Orthogonal frequency-division multiplexing
Encyclopedia
Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband
digital communication, whether wireless
or over copper
wires, used in applications such as digital television and audio broadcasting, DSL broadband internet access
, wireless networks, and 4G
mobile communications.
OFDM is essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT), and is a frequency-division multiplexing
(FDM) scheme used as a digital multi-carrier modulation
method. A large number of closely spaced orthogonal sub-carrier signals
are used to carry data
. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation
or phase-shift keying
) at a low symbol rate
, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
The primary 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 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 the use of a guard interval
between symbols affordable, making it possible to eliminate intersymbol interference
(ISI) and utilize echoes and time-spreading (that shows up as ghosting
on analogue TV) 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 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.
The OFDM based multiple access technology OFDMA
is also used in several 4G
and pre-4G cellular network
s and mobile broadband
standards:
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
, a separate filter for each sub-channel is not required.
The orthogonality requires that the sub-carrier spacing is
Hertz
, where TU second
s is the useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1. Therefore, with N sub-carriers, the total passband bandwidth will be B ≈ N·Δf (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 utilized. OFDM generally has 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 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 by Doppler shift due to movement. While Doppler shift alone may be compensated for by 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. Several techniques for ICI suppression are suggested, but they may increase the receiver complexity.
components that can efficiently calculate the FFT.
time characteristics) suffer less from intersymbol interference
caused by multipath 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 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 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.
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 sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant complex number, or a rarely changed value.
If differential modulation such as DPSK or DQPSK is applied to each sub-carrier, equalization can be completely omitted, since these non-coherent schemes are insensitive to slowly changing amplitude and phase distortion
.
In 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 the sub-carriers in some of the OFDM symbols may carry pilot signal
s for measurement of the channel conditions (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 and doppler shift is more significant. Over the last decade, research has been done on how to equalize OFDM transmission over doubly selective channels.
), 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 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 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 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 include turbo code
s and LDPC codes, which perform close to 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 code
s (on the DVB-S2
system) to improve upon an error floor
inherent to these codes at high signal-to-noise ratio
s.
, 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 or error coding to 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 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 is allocated to whichever subscriber needs it most.
, since it is utilized 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
is achieved by assigning different OFDM sub-channels to different users. OFDMA supports differentiated quality of service
by assigning different number of sub-carriers to different users in a similar fashion as in CDMA, and thus complex packet scheduling or Media 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 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.
(SFN), where many transmitters send the same signal simultaneously 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 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 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 network is a form of transmitter macrodiversity
. The concept can be further utilized 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 array
s and MIMO
channels. This is done in the IEEE802.11n Wireless LAN
standard.
because the independent phases of the sub-carriers 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 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.
An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, with baseband
data on each sub-carrier being independently modulated commonly using some type of quadrature amplitude modulation
(QAM) or phase-shift keying
(PSK). This composite baseband signal is typically used to modulate a main RF
carrier.
is a serial stream of binary digits. By inverse 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 inverse FFT is computed on each set of symbols, giving a set of complex time-domain samples. These samples are then quadrature-mixed to passband in the standard way. The real and imaginary components are first converted to the analogue domain using digital-to-analogue converters (DACs); the analogue signals are then used to modulate cosine and sine
waves at the carrier
frequency,
, respectively. These signals are then summed to give the transmission signal, 
.
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 using analogue-to-digital converters (ADCs), and a forward FFT is used to convert back to the frequency domain.
This returns
parallel streams, each of which is converted to a binary stream using an appropriate symbol detector
. These streams are then re-combined into a serial stream,
, which is an estimate of the original binary stream at the transmitter.
sub-carriers are used, and each sub-carrier is modulated using 
alternative symbols, the OFDM symbol alphabet consists of 
combined symbols.
The low-pass equivalent OFDM signal is expressed as:
where
are the data symbols, 
is the number of sub-carriers, and 
is the OFDM symbol time. The sub-carrier spacing of 
makes them orthogonal over each symbol period; this property is expressed as:
where
denotes the complex conjugate
operator and
is the Kronecker delta.
To avoid intersymbol interference in multipath fading channels, a guard interval of length
is inserted prior to the OFDM block. During this interval, a cyclic prefix is transmitted such that the signal in the interval 
equals the signal in the interval 
. The OFDM signal with cyclic prefix is thus:
The low-pass signal above can be either real or complex-valued. Real-valued low-pass equivalent signals are typically transmitted at baseband—wireline applications such as DSL use this approach. For wireless applications, the low-pass signal is typically complex-valued; in which case, the transmitted signal is up-converted to a carrier frequency
. In general, the transmitted signal can be represented as:
connections that follow the G.DMT (ITU G.992.1
) standard, in which existing copper wires are used to achieve high-speed data connections.
Long copper wires suffer from attenuation at high frequencies. The fact that OFDM can cope with this frequency selective attenuation and with narrow-band interference are the main reasons it is frequently used in applications such as ADSL modem
s. However, DSL cannot be used on every copper pair; interference may become significant if more than 25% of phone lines coming into a central office are used for DSL.
For experimental amateur radio
applications, users have even hooked up commercial off-the-shelf
ADSL equipment to radio transceivers which simply shift the bands used to the radio frequencies the user has licensed.
devices to extend Ethernet connections to other rooms in a home through its power wiring. Adaptive modulation is particularly important with such a noisy channel as electrical wiring.
