this ppt is focused on general information about OFDM, its types, advantages, disadvantages and applications

1. ORTHOGONAL
FREQUENCY DIVISION
MULTIPLEXING (OFDM)
DILIP MATHURIA
M.TECH (VLSI)
160137004

2. Objectives
 What is OFDM?
 How OFDM works?
 Idealized System Model
 Types of OFDM
 Difference between ODFM and OFDMA
 Advantages
 Disadvantages
 Conclusion
 Applications

3. What is OFDM?
• OFDM is a combination of Modulation and Multiplexing.
• Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data
on multiple carrier frequencies.
• OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier
modulation method.
• A large number of closely spaced orthogonal sub-carrier signals are used to carry data on
several parallel data streams or channels.
• Each sub-carrier is modulated with a conventional modulation scheme (such as
quadrature amplitude modulation or phase-shift keying) at a low symbol rate.

4. What is an OFDM System ?
• Data is transmitted in parallel on multiple carriers that overlap in frequency.
• Although the sidebands from each carrier overlap, they can still be received
without the interference because they are orthogonal to each another.

5. How OFDM works?
• In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal
to each other, meaning that cross-talk between the sub-channels is eliminated and
inter-carrier guard bands are not required.
• This greatly simplifies the design of both the transmitter and the receiver; unlike
conventional FDM, a separate filter for each sub-channel is not required.
• The orthogonality requires that the sub-carrier spacing is f=K/Tu Hertz, where TU
seconds is the useful symbol duration (the receiver-side window size), and k is a
positive integer, typically equal to 1.
• Therefore, with N sub-carriers, the total passband bandwidth will be B ≈ N·Δf (Hz).

6. Idealized system model
Transmitter
Transmitter
 s[n] is a serial stream of binary digits, these are first demultiplexed into N parallel streams,
 Each one mapped to a symbol stream using some modulation constellation (QAM, PSK, etc.)
 An inverse FFT is computed on each set of symbols, giving a set of complex time-domain samples.
 These samples are then quadrature-mixed to passband in the standard way.
 The real and imaginary components are first converted to the analogue domain using digital-to-analogue
converters (DACs);
 The analogue signals are then used to modulate cosine and sine waves at the carrier frequency, fc, respectively.
 These signals are then summed to give the transmission signal, s(t).

7. 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 also creates signals centered on 2fc, so low-pass filters are used to reject these.
 The baseband signals are then sampled and digitized using analog-to-digital converters (ADCs), and a forward FFT
is used to convert back to the frequency domain.
 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 s[n] which is an estimate of the original binary stream at
the transmitter.

8. Data on OFDM
 The data to be transmitted on an OFDM signal is
spread across the carriers of the signal.
 This reduces the data rate taken by each carrier.
 The lower data rate has the advantage that
interference from reflections is much less
critical.
 This is achieved by adding a guard interval into
the system.
 This ensures that the data is only sampled when
the signal is stable and no new delayed signals
arrive.

9. Types of OFDM
C-OFDM
V-OFDM
W-OFDM
Flash-OFDM

10. C-OFDM
 Coded Orthogonal frequency division multiplexing.
 A form of OFDM where error correction coding is incorporated into the
signal.
 Applications:
 Digital Audio Broadcasting (DAB)
 DigitalVideo Broadcasting (DVB-T)
 Advantages:
 -COFDM offers real benefit in the presence of isolated narrow-band
interfering signals

11. V-OFDM
 This form of OFDM uses the concept of MIMO technology.
 It is being developed by CISCO Systems.
 It uses multiple antennas to transmit and receive the signals so that multi-
path effects can be utilized to enhance the signal reception and improve
the transmission speeds that can be supported.
 Advantages:
 -Increases subscriber coverage.
 -Lowers the cost of provisioning and deploying infrastructure.
 -Employs both frequency and spatial diversity.
 -Creates a robust processing technique for multi-path fading and narrow
band interference.

12. W-OFDM
 The concept of this form of OFDM is that it uses a degree of spacing
between the channels that is large enough that any frequency errors
between transmitter and receiver do not affect the performance.
 It is particularly applicable toWi-Fi systems.
 Advantages:
 - Optimal performance against Multi-path
 - Less sensitive to carrier offset
 -Optimal power efficiency of the transmitter amplifier
 - More immune against fading

13. Flash-OFDM
 This is a variant of OFDM that was developed by Flarion.
 It is a fast hopped form of OFDM.
 It uses multiple tones and fast hopping to spread signals over a given
spectrum band.
 Wide-band spread-spectrum technology .
 Advantages:
 - Avoids the compromises inherent in other mobile data systems.
 - Capability to work around interfering signals.

14. OFDMVersus OFDMA
 OFDM support multiple users (Multiple
Access) viaTDMA basis only, while OFDMA
support either onTDMA or FDMA basis or
both at the same time.
 OFDMA supports simultaneous low data
rate transmission from several users, but
OFDM can only support one user at given
moment.
 OFDMA supports per channel or sub-
carrier power while OFDM needs to
maintain the same power for all sub-
carriers.

15. Advantages
 High spectral efficiency as compared to other double sideband modulation
schemes, spread spectrum, etc.
 Can easily adapt to severe channel conditions without complex time-domain
equalization.
 Robust against narrow-band co-channel interference
 Robust against inter symbol interference (ISI) and fading caused by
multipath propagation
 Efficient implementation using fast Fourier transform (FFT)
 Low sensitivity to time synchronization errors

16. Disadvantages
 Sensitive to Doppler shift
 Sensitive to frequency synchronization problems
 High peak-to-average-power ratio (PAPR), requiring linear transmitter
circuitry, which suffers from poor power efficiency
 Loss of efficiency caused by cyclic prefix/guard interval
 Sensitive to carrier offset and drift

17. Applications
 In theWi-Fi arena where the standards like 802.11a, 802.11n, 802.11ac and
more.
 In cellular telecommunications standard LTE / LTE
 Digital Audio andVideo Broadcasting
 Asymmetric Digital Subscriber Line (ADSL)
 Wireless Networking
 Power-lineTechnology
 DVB-C2, an enhanced version of the DVB-C digital cableTV standard
 Power line communication (PLC)
 Elastic Optical Networks (EON)

18. Conclusion
 OFDM, orthogonal frequency division multiplexing has gained a significant
presence in the wireless market place.
 The combination of high data capacity, high spectral efficiency, and its
resilience to interference as a result of multi-path effects means that it is
ideal for the high data applications.
 Become a major factor in today's communications scene.

19. THANKYOU!!!!