seminar on ofdm
TRANSCRIPT
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