OFDM-Wireless Communication Systems-Lecture Slides, Slides of Wireless Communication Systems

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Outline^ •^

What

is OFDM? (basic definition)

•^

Why

OFDM? (motivation)

•^

How

can we transmit via OFDM?

•^

When/Where

is OFDM used? (history & use)

•^

OFDM advantages and disadvantages

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What is OFDM?^ • Frequency-Division Multiplexing (FDM)

-^

Data sent across various frequency channels.

-^

Guard bands used to avoid interference between channels.

-^

Not very spectrally efficient.

-^

Examples are AM radio, and analog TV transmission.

3

frequency

A^

B^

C^

D^

E^

F^

G^

H^

I^

J

Guard Band

Subchannel BW

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What’s so great aboutOrthogonality?^ • Transmit waveform for a given sub-channel is orthogonal to

that of the remaining sub-channels.

• Same concept of CDMA signals having orthogonal spreading

codes.

• At the receiver, an individual sub-channel’s data can be

demodulated without interference from the others.

• Allows the receiver, in principle, to deal with each sub-

channel separately.

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Why OFDM?^ •

Next-generation systems are demanding higher and higher bitrates.^ •

i.e., requires more and more bandwidth (wider channel).

-^

wider channel increasingly less likely to be flat (spectrallyshaped).

-^

spectrally-shaped channels are the frequency-domainequivalent to time-domain dispersive channels.

•^

Multipath propagation effects in wireless channels limit theincrease of such rates.^ •

these effects cause Inter-symbol Interference (ISI).

-^

smearing of multiple adjacent data symbols with each otherwhich increase the bit-error rate.

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Why OFDM? A Simple Answer^ •

In theory, data transmitted over a given OFDM sub-channelcan be demodulated without interference from other sub-bands due to orthogonality.

-^

In theory, each sub-channel can be individually equalized witha simple complex scalar multiplication.^ •

High-rate single-carrier systems require very complicatedadaptive equalizers whose performance can degrade withfaster and faster data rates.

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How is OFDM Implemented?^ •

Difficult to use analog hardware to modulate data onto manysub-channels.

-^

Weinstein and Ebert (

) discovered that a digital complex-

baseband OFDM signal can be formed using the discreteFourier transform (DFT).^ •

DFT is an orthogonal transformation.

-^

Time domain

Discrete frequency domain.

•^

Better yet, let’s use FFTs (which are the same, butimplemented more efficiently!)

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Example: Frequency Symbol Domain

11

Real partof QPSKsymbols Imag partof QPSKsymbols

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Example: Magnitude in Time Domain

12

Magnitude of N-point time-domain signal after IFFT operation

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OFDM Transmitter

14

ChannelEncoding

IFFT

+ CP

D/A

RF Amplifier

bits

f c

Digital OFDM symbol

Pulse shaper

&

Essentially is a

time-to-frequency mapper

Major Design Issue!!!

Serial toParallel

Add Cyclic Prefix (CP)

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Some OFDM Maths?^ • OFDM uses a block transformation for modulation.• Complex (Random) Symbols:• Time-domain digital signal:• For large

N

,^

x

( n

) are approximately i.i.d complex Gaussian

with variance

•^

Gaussian distributed due to central limit theorem effect.

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OFDM Receiver

17

f c VCO

A/D

FFT

Data

Timing

CFO

Freq. OffsetEstimation

Synch.

Decoding

EqualizerChannel Est.

Parallelto Serial

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When did OFDM come about?^ •

1966

: R. W. Chang proposed OFDM for dispersive fading channels. Patent

issued

1970

.

-^

1971

: Weinstein and Ebert first proposed using the DFT for OFDM

transmission.

-^

1985

: Cimini looked at the feasibility of OFDM transmission. Does a proof

of concept design.

-^

1987

: Alard and Lasalle propose coded OFDM for digital broadcasting.

-^

1990s

: Standards and implementation of OFDM in

-^

Digital Audio Broadcasting (DAB).

-^

Asymmetric Digital Subscriber Lines (ADSL).

-^

Digital Video Broadcasting (DVB-T).

-^

Wireless LAN standards (HIPERLAN

2 , IEEE

a).

-^

What took so long for OFDM to come to realization?^ •

FFTs were too expensive to implement pre-1990’s. They are now cheapto implement, and OFDM can have less computational complexity thanconventional single-carrier systems in some systems.

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Advantages of OFDM^ •

OFDM is spectrally efficient (remember the overlap!)

-^

Robust to multipath interference.^ •

sub-channels are narrowband with essentially zero ISI.

-^

simple equalization compared to single-carrier systems.

•^

Robust to narrowband interference.^ •

can always not use (i.e. turn off) any bad sub-channels.

•^

Computationally efficient compared to single-carrier.

-^

Simple exploitation of frequency diversity (COFDM).

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Disadvantages of OFDM^ •

Very sensitive to carrier-frequency offset (CFOs)^ •

causes a loss in orthogonality resulting in inter-channelinterference (ICI).

•^

High peak-to-average power ratio (PAPR).^ •

occasional large peaks require an expensive high-poweredamplifier (HPA) for clean (i.e. linear) transmission.

-^

low power efficiency results to handle a large dynamicrange.

-^

any nonlinear amplification will destroy orthogonality!^ •

introduces out-of-band distortion, which is a big “No-No”!!

•^

Sensitive to clock frequency and timing offsets.

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