ofdm- an introduction
DESCRIPTION
Its an introduction to the very concept of OFDM.TRANSCRIPT
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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