Misplaced Pages

#774225

69-450: OFDM is a frequency-division multiplexing (FDM) scheme that was introduced by Robert W. Chang of Bell Labs in 1966. In OFDM, the incoming bitstream representing the data to be sent is divided into multiple streams. Multiple closely spaced orthogonal subcarrier signals with overlapping spectra are transmitted, with each carrier modulated with bits from the incoming stream so multiple bits are being transmitted in parallel. Demodulation

138-408: A carrier signal, a steady oscillating waveform at a single frequency that serves to "carry" information. The carrier is much higher in frequency than the baseband signal. The carrier signal and the baseband signal are combined in a modulator circuit. The modulator alters some aspect of the carrier signal, such as its amplitude , frequency , or phase, with the baseband signal, " piggybacking "

207-620: A common signal return path, electrical impedance in the return path creates common impedance coupling between the signals, resulting in crosstalk. Crosstalk is a significant issue in structured cabling , audio electronics , integrated circuit design , wireless communication and other communications systems . In structured cabling, crosstalk refers to electromagnetic interference from one unshielded twisted pair to another twisted pair, normally running in parallel. Signals traveling through adjacent pairs of wire create magnetic fields that interact with each other, inducing interference in

276-483: A comparison an Intel Pentium III CPU at 1.266 GHz is able to calculate a 8192 point FFT in 576 µs using FFTW . Intel Pentium M at 1.6 GHz does it in 387 µs. Intel Core Duo at 3.0 GHz does it in 96.8 µs . One key principle of OFDM is that since low symbol rate modulation schemes (i.e., where the symbols are relatively long compared to the channel time characteristics) suffer less from intersymbol interference caused by multipath propagation , it

345-571: A crest factor of 35.32 dB. Many PAPR (or crest factor ) reduction techniques have been developed, for instance, based on iterative clipping. Over the years, numerous model-driven approaches have been proposed to reduce the PAPR in communication systems. In recent years, there has been a growing interest in exploring data-driven models for PAPR reduction as part of ongoing research in end-to-end communication networks. These data-driven models offer innovative solutions and new avenues of exploration to address

414-423: A discussion of crosstalk conveyed through the integrated circuit substrate. There are a wide variety of repair solutions, with increased spacing, wire re-ordering, and shielding being the most common. In full-field optical coherence tomography , "crosstalk" refers to the phenomenon that due to highly scattering objects, multiple scattered photons reach the image plane and generate a coherent signal after traveling

483-451: A limited number of subcarriers, 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 national single-frequency networks (SFN), where many transmitters send the same signal simultaneously over the same channel frequency. SFNs use the available spectrum more effectively than conventional multi-frequency broadcast networks ( MFN ), where program content

552-442: A much more noticeable problem, as these are likely to be carrying very different programs or material. Crosstalk is an electrical effect and can be quantified with a crosstalk measurement. Crosstalk measurements are made on audio systems to determine the amount of signal leaking from one channel to another. The Independent Broadcasting Authority published a weighting curve for use in crosstalk measurement that gives due emphasis to

621-561: A nearly 'white' spectrum, giving it benign electromagnetic interference properties with respect to other co-channel users. OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the subcarriers will no longer be orthogonal, causing inter-carrier interference (ICI) (i.e., cross-talk between the subcarriers). Frequency offsets are typically caused by mismatched transmitter and receiver oscillators, or by Doppler shift due to movement. While Doppler shift alone may be compensated for by

690-438: A non-orthogonal subcarriers overlapping. For example, a low-complexity scheme referred to as WCP-OFDM ( Weighted Cyclic Prefix Orthogonal Frequency-Division Multiplexing ) consists of using short filters at the transmitter output in order to perform a potentially non-rectangular pulse shaping and a near perfect reconstruction using a single-tap per subcarrier equalization. Other ICI suppression techniques usually drastically increase

759-453: A pathlength that matches that of the sample depth within a coherence length. In stereoscopic 3D displays , crosstalk refers to the incomplete isolation of the left and right image channels so that one bleeds into the other - like a double exposure , which produces a ghosting effect. [REDACTED]  This article incorporates public domain material from Federal Standard 1037C . General Services Administration . Archived from

