Presented by :P.G.Ramya(11691a0480)

OFDM

Introduction about OFDM.

What is OFDM.

OFDM principle.

OFDM working.

Variations of OFDM.

Applications of OFDM.

OFDMA WiMAX Physical Layer.

Advantages and Drawbacks.

Conclusion.

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.

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:

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 :

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-0.2

0

0.2

0.4

0.6

0.8

Normalized Frequency (fT) --->

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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 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.

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).

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.

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.

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.

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.

SINGLE CARRIER SYSTEM MULTI-CARRIER SYSTEM

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.

• 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.

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.

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.

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.

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.

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.

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.