The IEEE 1901 standards include two incompatible physical layers that both use OFDM.
The ITU-T
G.hn
standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables) is based on a PHY layer that specifies OFDM with adaptive modulation and a Low-Density Parity-Check (LDPC) FEC code.
and WiMAX
.
IEEE 802.11a/g/n operating in the 2.4 and 5 GHz bands, specifies a per-stream airside data rates ranging from 6 to 54 Mbit/s. If both devices can utilize "HT mode" added with 802.11n then the top 20 MHz per-stream rate is increased to 72.2 Mbit/s with the option of data rates between 13.5 and 150 Mbit/s using a 40 MHz channel. Four different modulation schemes are used: BPSK, QPSK, 16-QAM, and 64-QAM, along with a set of error correcting rates (1/2–5/6). The multitude of choices allows the system to adapt the optimum data rate for the current signal conditions.
s in the 3.1–10.6 GHz ultrawideband spectrum (see MultiBand-OFDM).
, DVB-H
and T-DMB) and radio (EUREKA 147 DAB
, Digital Radio Mondiale
, HD Radio
and T-DMB).
(SDARS) systems used by XM Satellite Radio
and Sirius Satellite Radio
are transmitted using COFDM.
for terrestrial digital television
has been a subject of some controversy, especially between Europe and USA. The United States
has rejected several proposals to adopt the COFDM based DVB-T
system for its digital television services, and has instead opted for 8VSB
(vestigial sideband modulation) operation.
One of the major benefits provided by COFDM is in rendering radio broadcasts relatively immune to multipath
distortion and signal fading
due to atmospheric conditions or passing aircraft. Proponents of COFDM argue it resists multipath far better than 8VSB. Early 8VSB DTV
(digital television) receivers often had difficulty receiving a signal. Also, COFDM allows single-frequency networks, which is not possible with 8VSB.
However, newer 8VSB receivers are far better at dealing with multipath, hence the difference in performance may diminish with advances in equalizer
design. Moreover, 8VSB is nearly a single sideband transmission scheme, while OFDM can be described as a double sideband modulation scheme. This implies that 8VSB (with 3 bit/symbol) modulation offers similar bit rate and require similar bandwidth as 64QAM OFDM (with 6 bit per symbol and sub-carrier), i.e. similar spectral efficiency in (bit/s)/Hz. However, the small 8VSB alphabet of 8 symbols makes it less prone to noise than the 64QAM alphabet of 64 symbols, resulting in lower bit-error rate for the same carrier-to-noise
ratio in case of multipath propagation. 8VSB requires less power than 64QAM to transmit a signal the same distance (i.e., the received carrier-to-noise
threshold is lower for the same bit error rate).
(DAB), the standard for digital audio broadcasting at VHF frequencies, for Digital Radio Mondiale
(DRM), the standard for digital broadcasting at shortwave
and medium wave frequencies (below 30 MHz) and for DRM+ a more recently introduced standard for digital audio broadcasting at VHF frequencies. (30 to 174 MHz)
The USA again uses an alternate standard, a proprietary system developed by iBiquity
dubbed HD Radio
. However, it uses COFDM as the underlying broadcast technology to add digital audio to AM (medium wave) and FM broadcasts.
Both Digital Radio Mondiale and HD Radio are classified as in-band on-channel
systems, unlike Eureka 147 (DAB: Digital Audio Broadcasting
) which uses separate VHF or UHF
frequency bands instead.
, though BST-OFDM is intended to make it more flexible. The 6 MHz television channel may therefore be "segmented", with different segments being modulated differently and used for different services.
It is possible, for example, to send an audio service on a segment that includes a segment composed of a number of carriers, a data service on another segment and a television service on yet another segment—all within the same 6 MHz television channel. Furthermore, these may be modulated with different parameters so that, for example, the audio and data services could be optimized for mobile reception, while the television service is optimized for stationary reception in a high-multipath environment.
(UWB) wireless personal area network technology may also utilise OFDM, such as in Multiband OFDM (MB-OFDM). This UWB specification is advocated by the WiMedia Alliance
(formerly by both the Multiband OFDM Alliance [MBOA] and the WiMedia Alliance, but the two have now merged), and is one of the competing UWB radio interfaces.
. It was developed by Flarion, and purchased by Qualcomm
in January 2006. Flash-OFDM was marketed as a packet-switched cellular bearer, to compete with GSM and 3G
networks. As an example, 450 MHz frequency bands previously used by NMT-450
and C-Net C450
(both 1G analogue networks, now mostly decommissioned) in Europe are being licensed to Flash-OFDM operators.
In Finland
, the license holder Digita began deployment of a nationwide "@450" wireless network in parts of the country since April 2007. It was purchased by Datame in 2011.
T-Mobile Slovensko in Slovakia
offers Flash-OFDM connections with a maximum downstream speed of 5.3 Mbit/s, and a maximum upstream speed of 1.8 Mbit/s, with a coverage of over 70 percent of Slovak population.
T-Mobile Germany uses Flash-OFDM to backhaul Wi-Fi HotSpots on the Deutsche Bahn's ICE high speed trains.
American wireless carrier Nextel Communications
field tested wireless broadband network technologies including Flash-OFDM in 2005. Sprint
purchased the carrier in 2006 and decided to deploy the mobile version of WiMAX
, which is based on Scalable Orthogonal Frequency Division Multiple Access (SOFDMA) technology.