SECTION 10

#1733085750775

828-494: A rarely changed value. On a fundamental level, simpler digital equalizers are better because they require fewer operations, which translates to fewer round-off errors in the equalizer. Those round-off errors can be viewed as numerical noise and are inevitable. If differential modulation such as DPSK or DQPSK is applied to each subcarrier, equalization can be completely omitted, since these non-coherent schemes are insensitive to slowly changing amplitude and phase distortion . In

897-540: A sense, improvements in FIR equalization using FFTs or partial FFTs leads mathematically closer to OFDM, 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 different QAM constellation patterns and error-correction schemes to match individual sub-channel noise and interference characteristics. Some of

966-515: A separate filter for each sub-channel is not required. The orthogonality requires that the subcarrier spacing is Δ f = k T U {\displaystyle \scriptstyle \Delta f\,=\,{\frac {k}{T_{U}}}} Hertz , where T U seconds is the useful symbol duration (the receiver-side window size), and k is a positive integer, typically equal to 1. This stipulates that each carrier frequency undergoes k more complete cycles per symbol period than

1035-433: 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,

1104-474: Is convolutional coding , often concatenated with Reed-Solomon coding. 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 error bursts when there

1173-665: Is radio and television broadcasting, in which multiple radio signals at different frequencies pass through the air at the same time. Another example is cable television , in which many television channels are carried simultaneously on a single cable. FDM is also used by telephone systems to transmit multiple telephone calls through high capacity trunklines, communications satellites to transmit multiple channels of data on uplink and downlink radio beams, and broadband DSL modems to transmit large amounts of computer data through twisted pair telephone lines, among many other uses. An analogous technique called wavelength division multiplexing

1242-416: Is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal. An analog NTSC television channel is divided into subcarrier frequencies for video, color, and audio. DSL uses different frequencies for voice and for upstream and downstream data transmission on

1311-526: Is a form of transmitter macrodiversity . The concept can be further used in dynamic single-frequency networks (DSFN), where the SFN grouping is changed from timeslot to timeslot. OFDM may be combined with other forms of space diversity , for example antenna arrays and MIMO channels. This is done in the IEEE 802.11 Wireless LAN standards. An OFDM signal exhibits a high peak-to-average power ratio (PAPR) because

1380-411: 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 include turbo codes and LDPC codes, which perform close to

1449-508: Is achieved by assigning different OFDM sub-channels to different users. OFDMA supports differentiated quality of service by assigning different number of subcarriers to different users in a similar fashion as in CDMA , and thus complex packet scheduling or medium access control schemes can be avoided. OFDMA is used in: OFDMA is also a candidate access method for the IEEE 802.22 Wireless Regional Area Networks (WRAN). The project aims at designing

SECTION 20

#1733085750775

1518-436: 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 a guard interval between the OFDM symbols, thus eliminating the intersymbol interference. The guard interval also eliminates the need for a pulse-shaping filter , and it reduces the sensitivity to time synchronization problems. The cyclic prefix , which

1587-497: Is allocated to whichever subscriber needs it most. OFDM in its primary form is considered as a digital modulation technique, and not a multi-user channel access method , since it is used for transferring one bit stream over one communication channel using one sequence of OFDM symbols. However, OFDM can be combined with multiple access using time, frequency or coding separation of the users. In orthogonal frequency-division multiple access (OFDMA), frequency-division multiple access

1656-459: Is also used in several 4G and pre-4G cellular networks , mobile broadband standards, the next generation WLAN and the wired portion of Hybrid fiber-coaxial networks: The advantages and disadvantages listed below are further discussed in the Characteristics and principles of operation section below. Conceptually, OFDM is a specialized frequency-division multiplexing (FDM) method, with

