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

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
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)
Digital Video 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 to Wi-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. OFDM Versus OFDMA
OFDM support multiple users (Multiple Access) via TDMA basis only, while OFDMA support either on TDMA 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 the Wi-Fi arena where the standards like a, n, ac and more.
In cellular telecommunications standard LTE / LTE
Digital Audio and Video Broadcasting
Asymmetric Digital Subscriber Line (ADSL)
Wireless Networking
Power-line Technology
DVB-C2, an enhanced version of the DVB-C digital cable TV 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. THANK YOU!!!!