OFDM is a digital multi-carrier modulation technique that encodes data over multiple carrier frequencies. It splits a high-rate data stream into multiple lower-rate streams that are transmitted simultaneously over separate subcarriers. The subcarriers are chosen so they are orthogonal to each other, allowing them to be packed closely together without interference. OFDM provides high spectral efficiency, resilience to radio interference and multipath distortion, and works well for broadband applications including wireless networks, digital television and audio broadcasting.

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 Introduction about OFDM.
 What is OFDM.
 OFDM Mechanism.
 Variation of OFDM.
 Different Types of
Multiplexing.
 Applications of OFDM.
 Advantages and Drawbacks
 Conclusion.

3. In telecommunications, orthogonal
frequency-division multiplexing (OFDM)
is a method of encoding digital data on
multiple carrier frequencies.
OFDM is a modulation technique for
transmitting large amounts of digital
data over a radio wave.

4.  OFDM technology was first conceived of in the
1960s and 1970s during research into
minimizing interference among channels near
each other in frequency, and
 To achieve clean data transmission in situations
prone to interference and signal corruption when
more conventional modulation schemes are
used.

5. *Orthogonality:
The “orthogonal” part of the OFDM name
indicates that there is a precise
mathematical relationship between the
frequencies of the carriers in the system
Wireless, The OFDM modulation scheme
offers many advantages for broadband
wireless transport. -It supports high data
rates

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It is a special kind of FDM
The spacing between carriers are such
that they are orthogonal to one another
Therefore no need of guard band
between carriers.
One example makes the thing clear

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 Vector space
• A, B and C vectors in space
are orthogonal to each other
• A.B=B.C=C.A=0
• (A+B+C).A=(mod A)^2
• (A+B+C).B=(mod B)^2
• (A+B+C).C=(mod C)^2
A
B
C

8.  The use of the IFFT in OFDM- By the use of
the Inverse Fast Fourier Transform (IFFT)
algorithm. It can be better because it allows
precise control

9.  Certain types of broadband access through POTS
(plain old telephone service) copper wiring
 Power line communication (PLC)
 Multimedia over Coax Alliance (MoCA) home
networking
 Some digital radio systems
 Some digital TV systems
 Some mobile TV systems
 OFDM is used in Wi-Fi, DSL internet
access, 4Gwireless communications, and digital
televisionand radio broadcast services.

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 Main idea: split data stream into N parallel streams of
reduced data rate and transmit each on a separate
subcarrier.
 When the subcarriers have appropriate spacing to satisfy
orthogonality, their spectra will overlap. OFDM
modulation is equivalent to the IDFT:

11.  Orthogonal frequency division multiplexing (OFDM) is a
technique, method or scheme for digital multi-carrier
modulation using many closely spaced subcarriers - a
previously modulated signal modulated into another signal of
higher frequency and bandwidth.
 Each of these subcarriers contains numbers of parallel data
streams or channels and is modulated conventionally at a
low symbol rate; these are groups of bits of data related to
(but not the same as) gross bitrate, which is expressed in
bits/second.

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 Method of Digital Communication that breaks a large bandwidth into
small subcarriers using the Inverse Fast Fourier Transform (IFFT).
 Removes Intersymbol Interference (ISI) by having subcarrier
frequency be integer multiples of the symbol rate.
 By dividing total bandwidth into independent subchannels, multiple
access is achieved by distributing subchannels between users.
 Allows for higher data rates by allocating power and subchannels to
users through Adaptive Modulation.

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 Parallel Data Streams
 The available frequency spectrum is
divided into several sub-channels
 low-rate bit stream is transmitted
over one sub-channel by modulating
a sub-carrier using a standard
modelation scheme, for example 4-
QAM
 Multiple Carriers are combined
through the Fourier Series
• Computed by Inverse Fast
Fourier transform
4-QAM modulation

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 Spectra of Individual Sub-Carriers.
 Sub-Carrier Spacing = 1/(Symbol Duration)
 Slow-Roll off avoided using Raised Cosine (RC) Windowing.
-6 -4 -2 0 2 4 6
-0.2
0
0.2
0.4
0.6
0.8
Normalized Frequency (fT) --->
Normalized
Amplitude
--->

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 OFDM Transmitter :
An OFDM carrier signal is the sum of a number of orthogonal sub-carriers,
with base band data on each sub-carrier being independently modulated
commonly using some type of quadrature amplitude modulation (QAM) or
phase-shift keying (PSK).

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 OFDM Receiver:
The receiver picks up the signal r(t), which is then quadrature-
mixed down to baseband using cosine and sine waves at the
carrier frequency. This returns N parallel streams, each of
which is converted to a binary stream using an appropriate
symbol detector. These streams are then re-combined into a
serial stream, , which is an estimate of the original binary
stream at the transmitter.

