Orthogonal Frequency DIGITAL modulation

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Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier

frequencies. OFDM has developed into a popular scheme forwidebanddigital communication, whetherwirelessor

overcopperwires, used in applications such as digital television and audio broadcasting,DSLbroadband internet

access, wireless networks, and4Gmobile communications.

OFDM is essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT), and is

afrequency-division multiplexing(FDM) scheme used as a digital multi-carriermodulationmethod. The word "coded"

comes from the use offorward error correction(FEC).[1]

A large number of closely spacedorthogonalsub-carrier

signalsare used to carrydata[1]

on severalparalleldata streams or channels. Each sub-carrier is modulated with a

conventional modulation scheme (such asquadrature amplitude modulationorphase-shift keying) at a lowsymbol

rate, maintaining total data rates similar to conventional single-carriermodulation schemes in the same bandwidth.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with severechannelconditions (for

example,attenuationof high frequencies in a long copper wire, narrowbandinterferenceand frequency-

selectivefadingdue tomultipath) without complex equalization filters. Channelequalizationis simplified because

OFDM may be viewed as using many slowly modulatednarrowbandsignals rather than one rapidly

modulatedwidebandsignal. The low symbol rate makes the use of aguard intervalbetween symbols affordable,

making it possible to eliminateintersymbol interference(ISI) and utilize echoes and time-spreading (on analogue TV

these are visible asghostingand blurring, respectively) to achieve adiversity gain, i.e. asignal-to-noiseratioimprovement. This mechanism also facilitates the design ofsingle frequency networks(SFNs), where several

adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant

transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-

carrier system.

Contents

[hide]

1 Example of applications

o 1.1 Cable

o 1.2 Wireless

2 Key features

o 2.1 Summary of advantages

o 2.2 Summary of disadvantages

3 Characteristics and principles of operation

o 3.1 Orthogonality

o 3.2 Implementation using the FFT algorithm

o 3.3 Guard interval for elimination of intersymbol interference

o 3.4 Simplified equalization

o 3.5 Channel coding and interleaving

o 3.6 Adaptive transmission

o 3.7 OFDM extended with multiple access

o 3.8 Space diversity
http://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/Digital_communicationhttp://en.wikipedia.org/wiki/Digital_communicationhttp://en.wikipedia.org/wiki/Digital_communicationhttp://en.wikipedia.org/wiki/Wirelesshttp://en.wikipedia.org/wiki/Wirelesshttp://en.wikipedia.org/wiki/Wirelesshttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Digital_subscriber_linehttp://en.wikipedia.org/wiki/Digital_subscriber_linehttp://en.wikipedia.org/wiki/Broadband_internet_accesshttp://en.wikipedia.org/wiki/Broadband_internet_accesshttp://en.wikipedia.org/wiki/Broadband_internet_accesshttp://en.wikipedia.org/wiki/Broadband_internet_accesshttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Forward_error_correctionhttp://en.wikipedia.org/wiki/Forward_error_correctionhttp://en.wikipedia.org/wiki/Forward_error_correctionhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-cobas-1http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-cobas-1http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-cobas-1http://en.wikipedia.org/wiki/Orthogonality#Communicationshttp://en.wikipedia.org/wiki/Orthogonality#Communicationshttp://en.wikipedia.org/wiki/Subcarrierhttp://en.wikipedia.org/wiki/Subcarrierhttp://en.wikipedia.org/wiki/Subcarrierhttp://en.wikipedia.org/wiki/Subcarrierhttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Crosstalk_(electronics)http://en.wikipedia.org/wiki/Crosstalk_(electronics)http://en.wikipedia.org/wiki/Crosstalk_(electronics)http://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Phase-shift_keyinghttp://en.wikipedia.org/wiki/Phase-shift_keyinghttp://en.wikipedia.org/wiki/Phase-shift_keyinghttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Channel_(communications)http://en.wikipedia.org/wiki/Channel_(communications)http://en.wikipedia.org/wiki/Channel_(communications)http://en.wikipedia.org/wiki/Attenuation_distortionhttp://en.wikipedia.org/wiki/Attenuation_distortionhttp://en.wikipedia.org/wiki/Attenuation_distortionhttp://en.wikipedia.org/wiki/Interference_(communication)http://en.wikipedia.org/wiki/Interference_(communication)http://en.wikipedia.org/wiki/Interference_(communication)http://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Multipath_propagationhttp://en.wikipedia.org/wiki/Multipath_propagationhttp://en.wikipedia.org/wiki/Multipath_propagationhttp://en.wikipedia.org/wiki/Equalizationhttp://en.wikipedia.org/wiki/Equalizationhttp://en.wikipedia.org/wiki/Equalizationhttp://en.wikipedia.org/wiki/Narrowbandhttp://en.wikipedia.org/wiki/Narrowbandhttp://en.wikipedia.org/wiki/Narrowbandhttp://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/Guard_intervalhttp://en.wikipedia.org/wiki/Guard_intervalhttp://en.wikipedia.org/wiki/Guard_intervalhttp://en.wikipedia.org/wiki/Intersymbol