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# Orthogonal frequency division multiplexing (ofdm)

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### Orthogonal frequency division multiplexing (ofdm)

1. 1. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) DILIP MATHURIA M.TECH (VLSI) 160137004
2. 2. Objectives  What is OFDM?  How OFDM works?  Idealized System Model  Types of OFDM  Difference between ODFM and OFDMA  Advantages  Disadvantages  Conclusion  Applications
3. 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. 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. 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. 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. 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. 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. 9. Types of OFDM C-OFDM V-OFDM W-OFDM Flash-OFDM
10. 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. 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. 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. 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. 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. 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. 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. 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. 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. 19. THANKYOU!!!!