2. Introduction about OFDM.
What is OFDM.
OFDM principle.
OFDM working.
Variations of OFDM.
Applications of OFDM.
OFDMA WiMAX Physical Layer.
Advantages and Drawbacks.
Conclusion.
3. Orthogonal Frequency Division Multiplexing, is a digital
modulation technique for transmitting large amounts of
digital data over a radio wave.
OFDM works by splitting the signal into multiple smaller sub-
signals that are then transmitted simultaneously at different
frequencies to the receiver.
OFDM reduces the amount of crosstalk in signal
transmissions.
802.11a WLANS, 802.16 (WiMAX )technologies use OFDM.
4. Main idea: It is a process of splitting of higher data streams
into a number of lower data streams simultaneously.
In other words 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:
5. Orthogonality: The main aspect in OFDM is maintaining
orthogonality of the carriers.
If the integral of the product of two signals is zero over a time
period, then these two signals are said to be orthogonal to
each other.
Two sinusoids with frequencies that are integer multiples of a
common frequency can satisfy this criterion.
The orthogonality is defined as :
8. 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 by
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.
9. 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).
10. 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.
11.
12. Data to be transmitted is typically in the form of a serial data
stream. Serial to parallel conversion block is needed to
convert the input serial bit stream to the data to be transmitted
in each OFDM symbol.
The data allocated to each symbol depends on the modulation
scheme used and the number of subcarriers.
During symbol mapping the input data is converted into
complex value constellation points, according to a given
constellation. Typical constellations for wireless applications
are, BPSK, QAM, and 16 QAM.
The amount of data transmitted on each subcarrier depends
on the constellation.
13. Cyclic prefix is inserted in every block of data according to
the system specification and the data is multiplexed to a serial
fashion. At this point of time, the data is OFDM modulated
and ready to be transmitted.
A Digital-to-Analog Converter (DAC) is used to transform the
time domain digital data to time domain analog data. RF
modulation is performed and the signal is up-converted to
transmission frequency.
After the transmission of OFDM signal from the transmitter
antenna, the signals go through wireless channel.
After the receiving the signal, the receiver down converts the
signal and converts to digital domain using ADC.
14. At the time of down-conversion of received signal, carrier
frequency synchronization is performed.
After ADC conversion, symbol timing synchronization is
achieved. An FFT block is used to demodulate the OFDM
signal.
After that ,channel estimation is performed using the
demodulated pilots. Using the estimations, the complex
received data is obtained which are de-mapped
according to the transmission constellation diagram.
At this moment, FEC decoding and interleaving are used
to recover the originally transmitted bit stream.
17. MIMO-OFDM :Multiple-input, multiple-output OFDM was
developed by Iospan Wireless. Basically, MIMO-OFDM uses
OFDM to break up a signal and wirelessly transmit the pieces
simultaneously via multiple antennas.
Vector OFDM :Broadband silicon vendor Broadcom and
Cisco Systems developed VOFDM. VOFDM is most often used
in fixed-wireless metropolitan area networks (MANs).
Wideband OFDM: The OFDM Forum says Wi-LAN’s W-
OFDM should be the standard version. Rather than using
tightly packed orthogonal carriers.
F-OFDM : Flarion created F-OFDM by incorporating fast-
frequency-hopping spread spectrum technology, which
repeatedly switches frequencies during a radio transmission.
18. • Digital Audio and Video Broadcasting .
• Wireless Networking .
• Power-line Technology .
• Wireless LAN Networks.
• Wireless ATM transmission systems.
• Wi-Fi(IEEE 802.11a/g).
• IEEE 802.16 Broadband wireless access system (or) WiMAX.
19. Orthogonal Frequency Division Multiple Access (OFDMA) is
a multiple-access/multiplexing scheme that provides
multiplexing operation of data streams from multiple users
onto the downlink sub-channels and uplink multiple access by
means of uplink sub-channels.
OFDM is based on the FDM, which enables simultaneous
transmission of multiple signals by separating them into
different frequency band(subcarriers) and sending them in
Parallel and OFDM is spectrum efficient and cost effective.
For all of these reasons recent international standards such as
IEEE 802.16(WiMAX) have established OFDM as the
preferred technology of choice.
20. WiMAX stands for Worldwide Interoperability for Microwave
Access.
OFDM Parameters in WiMAX:
1.Fixed WiMAX OFDMA- Phy :The fixed and mobile
versions of WiMAX have slightly different implementations of
the OFDM physical layer. Fixed WiMAX, which is based on
IEEE 802.16- 2004, uses a 256 FFT-based OFDM physical
layer in which 192 subcarriers used for carrying data, 8 used
as pilot subcarriers for channel estimation and
synchronization purposes, and the rest used as guard band
sub-carriers. Since the FFT size is fixed, the subcarrier
spacing varies with channel bandwidth.
21. 2. Mobile WiMAX OFDMA-Phy:
It is based on the IEEE 802.16e standard, uses a scalable
OFDMA-based physical layer. In the case of mobile WiMAX,
the FFT sizes can vary from 128 bits to 2,048 bits.
Here, when the available bandwidth increases, the FFT
size is also increased such that the subcarrier spacing is
always 10.94 kHz.
This keeps the OFDM symbol duration, which is the
basic resource unit, fixed and therefore makes scaling
have minimal impact on higher layers.
22. Efficient modulation and demodulation.
High transmission bitrates.
Flexibility.
Easy equalization.
High spectral efficiency.
Low multipath distortion.
Efficiently Deals With Channel Delay Spread and Enhanced
Channel Capacity.
Efficiently Deals With Multi-path Fading.
Adaptively Modifies Modulation Density.
23. High sensitivity inter-channel interference:
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
and non-linear amplification destroys the orthogonality of
the OFDM signal and introduced out-of-band radiation.
Low noise receiver amplifiers need large dynamic range.
Capacity and power loss due to guard interval.
Bandwidth and power loss due to the guard interval can be
significant.
24. OFDM produces low hardware complexity.
It is a suitable modulation technique for high performance
wireless telecommunication.
The IEEE 802.16 Broadband wireless access system (or)
WiMAX uses the OFDMA systems.
But the goal of the WiMAX is to get 100 M bit/s for mobile
devices and 1 Giga bits per second fixed type of WiMAX
devices and 4G for next generation mobile network.
In OFDMA the frequency reuse is possible.
