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. 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. 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. 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. 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. 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. 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. 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