Ee359 lecture 18 outline
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EE359 – Lecture 18 Outline. 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

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EE359 – Lecture 18 Outline

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Ee359 lecture 18 outline

EE359 – Lecture 18 Outline

  • 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


End of quarter schedule

End of Quarter Schedule

  • 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

  • No regular lectures Dec 3 or Dec. 5

    • Makeup 11/22 (Fri) 12:50-2:05 ([email protected]:30): SS/CSMA

    • Makeup 12/6 (Fri): 12:30-2:05 ([email protected]): SS/359 wrapup

  • Wrapup includes summary & advanced topics

  • Final Exam: Dec 13, 12:15-3:15pm


Review of last lecture

S

cos(2pf0t)

cos(2pfNt)

x

x

Review of Last Lecture

  • 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


Fft implementation of ofdm

cos(2pfct)

cos(2pfct)

LPF

A/D

D/A

Serial

To

Parallel

Converter

x

x

+

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

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


Ofdm design issues

OFDM Design Issues

  • 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


Intro to spread spectrum

Intro. to Spread Spectrum

  • 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


Direct sequence spread spectrum

Tc

Direct Sequence Spread Spectrum

  • 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


Isi and interference rejection

S(f)

I(f)

S(f)

S(f)*Sc(f)

I(f)*Sc(f)

Despread Signal

Receiver Input

Info. Signal

ISI and Interference Rejection

  • 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


Main points

Main Points

  • 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


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