the symbol rate is measured in baud

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Document Title Security Level

2013-08-17 Huawei Proprietary - Restricted Distribution Page1, Total33

The Symbol rate is measured in baud (Bd) or symbols/second.

Each symbol can represent or convey one or several bits of data

A sending device places symbols on the channel at a fixed and known symbol rate, and the

receiving device has the job of detecting the sequence of symbols in order to reconstruct the

transmitted data. There may be a direct correspondence between a symbol and a small unit of data

(for example, each symbol may encode one or several binary digits or 'bits') or the data may be

represented by the transitions between symbols or even by a sequence of many symbols.

The symbol duration time Ts can be calculated as: Ts=1/fs, where fs is the symbol rate.

A simple example: A baud rate of 1 kBd = 1,000 Bd is synonymous to a symbol rate of 1,000

symbols per second. In case of a modem, this corresponds to 1,000 tones per second, and in case

of a line code, this corresponds to 1,000 pulses per second. The symbol duration time is 1/1,000

second = 1 millisecond.

Relationship to gross bitrate

The term baud rate has sometimes incorrectly been used to mean bit rate, since these rates are the

same in old modems as well as in the simplest digital communication links using only one bit per

symbol, such that binary "0" is represented by one symbol, and binary "1" by another symbol. In

more advanced modems and data transmission techniques, a symbol may have more than two

states, so it may represent more than one binary digit (a binary digit always represents exactly two

states). For this reason, the baud rate value will often be lower than the gross bit rate.

If N bits are conveyed per symbol, and the gross bit rate is R, inclusive of channel coding

overhead, the symbol rate can be calculated as:

fs=R/N

1 Digital television and OFDM example

Indigital televisiontransmission the symbol rate calculation is:

symbol rate in symbols per second = (Data rate in bits per second * 204) / (188 * bits

per symbol)

The 204 is the number of bytes in a packet including the 16 trailingReed-Solomonerror

checking and correctionbytes. The 188 is the number of data bytes (187 bytes) plus the

leading packetsync byte(0x47).
http://en.wikipedia.org/wiki/Digital_televisionhttp://en.wikipedia.org/wiki/Digital_televisionhttp://en.wikipedia.org/wiki/Digital_televisionhttp://en.wikipedia.org/wiki/Reed-Solomonhttp://en.wikipedia.org/wiki/Reed-Solomonhttp://en.wikipedia.org/wiki/Error_detection_and_correctionhttp://en.wikipedia.org/wiki/Error_detection_and_correctionhttp://en.wikipedia.org/wiki/Error_detection_and_correctionhttp://en.wikipedia.org/wiki/Error_detection_and_correctionhttp://en.wikipedia.org/wiki/Syncwordhttp://en.wikipedia.org/wiki/Syncwordhttp://en.wikipedia.org/wiki/Syncwordhttp://en.wikipedia.org/wiki/Syncwordhttp://en.wikipedia.org/wiki/Error_detection_and_correctionhttp://en.wikipedia.org/wiki/Error_detection_and_correctionhttp://en.wikipedia.org/wiki/Reed-Solomonhttp://en.wikipedia.org/wiki/Digital_television


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The bits per symbol is the (modulation's power of 2)*(Forward Error Correction). So for

example in 64-QAM modulation 64 = 26

so the bits per symbol is 6. The Forward Error

Correction (FEC) is usually expressed as a fraction, i.e., 1/2, 3/4, etc. In the case of 3/4

FEC, for every 3 bits of data, you are sending out 4 bits, one of which is for errorcorrection.

Example:

given bit rate = 18096263

Modulation type = 64-QAM

FEC = 3/4

then

In digital terrestrial digital television (DVB-T,DVB-Hand similar

techniques)OFDMmodulation is used, i.e. multi-carrier modulation. The above

symbol rate should then be divided by the number of OFDM sub-carriers in view to

achieve the OFDM symbol rate. See theOFDM system comparison tablefor further

numerical details.

2 [edit]Relationship to chip rate

3 Relationship to chip rate

Some communication links (such asGPStransmissions,CDMAcell phones, and

otherspread spectrumlinks) have a symbol rate much higher than the data rate (they

transmit many symbols calledchipsper data bit. Representing one bit by a chip sequence of

many symbols overcomesco-channel interferencefrom other transmitters sharing the same

frequency channel, includingradio jamming, and is common inmilitary radioandcell phones.

Despite the fact that using morebandwidthto carry the same bit rate gives lowchannel

spectral efficiencyin (bit/s)/Hz, it allows many simultaneous users, which results in

highsystem spectral efficiencyin (bit/s)/Hz per unit of area.

In these systems, the symbol rate of the physically transmitted high-frequency signal rate

is calledchip rate, which also is the pulse rate of the equivalentbase bandsignal.