17. The IEEE 802.16e/ WiMax use OFDMA as
Multiple access technique
• Bandwidth options 1.25, 5, 10, or 20 MHz
• Entire bandwidth divided into 128, 512, 1024 or
2048 sub carriers
• 20 MHz bandwidth with 2048 sub carriers has 9.8
KHz spacing between sub carriers

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MIMO-OFDM
VOFDM (Vector OFDM)
WOFDM - Wideband OFDM
 Flash OFDM - Flarion (Lucent/Bell Labs
spinoff)

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 FDM frequency division multiplexing
(FDM) is a technology that transmits multiple signals simultaneously over a
single transmission path.
 TDM (synchronous)
Time-division multiplexing (TDM) is a method of putting multiple data
streams in a single signal by separating the signal into many segments,
each having a very short duration.
 Wavelength-Division Multiplexing (WDM)
technology which multiplexes multiple optical carrier signals on a single
optical fibre by using different wavelengths (colours) of laser light to carry
different signals.

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 Digital Video Broadcasting
 Digital Audio Broadcasting
 ADSL
 Wireless LANs
 OFDMA -Multiple Access.

22.  OFDM is used for wideband digital communication,
which is commonly used for digital television and audio
broadcasting (radio) as well as broadband Internet
access and wireless networking.
 OFDM is very similar to FDM (frequency division
multiplexing) but with technology purposely
emphasizing the minimization of crosstalk or signal
interference from other nearby signal carrying
communication mediums.
 OFDM uses many narrow band signals as opposed to
a signal modulated at a high symbol rate and a large
bandwidth.

23.  Allows carriers to overlap (no guard band), resulting in
lesser wasted bandwidth without any Inter Carrier
Interference (ICI)
 High data rate distributed over multiple carriers resulting
in lower symbol rate (more immune to ISI)
 Permits higher data rate as compared to FDM
 Increased security and bandwidth efficiency possible
using CDMA – OFDM (MC-CDMA)
 Simple guard intervals make the system more robust to
multipath effects.

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 OFDM is spectrally efficient
• IFFT/FFT operation ensures that sub-carriers do not interfere
with each other.
 OFDM has an inherent robustness against narrowband interference.
• Narrowband interference will affect at most a couple
of subchannels.
• Information from the affected subchannels can be
erased and recovered via the forward error
correction (FEC) codes.
 Equalization is very simple compared to Single-Carrier systems

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 OFDM has excellent robustness in multi-path environments.
• Cyclic prefix preserves orthogonality between sub-
carriers.
• Cyclic prefix allows the receiver to capture multi-
path energy more efficiently.
 Ability to comply with world-wide regulations:
• Bands and tones can be dynamically turned on/off
to comply with changing regulations.
 Coexistence with current and future systems:
• Bands and tones can be dynamically turned on/off
for enhanced coexistence with the other devices.

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Multi-user Diversity
• broadband signals experience frequency
selective fading
• OFDMA allows different users to transmit over
different portions of the broadband spectrum
(traffic channel)
• Different users perceive different channel
qualities, a deep faded channel for one user
may still be favorable to others

27. Multi-user Diversity

28. Efficient use of Spectrum
4/3 Hz per symbol
6/5 Hz per symbol

29.  Receiver Simplicity
• It eliminates the intra-cell interference avoiding
CDMA type of multi-user detection
• Orthogonality of code destroyed by selective
fading
• Only FFT processor is required
 Bit Error Rate performance is better only in Fading
environment

30.  Peak to average power
ratio (PAPR)
avg
P
t
x
PAPR
2
)
(

The large amplitude variation increases in-band noise and increases the
BER when the signal has to go through amplifier nonlinearities.

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 Synchronization
• Tight Synchronization between users are required
for FFT in receiver
• Pilot signals are used for synchronizations
 Co-channel interference
• Dealing with this is more complex in OFDM than in CDMA
• Dynamic channel allocation with advanced
coordination among adjacent base stations

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 High sensitivity inter-channel interference, ICI
 OFDM is sensitive to frequency, clock and phase offset
 The OFDM time-domain signal has a relatively large peak-to-average
ratio
• 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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 OFDM and Adaptive Modulation allow for increased
performance in a time-varying channel
 Complicated communications system between three
software applications on two different processors
 Root-finding and Linear methods handle allocations with
clear tradeoffs.

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 Future works
• Peak-to-average power reduction in OFDM
• Timing and Frequency Synchronization
• Efficient digital signal processing Implementation of OFDM
• Multiple input/Multiple output (MIMO) OFDM
Conclusion
• Different variations of OFDMA are proposed
and have different pros and cons

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Thank you for listening
Any Questions?