_interferencehttp://en.wikipedia.org/wiki/Intersymbol_interferencehttp://en.wikipedia.org/wiki/Intersymbol_interferencehttp://en.wikipedia.org/wiki/Ghosting_(television)http://en.wikipedia.org/wiki/Ghosting_(television)http://en.wikipedia.org/wiki/Ghosting_(television)http://en.wikipedia.org/wiki/Diversity_gainhttp://en.wikipedia.org/wiki/Diversity_gainhttp://en.wikipedia.org/wiki/Diversity_gainhttp://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://en.wikipedia.org/wiki/Single_frequency_networkhttp://en.wikipedia.org/wiki/Single_frequency_networkhttp://en.wikipedia.org/wiki/Single_frequency_networkhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Example_of_applicationshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Example_of_applicationshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Cablehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Cablehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wirelesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wirelesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Key_featureshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Key_featureshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Summary_of_advantageshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Summary_of_advantageshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Summary_of_disadvantageshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Summary_of_disadvantageshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Orthogonalityhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Orthogonalityhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Implementation_using_the_FFT_algorithmhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Implementation_using_the_FFT_algorithmhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Guard_interval_for_elimination_of_intersymbol_interferencehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Guard_interval_for_elimination_of_intersymbol_interferencehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Simplified_equalizationhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Simplified_equalizationhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Channel_coding_and_interleavinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Channel_coding_and_interleavinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Adaptive_transmissionhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Adaptive_transmissionhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#OFDM_extended_with_multiple_accesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#OFDM_extended_with_multiple_accesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Space_diversityhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Space_diversityhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Space_diversityhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#OFDM_extended_with_multiple_accesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Adaptive_transmissionhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Channel_coding_and_interleavinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Simplified_equalizationhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Guard_interval_for_elimination_of_intersymbol_interferencehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Implementation_using_the_FFT_algorithmhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Orthogonalityhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Summary_of_disadvantageshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Summary_of_advantageshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Key_featureshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wirelesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Cablehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Example_of_applicationshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Single_frequency_networkhttp://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://en.wikipedia.org/wiki/Diversity_gainhttp://en.wikipedia.org/wiki/Ghosting_(television)http://en.wikipedia.org/wiki/Intersymbol_interferencehttp://en.wikipedia.org/wiki/Guard_intervalhttp://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/Narrowbandhttp://en.wikipedia.org/wiki/Equalizationhttp://en.wikipedia.org/wiki/Multipath_propagationhttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Interference_(communication)http://en.wikipedia.org/wiki/Attenuation_distortionhttp://en.wikipedia.org/wiki/Channel_(communications)http://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Phase-shift_keyinghttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Crosstalk_(electronics)http://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Subcarrierhttp://en.wikipedia.org/wiki/Subcarrierhttp://en.wikipedia.org/wiki/Orthogonality#Communicationshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-cobas-1http://en.wikipedia.org/wiki/Forward_error_correctionhttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/Broadband_internet_accesshttp://en.wikipedia.org/wiki/Broadband_internet_accesshttp://en.wikipedia.org/wiki/Digital_subscriber_linehttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Wirelesshttp://en.wikipedia.org/wiki/Digital_communicationhttp://en.wikipedia.org/wiki/Wideband


