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Modulation

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

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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

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