Digital Modulation Two general classifications modulation methods: of digital 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 1 Constant Envelope Modulation BFSK Binary Frequency Shift Keying Frequency of constant amplitude carrier shifted between two possible states » fH = “1” and fL = “0” 0 Df fL 0 1 0 Df fc t fH 1 f Tb Continuous Phase Transitions ECE 4730: Lecture #15 2 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 Rx’s signals can be demodulated with non-coherent » Simple and cheap » Unlike BPSK which requires coherent detection ECE 4730: Lecture #15 3 Constant Envelope Modulation BFSK Binary Frequency Shift Keying Coherent detection implemented of BFSK can also be » Better BER for same Eb / No as non-coherent Rx » Never done in practice b/c coherent detection of BPSK has best possible BER vs. Eb / No Bandwidth » RF BW = BT = 2 Df + 2 B (Carson’s rule like FM!) where B = baseband BW (single null) ECE 4730: Lecture #15 4 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 5 MSK vs. QPSK PSD Sidelobe Levels FNBWs ECE 4730: Lecture #15 6 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 7 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 BWbit duration product BG = 3 dB filter BW ECE 4730: Lecture #15 8 GMSK Bandwidth ECE 4730: Lecture #15 9 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 10 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 Tx PSD Both have same Pav Tx PSD f fc f fc ECE 4730: Lecture #15 11 Spread Spectrum Modulation SSM signal spreading done by multiplying baseband data signal by pseudo-noise (PN) code or sequence Ts 0 1 0 1 0 1 to RF Mod 0 Data Signal Ts = 100 msec Data Rate = 10 kbps “chip” bw 1 / Ts Tc 0 1 0 1 0 1 0 Spreading Sequence Tc = 1 msec Chip Rate = 1 Mcps BW 1 / Tc f fc 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 Spread Spectrum Modulation Pseudo-random Noise (PN) Codes Code Division 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 14 Spread Spectrum Modulation White noise properties Autocorrelation in Time d(t) t Frequency PSD f Delta Function Autocorrelation in Time Flat PSD in Frequency equal amount of energy at all frequencies ECE 4730: Lecture #15 15 Spread Spectrum Modulation PN Spreading Codes Example: 0 0 0 1 1 0 1 + + + let “0” = & “1” = + Matched + + + + + + Time shifted by 1 step + + + + + + 1 0 1 1 1 1 1 1 S=7 1 1 -1 1 -1 -1 0 S=0! Uncorrelated !! ECE 4730: Lecture #15 16 Spread Spectrum Modulation Auto-correlation of PN code noise-like! Time d(t) t Frequency PSD f 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 17 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 18 Spread Spectrum Modulation PG Ts Tc Rs Rc Wss B where Wss : SS BW and B : signal BW ECE 4730: Lecture #15 19 Spread Spectrum Modulation Sprint PCS and Verizon Wireless Both used 2G DS-SSM (CDMA) technology Sprint PCS first nationwide deployment of 2G CDMA system in the world in 1998-99 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 20