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o 3.9 Linear transmitter power amplifier

4 Idealized system model

o 4.1 Transmitter

o 4.2 Receiver

5 Mathematical description

6 Usage

o 6.1 OFDM system comparison table

o 6.2 ADSL

o 6.3 Powerline Technology

o 6.4 Wireless local area networks (LAN) and metropolitan area networks (MAN)

o 6.5 Wireless personal area networks (PAN)

o 6.6 Terrestrial digital radio and television broadcasting

6.6.1 DVB-T

6.6.2 SDARS

6.6.3 COFDM vs VSB

6.6.4 Digital radio

6.6.5 BST-OFDM used in ISDB

o 6.7 Ultra-wideband

o 6.8 FLASH-OFDM

7 History

8 See also

9 References

10 External links

Example of applications [edit]

The following list is a summary of existing OFDM based standards and products. For further details, see

theUsagesection at the end of the article.

Cable [edit]

ADSLandVDSLbroadband access viaPOTScopperwiring.

DVB-C2, an enhanced version of theDVB-Cdigital cable TV standard.

Power line communication(PLC).

ITU-TG.hn, a standard which provides high-speed local area networking of existing home wiring (power lines,

phone lines and coaxial cables).

TrailBlazertelephone linemodems.

Multimedia over Coax Alliance(MoCA) home networking.
http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Linear_transmitter_power_amplifierhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Linear_transmitter_power_amplifierhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Idealized_system_modelhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Idealized_system_modelhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Transmitterhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Transmitterhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Receiverhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Receiverhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Mathematical_descriptionhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Mathematical_descriptionhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Usagehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Usagehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#ADSLhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#ADSLhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Powerline_Technologyhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Powerline_Technologyhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wireless_local_area_networks_.28LAN.29_and_metropolitan_area_networks_.28MAN.29http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wireless_local_area_networks_.28LAN.29_and_metropolitan_area_networks_.28MAN.29http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wireless_personal_area_networks_.28PAN.29http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wireless_personal_area_networks_.28PAN.29http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Terrestrial_digital_radio_and_television_broadcastinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Terrestrial_digital_radio_and_television_broadcastinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#DVB-Thttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#DVB-Thttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#SDARShttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#SDARShttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#COFDM_vs_VSBhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#COFDM_vs_VSBhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Digital_radiohttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Digital_radiohttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#BST-OFDM_used_in_ISDBhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#BST-OFDM_used_in_ISDBhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Ultra-widebandhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Ultra-widebandhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#FLASH-OFDMhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#FLASH-OFDMhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Historyhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Historyhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#See_alsohttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#See_alsohttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Referenceshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Referenceshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#External_linkshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#External_linkshttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=1http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Usagehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Usagehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Usagehttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=2http://en.wikipedia.org/wiki/ADSLhttp://en.wikipedia.org/wiki/ADSLhttp://en.wikipedia.org/wiki/VDSLhttp://en.wikipedia.org/wiki/VDSLhttp://en.wikipedia.org/wiki/VDSLhttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/DVB-Chttp://en.wikipedia.org/wiki/DVB-Chttp://en.wikipedia.org/wiki/DVB-Chttp://en.wikipedia.org/wiki/DVB-Chttp://en.wikipedia.org/wiki/DVB-Chttp://en.wikipedia.org/wiki/Power_line_communicationhttp://en.wikipedia.org/wiki/Power_line_communicationhttp://en.wikipedia.org/wiki/ITU-Thttp://en.wikipedia.org/wiki/G.hnhttp://en.wikipedia.org/wiki/G.hnhttp://en.wikipedia.org/wiki/G.hnhttp://en.wikipedia.org/wiki/TrailBlazerhttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliancehttp://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliancehttp://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliancehttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/TrailBlazerhttp://en.wikipedia.org/wiki/G.hnhttp://en.wikipedia.org/wiki/ITU-Thttp://en.wikipedia.org/wiki/Power_line_communicationhttp://en.wikipedia.org/wiki/DVB-Chttp://en.wikipedia.org/wiki/DVB-Chttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/VDSLhttp://en.wikipedia.org/wiki/ADSLhttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=2http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Usagehttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=1http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#External_linkshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Referenceshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#See_alsohttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Historyhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#FLASH-OFDMhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Ultra-widebandhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#BST-OFDM_used_in_ISDBhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Digital_radiohttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#COFDM_vs_VSBhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#SDARShttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#DVB-Thttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Terrestrial_digital_radio_and_television_broadcastinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wireless_personal_area_networks_.28PAN.29http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Wireless_local_area_networks_.28LAN.29_and_metropolitan_area_networks_.28MAN.29http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Powerline_Technologyhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#ADSLhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Usagehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Mathematical_descriptionhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Receiverhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Transmitterhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Idealized_system_modelhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Linear_transmitter_power_amplifier


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Wireless [edit]

Thewireless LAN(WLAN) radio interfacesIEEE 802.11a,g,nandHIPERLAN/2.

Thedigital radiosystemsDAB/EUREKA 147,DAB+,Digital Radio Mondiale,HD Radio,T-DMBandISDB-TSB.

The terrestrialdigital TVsystemsDVB-TandISDB-T.

The terrestrialmobile TVsystemsDVB-H,T-DMB,ISDB-TandMediaFLOforward link. The wirelesspersonal area network(PAN)ultra-wideband(UWB)IEEE 802.15.3aimplementation suggested

byWiMedia Alliance.

The OFDM basedmultiple accesstechnologyOFDMAis also used in several4Gand pre-4Gcellular

networksandmobile broadbandstandards:

The mobility mode of thewireless MAN/broadband wireless access(BWA) standardIEEE 802.16e(or Mobile-

WiMAX).

Themobile broadband wireless access(MBWA) standardIEEE 802.20.

the downlink of the3GPPLong Term Evolution(LTE) fourth generation mobile broadband standard. The radio

interface was formerly named High Speed OFDM Packet Access (HSOPA), now namedEvolved UMTS

Terrestrial Radio Access(E-UTRA).Key features [edit]

The advantages and disadvantages listed below are further discussed in theCharacteristics and principles of

operationsection below.

Summary of advantages [edit]

Highspectral efficiencyas compared to other doublesidebandmodulation schemes, spread spectrum, etc.

Can easily adapt to severe channel conditions without complex time-domain equalization.

Robust against narrow-band co-channel interference.

Robust againstintersymbol interference(ISI) and fading caused by multipath propagation.

Efficient implementation usingFast Fourier Transform(FFT).

Low sensitivity to time synchronization errors.

Tuned sub-channel receiver filters are not required (unlike conventionalFDM).

Facilitatessingle frequency networks(SFNs); i.e., transmittermacrodiversity.

Summary of disadvantages [edit]

Sensitive toDoppler shift.

Sensitive to frequency synchronization problems.

Highpeak-to-average-power ratio(PAPR), requiring linear transmitter circuitry, which suffers from poor power

efficiency.

Loss of efficiency caused bycyclic prefix/guard interval.

Characteristics and principles of operation [edit]

Orthogonality [edit]

Conceptually, OFDM is a specialized FDM, the additional constraint being: all the carrier signals are orthogonal to

each other.

