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OFDM with pulse shaping

Gang LIN

Department of Electronics and telecommunicationsAugust 26, 2004

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Traditional Frequency Division

Multiplex (FDM)

Sub-channels are completely separated in

frequency

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Orthogonal Frequency Division

Multiplex (OFDM)

There exists intersection between sub-channels but

sub-channels are orthogonal to each other.

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Why we use OFDM in wireless channel?

OFDM is much more robust in wireless fading

channel than single carrier system

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Two main categories of OFDM

OFDM/QAM with guard interval:

- The most common type of OFDM

- Used in ADSL, DVB, IEEE 802.11, etc. OFDM/OQAM with pulse shaping:

- Improved power and bandwidth efficiency

- Lower side lobes in frequency domain

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OFDM/QAM with guard interval

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Guard interval to eliminate ISI in

OFDM/QAM

If the maximum delay of multi-path channel is lessthan the guard interval , there will be no inter-

symbol interference (ISI) and inter-channel

interference (ICI); The guard interval can beim lemented b insertin c clic refix or suffix.

Effi i h f OFDM/QAM

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Efficient scheme for OFDM/QAM

based on DFT

S h i i f OFDM/QAM

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Some characteristics of OFDM/QAM

The DFT based scheme has low complexity; The receiver pulse is not matched with transmitter

pulse. SNR loss:

The guard interval will reduce the information rate:

Usually,

)1log(10 TSNR

loss

+=

)1(1 TBWloss +=

T2.0=

B i bl k di f OFDM/OQAM

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Basic block diagram of OFDM/OQAM

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Pulse design for OFDM/OQAM

Principles for the design of pulse:

- Zero ISI and ICI

- Concentrated in time and/or frequency

Example:(1) Square root raised cosine pulse with a roll off factor less

than 1, which is strictly band limited to [-1/T,1/T];

(2) Optimal finite duration pulses with maximum energy

concentration in [-1/T,1/T].

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Modulator of OFDM/OQAM based on DFT

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Demodulator of OFDM/OQAM based on DFT

E li ti f OFDM/OQAM

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Equalization for OFDM/OQAM

OFDM/OQAM need equalizer in multi-path

channel; If the equalizer eliminates ISI, then ICI is also

eliminated;

If the number of sub-channel is large enough, the

channel can be viewed as flat fading. Then a one-

tap equalizer is sufficient.

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A rough Comparison of OFDM/QAM

and OFDM/OQAM

No reductionInformation rate

reduction

YesOne-tap equalizerEqualizer

(Cariolanos scheme)

Total computations

per complexsymbol period

SNR loss (dB)

OFDM/OQAMOFDM/QAM

)1(log5 2 NN )1(log54 2 + NNP

)1log(10T

+ 0

)1(1 T+

P: the length of shaping pulse with respect toan over sam lin interval T/N

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Work done so far

Implementation of OFDM/OQAM;

Equalization for OFDM/OQAM over multi-path

fading channel; Design of pulses robust to carrier frequency offset.

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Future research

Synchronization;

- Timing and frequency offset estimation

- Joint timing and frequency offset and channel estimation

A comparison of OFDM/QAM and

OFDM/OQAM;

Wireless MIMO system based on OFDM/OQAM.