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EE359 – Lecture 18 Outline

EE359 – Lecture 18 Outline. Announcements Today’s lecture will end ~10 minutes early Nima’s OH Thur moved to Sun 10am, Packard 3 rd floor HW due Sunday 5pm. No late HWs accepted Last HW will be assigned Friday, due 1+ week later Bonus lecture info

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EE359 – Lecture 18 Outline

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  1. EE359 – Lecture 18 Outline • Announcements • Today’s lecture will end ~10 minutes early • Nima’s OH Thur moved to Sun 10am, Packard 3rd floor • HW due Sunday 5pm. No late HWs accepted • Last HW will be assigned Friday, due 1+ week later • Bonus lecture info • Final info (coverage, review, extra OHs, etc) given 12/5 • Review of Last Lecture • Fading across subcarriers in multicarrier • OFDM FFT Implementation • OFDM Design Issues

  2. Remainder of Course • Today: OFDM • Lecture will end around 10:35am • Dec. 5: Spread spectrum • Bonus lecture: Dec. 5 6-8pm, Hewlett 103 • Finish Spread Spectrum • Class summary • Advanced topics in wireless • Class Ends  • Except for final Dec. 14  • No lecture Dec. 7

  3. Review of Last Lecture • Sphere Decoder (near ML): • Only searches within a sphere of received symbol. • Other MIMO design issues • Space-Time Codes, Adaptive techniques, Limited FB • ISI Countermeasures • Equalization, multicarrier (OFDM), spread spectrum • Multicarrier Modulation • Breaks data into N substreams such that B/N<Bc • Substream modulated onto separate carriers • Overlapping substreams: Dfi=1/TN

  4. Fading Across Subcarriers • Leads to different BERS • Compensation techniques • Frequency equalization (noise enhancement) • Precoding • Coding across subcarriers • Adaptive loading (power and rate)

  5. 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 TX Add cyclic prefix and Parallel To Serial Convert R bps QAM Modulator 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

  6. OFDM Design Issues • Timing/frequency offset: • Impacts subcarrier orthogonality; self-interference • Peak-to-Average Power Ratio (PAPR) • Adding subcarrier signals creates large signal peaks • Different fading across subcarriers • Same mitigation techniques as in MCM: Precoding to invert fading, coding across subcarriers, and adaptative loading over time most common • MIMO/OFDM • Apply OFDM across each spatial dimension • Can adapt across space, time, and frequency

  7. Main Points • Subcarrier fading degrades MCM performance • Compensate through precoding (channel inversion), coding across subcarriers, or adaptation • OFDM efficiently implemented using IFFTs/FFTs • Block size depends on data rate relative to delay spread • OFDM challenges: • PAPR; timing/frequency offset; fading across subcarriers • 4G Cellular, Wimax, 802.11n all use OFDM+MIMO • Adapt across space, time, and frequency

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