Citizens Telephone Cooperative launched a mobile broadband service based on Flash-OFDM technology to subscribers in parts of Virginia
in March 2006. The maximum speed available was 1.5 Mbit/s. The service was discontinued on April 30, 2009.
Digiweb Ltd.
launched a mobile broadband network using Flash-OFDM technology at 872 MHz in July 2007 in Ireland and Digiweb also owns a national 872MHz license in Norway. Voice handsets are not yet available as of November 2007. The deployment is live in a small area north of Dublin only.
Butler Networks operates a Flash-OFDM network in Denmark at 872 MHz.
In The Netherlands, KPN-telecom will start a pilot around July 2007.
Wideband
In communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
digital communication, whether wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
or over copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
wires, used in applications such as digital television and audio broadcasting, DSL broadband internet access
Broadband Internet access
Broadband Internet access, often shortened to just "broadband", is a high data rate, low-latency connection to the Internet— typically contrasted with dial-up access using a 56 kbit/s modem or satellite Internet with inherently high latency....
, wireless networks, and 4G
4G
In telecommunications, 4G is the fourth generation of cellular wireless standards. It is a successor to the 3G and 2G families of standards. In 2009, the ITU-R organization specified the IMT-Advanced requirements for 4G standards, setting peak speed requirements for 4G service at 100 Mbit/s...
mobile communications.
OFDM is essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT), and is a frequency-division multiplexing
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
(FDM) scheme used as a digital multi-carrier modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
method. A large number of closely spaced orthogonal sub-carrier signals
Subcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
are used to carry data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
or phase-shift keying
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
) at a low symbol rate
Symbol rate
In digital communications, symbol rate is the number of symbol changes made to the transmission medium per second using a digitally modulated signal or a line code. The Symbol rate is measured in baud or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second...
, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
conditions (for example, attenuation
Attenuation Distortion
Attenuation distortion is the distortion of an analog signal that occurs during transmission when the transmission medium does not have a flat frequency response across the bandwidth of the medium or the frequency spectrum of the signal....
of high frequencies in a long copper wire, narrowband interference
Interference (communication)
In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
and frequency-selective fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
due to multipath) without complex equalization filters. Channel equalization
Equalization
Equalization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
is simplified because OFDM may be viewed as using many slowly modulated narrowband
Narrowband
In radio, narrowband describes a channel in which the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.The opposite of...
signals rather than one rapidly modulated wideband
Wideband
In communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
signal. The low symbol rate makes the use of a guard interval
Guard interval
In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users or to the same user ....
between symbols affordable, making it possible to eliminate intersymbol interference
Intersymbol interference
In telecommunication, intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable...
(ISI) and utilize echoes and time-spreading (that shows up as ghosting
Ghosting (television)
In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image on an analogue broadcast.-Common causes:Common causes of ghosts are:...
on analogue TV) to achieve a diversity gain
Diversity gain
In wireless communications, diversity gain is the increase in signal-to-interference ratio due to some diversity scheme, or how much the transmission power can be reduced when a diversity scheme is introduced, without a performance loss. Diversity gain is usually expressed in decibel, and sometimes...
, i.e. a signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
improvement. This mechanism also facilitates the design of single 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.
Example of 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.Cable
- ADSL and VDSL broadband access via POTSPlain old telephone servicePlain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
copperCopperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
wiring. - DVB-C2DVB-CDVB-C stands for Digital Video Broadcasting - Cable and it is the DVB European consortium standard for the broadcast transmission of digital television over cable...
, an enhanced version of the DVB-CDVB-CDVB-C stands for Digital Video Broadcasting - Cable and it is the DVB European consortium standard for the broadcast transmission of digital television over cable...
digital cable TV standard. - Power line communicationPower line communicationPower line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...
(PLC). - ITU-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
G.hnG.hnG.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
, a standard which provides high-speed local area networking of existing home wiring (power lines, phone lines and coaxial cables). - TrailBlazerTrailblazerTrailblazer may refer to:* Trail blazing, a person who marks a trail through wilderness areasIn sports:* Portland Trail Blazers, a basketball team based in Portland, Oregon* North Carolina Trailblazers, a US women's recreational ice hockey association...
telephone linePlain old telephone servicePlain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
modems. - Multimedia over Coax AllianceMultimedia over Coax AllianceMultimedia over Coax Alliance is a trade group promoting a standard that uses coaxial cables to connect consumer electronics and home networking devices in homes. It allows both data communication and the transfer of audio and video streams....
(MoCA) home networking.
Wireless
- The wireless LANWireless LANA wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
(WLAN) radio interfaces IEEE 802.11a, gIEEE 802.11g-2003IEEE 802.11g-2003 or 802.11g is an amendment to the IEEE 802.11 specification that extended throughput to up to 54 Mbit/s using the same 2.4 GHz band as 802.11b. This specification under the marketing name of Wi-Fi has been implemented all over the world...
, n and HIPERLAN/2. - The digital radioDigital radioDigital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
systems DAB/EUREKA 147, DAB+, Digital Radio MondialeDigital Radio MondialeDigital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly shortwave...
, HD RadioHD RadioHD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals...