However, in spread spectrum systems, the term symbol may also be used at a higher

layer and refer to one information bit, or a block of information bits that are modulated

using for example conventional QAM modulation, before the CDMA spreading code is

applied. Using the latter definition, the symbol rate is equal to or lower than the bit rate.

4 Orthogonal frequency-division multiplexing

From Wikipedia, the free encyclopedia(Redirected fromOFDM system comparison table)
http://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/DVB-Thttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/w/index.php?title=Symbol_rate&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Symbol_rate&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Symbol_rate&action=edit&section=6http://en.wikipedia.org/wiki/GPShttp://en.wikipedia.org/wiki/GPShttp://en.wikipedia.org/wiki/GPShttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/Spread_spectrumhttp://en.wikipedia.org/wiki/Spread_spectrumhttp://en.wikipedia.org/wiki/Spread_spectrumhttp://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Co-channel_interferencehttp://en.wikipedia.org/wiki/Co-channel_interferencehttp://en.wikipedia.org/wiki/Co-channel_interferencehttp://en.wikipedia.org/wiki/Radio_jamminghttp://en.wikipedia.org/wiki/Radio_jamminghttp://en.wikipedia.org/wiki/Radio_jamminghttp://en.wikipedia.org/wiki/Military_radiohttp://en.wikipedia.org/wiki/Military_radiohttp://en.wikipedia.org/wiki/Military_radiohttp://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Channel_spectral_efficiencyhttp://en.wikipedia.org/wiki/Channel_spectral_efficiencyhttp://en.wikipedia.org/wiki/Channel_spectral_efficiencyhttp://en.wikipedia.org/wiki/System_spectral_efficiencyhttp://en.wikipedia.org/wiki/System_spectral_efficiencyhttp://en.wikipedia.org/wiki/System_spectral_efficiencyhttp://en.wikipedia.org/wiki/Chip_ratehttp://en.wikipedia.org/wiki/Chip_ratehttp://en.wikipedia.org/wiki/Chip_ratehttp://en.wikipedia.org/wiki/Base_bandhttp://en.wikipedia.org/wiki/Base_bandhttp://en.wikipedia.org/wiki/Base_bandhttp://en.wikipedia.org/w/index.php?title=OFDM_system_comparison_table&redirect=nohttp://en.wikipedia.org/w/index.php?title=OFDM_system_comparison_table&redirect=nohttp://en.wikipedia.org/w/index.php?title=OFDM_system_comparison_table&redirect=nohttp://en.wikipedia.org/w/index.php?title=OFDM_system_comparison_table&redirect=nohttp://en.wikipedia.org/wiki/Base_bandhttp://en.wikipedia.org/wiki/Chip_ratehttp://en.wikipedia.org/wiki/System_spectral_efficiencyhttp://en.wikipedia.org/wiki/Channel_spectral_efficiencyhttp://en.wikipedia.org/wiki/Channel_spectral_efficiencyhttp://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Military_radiohttp://en.wikipedia.org/wiki/Radio_jamminghttp://en.wikipedia.org/wiki/Co-channel_interferencehttp://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Spread_spectrumhttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/GPShttp://en.wikipedia.org/w/index.php?title=Symbol_rate&action=edit&section=6http://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/DVB-Hhttp://en.wikipedia.org/wiki/DVB-T


