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EE359 – Lecture 18 Outline

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- Announcements
- HW posted, due Friday. One more HW due last week of classes
- 3 Lectures this week (last Tues, now, tomorrow 12:50-2:05)
- Final info (coverage, review, extra OHs, etc) given in Friday lecture

- Review of Last Lecture
- FFT implementation of multicarrier (OFDM)
- Introduction to Spread Spectrum
- ISI and Interference Rejection

- Remaining lectures:
- 11/18 week (3 lectures): Should complete all EE359 material
- Multicarrier modulation/OFDM, SS/CDMA

- 11/25 week: Thanksgiving
- 12/2 week (1 lecture): Course Summary, Advanced topics

- 11/18 week (3 lectures): Should complete all EE359 material
- No regular lectures Dec 3 or Dec. 5
- Makeup 11/22 (Fri) 12:50-2:05 (pizza@12:30): SS/CSMA
- Makeup 12/6 (Fri): 12:30-2:05 (pizza@12): SS/359 wrapup

- Wrapup includes summary & advanced topics
- Final Exam: Dec 13, 12:15-3:15pm

S

cos(2pf0t)

cos(2pfNt)

x

x

- Multicarrier Modulation: breaks data into N substreams (B/N<Bc); Substream modulated onto separate carriers
- Overlapping substreams
- Minimum substream separation is BN=1/TN

- DFT Review
- Use cyclic prefix to make linear convolution circular

R/N bps

QAM

Modulator

R bps

Serial

To

Parallel

Converter

R/N bps

QAM

Modulator

cos(2pfct)

cos(2pfct)

LPF

A/D

D/A

Serial

To

Parallel

Converter

x

x

+

- 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

n(t)

TX

R bps

Add cyclic

prefix and

Parallel

To Serial

Convert

QAM

Modulator

h(t)

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

Yi=HiXi+ni

- Timing/frequency offset:
- Impacts subcarrier orthogonality; self-interference

- Peak-to-Average Power Ratio (PAPR)
- Adding subcarrier signals creates large signal peaks
- Solve with clipping or PAPR-optimized coding

- MIMO/OFDM
- Apply OFDM across each spatial dimension
- Can adapt across space, time, and frequency

- Different fading across subcarriers
- Mitigate by precoding (fading inversion), adaptive modulation over frequency, and coding across subcarriers

- Modulation that increases signal BW
- Mitigates or coherently combines ISI
- Mitigates narrowband interference/jamming
- Hides signal below noise (DSSS) or makes it hard to track (FH)
- Also used as a multiple access technique

- Two types
- Frequency Hopping:
- Narrowband signal hopped over wide bandwidth

- Direction Sequence:
- Modulated signal multiplied by faster chip sequence

- Frequency Hopping:

Tc

- Bit sequence modulated by chip sequence
- Spreads bandwidth by large factor (G)
- Despread by multiplying by sc(t) again (sc(t)=1)
- Mitigates ISI and narrowband interference

S(f)

s(t)

sc(t)

Sc(f)

S(f)*Sc(f)

1/Tb

1/Tc

Tb=KTc

2

S(f)

I(f)

S(f)

S(f)*Sc(f)

I(f)*Sc(f)

Despread Signal

Receiver Input

Info. Signal

- Narrowband Interference Rejection (1/K)
- Multipath Rejection (Autocorrelation r(t))

aS(f)

S(f)*Sc(f)[ad(t)+b(t-t)]

S(f)

brS’(f)

Despread Signal

Receiver Input

Info. Signal

- 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 and 802.11n/ac all use OFDM+MIMO
- Spread spectrum increases signal bandwidth above that required for information transmission
- Benefits: ISI/interference rejection, multiuser technique