, T-DMB and ISDB-TSB. - The terrestrial digital TV systems DVB-TDVB-TDVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
and ISDB-T. - The terrestrial mobile TVMobile TVMobile television usually means television watched on a small handheld device. It may be a pay TV service broadcast on mobile phone networks or received free-to-air via terrestrial television stations from either regular broadcast or a special mobile TV transmission format...
systems DVB-HDVB-HDVB-H is one of three prevalent mobile TV formats. It is a technical specification for bringing broadcast services to mobile handsets. DVB-H was formally adopted as ETSI standard EN 302 304 in November 2004. The DVB-H specification can be downloaded from the official DVB-H website...
, T-DMB, ISDB-T and MediaFLOMediaFLOMediaFLO is a technology developed by Qualcomm for transmitting audio, video and data to portable devices such as mobile phones and personal televisions, used for mobile television...
forward link. - The wireless personal area networkPersonal area networkA personal area network is a computer network used for communication among computer devices, including telephones and personal digital assistants, in proximity to an individual's body. The devices may or may not belong to the person in question. The reach of a PAN is typically a few meters...
(PAN) ultra-widebandUltra-widebandUltra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...
(UWB) IEEE 802.15.3a implementation suggested by WiMedia AllianceWiMedia AllianceThe WiMedia Alliance is a non-profit open industry association that promotes and enables the rapid adoption, regulation, standardization and multi-vendor interoperability of ultra-wideband worldwide....
.
The OFDM based multiple access technology OFDMA
OFDMA
Orthogonal Frequency-Division Multiple Access is a multi-user version of the popular Orthogonal frequency-division multiplexing digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the illustration below...
is also used in several 4G
4G
In telecommunications, 4G is the fourth generation of cellular wireless standards. It is a successor to the 3G and 2G families of standards. In 2009, the ITU-R organization specified the IMT-Advanced requirements for 4G standards, setting peak speed requirements for 4G service at 100 Mbit/s...
and pre-4G cellular network
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...
s and mobile broadband
Mobile Broadband
Mobile broadband is the marketing term for wireless Internet access through a portable modem, mobile phone or other mobile device.-Description:...
standards:
- The mobility mode of the wireless MAN/broadband wireless accessBroadband wireless accessWireless Broadband refers to technology that provides high-speed wireless Internet access or computer networking access over a wide area.-The term broadband:...
(BWA) standard IEEE 802.16e (or Mobile-WiMAXWiMAXWiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
). - The mobile broadband wireless access (MBWA) standard IEEE 802.20IEEE 802.20IEEE 802.20 or Mobile Broadband Wireless Access was a specification by the standard association of the Institute of Electrical and Electronics Engineers for mobile wireless Internet access networks...
. - the downlink of the 3GPP3GPPThe 3rd Generation Partnership Project is a collaboration between groups of telecommunications associations, known as the Organizational Partners...
Long Term Evolution (LTE) fourth generation mobile broadband standard. The radio interface was formerly named High Speed OFDM Packet Access (HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).
Key features
The advantages and disadvantages listed below are further discussed in the Characteristics and principles of operation section below.Summary of advantages
- Can easily adapt to severe channel conditions without complex time-domain equalization.
- Robust against narrow-band co-channel interference.
- Robust against intersymbol interferenceIntersymbol interferenceIn telecommunication, intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable...
(ISI) and fading caused by multipath propagation. - High spectral efficiencySpectral efficiencySpectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system...
as compared to conventional modulation schemes, spread spectrum, etc. - Efficient implementation using Fast Fourier TransformFast Fourier transformA fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform and its inverse. "The FFT has been called the most important numerical algorithm of our lifetime ." There are many distinct FFT algorithms involving a wide range of mathematics, from simple...
(FFT). - Low sensitivity to time synchronization errors.
- Tuned sub-channel receiver filters are not required (unlike conventional FDMFrequency-division multiplexingFrequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
). - Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversityMacrodiversityIn the field of wireless communication, Macrodiversity is a kind of space diversity scheme using several receiver antennas and/or transmitter antennas for transferring the same signal...
.
Summary of disadvantages
- Sensitive to Doppler shiftDoppler effectThe Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
. - Sensitive to frequency synchronization problems.
- High peak-to-average-power ratioCrest factorThe crest factor or peak-to-average ratio or peak-to-average power ratio is a measurement of a waveform, calculated from the peak amplitude of the waveform divided by the RMS value of the waveform.C =...
(PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency. - Loss of efficiency caused by cyclic prefixCyclic prefixIn telecommunications, the term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes....
/guard intervalGuard intervalIn telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users or to the same user ....
.
Orthogonality
In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talkCrosstalk (electronics)
In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel...
between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
and the receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
; unlike conventional FDM
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
, a separate filter for each sub-channel is not required.
The orthogonality requires that the sub-carrier spacing is
HertzHertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, where TU second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s is the useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1. Therefore, with N sub-carriers, the total passband bandwidth will be B ≈ N·Δf (Hz).
The orthogonality also allows high spectral efficiency
Spectral efficiency
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system...
, with a total symbol rate near the Nyquist rate
Nyquist rate
In signal processing, the Nyquist rate, named after Harry Nyquist, is two times the bandwidth of a bandlimited signal or a bandlimited channel...
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 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 of NΔf = 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 by Doppler shift due to movement. While Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined with multipath
Multipath interference
Multipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two components of the wave interfere...
, 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. Several techniques for ICI suppression are suggested, but they may increase the receiver complexity.
Implementation using the FFT algorithm
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 processingDigital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
components that can efficiently calculate the FFT.
