thru origin !! • Amplitude variations less than QPSK & much less than BPSK • Non-linear Tx amplifiers can be used to achieve better utilization of battery power in mobile unit • Retains same BW efficiency as QPSK • Very popular modulation method for reverse-link in wireless systems ECE 4730: Lecture #14

  18. QPSK vs. OQPSK QPSK OQPSK Q Q AM!! “01” “01” No 180° Phase Transitions I I “00” “00” “11” “11” “10” “10” ECE 4730: Lecture #14

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