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Modulation

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Modulation

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  1. Modulation • Modulation: Encoding information in baseband signal and then translating (shifting) signal to much higher frequency prior to transmission • Primary goal is to transport information thru MRC with the best quality (low BER), lowest power, & least amount of frequency spectrum • Must overcome difficult impairments introduced by MRC: • Fading/multipath • Doppler Spread • ACI & CCI ECE 4730: Lecture #11

  2. Modulation • Information signal  Baseband Signal • Signal has original frequency domain characteristics • Modulated signal  Bandpass or RF signal • Baseband signal spectrum shifted to much higher frequency  spectrum often changes based on modulation method • Higher frequency  carrier, RF, or channel frequency ECE 4730: Lecture #11

  3. f -W 0 +W f -fc-W -fc -fc+W fc-W fcfc+W Modulation • Baseband signal • s(t) = m(t)cos(2pfc t) • Bandpass or RF signal ECE 4730: Lecture #11

  4. Frequency Modulation • Frequency Modulation  FM • Most widely used form of analog modulation for mobile radio applications • 1G AMPS • Police/Fire/Ambulance Two-Way Radios • Many unique characteristics ECE 4730: Lecture #11

  5. Frequency Modulation • Unlike AM the amplitude of FM carrier is kept constant (constant envelope) & the frequency is varied  to modulating signal m(t) : • Ac and Am : amplitudes of carrier & modulating signals (V) • fm : modulating signal frequency (Hz) • kf : frequency deviation constant (Hz/V) ECE 4730: Lecture #11

  6. Carrier | SFM(f ) | Sidebands f fc BT Frequency Modulation • Frequency Modulation Index  • Df : peak frequency deviation of Tx • W: maximum bandwidth of m(t) • FM signal spectrum  carrier + infinite # of sidebands ECE 4730: Lecture #11

  7. Frequency Modulation • FM signal spectrum • RF BW determined by Carson’s rule: • Contains 98% of Tx power • Example: AMPS uses bf = 3 and fm = 4 kHz then • 24 kHz < BT < 32 kHz • Poor spectral efficiency • Allocated channel BW = 30 kHz • Actual FCC standard uses threshold specification ECE 4730: Lecture #11

  8. 45 kHz 20 kHz 26 dB 45 dB | SFM( f ) | f fc Frequency Modulation FCC AMPS Frequency Specification ECE 4730: Lecture #11

  9. Frequency Modulation • SNR vs. BW tradeoff • For FM one can increase RF BW to improve SNR : • where SNRi is @ Rx input to demodulator • Rapid non-linear improvement in output signal quality (SNRo) for increases in input signal power (SNRi) • “Capture effect” : FM Rx rejects weaker of two FM signals in same RF BW  resistant to CCI!! • Requires that SNRi 10 dB (capture threshold) • AM only has linear improvement in SNRo for increasing Rx signal power ECE 4730: Lecture #11

  10. FM vs. AM • FM Advantages 1) Signal information contained in frequency variations of carrier • Resistant to thermal/impulse noise & signal fading • Both cause amplitude variations • Constant Tx carrier power  constant envelope • Can use efficient non-linear Class C power amplifiers @ Tx output • 80-85% DC to RF conversion efficiencies • Improved battery life in mobile units!! major reason why FM was adopted for AMPS ECE 4730: Lecture #11

  11. FM vs. AM • FM Advantages 2) Non-Linear Modulation • Rapid  in SNRofor modest increase in SNRi • Trade  BW for SNRo • Smaller Tx power requirements  longer battery life • Capture effect  FM resistant to CCI ECE 4730: Lecture #11

  12. FM vs. AM • FM Disadvantages 1) Poor spectral efficiency • Much wider occupied BW than AM • Fewer # of users in given frequency band • AMPS: fm = 4 kHz (information BW) but BT = 30 kHz!! 2) Poor signal detection for signals < capture threshold 3) Tx/Rx more complex than AM  more costly ECE 4730: Lecture #11

  13. FM vs. AM • AM Advantages 1) Good spectral efficiency • Only 3-4 kHz signal BW required for voice vs. 30 kHz for FM!! 2) Simple Tx/Rx  less costly than FM ECE 4730: Lecture #11

  14. FM vs. AM • AM Disadvantages 1) Linear amplitude modulation • Susceptible to noise, fading, & CCI 2) Requires linear output amplifiers • Information contained in amplitude!! • Non-linear output amplifier would distort signal • Must use linear Class A output amplifiers in Tx • 30-40% DC to RF efficiency • Poor battery life in mobile units ECE 4730: Lecture #11

  15. Digital Modulation • Better performance and more cost effective than analog modulation methods (AM, FM, etc.) • Used in 2nd generation (2G) cellular systems deployed in U.S. since 1998/99 • Cingular, AT&T Wireless, Verizon Wireless, Sprint PCS, T-Mobile, etc. • Advancements in VLSI, DSP, ASICs, etc. made digital solutions practical and affordable ECE 4730: Lecture #11

  16. Digital Modulation • Performance advantages: 1) Resistant to noise, fading, & interference 2) Combine multiple information types (voice, data, & video) in single transmission channel 3) Improved security (e.g. encryption)  deters phone cloning + eavesdropping 4) Error coding to detect / correct transmission errors 5) Signal conditioning to combat hostile MRC environment 6) Implement mod/dem functions using DSP software ECE 4730: Lecture #11