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 the channelChannel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
time characteristics) suffer less from intersymbol interference
Intersymbol interference
In telecommunication, intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable...
caused by multipath 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 a guard interval
Guard interval
In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users or to the same user ....
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.
- 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.
The cyclic prefix
Cyclic prefix
In telecommunications, the term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes....
, 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.
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 domain equalizationEqualization
Equalization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
possible at the receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
, 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 a FIR filter would 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 an FIR filter based 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 as DPSK or DQPSK is applied to each sub-carrier, equalization can be completely omitted, since these non-coherent schemes are insensitive to slowly changing amplitude and phase distortion
Phase distortion
In signal processing, phase distortion or phase-frequency distortion is distortion that occurs when a filter's phase response is not linear over the frequency range of interest, that is, the phase shift introduced by a circuit or device is not directly proportional to frequency, or the...
.
In 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 the sub-carriers in some of the OFDM symbols may carry pilot signal
Pilot signal
In telecommunications, a pilot is a signal, usually a single frequency, transmitted over a communications system for supervisory, control, equalization, continuity, synchronization, or reference purposes....
s for measurement of the channel conditions (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 and doppler shift is more significant. Over the last decade, research has been done on how to equalize OFDM transmission over doubly selective channels.
Channel coding and interleaving
OFDM is invariably used in conjunction with channel coding (forward error correctionForward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
), and almost always uses frequency and/or time interleaving
Interleaving
In computer science and telecommunication, interleaving is a way to arrange data in a non-contiguous way to increase performance.It is typically used:* In error-correction coding, particularly within data transmission, disk storage, and computer memory....
.
Frequency (subcarrier) interleaving
Interleaving
In computer science and telecommunication, interleaving is a way to arrange data in a non-contiguous way to increase performance.It is typically used:* In error-correction coding, particularly within data transmission, disk storage, and computer memory....
increases resistance to frequency-selective channel conditions such as fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
. 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 is convolutional coding
Convolutional code
In telecommunication, a convolutional code is a type of error-correcting code in which* each m-bit information symbol to be encoded is transformed into an n-bit symbol, where m/n is the code rate and...
, often concatenated
Concatenated error correction codes
In coding theory, concatenated codes form a class of error-correcting codes that are derived by combining an inner code and an outer code...
with Reed-Solomon
Reed–Solomon error correction
In coding theory, Reed–Solomon codes are non-binary cyclic error-correcting codes invented by Irving S. Reed and Gustave Solomon. They described a systematic way of building codes that could detect and correct multiple random symbol errors...
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 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 include turbo code
Turbo code
In information theory, turbo codes are a class of high-performance forward error correction codes developed in 1993, which were the first practical codes to closely approach the channel capacity, a theoretical maximum for the code rate at which reliable communication is still possible given a...
s and LDPC codes, which perform close to the Shannon limit for the Additive White Gaussian Noise (AWGN
Additive white Gaussian noise
Additive white Gaussian noise is a channel model in which the only impairment to communication is a linear addition of wideband or white noise with a constant spectral density and a Gaussian distribution of amplitude. The model does not account for fading, frequency selectivity, interference,...
) channel. Some systems that have implemented these codes have concatenated them with either Reed-Solomon (for example on the MediaFLO
MediaFLO
MediaFLO is a technology developed by Qualcomm for transmitting audio, video and data to portable devices such as mobile phones and personal televisions, used for mobile television...
system) or BCH code
BCH code
In coding theory the BCH codes form a class of parameterised error-correcting codes which have been the subject of much academic attention in the last fifty years. BCH codes were invented in 1959 by Hocquenghem, and independently in 1960 by Bose and Ray-Chaudhuri...
s (on the DVB-S2
DVB-S2
Digital Video Broadcasting - Satellite - Second Generation is a digital television broadcast standard that has been designed as a successor for the popular DVB-S system. It was developed in 2003 by the , an international industry consortium, and ratified by ETSI in March 2005...
system) to improve upon an error floor
Error floor
The error floor is a phenomenon encountered in modern iterated sparse graph-based error correcting codes like LDPC codes and turbo codes. When the bit error ratio curve is plotted for conventional codes like Reed Solomon codes under algebraic decoding or for convolutional codes under Viterbi...
inherent to these codes at high signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
s.
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, adaptive modulationModulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
, 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 or error coding to 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 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 is allocated to whichever subscriber needs it most.
OFDM extended with multiple access
OFDM in its primary form is considered as a digital modulation technique, and not a multi-user channel access methodChannel access method
In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same multi-point transmission medium to transmit over it and to share its capacity...
, since it is utilized for transferring one bit stream over one communication channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio 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
Frequency-division multiple access
Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication. FDMA, like...
is achieved by assigning different OFDM sub-channels to different users. OFDMA supports differentiated quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
by assigning different number of sub-carriers to different users in a similar fashion as in CDMA, and thus complex packet scheduling or Media Access Control
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
schemes can be avoided. OFDMA is used in:
- the mobility mode of the IEEE 802.16IEEE 802.16IEEE 802.16 is a series of Wireless Broadband standards authored by the Institute of Electrical and Electronics Engineers . The IEEE Standards Board in established a working group in 1999 to develop standards for broadband Wireless Metropolitan Area Networks...