In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers areorthogonalto each other, meaning

thatcross-talkbetween the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly
http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=3http://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/IEEE_802.11ahttp://en.wikipedia.org/wiki/IEEE_802.11ahttp://en.wikipedia.org/wiki/IEEE_802.11ahttp://en.wikipedia.org/wiki/IEEE_802.11g-2003http://en.wikipedia.org/wiki/IEEE_802.11g-2003http://en.wikipedia.org/wiki/IEEE_802.11g-2003http://en.wikipedia.org/wiki/IEEE_802.11n-2009http://en.wikipedia.org/wiki/IEEE_802.11n-2009http://en.wikipedia.org/wiki/IEEE_802.11n-2009http://en.wikipedia.org/wiki/HIPERLAN/2http://en.wikipedia.org/wiki/HIPERLAN/2http://en.wikipedia.org/wiki/HIPERLAN/2http://en.wikipedia.org/wiki/Digital_radiohttp://en.wikipedia.org/wiki/Digital_radiohttp://en.wikipedia.org/wiki/Digital_radiohttp://en.wikipedia.org/wiki/Eureka_147http://en.wikipedia.org/wiki/Eureka_147http://en.wikipedia.org/wiki/Eureka_147http://en.wikipedia.org/wiki/DAB%2Bhttp://en.wikipedia.org/wiki/DAB%2Bhttp://en.wikipedia.org/wiki/DAB%2Bhttp://en.wikipedia.org/wiki/Digital_Radio_Mondialehttp://en.wikipedia.org/wiki/Digital_Radio_Mondialehttp://en.wikipedia.org/wiki/Digital_Radio_Mondialehttp://en.wikipedia.org/wiki/HD_Radiohttp://en.wikipedia.org/wiki/HD_Radiohttp://en.wikipedia.org/wiki/HD_Radiohttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/ISDB-TSBhttp://en.wikipedia.org/wiki/ISDB-TSBhttp://en.wikipedia.org/wiki/ISDB-TSBhttp://en.wikipedia.org/wiki/Digital_TVhttp://en.wikipedia.org/wiki/Digital_TVhttp://en.wikipedia.org/wiki/Digital_TVhttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/Mobile_TVhttp://en.wikipedia.org/wiki/Mobile_TVhttp://en.wikipedia.org/wiki/Mobile_TVhttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/MediaFLOhttp://en.wikipedia.org/wiki/MediaFLOhttp://en.wikipedia.org/wiki/MediaFLOhttp://en.wikipedia.org/wiki/Personal_area_networkhttp://en.wikipedia.org/wiki/Personal_area_networkhttp://en.wikipedia.org/wiki/Personal_area_networkhttp://en.wikipedia.org/wiki/Ultra-widebandhttp://en.wikipedia.org/wiki/Ultra-widebandhttp://en.wikipedia.org/wiki/Ultra-widebandhttp://en.wikipedia.org/wiki/IEEE_802.15.3ahttp://en.wikipedia.org/wiki/IEEE_802.15.3ahttp://en.wikipedia.org/wiki/IEEE_802.15.3ahttp://en.wikipedia.org/wiki/WiMedia_Alliancehttp://en.wikipedia.org/wiki/WiMedia_Alliancehttp://en.wikipedia.org/wiki/WiMedia_Alliancehttp://en.wikipedia.org/wiki/Multiple_accesshttp://en.wikipedia.org/wiki/Multiple_accesshttp://en.wikipedia.org/wiki/Multiple_accesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiple_accesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiple_accesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiple_accesshttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Mobile_broadbandhttp://en.wikipedia.org/wiki/Mobile_broadbandhttp://en.wikipedia.org/wiki/Mobile_broadbandhttp://en.wikipedia.org/wiki/Wireless_MANhttp://en.wikipedia.org/wiki/Wireless_MANhttp://en.wikipedia.org/wiki/Broadband_wireless_accesshttp://en.wikipedia.org/wiki/Broadband_wireless_accesshttp://en.wikipedia.org/wiki/Broadband_wireless_accesshttp://en.wikipedia.org/wiki/IEEE_802.16ehttp://en.wikipedia.org/wiki/IEEE_802.16ehttp://en.wikipedia.org/wiki/IEEE_802.16ehttp://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/Mobile_Broadband_Wireless_Accesshttp://en.wikipedia.org/wiki/Mobile_Broadband_Wireless_Accesshttp://en.wikipedia.org/wiki/Mobile_Broadband_Wireless_Accesshttp://en.wikipedia.org/wiki/IEEE_802.20http://en.wikipedia.org/wiki/IEEE_802.20http://en.wikipedia.org/wiki/IEEE_802.20http://en.wikipedia.org/wiki/3GPPhttp://en.wikipedia.org/wiki/3GPPhttp://en.wikipedia.org/wiki/Long_Term_Evolutionhttp://en.wikipedia.org/wiki/Long_Term_Evolutionhttp://en.wikipedia.org/wiki/Long_Term_Evolutionhttp://en.wikipedia.org/wiki/Evolved_UMTS_Terrestrial_Radio_Accesshttp://en.wikipedia.org/wiki/Evolved_UMTS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/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=5http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=4http://en.wikipedia.org/wiki/Evolved_UMTS_Terrestrial_Radio_Accesshttp://en.wikipedia.org/wiki/Evolved_UMTS_Terrestrial_Radio_Accesshttp://en.wikipedia.org/wiki/Long_Term_Evolutionhttp://en.wikipedia.org/wiki/3GPPhttp://en.wikipedia.org/wiki/IEEE_802.20http://en.wikipedia.org/wiki/Mobile_Broadband_Wireless_Accesshttp://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/IEEE_802.16ehttp://en.wikipedia.org/wiki/Broadband_wireless_accesshttp://en.wikipedia.org/wiki/Wireless_MANhttp://en.wikipedia.org/wiki/Mobile_broadbandhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiple_accesshttp://en.wikipedia.org/wiki/Multiple_accesshttp://en.wikipedia.org/wiki/WiMedia_Alliancehttp://en.wikipedia.org/wiki/IEEE_802.15.3ahttp://en.wikipedia.org/wiki/Ultra-widebandhttp://en.wikipedia.org/wiki/Personal_area_networkhttp://en.wikipedia.org/wiki/MediaFLOhttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/Mobile_TVhttp://en.wikipedia.org/wiki/ISDB-Thttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/Digital_TVhttp://en.wikipedia.org/wiki/ISDB-TSBhttp://en.wikipedia.org/wiki/T-DMBhttp://en.wikipedia.org/wiki/HD_Radiohttp://en.wikipedia.org/wiki/Digital_Radio_Mondialehttp://en.wikipedia.org/wiki/DAB%2Bhttp://en.wikipedia.org/wiki/Eureka_147http://en.wikipedia.org/wiki/Digital_radiohttp://en.wikipedia.org/wiki/HIPERLAN/2http://en.wikipedia.org/wiki/IEEE_802.11n-2009http://en.wikipedia.org/wiki/IEEE_802.11g-2003http://en.wikipedia.org/wiki/IEEE_802.11ahttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=3


