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EE359 – Lecture 18 Outline

EE359 – Lecture 18 Outline. Review of Last Lecture Multicarrier Modulation Overlapping Substreams OFDM FFT Implementation OFDM Design Issues. Review of Last Lecture. Decision-Feedback Decoders Linear ML receiver except for error propagation Sphere Decoder (near ML):

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EE359 – Lecture 18 Outline

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  1. EE359 – Lecture 18 Outline • Review of Last Lecture • Multicarrier Modulation • Overlapping Substreams • OFDM FFT Implementation • OFDM Design Issues

  2. Review of Last Lecture • Decision-Feedback Decoders • Linear ML receiver except for error propagation • 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

  3. S cos(2pf0t) cos(2pfNt) x x Multicarrier Modulation R/N bps • Breaks data into N substreams • Substream modulated onto separate carriers • Substream bandwidth is B/N for B total bandwidth • B/N<Bc implies flat fading on each subcarrier (no ISI) QAM Modulator R bps Serial To Parallel Converter R/N bps QAM Modulator

  4. Overlapping Substreams • Can have completely separate subchannels • Required passband bandwidth is B. • OFDM overlaps substreams • Substreams (symbol time TN) separated in RX • Minimum substream separation is BN/(1+b). • Total required bandwidth is B/2 (for TN=1/BN) B/N fN-1 f0

  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 • MIMO/OFDM • Apply OFDM across each spatial dimension • Can adapt across space, time, and frequency • Different fading across subcarriers • Mitigate by precoding (fading inversion), coding across subcarriers, and adaptative loading over time

  7. Main Points • MCM splits channel into NB flat fading subchannels • Can overlap subcarriers to preserve bandwidth • 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, Wimax, 802.11n all use OFDM+MIMO • Adapt across space, time, and frequency

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