1725-494: Is based on fast Fourier transform algorithms. OFDM was improved by Weinstein and Ebert in 1971 with the introduction of a guard interval , providing better orthogonality in transmission channels affected by multipath propagation. Each subcarrier (signal) is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying ) at a low symbol rate . This maintains total data rates similar to conventional single-carrier modulation schemes in

1794-413: Is divided into a series of non-overlapping frequency bands , each of which is used to carry a separate signal. This allows a single transmission medium such as a microwave radio link, cable or optical fiber to be shared by multiple independent signals. Another use is to carry separate serial bits or segments of a higher rate signal in parallel . The most common example of frequency-division multiplexing

1863-419: Is invariably used in conjunction with channel coding ( forward error correction ), and almost always uses frequency and/or time interleaving . Frequency (subcarrier) interleaving increases resistance to frequency-selective channel conditions such as fading . For example, when a part of the channel bandwidth fades, frequency interleaving ensures that the bit errors that would result from those subcarriers in

1932-406: Is often distinguishable as pieces of speech or in-band signaling tones leaking from other people's connections. If the connection is analog, twisted pair cabling can often be used to reduce crosstalk. Alternatively, the signals can be converted to digital form, which is typically less susceptible to crosstalk. In wireless communication , crosstalk is often denoted co-channel interference , and

2001-439: Is related to adjacent-channel interference . In integrated circuit design , crosstalk normally refers to a signal affecting another nearby signal. Usually, the coupling is capacitive, and to the nearest neighbor, but other forms of coupling and effects on signal further away are sometimes important, especially in analog designs. See signal integrity for tools used to measure and prevent this problem, and substrate coupling for

2070-450: Is replicated on different carrier frequencies. SFNs also result in a diversity gain in 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 subcarriers. Although the guard interval only contains redundant data, which means that it reduces the capacity, some OFDM-based systems, such as some of

2139-411: 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

Orthogonal frequency-division multiplexing - Misplaced Pages Continue

2208-465: Is used in fiber-optic communication , in which multiple channels of data are transmitted over a single optical fiber using different wavelengths (frequencies). The multiple separate information (modulation) signals that are sent over an FDM system, such as the video signals of the television channels that are sent over a cable TV system, are called baseband signals. At the source end, for each frequency channel, an electronic oscillator generates

2277-404: Is usually caused by undesired capacitive , inductive , or conductive coupling from one circuit or channel to another. Where the electric, magnetic, or traveling fields of two electric signals overlap, the electromagnetic interference created causes crosstalk. For example, crosstalk can comprise magnetic fields that induce a smaller signal in neighboring wires. In electrical circuits sharing

2346-650: The Shannon limit for the Additive White Gaussian Noise ( AWGN ) channel. Some systems that have implemented these codes have concatenated them with either Reed-Solomon (for example on the MediaFLO system) or BCH codes (on the DVB-S2 system) to improve upon an error floor inherent to these codes at high signal-to-noise ratios . The resilience to severe channel conditions can be further enhanced if information about

2415-399: The coaxial cable used by cable television systems has a bandwidth of about 1000 MHz , but the passband of each television channel is only 6 MHz wide, so there is room for many channels on the cable (in modern digital cable systems each channel in turn is subdivided into subchannels and can carry up to 10 digital television channels). At the destination end of the cable or fiber, or

2484-501: The 8 MHz range to form a Channel Group of 12 channels, 48 kHz bandwidth in the range 8140 to 8188 kHz by selecting carriers in the range 8140 to 8184 kHz selecting upper sideband this group can then be translated to the standard range 60 to 108 kHz by a carrier of 8248 kHz. Such systems are used in DTL (Direct To Line) and DFSG (Directly formed super group). 132 voice channels (2SG + 1G) can be formed using DTL plane

2553-535: The FFT. In some standards such as Ultrawideband , 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. 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

2622-400: The FFT. The time to compute the inverse-FFT or FFT has to take less than the time for each symbol, which for example for DVB-T (FFT 8k) means the computation has to be done in 896 µs or less. For an 8192 -point FFT this may be approximated to: The computational demand approximately scales linearly with FFT size so a double size FFT needs double the amount of time and vice versa. As