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simplifies the design of both thetransmitterand thereceiver; unlike conventionalFDM, a separate filter for each sub-

channel is not required.

The orthogonality requires that the sub-carrier spacing is Hertz, where TUsecondsis the useful symbol

duration (the receiver side window size), and kis a positive integer, typically equal to 1. Therefore, with Nsub-

carriers, the total passband bandwidth will be BNf(Hz).

The orthogonality also allows highspectral efficiency, with a total symbol rate near theNyquist ratefor the equivalent

baseband signal (i.e. near half the Nyquist rate for the double-side band physical passband signal). Almost the whole

available frequency band can be utilized. OFDM generally has a nearly 'white' spectrum, giving it benign

electromagnetic interference properties with respect to other co-channel users.

A simple example: A useful symbol duration TU = 1 ms would require a sub-carrier spacing

of (or an integer multiple of that) for orthogonality. N= 1,000 sub-carriers would result

in a total passband bandwidth ofNf = 1 MHz. For this symbol time, the required bandwidth in theory

according to Nyquist is N/2TU = 0.5 MHz (i.e., half of the achieved bandwidth required by our scheme). If a

guard interval is applied (see below), Nyquist bandwidth requirement would be even lower. The FFT would

result in N= 1,000 samples per symbol. If no guard interval was applied, this would result in a base band

complex valued signal with a sample rate of 1 MHz, which would require a baseband bandwidth of 0.5 MHz

according to Nyquist. However, the passband RF signal is produced by multiplying the baseband signal with

a carrier waveform (i.e., double-sideband quadrature amplitude-modulation) resulting in a passband

bandwidth of 1 MHz. A single-side band (SSB) or vestigial sideband (VSB) modulation scheme would

achieve almost half that bandwidth for the same symbol rate (i.e., twice as high spectral efficiency for the

same symbol alphabet length). It is however more sensitive to multipath interference.

OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with

frequency deviation the sub-carriers will no longer be orthogonal, causing inter-carrier interference (ICI) (i.e.,cross-talk between the sub-carriers). Frequency offsets are typically caused by mismatched transmitter and

receiver oscillators, or byDoppler shiftdue to movement. While Doppler shift alone may be compensated for by

the receiver, the situation is worsened when combined withmultipath, as reflections will appear at various

frequency offsets, which is much harder to correct. This effect typically worsens as speed increases,[2]

and is an

important factor limiting the use of OFDM in high-speed vehicles. Several techniques for ICI suppression are

suggested, but they may increase the receiver complexity.

Implementation using the FFT algorithm [edit]

The orthogonality allows for efficient modulator and demodulator implementation using theFFTalgorithm on the

receiver side, and inverse FFT on the sender side. Although the principles and some of the benefits have been

known since the 1960s, OFDM is popular for wideband communications today by way of low-costdigital signal

processingcomponents that can efficiently calculate the FFT.

The time to compute the inverse-FFT or FFT transform has to take less than the time for each symbol.[3]

Which

for example forDVB-T(FFT 8k) means the computation has to be done in 896 sor less.

