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EE359 – Lecture 18 Outline

EE359 – Lecture 18 Outline. Announcements HW posted, due Friday. One more HW due last week of classes 3 Lectures this week (last Tues, now, tomorrow 12:50-2:05) Final info (coverage, review, extra OHs, etc) given in Friday lecture Review of Last Lecture

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EE359 – Lecture 18 Outline

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  1. EE359 – Lecture 18 Outline • Announcements • HW posted, due Friday. One more HW due last week of classes • 3 Lectures this week (last Tues, now, tomorrow 12:50-2:05) • Final info (coverage, review, extra OHs, etc) given in Friday lecture • Review of Last Lecture • FFT implementation of multicarrier (OFDM) • Introduction to Spread Spectrum • ISI and Interference Rejection

  2. End of Quarter Schedule • Remaining lectures: • 11/18 week (3 lectures): Should complete all EE359 material • Multicarrier modulation/OFDM, SS/CDMA • 11/25 week: Thanksgiving • 12/2 week (1 lecture): Course Summary, Advanced topics • No regular lectures Dec 3 or Dec. 5 • Makeup 11/22 (Fri) 12:50-2:05 (pizza@12:30): SS/CSMA • Makeup 12/6 (Fri): 12:30-2:05 (pizza@12): SS/359 wrapup • Wrapup includes summary & advanced topics • Final Exam: Dec 13, 12:15-3:15pm

  3. S cos(2pf0t) cos(2pfNt) x x Review of Last Lecture • Multicarrier Modulation: breaks data into N substreams (B/N<Bc); Substream modulated onto separate carriers • Overlapping substreams • Minimum substream separation is BN=1/TN • DFT Review • Use cyclic prefix to make linear convolution circular R/N bps QAM Modulator R bps Serial To Parallel Converter R/N bps QAM Modulator

  4. cos(2pfct) cos(2pfct) LPF A/D D/A Serial To Parallel Converter x x + FFT Implementation of OFDM • Use IFFT at TX to modulate symbols on each subcarrier • Cyclic prefix makes linear convolution of channel circular, so no interference between FFT blocks in RX processing • Reverse structure (with FFT) at receiver X0 x0 n(t) TX R bps Add cyclic prefix and Parallel To Serial Convert QAM Modulator h(t) IFFT XN-1 xN-1 RX Y0 y0 Remove cyclic prefix and Serial to Parallel Convert R bps QAM Modulator Parallel To Serial Convert FFT yN-1 YN-1 Yi=HiXi+ni

  5. OFDM Design Issues • Timing/frequency offset: • Impacts subcarrier orthogonality; self-interference • Peak-to-Average Power Ratio (PAPR) • Adding subcarrier signals creates large signal peaks • Solve with clipping or PAPR-optimized coding • MIMO/OFDM • Apply OFDM across each spatial dimension • Can adapt across space, time, and frequency • Different fading across subcarriers • Mitigate by precoding (fading inversion), adaptive modulation over frequency, and coding across subcarriers

  6. Intro. to Spread Spectrum • Modulation that increases signal BW • Mitigates or coherently combines ISI • Mitigates narrowband interference/jamming • Hides signal below noise (DSSS) or makes it hard to track (FH) • Also used as a multiple access technique • Two types • Frequency Hopping: • Narrowband signal hopped over wide bandwidth • Direction Sequence: • Modulated signal multiplied by faster chip sequence

  7. Tc Direct Sequence Spread Spectrum • Bit sequence modulated by chip sequence • Spreads bandwidth by large factor (G) • Despread by multiplying by sc(t) again (sc(t)=1) • Mitigates ISI and narrowband interference S(f) s(t) sc(t) Sc(f) S(f)*Sc(f) 1/Tb 1/Tc Tb=KTc 2

  8. S(f) I(f) S(f) S(f)*Sc(f) I(f)*Sc(f) Despread Signal Receiver Input Info. Signal ISI and Interference Rejection • Narrowband Interference Rejection (1/K) • Multipath Rejection (Autocorrelation r(t)) aS(f) S(f)*Sc(f)[ad(t)+b(t-t)] S(f) brS’(f) Despread Signal Receiver Input Info. Signal

  9. Main Points • OFDM efficiently implemented using IFFTs/FFTs • Block size depends on data rate relative to delay spread • OFDM challenges: PAPR; timing/frequency offset, MIMO • Subcarrier fading degrades OFDM performance • Compensate through precoding (channel inversion), coding across subcarriers, or adaptation • 4G Cellular and 802.11n/ac all use OFDM+MIMO • Spread spectrum increases signal bandwidth above that required for information transmission • Benefits: ISI/interference rejection, multiuser technique

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