OFDMOrthogonal Frequency Division

Multiplexing

By

Priyadarshi Mukherjee (Roll :08102003008)

Ravi Kumar (Roll :08102003044)

Ankit Kr Khetan (Roll 1

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Multi-path Propagation…

r(t) = a0 s(t-t0) + a1 s(t-t1) + a2 s(t-t2) + a3 s(t-t3)

Multi-path Propagation --contd.

Now ,this multi-path channel can be

represented as a linear function h(t). Due to

the different propagation delays ,the

channel impulse response is the

superposition of delayed delta functions :

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Multi-path Propagation -- contd.

r(t) = a0 s(t-t0) + a1 s(t-t1) + a2 s(t-t2) + a3 s(t-t3)

channelInput

(Tx signal)

Output

(Rx signal)

Impulse

Response h(t)

t3 - t0

time

a3

a0

freq.

Frequency

Response H(f)

Multi-path Propagation --contd.

Since the multi-path delays are distinct ,the

frequency response of the medium will

introduce amplitude fluctuations in the

broadband signal .Such fluctuations in the

frequency domain will distort the waveform of a

signal .

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Frequency Selective Fading...

Time2.0 secs 2.5 secs 3.0

secs

Fading

Frequency Selective Fading Channels provide

-- time diversity (can be exploited in DS-CDMA)

-- frequency diversity (can be exploited in OFDM ,which is our SEMINAR topic)

Multi-carrier Transmission…

This very problem of “frequency selective

fading” gets solved when data is transmitted in

parallel :on multiple carriers that overlap in

frequency ,but are orthogonal to each other

.Here arrives the very concept of Orthogonal

Frequency Division Multiplexing (OFDM).

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What is an OFDM System ?

Data is transmitted in parallel :on multiple carriers that

overlap in frequency and also are orthogonal to each

other .

F

A

OFDM :definition…

OFDM = Orthogonal FDM

Carrier centers are put on orthogonal frequencies

ORTHOGONALITY - The peak of each signal coincides

with trough of other signals

Subcarriers are spaced by 1/Ts

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FDM OFDM

Frequency Division Multiplexing

OFDM frequency dividing

EARN IN SPECTRAL EFFICIENCY

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How OFDM works…

BASIC IDEA : Channel bandwidth is divided into

multiple sub channels to reduce ISI and

frequency-selective fading.

Multicarrier transmission : Subcarriers are

orthogonal each other in frequency domain.

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Basic OFDM transceiver…

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OFDM advantages …

OFDM is spectrally efficient

◦ IFFT/FFT operation ensures that sub-carriers do not

interfere with each other.

Equalization is very simple compared to Single-Carrier

systems.

OFDM has excellent robustness in multi-path

environments.

Cyclic prefix acts like a guard interval and it compensate

delay spread.

Suited for high-bit rate applications.

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OFDM drawbacks…

The tightly packed subcarriers give rise to

increased sensitivity with respect to carrier

frequency errors (oscillator impairments etc.) and

interchannel interference (ICI) .

The OFDM time-domain signal has a relatively large peak-to-average power ratio(PAPR)

◦ tends to reduce the power efficiency of the RF amplifier.

◦ non-linear amplification destroys the orthogonality of the OFDM signal and introduced out-of-band radiation.

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Applications…

Asymmetric Digital Subscriber Line (ADSL): provides internet service, use large BW upto1MHz(255 equally spaced subcarriers).

Digital Audio Broadcasting (DAB):covers both terrestrial digital & satellite audio broadcasting without directional receiving antennas.

Digital Video Broadcasting (DVB):specifies

three OFDM transmission modes-2K

mode(2048 subcarriers), 4K mode(4096

subcarriers), 8K mode(8196 subcarriers).

Each subcarriers can have three modulation

formats:QPSK,16-QAM,or 64-QAM.15

Conclusion…

OFDM is emerging as popular solution forwireless LAN, and also for fixed broad-bandaccess.

BUT….Will OFDM be cost-effective? If not right now,

when?

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References…

OFDM-Based Broadband Wireless Networks

Design and Optimization

-- Hui Liu and Guoqing Li.

Communication Systems (4th edition)

--Simon Haykin.

Theory and Applications of OFDM and CDMA

--Henrik Schulze and Christian Lűders.

Modern Digital and Analog Communication Systems (International Edition)

--B.P Lathi and Zhi Ding.

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Acknowledgement…

We are greatly indebted to Sir Anup Dey for sharing with us his invaluable knowledge on this topic .Moreover we are also thankful to all our friends for having the patience to listen to whatever we have said, or at least tried to say .

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