For an 8192-pointFFTthis may be approximated to:[3][clarification needed]
http://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Spectral_efficiencyhttp://en.wikipedia.org/wiki/Spectral_efficiencyhttp://en.wikipedia.org/wiki/Spectral_efficiencyhttp://en.wikipedia.org/wiki/Nyquist_ratehttp://en.wikipedia.org/wiki/Nyquist_ratehttp://en.wikipedia.org/wiki/Nyquist_ratehttp://en.wikipedia.org/wiki/Doppler_shifthttp://en.wikipedia.org/wiki/Doppler_shifthttp://en.wikipedia.org/wiki/Doppler_shifthttp://en.wikipedia.org/wiki/Multipath_interferencehttp://en.wikipedia.org/wiki/Multipath_interferencehttp://en.wikipedia.org/wiki/Multipath_interferencehttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-2http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-2http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-2http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=9http://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-ce883-3http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-ce883-3http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-ce883-3http://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-ce883-3http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-ce883-3http://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-ce883-3http://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-ce883-3http://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Fast_Fourier_transformhttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=9http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#cite_note-2http://en.wikipedia.org/wiki/Multipath_interferencehttp://en.wikipedia.org/wiki/Doppler_shifthttp://en.wikipedia.org/wiki/Nyquist_ratehttp://en.wikipedia.org/wiki/Spectral_efficiencyhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Transmitter


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[3]

MIPS =Million instructions per second

The computational demand approximately scales linearly with FFT size so a double size FFT needs double

the amount of time and vice versa.[3]

As a comparison anIntel Pentium IIICPU at 1.266 GHz is able to

calculate a 8 192 point FFT in 576 s usingFFTW.[4]

Intel Pentium Mat 1.6 GHz does it in 387 s.[5]

Intel

Core Duoat 3.0 GHz does it in 96.8 s.[6]

Guard interval for elimination of intersymbol interference [edit]

One key principle of OFDM is that since low symbol rate modulation schemes (i.e., where the symbols are

relatively long compared to thechanneltime characteristics) suffer less fromintersymbol

interferencecaused bymultipath propagation, it is advantageous to transmit a number of low-rate streams

in parallel instead of a single high-rate stream. Since the duration of each symbol is long, it is feasible to

insert aguard intervalbetween the OFDM symbols, thus eliminating the intersymbol interference.

The guard interval also eliminates the need for apulse-shaping filter, and it reduces the sensitivity to time

synchronization problems.

A simple example: If one sends a million symbols per second using conventional single-carrier modulation

over a wireless channel, then the duration of each symbol would be one microsecond or less. This imposes

severe constraints on synchronization and necessitates the removal of multipath interference. If the same

million symbols per second are spread among one thousand sub-channels, the duration of each symbol can

be longer by a factor of a thousand (i.e., one millisecond) for orthogonality with approximately the same

bandwidth. Assume that a guard interval of 1/8 of the symbol length is inserted between each symbol.

Intersymbol interference can be avoided if the multipath time-spreading (the time between the reception of

the first and the last echo) is shorter than the guard interval (i.e., 125 microseconds). This corresponds to a

maximum difference of 37.5 kilometers between the lengths of the paths.

Thecyclic prefix, which is transmitted during the guard interval, consists of the end of the OFDM

symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM

symbol. The reason that the guard interval consists of a copy of the end of the OFDM symbol is so that

the receiver will integrate over an integer number of sinusoid cycles for each of the multipaths when it

performs OFDM demodulation with the FFT. In some standards such asUltrawideband, in the interest

of transmitted power, cyclic prefix is skipped and nothing is sent during the guard interval. The receiver

will then have to mimic the cyclic prefix functionality by copying the end part of the OFDM symbol and

adding it to the beginning portion.

Simplified equalization [edit]

The effects of frequency-selective channel conditions, for example fading caused by multipath

propagation, can be considered as constant (flat) over an OFDM sub-channel if the sub-channel is

sufficiently narrow-banded (i.e., if the number of sub-channels is sufficiently large). This makes
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frequency domainequalizationpossible at thereceiver, which is far simpler than the time-domain

equalization used in conventional single-carrier modulation. In OFDM, the equalizer only has to

multiply each detected sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant

complex number, or a rarely changed value.

Our example: The OFDM equalization in the above numerical example would require one complex valued

multiplication per subcarrier and symbol (i.e., complex multiplications per OFDM symbol; i.e.,

one million multiplications per second, at the receiver). The FFT algorithm requires

[this is imprecise: over half of these complex multiplications are trivial, i.e. = to 1 and are not implemented in

software or HW]. complex-valued multiplications per OFDM symbol (i.e., 10 million multiplications per

second), at both the receiver and transmitter side. This should be compared with the corresponding one

million symbols/second single-carrier modulation case mentioned in the example, where the equalization of

125 microseconds time-spreading using aFIR filterwould require, in a naive implementation, 125

multiplications per symbol (i.e., 125 million multiplications per second). FFT techniques can be used to

reduce the number of multiplications for anFIR filterbased time-domain equalizer to a number comparable

with OFDM, at the cost of delay between reception and decoding which also becomes comparable with

OFDM.

If differential modulation such asDPSKorDQPSKis applied to each sub-carrier, equalization can

be completely omitted, since these non-coherent schemes are insensitive to slowly changing

amplitude andphase distortion.

In a sense, improvements in FIR equalization using FFTs or partial FFTs leads mathematically

closer to OFDM,[citation needed]

but the OFDM technique is easier to understand and implement, and

the sub-channels can be independently adapted in other ways than varying equalization

coefficients, such as switching between differentQAMconstellation patterns and error-correction

schemes to match individual sub-channel noise and interference characteristics.[clarification needed]

Some of the sub-carriers in some of the OFDM symbols may carrypilot signalsfor measurement

of the channel conditions[7][8]

(i.e., the equalizer gain and phase shift for each sub-carrier). Pilot

signals and training symbols (preambles) may also be used for time synchronization (to avoid

intersymbol interference, ISI) and frequency synchronization (to avoid inter-carrier interference,

ICI, caused by Doppler shift).