Wireless MAN standard, commonly referred to as WiMAX, - the IEEE 802.20IEEE 802.20IEEE 802.20 or Mobile Broadband Wireless Access was a specification by the standard association of the Institute of Electrical and Electronics Engineers for mobile wireless Internet access networks...
mobile Wireless MAN standard, commonly referred to as MBWA, - the 3GPP Long Term Evolution3GPP Long Term Evolution3GPP Long Term Evolution, usually referred to as LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals. It is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using new modulation techniques...
(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-UTRAE-UTRAe-UTRAN or eUTRAN is the air interface of 3GPP's Long Term Evolution upgrade path for mobile networks. It is the abbreviation for evolved UMTS Terrestrial Radio Access Network, also referred to as the 3GPP work item on the Long Term Evolution also known as the Evolved Universal Terrestrial Radio...
). - the now defunct QualcommQualcommQualcomm is an American global telecommunication corporation that designs, manufactures and markets digital wireless telecommunications products and services based on its code division multiple access technology and other technologies. Headquartered in San Diego, CA, USA...
/3GPP2 Ultra Mobile BroadbandUltra Mobile BroadbandUMB was the brand name for a project within 3GPP2 to improve the CDMA2000 mobile phone standard for next generation applications and requirements...
(UMB) project, intended as a successor of CDMA2000CDMA2000CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
, but replaced by LTE.
OFDMA is also a candidate access method for the IEEE 802.22
IEEE 802.22
IEEE 802.22 is a standard for Wireless Regional Area Network using white spaces in the TV frequency spectrum. The development of the IEEE 802.22 WRAN standard is aimed at using cognitive radio techniques to allow sharing of geographically unused spectrum allocated to the Television Broadcast...
Wireless Regional Area Networks (WRAN). The project aims at designing the first cognitive radio
Cognitive radio
A cognitive radio is a kind of two-way radio that automatically changes its transmission or reception parameters, in a way where the entire wireless communication network -- of which it is a node -- communicates efficiently, while avoiding interference with licensed or licensed exempt users...
based standard operating in the VHF-low UHF spectrum (TV spectrum).
In Multi-carrier code division multiple access
Multi-carrier code division multiple access
Multi-Carrier Code Division Multiple Access is a multiple access scheme used in OFDM-based telecommunication systems, allowing the system to support multiple users at the same time.MC-CDMA spreads each user symbol in the frequency domain...
(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.
Space diversity
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 national single-frequency networksSingle-frequency network
A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.-Overview:...
(SFN), where many transmitters send the same signal simultaneously over the same channel frequency. SFNs utilise the available spectrum more effectively than conventional multi-frequency broadcast networks (MFN
Multi-frequency network
-Introduction:Data networks, such as wireless communication networks, have to trade off between services customized for a single terminal and services provided to a large number of terminals. For example, the distribution of multimedia content to a large number of resource limited portable devices ...
), where program content is replicated on different carrier frequencies. SFNs also result in a diversity gain
Diversity gain
In wireless communications, diversity gain is the increase in signal-to-interference ratio due to some diversity scheme, or how much the transmission power can be reduced when a diversity scheme is introduced, without a performance loss. Diversity gain is usually expressed in decibel, and sometimes...
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 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 network is a form of transmitter macrodiversity
Macrodiversity
In the field of wireless communication, Macrodiversity is a kind of space diversity scheme using several receiver antennas and/or transmitter antennas for transferring the same signal...
. The concept can be further utilized in dynamic single-frequency networks
Dynamic single-frequency networks
Dynamic Single Frequency Networks is a transmitter macrodiversity technique for OFDM based cellular networks.DSFN is based on the idea of single frequency networks , which is a group of radio transmitters that send the same signal simultaneously over the same frequency...
(DSFN), where the SFN grouping is changed from timeslot to timeslot.
OFDM may be combined with other forms of space diversity, for example antenna array
Antenna array
Antenna array may refer to:*Antenna array a group of isotropic radiators such that the currents running through them are of different amplitudes and phases*Interferometric array of radio telescopes used in radio astronomy...
s and MIMO
MIMO
In radio, multiple-input and multiple-output, or MIMO , is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology...
channels. This is done in the IEEE802.11n Wireless LAN
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
standard.
Linear transmitter power amplifier
An OFDM signal exhibits a high peak-to-average power ratio (PAPR)Crest factor
The crest factor or peak-to-average ratio or peak-to-average power ratio is a measurement of a waveform, calculated from the peak amplitude of the waveform divided by the RMS value of the waveform.C =...
because the independent phases of the sub-carriers mean that they will often combine constructively. Handling this high PAPR requires:
- a high-resolution digital-to-analogue converter (DAC) in the transmitter
- a high-resolution analogue-to-digital converter (ADC) in the receiver
- a linear signal chainSignal chainSignal chain, or signal-processing chain is a term used in signal processing and mixed-signal system design to describe a series of signal-conditioning electronic components that receive input in tandem, with the output of one portion of the chain supplying input to the next...
.
Any non-linearity in the signal chain will cause intermodulation distortion that
- 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.
Idealized system model
This section describes a simple idealized OFDM system model suitable for a time-invariant AWGN channel.Transmitter
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
data on each sub-carrier being independently modulated commonly using some type of quadrature amplitude modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
(QAM) or phase-shift keying
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
(PSK). This composite baseband signal is typically used to modulate a main RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
carrier.
is a serial stream of binary digits. By inverse multiplexing, these are first demultiplexed into parallel streams, and each one mapped to a (possibly complex) symbol stream using some modulation constellation (QAM, PSKPhase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
, etc.). Note that the constellations may be different, so some streams may carry a higher bit-rate than others.