2691-438: The additional constraint that all subcarrier signals within a communication channel are orthogonal to one another. In OFDM, the subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other, meaning that crosstalk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver ; unlike conventional FDM,

2760-472: 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 network

2829-398: The challenges posed by high PAPR effectively. By leveraging data-driven techniques, researchers aim to enhance the performance and efficiency of communication networks by optimizing power utilization. Frequency-division multiplexing In telecommunications , frequency-division multiplexing ( FDM ) is a technique by which the total bandwidth available in a communication medium

Orthogonal frequency-division multiplexing - Misplaced Pages Continue

2898-436: The channel is in a narrow band of frequencies clustered around the carrier frequency, this is called the passband of the channel. Similarly, additional baseband signals are used to modulate carriers at other frequencies, creating other channels of information. The carriers are spaced far enough apart in frequency that the band of frequencies occupied by each channel, the passbands of the separate channels, do not overlap. All

2967-562: The channel is sent over a return-channel. Based on this feedback information, adaptive modulation , channel coding and power allocation may be applied across all subcarriers, or individually to each subcarrier. 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 or error coding to those subcarriers. The term discrete multitone modulation ( DMT ) denotes OFDM-based communication systems that adapt

3036-435: The channels are sent through the transmission medium, such as a coaxial cable, optical fiber, or through the air using a radio transmitter . As long as the channel frequencies are spaced far enough apart that none of the passbands overlap, the separate channels will not interfere with each other. Thus the available bandwidth is divided into "slots" or channels, each of which can carry a separate modulated signal. For example,

3105-403: The data onto the carrier. The result of modulating (mixing) the carrier with the baseband signal is to generate sub-frequencies near the carrier frequency , at the sum ( f C + f B ) and difference ( f C − f B ) of the frequencies. The information from the modulated signal is carried in sidebands on each side of the carrier frequency. Therefore, all the information carried by

3174-501: The end of the 20th century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM. Since the late 20th century, digital subscriber lines (DSL) have used a Discrete multitone (DMT) system to divide their spectrum into frequency channels. A once commonplace FDM system, used for example in L-carrier , uses crystal filters which operate at

3243-542: 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

3312-744: The first cognitive radio -based standard operating in the VHF-low UHF spectrum (TV spectrum). In multi-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 manual fixed channel allocation (FCA) frequency planning is simplified, or complex dynamic channel allocation (DCA) schemes are avoided. 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

3381-479: 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 subcarriers is where CF c is the crest factor (in dB) for each subcarrier. (CF c 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 subcarriers that are each QPSK-modulated, giving

3450-418: The independent phases of the subcarriers mean that they will often combine constructively. Handling this high PAPR requires: Any non-linearity in the signal chain will cause intermodulation distortion that 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

3519-517: The modulation and frequency plan are given in FIG1 and FIG2 use of DTL technique allows the formation of a maximum of 132 voice channels that can be placed direct to line. DTL eliminates group and super group equipment. DFSG can take similar steps where a direct formation of a number of super groups can be obtained in the 8 kHz the DFSG also eliminates group equipment and can offer: Both DTL and DFSG can fit

SECTION 50

#1733085750775

3588-480: The names of acoustic telegraphy and harmonic telegraphy. Practical FDM was only achieved in the electronic age. Meanwhile, their efforts led to an elementary understanding of electroacoustic technology, resulting in the invention of the telephone . Crosstalk In electronics , crosstalk is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk

3657-399: The neighboring pair. The pair causing the interference is called the disturbing pair , while the pair experiencing the interference is the disturbed pair . In stereo audio reproduction, crosstalk can refer to signal leakage across from one program channel to another, reducing channel separation and stereo imaging . Crosstalk between channels in mixing consoles, and between studio feeds is

3726-515: The previous carrier. Therefore, with N subcarriers, the total passband bandwidth will be B ≈ N ·Δ f (Hz). The orthogonality also allows high spectral efficiency , with a total symbol rate near the Nyquist rate for 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 used. OFDM generally has

