[2024]Digital Global Overview Report 2024 Meltwater.pdf
Lecture18
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