An inverse FFT is computed on each set of symbols, giving a set of complex time-domain samples. These samples are then quadrature-mixed to passband in the standard way. The real and imaginary components are first converted to the analogue domain using digital-to-analogue converters (DACs); the analogue signals are then used to modulate cosine and sine
Sine
In mathematics, the sine function is a function of an angle. In a right triangle, sine gives the ratio of the length of the side opposite to an angle to the length of the hypotenuse.Sine is usually listed first amongst the trigonometric functions....
waves at the carrier
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
frequency,
, respectively. These signals are then summed to give the transmission signal, .Receiver
, 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 using analogue-to-digital converters (ADCs), and a forward FFT is used to convert back to the frequency domain.This returns
parallel streams, each of which is converted to a binary stream using an appropriate symbol detectorDetector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
. These streams are then re-combined into a serial stream,
, which is an estimate of the original binary stream at the transmitter.Mathematical description
If sub-carriers are used, and each sub-carrier is modulated using alternative symbols, the OFDM symbol alphabet consists of combined symbols.The low-pass equivalent OFDM signal is expressed as:
where
are the data symbols, is the number of sub-carriers, and is the OFDM symbol time. The sub-carrier spacing of makes them orthogonal over each symbol period; this property is expressed as:where
denotes the complex conjugateComplex conjugate
In mathematics, complex conjugates are a pair of complex numbers, both having the same real part, but with imaginary parts of equal magnitude and opposite signs...
operator and
is the Kronecker delta.To avoid intersymbol interference in multipath fading channels, a guard interval of length
is inserted prior to the OFDM block. During this interval, a cyclic prefix is transmitted such that the signal in the interval equals the signal in the interval . The OFDM signal with cyclic prefix is thus:The low-pass signal above can be either real or complex-valued. Real-valued low-pass equivalent signals are typically transmitted at baseband—wireline applications such as DSL use this approach. For wireless applications, the low-pass signal is typically complex-valued; in which case, the transmitted signal is up-converted to a carrier frequency
. In general, the transmitted signal can be represented as:OFDM system comparison table
Key features of some common OFDM based systems are presented in the following table.| Standard name | DAB Digital audio broadcasting Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format.... Eureka 147 |
DVB-T DVB-T DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998... |
DVB-H DVB-H DVB-H is one of three prevalent mobile TV formats. It is a technical specification for bringing broadcast services to mobile handsets. DVB-H was formally adopted as ETSI standard EN 302 304 in November 2004. The DVB-H specification can be downloaded from the official DVB-H website... |
DMB-T/H DMB-T/H DTMB is the TV standard for mobile and fixed terminals used in the People's Republic of China, Hong Kong and Macau. Although at first this standard was called DMB-T/H , the official name is DTMB.DTT broadcasting systems... |
DVB-T2 DVB-T2 DVB-T2 is an abbreviation for Digital Video Broadcasting – Second Generation Terrestrial; it is the extension of the television standard DVB-T, issued by the consortium DVB, devised for the broadcast transmission of digital terrestrial television.... |
IEEE 802.11a |
|---|---|---|---|---|---|---|
| Ratified year | 1995 | 1997 | 2004 | 2006 | 2007 | 1999 |
| Frequency range of today's equipment (MHz) |
174–240 1,452–1,492 |
470–862 174–230 |
470–862 | 470–862 | 4,915–5,825 | |
| Channel spacing, B (MHz) |
1.712 | 6, 7, 8 | 5, 6, 7, 8 | 8 | 1.7, 5, 6, 7, 8, 10 | 20 |
| FFT size (k=1,024) | Mode I: 2k Mode II: 512 Mode III: 256 Mode IV: 1k |
2k, 8k | 2k, 4k, 8k | 1 (single-carrier) 4k (multi-carrier) |
1k, 2k, 4k, 8k, 16k, 32k | 64 |
| Number of non-silent sub-carriers, N | Mode I: 1,536 Mode II: 384 Mode III: 192 Mode IV: 768 |
2K mode: 1,705 8K mode: 6,817 |
1,705, 3,409, 6,817 | 1 (single-carrier) 3,780 (multi-carrier) |
853-27,841 (1K normal to 32K extended carrier mode) | 52 |
| Sub-carrier modulation scheme | -DQPSK | QPSK, 16QAM or 64QAM | QPSK, 16QAM or 64QAM | 4QAM, 4QAM-NR, 16QAM, 32QAM and 64QAM. |
QPSK, 16QAM, 64QAM, 256QAM | BPSK, QPSK, 16QAM or 64QAM |
| Useful symbol length, TU (μs) |
Mode I: 1,000 Mode II: 250 Mode III: 125 Mode IV: 500 |
2K mode: 224 8K mode: 896 |
224, 448, 896 | 500 (multi-carrier) | 112-3,584 (1K to 32K mode on 8 MHz channel) | 3.2 |
| Additional guard interval, TG (fraction of TU) |
24.6% (all modes) | , , , | , , , | , , | 1/128, 1/32, 1/16, 19/256, 1/8, 19/128, 1/4. (For 32k mode maximum 1/8) |
|
Sub-carrier spacing (Hz) |
Mode I: 1,000 Mode II: 4,000 Mode III: 8,000 Mode IV: 2,000 |
2K mode: 4,464 8K mode: 1,116 |
4,464, 2,232, 1,116 | 8 M (single-carrier) 2,000 (multi-carrier) |
279-8,929 (32K down to 1K mode) | 312.5K |
| Net bit rate, R (Mbit/s) |
