EE359 – Lecture 18 Outline
Announcements
 Makeup lectures Fri 11/30 9:30am & Mon 5pm, Gates B12
 Alexandros OHs/discuss...
Remainder of Course
Today: Multicarrier modulation and OFDM
Friday 9:30am: OFDM & Spread Spectrum
Dec. 3 (last lecture):...
Review of Last Lecture
 Decision-Feedback Decoders
 Linear ML receiver except for error propagation
 Sphere Decoder (ne...
Multicarrier Modulation
 Breaks data into N substreams
 Substream modulated onto separate carriers
 Substream bandwidth...
Overlapping Substreams
Can have completely separate subchannels
Required passband bandwidth is B.
OFDM overlaps substre...
FFT Implementation of OFDM
 Use IFFT at TX to modulate symbols on each subcarrier
 Cyclic prefix makes linear convolutio...
OFDM Design Issues
Timing/frequency offset:
 Impacts subcarrier orthogonality; self-interference
Peak-to-Average Power ...
Main Points
 MCM splits channel into NB flat fading subchannels
 Can overlap subcarriers to preserve bandwidth
 OFDM ef...
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Lecture18

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Lecture18

  1. 1. EE359 – Lecture 18 Outline Announcements  Makeup lectures Fri 11/30 9:30am & Mon 5pm, Gates B12  Alexandros OHs/discussion next week will Wed. 7pm.  Last HW will be assigned Thursday, due 1+ week later  Final info (coverage, review, extra OHs, etc) given Friday Review of Last Lecture Multicarrier Modulation Overlapping Substreams OFDM FFT Implementation OFDM Design Issues
  2. 2. Remainder of Course Today: Multicarrier modulation and OFDM Friday 9:30am: OFDM & Spread Spectrum Dec. 3 (last lecture): 5pm (no lecture at 9:30am) Finish spread spectrum (30-45 min) Class summary (30-45 min) Pizza break Optional: Advanced topics in wireless (45-60 min) Cake Class Ends  (no lecture 12/5) Final review: Sunday 12/9? Another day? Final 12/13 8:30
  3. 3. 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
  4. 4. Multicarrier Modulation  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) x cos(2πf0t) x cos(2πfNt) Σ R bps R/N bps R/N bps QAM Modulator QAM Modulator Serial To Parallel Converter
  5. 5. 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+β). Total required bandwidth is B/2 (for TN=1/BN) f0 fN-1 B/N
  6. 6. 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 x cos(2πfct) R bps QAM Modulator Serial To Parallel Converter IFFT X0 XN-1 x0 xN-1 Add cyclic prefix and Parallel To Serial Convert D/A TX x cos(2πfct) R bpsQAM ModulatorFFT Y0 YN-1 y0 yN-1 Remove cyclic prefix and Serial to Parallel Convert A/DLPF Parallel To Serial Convert RX
  7. 7. 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
  8. 8. 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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