3795-1019: The radio receiver, for each channel a local oscillator produces a signal at the carrier frequency of that channel, that is mixed with the incoming modulated signal. The frequencies subtract, producing the baseband signal for that channel again. This is called demodulation . The resulting baseband signal is filtered out of the other frequencies and output to the user. For long distance telephone connections , 20th century telephone companies used L-carrier and similar coaxial cable systems carrying thousands of voice circuits multiplexed in multiple stages by channel banks . For shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier. Those cables did not allow such large bandwidths, so only 12 voice channels ( double sideband ) and later 24 ( single sideband ) were multiplexed into four wires , one pair for each direction with repeaters every several miles, approximately 10 km. See 12-channel carrier system . By

3864-456: The receiver complexity. The orthogonality allows for efficient modulator and demodulator implementation using the FFT algorithm 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-cost digital signal processing components that can efficiently calculate

3933-412: The receiver, the situation is worsened when combined with multipath , as reflections will appear at various frequency offsets, which is much harder to correct. This effect typically worsens as speed increases, and is an important factor limiting the use of OFDM in high-speed vehicles. In order to mitigate ICI in such scenarios, one can shape each subcarrier in order to minimize the interference resulting in

4002-445: The requirement of low density system (using DTL) and higher density system (using DFSG). The DFSG terminal is similar to DTL terminal except instead of two super groups many super groups are combined. A Mastergroup of 600 channels (10 super-groups) is an example based on DFSG. FDM can also be used to combine signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers : an example

4071-528: The same bandwidth. The main advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath ) without the need for complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes

4140-399: The same conductors, which is also an example of frequency duplex . Where frequency-division multiplexing is used as to allow multiple users to share a physical communications channel , it is called frequency-division multiple access (FDMA). FDMA is the traditional way of separating radio signals from different transmitters. In the 1860s and 70s, several inventors attempted FDM under

4209-521: The same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be re-combined constructively, sparing interference of a traditional single-carrier system. In coded orthogonal frequency-division multiplexing ( COFDM ), forward error correction (convolutional coding) and time/frequency interleaving are applied to the signal being transmitted. This is done to overcome errors in mobile communication channels affected by multipath propagation and Doppler effects . COFDM

SECTION 60

#1733085750775

4278-429: The sub-channel is sufficiently narrow-banded (i.e., if the number of sub-channels is sufficiently large). This makes frequency domain equalization possible at the receiver , 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 subcarrier (each Fourier coefficient) in each OFDM symbol by a constant complex number , or

4347-413: The subcarriers in some of the OFDM symbols may carry pilot signals for measurement of the channel conditions (i.e., the equalizer gain and phase shift for each subcarrier). 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

4416-472: The subjective audibility of different frequencies. In the absence of any international standards, this is still in use despite the demise of the IBA. Good crosstalk performance for a stereo system is not difficult to achieve in today's digital audio systems, though it is hard to keep below the desired figure of -30 dB or so on vinyl recordings and FM radio . In telecommunication or telephony , crosstalk

4485-399: The transmission to the channel conditions individually for each subcarrier, by means of so-called bit-loading . Examples are ADSL and VDSL . The upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose. Some forms of rate-adaptive DSL use this feature in real time, so that the bitrate is adapted to the co-channel interference and bandwidth

4554-425: The use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and use echoes and time-spreading (in analog television visible as ghosting and blurring, respectively) to achieve a diversity gain , i.e. a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs) where several adjacent transmitters send

4623-534: 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

4692-404: 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 and doppler shift is more significant. Over the last decade, research has been done on how to equalize OFDM transmission over doubly selective channels. OFDM

4761-493: Was introduced by Alard in 1986 for Digital Audio Broadcasting for Eureka Project 147. In practice, OFDM has become used in combination with such coding and interleaving, so that the terms COFDM and OFDM co-apply to common applications. The following list is a summary of existing OFDM-based standards and products. For further details, see the Usage section at the end of the article. The OFDM-based multiple access technology OFDMA

#774225