OFDM was initially used for wired and stationary wireless communications. However, with an

increasing number of applications operating in highly mobile environments, the effect of dispersive

fading caused by a combination of multi-path propagation anddoppler shiftis more significant.

Over the last decade, research has been done on how to equalize OFDM transmission over

doubly selective channels.[9][10][11]

Channel coding and interleaving [edit]

OFDM is invariably used in conjunction withchannel coding(forward error correction), and almost

always uses frequency and/or timeinterleaving.

Frequency (subcarrier)interleavingincreases resistance to frequency-selective channel conditions

such asfading. For example, when a part of the channel bandwidth fades, frequency interleaving

ensures that the bit errors that would result from those subcarriers in the faded part of the

bandwidth are spread out in the bit-stream rather than being concentrated. Similarly, time
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interleaving ensures that bits that are originally close together in the bit-stream are transmitted far

apart in time, thus mitigating against severe fading as would happen when travelling at high

speed.

However, time interleaving is of little benefit in slowly fading channels, such as for stationary

reception, and frequency interleaving offers little to no benefit for narrowband channels that suffer

from flat-fading (where the whole channel bandwidth fades at the same time).

The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-

stream that is presented to the error correction decoder, because when such decoders are

presented with a high concentration of errors the decoder is unable to correct all the bit errors,

and a burst of uncorrected errors occurs. A similar design of audio data encoding makes compact

disc (CD) playback robust.

A classical type of error correction coding used with OFDM-based systems isconvolutional

coding, oftenconcatenatedwithReed-Solomoncoding. Usually, additional interleaving (on top of

the time and frequency interleaving mentioned above) in between the two layers of coding is

implemented. The choice for Reed-Solomon coding as the outer error correction code is based on

the observation that the Viterbi decoder used for inner convolutional decoding produces short

errors bursts when there is a high concentration of errors, and Reed-Solomon codes are

inherently well-suited to correcting bursts of errors.

Newer systems, however, usually now adopt near-optimal types of error correction codes that use

the turbo decoding principle, where the decoder iterates towards the desired solution. Examples

of such error correction coding types includeturbo codesandLDPCcodes, which perform close

to theShannon limitfor the Additive White Gaussian Noise (AWGN) channel. Some systems that

have implemented these codes have concatenated them with either Reed-Solomon (for example

on theMediaFLOsystem) orBCH codes(on theDVB-S2system) to improve upon anerror

floorinherent to these codes at highsignal-to-noise ratios.

Adaptive transmission [edit]

The resilience to severe channel conditions can be further enhanced if information about thechannel is sent over a return-channel. Based on this feedback information, adaptivemodulation,

channel coding and power allocation may be applied across all sub-carriers, or individually to

each sub-carrier. In the latter case, if a particular range of frequencies suffers from interference or

attenuation, the carriers within that range can be disabled or made to run slower by applying more

robust modulation orerror codingto those sub-carriers.

The term discrete multitone modulation (DMT) denotes OFDM based communication systems that

adapt the transmission to the channel conditions individually for each sub-carrier, by means of so-

called bit-loading. Examples areADSLandVDSL.

The upstream and downstream speeds can be varied by allocating either more or fewer carriers

for each purpose. Some forms ofrate-adaptive DSLuse this feature in real time, so that the

bitrate is adapted to the co-channel interference and bandwidth is allocated to whicheversubscriber needs it most.

OFDM extended with multiple access [edit]

OFDM in its primary form is considered as a digital modulation technique, and not a multi-

userchannel access method, since it is utilized for transferring one bit stream over one

communicationchannelusing one sequence of OFDM symbols. However, OFDM can be

combined withmultiple accessusing time, frequency or coding separation of the users.
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Inorthogonal frequency-division multiple access(OFDMA),frequency-division multiple accessis

achieved by assigning different OFDM sub-channels to different users. OFDMA supports

differentiatedquality of serviceby assigning different number of sub-carriers to different users in a

similar fashion as inCDMA, and thus complex packet scheduling orMedia Access

Controlschemes can be avoided. OFDMA is used in:

the mobility mode of theIEEE 802.16Wireless MAN standard, commonly referred to as

WiMAX,

theIEEE 802.20mobile Wireless MAN standard, commonly referred to as MBWA,

the3GPP Long Term Evolution(LTE) fourth generation mobile broadband standard

downlink. The radio interface was formerly named High Speed OFDM Packet Access

(HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).

the now defunctQualcomm/3GPP2Ultra Mobile Broadband(UMB) project, intended as a

successor ofCDMA2000, but replaced by LTE.

OFDMA is also a candidate access method for theIEEE 802.22Wireless Regional Area

Networks (WRAN). The project aims at designing the firstcognitive radiobased standard

operating in the VHF-low UHF spectrum (TV spectrum).

