Ee359 lecture 18 outline
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EE359 – Lecture 18 Outline. Review of Last Lecture Multicarrier Modulation Overlapping Substreams OFDM FFT Implementation OFDM Design Issues. Review of Last Lecture. Decision-Feedback Decoders Linear ML receiver except for error propagation Sphere Decoder (near ML):

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EE359 – Lecture 18 Outline

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Ee359 lecture 18 outline

EE359 – Lecture 18 Outline

  • Review of Last Lecture

  • Multicarrier Modulation

  • Overlapping Substreams

  • OFDM FFT Implementation

  • OFDM Design Issues


Review of last lecture

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


Multicarrier modulation

S

cos(2pf0t)

cos(2pfNt)

x

x

Multicarrier Modulation

R/N bps

  • 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)

QAM

Modulator

R bps

Serial

To

Parallel

Converter

R/N bps

QAM

Modulator


Overlapping substreams

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+b).

    • Total required bandwidth is B/2 (for TN=1/BN)

B/N

fN-1

f0


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

TX

Add cyclic

prefix and

Parallel

To Serial

Convert

R bps

QAM

Modulator

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


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

  • 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


Main points

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


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