1 / 9

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

webb
Download Presentation

EE359 – Lecture 18 Outline

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related