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. Multicarrier Modulation Breaks data into N substreams Substream modulated onto separate carriers Substream bandwidth is B/N for B total bandwidth B/N<B c implies flat fading on each subcarrier (no ISI) x cos(2  f 0 t) x cos(2  f N t)  R bps R/N bps QAM Modulator QAM Modulator Serial To Parallel Converter

4. Overlapping Substreams Can have completely separate subchannels Required passband bandwidth is B. OFDM overlaps substreams Substreams (symbol time T N ) separated in RX Minimum substream separation is B N /(1+  ). Total required bandwidth is B/2 (for T N =1/B N ) f0f0 f N-1 B/N

5. 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(2  f c t) R bps QAM Modulator Serial To Parallel Converter IFFT X0X0 X N-1 x0x0 x N-1 Add cyclic prefix and Parallel To Serial Convert D/A TX x cos(2  f c t) R bps QAM Modulator FFT Y0Y0 Y N-1 y0y0 y N-1 Remove cyclic prefix and Serial to Parallel Convert A/D LPF Parallel To Serial Convert RX

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