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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 #15

  2. 0 1 0 Df Df t f fL fc fH Tb 0 1 Continuous Phase Transitions Constant Envelope Modulation • BFSK Binary Frequency Shift Keying • Frequency of constant amplitude carrier shifted between two possible states • fH= “1” and fL= “0” ECE 4730: Lecture #15

  3. Constant Envelope Modulation • BFSK Binary Frequency Shift Keying • Df = frequency offset from fc • Note that phase between bits can be continuous • No discontinuity  constant envelope retained !! • Discontinuous phase  can be allowed but leads to envelope variations in bandlimited system and spectral broadening when non-linear amplifiers are used • BFSK signals can be demodulated with non-coherent Rx’s • Simple and cheap • Unlike BPSK which requires coherent detection ECE 4730: Lecture #15

  4. Constant Envelope Modulation • BFSK Binary Frequency Shift Keying • Coherent detection of BFSK can also be implemented • Better BER for same Eb / No as non-coherent Rx • Bandwidth • RF BW = BT = 2Df + 2B (Carson’s rule like FM!) where B = baseband BW (single null) ECE 4730: Lecture #15

  5. Constant Envelope Modulation • MSK Minimum Shift Keying • Specific type of continuous phase (CP) FSK • Choose minimum allowed frequency spacing such that high & low FSK tones are orthogonal • Orthogonal  no ISI due to demodulation (other ISI still present) • Modulation index = 0.5 = 2 Df / RbDf = 0.25 Rb • Fig. 6.38, pg. 317  MSK RF signal BW • MSK has lower sidelobes than QPSK  –23 dB vs. –10 dB • Larger null-to-null BW than QPSK  1.5 Rb vs. 1.0 Rb • 99% RF BW much better than QPSK (1.2 Rb vs. 8.0 Rb !!) • Very low ACI!! ECE 4730: Lecture #15

  6. Sidelobe Levels FNBWs MSK vs. QPSK PSD ECE 4730: Lecture #15

  7. Constant Envelope Modulation • MSK Minimum Shift Keying • Constant envelope achieved due to continuous phase • DC/RF efficient non-linear Tx amplifiers (Class C) • Long battery life for mobile units • Non-coherent detection • Simple & inexpensive Rx’s • Very popular modulation scheme for mobile radio ECE 4730: Lecture #15

  8. Constant Envelope Modulation • GMSK Gaussian MSK • Spectral efficiency of MSK further enhanced using baseband Gaussian pulse-shaping filter • Reduce signal BW • Gaussian filter will introduce some ISI • Does NOT satisfy Nyquist criterion • ISI not severe if BG Tb > 0.5 • BWbit duration product BG = 3 dB filter BW ECE 4730: Lecture #15

  9. GMSK Bandwidth ECE 4730: Lecture #15

  10. Constant Envelope Modulation • GMSK RF BW • BW  as BG Tb but ISI  • GMSK with BG Tb < 0.5 used with no adverse effects if BER < irreducible MRC BER • MRC BER caused by multipath delay + mobile velocity • BER floor inherent in MRC • T-Mobile, Cingular, and AT&T Wireless all used GSM standard  0.3 GMSK (BG Tb = 0.3) ECE 4730: Lecture #15

  11. Both have samePav Tx PSD Tx PSD f f fc fc Spread Spectrum Modulation • Spread Spectrum Modulation (SSM) read pgs. 329-334 • Tx expands (spreads) signal BW many times and the signal is then collapsed (despread) in Rx • Trade BW for signal power like in FM ECE 4730: Lecture #15

  12. Spread Spectrum Modulation • SSM Advantages 1)Combats multipath fading  no equalization needed 2)Resistant to narrowband interference 3)Allows multiple users with different codes to share same MRC  No frequency reuse!! 4) As # simultaneous users  the bandwidth efficiency  ECE 4730: Lecture #15

  13. Ts to RF Mod 0 0 0 0 1 1 1 Data Signal Ts = 100 msec Data Rate = 10 kbps “chip” bw  1 / Ts Tc 0 0 0 0 1 1 1 Spreading Sequence Tc = 1 msec Chip Rate = 1 Mcps BW  1 / Tc f fc Spread Spectrum Modulation • SSM signal spreading done by multiplying baseband data signal by pseudo-noise (PN) code or sequence ECE 4730: Lecture #15

  14. Spread Spectrum Modulation • Pseudo-random Noise (PN) Codes • CodeDivision Multiple Access (CDMA) • Mobiles users share spectrum using codes • Binary sequence with random properties  noise-like •  equal #’s of 1’s and 0’s • Very low correlation between time-shifted versions of same sequence (high-correlation at exact time overlap) • Very low cross-correlation between different codes • Each user assigned unique code • Other user’s signal appears (approximately) like random noise! • White noise properties ECE 4730: Lecture #15

  15. Frequency Time  PSD t f d(t) Spread Spectrum Modulation • White noise properties • Delta Function Autocorrelation in Time • Flat PSD in Frequency equal amount of energy at all frequencies ECE 4730: Lecture #15

  16. Spread Spectrum Modulation • PN Spreading Codes Example: 0 0 0 1 1 0 1 let “0” = & “1” = + + + + Matched Time shifted by 1 step + + + + + + + + ++ + + 1 1 1 1 1 1 1 0 1 1 -1 1 -1 -1 0 S = 7S = 0 ! Uncorrelated !! ECE 4730: Lecture #15

  17. Frequency Time  PSD t f d(t) Spread Spectrum Modulation • Auto-correlation of PN code  noise-like! • Cross-correlation between different users’ codes has similar noise-like properties • Spread Spectrum Modulation (SSM) must be used with PSK or FSK to encode data bits ECE 4730: Lecture #15

  18. Spread Spectrum Modulation • Two types of SSM 1) Direct Sequence (DS)  used with PSK • Multiply baseband data by PN code (same as diagram above) 2) Frequency Hopping (FH)  used with FSK • Randomly change fc with time • Processing Gain = PG • SSM resistant to narrowband interfering signals • Narrowband interfering signal converted to wideband energy in SS Rx after despreading + LPF • Fig. 6.50, pg. 333 ECE 4730: Lecture #15

  19. Spread Spectrum Modulation ECE 4730: Lecture #15

  20. Spread Spectrum Modulation • Sprint PCS and Verizon Wireless • Both use DS-SSM (CDMA) technology • Sprint PCS  first nationwide deployment of CDMA system • Main disadvantage of DS-SSM is that perfect power control of mobiles is required to maximize capacity • Near/far problem where one mobile unit can dominate base station Rx thereby wiping out other users!! ECE 4730: Lecture #15

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