InMulti-carrier code division multiple access(MC-CDMA), also known as OFDM-CDMA, OFDM is

combined with CDMA spread spectrum communication for coding separation of the users. Co-

channel interference can be mitigated, meaning that manualfixed channel allocation(FCA)

frequency planning is simplified, or complexdynamic channel allocation(DCA) schemes are

avoided.

Space diversity [edit]

In OFDM based wide area broadcasting, receivers can benefit from receiving signals from several

spatially dispersed transmitters simultaneously, since transmitters will only destructively interfere

with each other on a limited number of sub-carriers, whereas in general they will actually reinforce

coverage over a wide area. This is very beneficial in many countries, as it permits the operation of

nationalsingle-frequency networks(SFN), where many transmitters send the same signalsimultaneously over the same channel frequency. SFNs utilise the available spectrum more

effectively than conventional multi-frequency broadcast networks (MFN), where program content

is replicated on different carrier frequencies. SFNs also result in adiversity gainin receivers

situated midway between the transmitters. The coverage area is increased and the outage

probability decreased in comparison to an MFN, due to increased received signal strength

averaged over all sub-carriers.

Although the guard interval only contains redundant data, which means that it reduces the

capacity, some OFDM-based systems, such as some of the broadcasting systems, deliberately

use a long guard interval in order to allow the transmitters to be spaced farther apart in an SFN,

and longer guard intervals allow larger SFN cell-sizes. A rule of thumb for the maximum distance

between transmitters in an SFN is equal to the distance a signal travels during the guard interval for instance, a guard interval of 200 microseconds would allow transmitters to be spaced 60 km

apart.

A single frequency networkis a form of transmittermacrodiversity. The concept can be further

utilized indynamic single-frequency networks(DSFN), where the SFN grouping is changed from

timeslot to timeslot.

OFDM may be combined with other forms ofspace diversity, for exampleantenna

arraysandMIMOchannels. This is done in the IEEE802.11Wireless LANstandard.
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CDMA2000http://en.wikipedia.org/wiki/Ultra_Mobile_Broadbandhttp://en.wikipedia.org/wiki/3GPP2http://en.wikipedia.org/wiki/Qualcommhttp://en.wikipedia.org/wiki/E-UTRAhttp://en.wikipedia.org/wiki/3GPP_Long_Term_Evolutionhttp://en.wikipedia.org/wiki/IEEE_802.20http://en.wikipedia.org/wiki/IEEE_802.16http://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Frequency-division_multiple_accesshttp://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiple_access


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Linear transmitter power amplifier[edit]

An OFDM signal exhibits a highpeak-to-average power ratio (PAPR)because the independent

phases of the sub-carriers mean that they will often combine constructively. Handling this high

PAPR requires:

a high-resolutiondigital-to-analogue converter(DAC) in the transmitter a high-resolutionanalogue-to-digital converter(ADC) in the receiver

a linearsignal chain.

Any non-linearity in the signal chain will causeintermodulation distortionthat

raises the noise floor

may cause inter-carrier interference

generates out-of-band spurious radiation.

The linearity requirement is demanding, especially for transmitter RF output circuitry where

amplifiers are often designed to be non-linear in order to minimise power consumption. In practical

OFDM systems a small amount of peak clipping is allowed to limit the PAPR in a judicious trade-

off against the above consequences. However, the transmitter output filter which is required to

reduce out-of-band spurs to legal levels has the effect of restoring peak levels that were clipped,

so clipping is not an effective way to reduce PAPR.

Although the spectral efficiency of OFDM is attractive for both terrestrial and space

communications, the high PAPR requirements have so far limited OFDM applications to terrestrial

systems.

The crest factor CF (in dB) for an OFDM system with n uncorrelated sub-carriers is[12]

CF = 10 log( n ) + CFc ...

where CFc is the crest factor (in dB) for each sub-carrier. (CFc is 3.01 dB for the sine waves used

for BPSK and QPSK modulation).

For example, the DVB-T signal in 2K mode is composed of 1705 sub-carriers that are each

QPSK-modulated, giving a crest factor of 35.32 dB.[12]

Manycrest factorreduction techniques have been developed.

Idealized system model [edit]

This section describes a simple idealized OFDM system model suitable for a time-

invariantAWGNchannel.

Transmitter[edit]
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An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, withbasebanddata on

each sub-carrier being independently modulated commonly using some type ofquadrature

amplitude modulation(QAM) orphase-shift keying(PSK). This composite baseband signal is

typically used to modulate a mainRFcarrier.

is a serial stream of binary digits. Byinverse multiplexing, these are first demultiplexed

into parallel streams, and each one mapped to a (possibly complex) symbol stream using

some modulation constellation (QAM,PSK, etc.). Note that the constellations may be different, so

some streams may carry a higher bit-rate than others.

An inverseFFTis computed on each set of symbols, giving a set of complex time-domain

samples. These samples are thenquadrature-mixed to passband in the standard way. The real

and imaginary components are first converted to the analogue domain usingdigital-to-analogue

converters(DACs); the analogue signals are then used to modulatecosineandsinewaves at

thecarrierfrequency, , respectively. These signals are then summed to give the transmission

signal, .

Receiver[edit]

The receiver picks up the signal , which is then quadrature-mixed down to baseband using

cosine and sine waves at the carrier frequency. This also creates signals centered on , so

low-pass filters are used to reject these. The baseband signals are then sampled and digitised
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