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Passbandmodulation

Analog modulation

AM

FM

PM

QAM

SM

SSB

Digital modulation

ASK

CPM

FSK

MFSK

MSK

OOK

PPM

PSK

QAM

SC-FDE

TCM
http://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Modulation#Analog_modulation_methodshttp://en.wikipedia.org/wiki/Modulation#Analog_modulation_methodshttp://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Frequency_modulationhttp://en.wikipedia.org/wiki/Frequency_modulationhttp://en.wikipedia.org/wiki/Phase_modulationhttp://en.wikipedia.org/wiki/Phase_modulationhttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Space_modulationhttp://en.wikipedia.org/wiki/Space_modulationhttp://en.wikipedia.org/wiki/Single-sideband_modulationhttp://en.wikipedia.org/wiki/Single-sideband_modulationhttp://en.wikipedia.org/wiki/Digital_modulationhttp://en.wikipedia.org/wiki/Digital_modulationhttp://en.wikipedia.org/wiki/Amplitude-shift_keyinghttp://en.wikipedia.org/wiki/Amplitude-shift_keyinghttp://en.wikipedia.org/wiki/Continuous_phase_modulationhttp://en.wikipedia.org/wiki/Continuous_phase_modulationhttp://en.wikipedia.org/wiki/Frequency-shift_keyinghttp://en.wikipedia.org/wiki/Frequency-shift_keyinghttp://en.wikipedia.org/wiki/Multiple_frequency-shift_keyinghttp://en.wikipedia.org/wiki/Multiple_frequency-shift_keyinghttp://en.wikipedia.org/wiki/Minimum-shift_keyinghttp://en.wikipedia.org/wiki/Minimum-shift_keyinghttp://en.wikipedia.org/wiki/On-off_keyinghttp://en.wikipedia.org/wiki/On-off_keyinghttp://en.wikipedia.org/wiki/Pulse-position_modulationhttp://en.wikipedia.org/wiki/Pulse-position_modulationhttp://en.wikipedia.org/wiki/Phase-shift_keyinghttp://en.wikipedia.org/wiki/Phase-shift_keyinghttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Single-carrier_FDMAhttp://en.wikipedia.org/wiki/Single-carrier_FDMAhttp://en.wikipedia.org/wiki/Trellis_modulationhttp://en.wikipedia.org/wiki/Trellis_modulationhttp://en.wikipedia.org/wiki/Trellis_modulationhttp://en.wikipedia.org/wiki/Single-carrier_FDMAhttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Phase-shift_keyinghttp://en.wikipedia.org/wiki/Pulse-position_modulationhttp://en.wikipedia.org/wiki/On-off_keyinghttp://en.wikipedia.org/wiki/Minimum-shift_keyinghttp://en.wikipedia.org/wiki/Multiple_frequency-shift_keyinghttp://en.wikipedia.org/wiki/Frequency-shift_keyinghttp://en.wikipedia.org/wiki/Continuous_phase_modulationhttp://en.wikipedia.org/wiki/Amplitude-shift_keyinghttp://en.wikipedia.org/wiki/Digital_modulationhttp://en.wikipedia.org/wiki/Single-sideband_modulationhttp://en.wikipedia.org/wiki/Space_modulationhttp://en.wikipedia.org/wiki/Quadrature_amplitude_modulationhttp://en.wikipedia.org/wiki/Phase_modulationhttp://en.wikipedia.org/wiki/Frequency_modulationhttp://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Modulation#Analog_modulation_methodshttp://en.wikipedia.org/wiki/Modulation


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Spread spectrum

CSS

DSSS

FHSS

THSS

See also

Capacity-approaching codes Demodulation

Line coding

Modem

PAM

PCM

PWM

V

T

E

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
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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 (that

shows up asghostingon analogue TV) to achieve adiversity gain, i.e. asignal-to-noise

ratioimprovement. 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.

5 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/Forward_error_correctionhttp://en.wikipedia.org/wiki/Forward_error_correctionhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#cite_note-cobas-0http://en.wikipedia.org/wiki/OFDM_system_comparison_table#cite_note-cobas-0http://en.wikipedia.org/wiki/OFDM_system_comparison_table#cite_note-cobas-0http://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/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/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/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/Single_frequency_networkhttp://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Example_of_applicationshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Example_of_applicationshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Cablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Cablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wirelesshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wirelesshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Key_featureshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Key_featureshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Summary_of_advantageshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Summary_of_advantageshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Summary_of_disadvantageshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Summary_of_disadvantageshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Orthogonalityhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Orthogonalityhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Implementation_using_the_FFT_algorithmhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Implementation_using_the_FFT_algorithmhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Guard_interval_for_elimination_of_intersymbol_interferencehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Guard_interval_for_elimination_of_intersymbol_interferencehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Simplified_equalizationhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Simplified_equalizationhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Channel_coding_and_interleavinghttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Channel_coding_and_interleavinghttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Adaptive_transmissionhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Adaptive_transmissionhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#OFDM_extended_with_multiple_accesshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#OFDM_extended_with_multiple_accesshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Space_diversityhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Space_diversityhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Space_diversityhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#OFDM_extended_with_multiple_accesshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Adaptive_transmissionhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Channel_coding_and_interleavinghttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Simplified_equalizationhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Guard_interval_for_elimination_of_intersymbol_interferencehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Implementation_using_the_FFT_algorithmhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Orthogonalityhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Characteristics_and_principles_of_operationhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Summary_of_disadvantageshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Summary_of_advantageshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Key_featureshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wirelesshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Cablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Example_of_applicationshttp://en.wikipedia.org/wiki/OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/Single_frequency_networkhttp://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/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/Orthogonality#Communicationshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#cite_note-cobas-0http://en.wikipedia.org/wiki/Forward_error_correction


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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

6 [edit]Example of applications

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

details, see theUsagesection at the end of the article.

