JNIT

Seminar on OFDMOrthogonal Frequency Division Multiplexing

Presentation by:Deepesh jain ECE 07/33/122

JNIT

ObjectivesWhat is OFDM How OFDM works Principle & modulation Transmitter & receiver Types of OFDM Applications Advantage Drawback

DefinitionThe first OFDM schemes presented in 1966 . OFDM is a broadband multicarrier modulation method that offers

superior performance and benefits over older, more traditional OFDM, Quick Intro single-carrier modulation methods because it is a better fit with todays high-speed data requirements and operation in the UHF and microwave spectrum.

WHY OFDM ?High bit rate needs are clumped by the nature of communication

channels.Multi-path Propagation effects forbid increasing of transmission

rates.

A

F

How OFDM works.

It distributes the data over a large number of carriers that

are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own.

OFDM spectrum. OFDM = Orthogonal FDM Carrier centers are put on orthogonal frequencies ORTHOGONALITY - The peak of each signal coincides with trough of other signals Subcarriers are spaced by 1/Ts

Modulation

PRINCIPLEBASIC IDEA : Channel bandwidth is divided into multiple sub

channels to reduce ISI and frequency-selective fading.Multicarrier transmission : Subcarriers are orthogonal each other in

frequency domain. Time-domain spreading: Spreading is achieved in the time-domain by repeating the same information in an OFDM symbol on two different sub-bands => Frequency Diversity.Frequency-domain spreading: Spreading is achieved by choosing conjugate symmetric inputs

for the input to the IFFT (real output) Exploits frequency diversity and helps reduce the transmitter complexity/power consumption

FDM OFDMFrequency Division Multiplexing

OFDM frequency dividing

EARN IN SPECTRAL EFFICIENCY

Generic OFDM Transmitter

OFDM symbolFEC

bits

Serial to Parallel

IFFT

Pulse shaper &

DAC add cyclic extensionview this as a time to frequency mapper

Linear PA

fc

Complexity (cost) is transferred back from the digital to the analog domain!

Generic OFDM ReceiverSlot & Timing AGCSync.

Sampler

FFT

fcVCO gross offset

P/S and Detection

ErrorRecovery

fine offset

Freq. Offset Estimation(of all tones sent in one OFDM symbol)

Types of OFDMC-OFDM MIMO-OFDM V-OFDM W-OFDM Flash-OFDM

Coded OFDMDigital Audio Broadcasting (DAB) Digital Video Broadcasting (DVBT) COFDM offers real benefit in the presence of isolated narrow-band interfering signals

Multiple Input, Multiple Output OFDM (MIMO-OFDM)

Developed by Iospan Wireless Uses multiple antennas to transmit and receive radio signals Spatial multiplexing

Vector OFDMDeveloped by CISCO 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 interferencePaths

Base Station

Subscriber

Wideband OFDMInvented by Wi-LAN Large spacing between carriers Advantages: - Optimal performance against Multi-path - Less sensitive to carrier offset -Optimal power efficiency of the transmitter amplifier - More immune against fading

Flash-OFDM

Fast-hopped OFDM Wide-band spread-spectrum technology Avoids the compromises inherent in other mobile data systems Capability to work around interfering signals

Applications

Digital Audio and Video Broadcasting Asymmetric Digital Subscriber Line (ADSL) Wireless Networking Power-line Technology

AdvantageEliminates ISI Makes efficient use of the spectrum by allowing overlap. By dividing the channel into narrowband flat fading sub channels, OFDM is more resistant to frequency selective fading than single carrier systems are.

Drawback

1. The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it

2. requires RF power amplifiers with a high peak to average power ratio. It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.

THANKING YOU