7 [edit]Cable

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).
http://en.wikipedia.org/wiki/OFDM_system_comparison_table#Linear_transmitter_power_amplifierhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Linear_transmitter_power_amplifierhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Idealized_system_modelhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Idealized_system_modelhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Transmitterhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Transmitterhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Receiverhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Receiverhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Mathematical_descriptionhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Mathematical_descriptionhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Usagehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Usagehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#ADSLhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#ADSLhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Powerline_Technologyhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Powerline_Technologyhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wireless_local_area_networks_.28LAN.29_and_metropolitan_area_networks_.28MAN.29http://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wireless_local_area_networks_.28LAN.29_and_metropolitan_area_networks_.28MAN.29http://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wireless_personal_area_networks_.28PAN.29http://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wireless_personal_area_networks_.28PAN.29http://en.wikipedia.org/wiki/OFDM_system_comparison_table#Terrestrial_digital_radio_and_television_broadcastinghttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Terrestrial_digital_radio_and_television_broadcastinghttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#DVB-Thttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#DVB-Thttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#SDARShttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#SDARShttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#COFDM_vs_VSBhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#COFDM_vs_VSBhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#DIGITAL_RADIOhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#DIGITAL_RADIOhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#BST-OFDM_used_in_ISDBhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#BST-OFDM_used_in_ISDBhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Ultra-widebandhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Ultra-widebandhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#FLASH-OFDMhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#FLASH-OFDMhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Historyhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Historyhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#See_alsohttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#See_alsohttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Referenceshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Referenceshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#External_linkshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#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/OFDM_system_comparison_table#Usagehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Usagehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#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/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/OFDM_system_comparison_table#Usagehttp://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&action=edit&section=1http://en.wikipedia.org/wiki/OFDM_system_comparison_table#External_linkshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Referenceshttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#See_alsohttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Historyhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#FLASH-OFDMhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Ultra-widebandhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#BST-OFDM_used_in_ISDBhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#DIGITAL_RADIOhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#COFDM_vs_VSBhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#SDARShttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#DVB-Thttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Terrestrial_digital_radio_and_television_broadcastinghttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wireless_personal_area_networks_.28PAN.29http://en.wikipedia.org/wiki/OFDM_system_comparison_table#Wireless_local_area_networks_.28LAN.29_and_metropolitan_area_networks_.28MAN.29http://en.wikipedia.org/wiki/OFDM_system_comparison_table#Powerline_Technologyhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#ADSLhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#OFDM_system_comparison_tablehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Usagehttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Mathematical_descriptionhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Receiverhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Transmitterhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Idealized_system_modelhttp://en.wikipedia.org/wiki/OFDM_system_comparison_table#Linear_transmitter_power_amplifier


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TrailBlazertelephone linemodems.

Multimedia over Coax Alliance(MoCA) home networking.

8[edit]

Wireless

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).

9 [edit]Key features

The advantages and disadvantages listed below are further discussed in the Characteristics and

principles of operationsection below.

10 [edit]Summary of advantages

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.

Highspectral efficiencyas compared to conventional modulation schemes, spread spectrum,

etc.

Efficient implementation usingFast Fourier Transform(FFT).

Low sensitivity to time synchronization errors.

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

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

11 [edit]Summary of disadvantages
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ikipedia.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=3http://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliancehttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/TrailBlazer


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Sensitive toDoppler shift.

Sensitive to frequency synchronization problems.

Highpeak-to-average-power ratio(PAPR), requiring linear transmitter circuitry, which suffersfrom poor power efficiency.

Loss of efficiency caused bycyclic prefix/guard interval.

12 [edit]Characteristics and principles of operation

13 [edit]Orthogonality

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 are orthogonalto each

other, meaning thatcross-talkbetween the sub-channels is eliminated and inter-carrier guard

bands are not required. This greatly 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
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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,

causinginter-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.

14 [edit]Implementation using the FFT algorithm

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 s or less.

For an 8 192 pointFFTthis may be approximated to:[3][clarification needed]

[3]

MIPS =Million instructions per second

The computational demand approximately scales linear with FFT size so a double size

FFT needs the double amount of time and vice versa.[3]

As a comparison aIntel 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]
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15 [edit]Guard interval for elimination of intersymbol

interference

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.

16 [edit]Simplified equalization

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 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
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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,
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research has been done on how to equalize OFDM transmission over doubly

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

17 [edit]Channel coding and interleavingOFDM 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 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 mentionedabove) 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
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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.

18 [edit]Adaptive transmission

The resilience to severe channel conditions can be further enhanced if

information about the channel 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 whichever subscriber needs it most.

19 [edit]OFDM extended with multiple access

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.

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:
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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 802.22WirelessRegional 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 manualfixe

the symbol rate is measured in baud

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