0.576–1.152 | 4.98–31.67 (typically 24.13) |
3.7–23.8 | 4.81–32.49 | Typically 35.4 | 6–54 |
| Link spectral efficiency R/B (bit/s/Hz) |
0.34–0.67 | 0.62–4.0 (Typ 3.0) | 0.62–4.0 | 0.60–4.1 | 0.87-6.65 | 0.30–2.7 |
| Inner FEC Forward error correction In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels.... |
Conv coding Convolutional code In telecommunication, a convolutional code is a type of error-correcting code in which* each m-bit information symbol to be encoded is transformed into an n-bit symbol, where m/n is the code rate and... with Equal error protection code rates , , , , , , , Unequal error protection with av. code rates of ~0.34, 0.41, 0.50, 0.60, and 0.75 |
Conv. coding Convolutional code In telecommunication, a convolutional code is a type of error-correcting code in which* each m-bit information symbol to be encoded is transformed into an n-bit symbol, where m/n is the code rate and... with code rates , , , , or |
Conv. coding Convolutional code In telecommunication, a convolutional code is a type of error-correcting code in which* each m-bit information symbol to be encoded is transformed into an n-bit symbol, where m/n is the code rate and... with code rates , , , , or |
LDPC Low-density parity-check code In information theory, a low-density parity-check code is a linear error correcting code, a method of transmitting a message over a noisy transmission channel, and is constructed using a sparse bipartite graph... with code rates 0.4, 0.6 or 0.8 |
LDPC 1/2, 3/5, 2/3, 3/4, 4/5, 5/6 | Conv. coding Convolutional code In telecommunication, a convolutional code is a type of error-correcting code in which* each m-bit information symbol to be encoded is transformed into an n-bit symbol, where m/n is the code rate and... with code rates , , or |
| Outer FEC Forward error correction In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels.... (if any) |
Optional RS Reed–Solomon error correction In coding theory, Reed–Solomon codes are non-binary cyclic error-correcting codes invented by Irving S. Reed and Gustave Solomon. They described a systematic way of building codes that could detect and correct multiple random symbol errors... (120, 110, t = 5) |
RS Reed–Solomon error correction In coding theory, Reed–Solomon codes are non-binary cyclic error-correcting codes invented by Irving S. Reed and Gustave Solomon. They described a systematic way of building codes that could detect and correct multiple random symbol errors... (204, 188, t=8) |
RS Reed–Solomon error correction In coding theory, Reed–Solomon codes are non-binary cyclic error-correcting codes invented by Irving S. Reed and Gustave Solomon. They described a systematic way of building codes that could detect and correct multiple random symbol errors... (204, 188, t=8) + MPE-FEC |
BCH code BCH code In coding theory the BCH codes form a class of parameterised error-correcting codes which have been the subject of much academic attention in the last fifty years. BCH codes were invented in 1959 by Hocquenghem, and independently in 1960 by Bose and Ray-Chaudhuri... (762, 752) |
BCH code BCH code In coding theory the BCH codes form a class of parameterised error-correcting codes which have been the subject of much academic attention in the last fifty years. BCH codes were invented in 1959 by Hocquenghem, and independently in 1960 by Bose and Ray-Chaudhuri... |
|
| Maximum travelling speed (km/h) |
200–600 | 53–185 depends on transmission frequency |
||||
| Time interleaving Interleaving In computer science and telecommunication, interleaving is a way to arrange data in a non-contiguous way to increase performance.It is typically used:* In error-correction coding, particularly within data transmission, disk storage, and computer memory.... depth (ms) |
384 | 0.6–3.5 | 0.6–3.5 | 200–500 | up to 250 (500 with extension frame) | |
| Adaptive transmission (if any) |
None | None | None | None | ||
| Multiple access method (if any) |
None | None | None | None | ||
| Typical source coding Source coding In information theory, Shannon's source coding theorem establishes the limits to possible data compression, and the operational meaning of the Shannon entropy.... |
192 kbit/s MPEG2 Audio layer 2 |
2–18 Mbit/s Standard - HDTV H.264 or MPEG2 |
H.264 | Not defined (Video: MPEG-2, H.264 and/or AVS Audio Video Standard Audio Video Standard is a compression standard for digital audio and video, and is competing with H.264/AAC to potentially replace MPEG-2. Chinese companies own 90% of AVS patents. The audio and video files have an .avs extension as a container format.-Overview:Development of AVS was initiated by... Audio: MP2 MPEG-1 Audio Layer II MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III... or AC-3 AC-3 AC-3 can be:* AC-3 algorithm , one of a series of algorithms used for the solution of constraint satisfaction problems* Comte AC-3, a 1920s Swiss bomber/transport aircraft.* Dolby AC-3, a Dolby Digital, audio codec... ) |
H.264 or MPEG2 |
ADSL
OFDM is used in ADSLAsymmetric Digital Subscriber Line
Asymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...
connections that follow the G.DMT (ITU G.992.1
ITU G.992.1
In telecommunications, ITU G.992.1 is an ITU standard for ADSL using discrete multitone modulation. G.DMT full-rate ADSL expands the usable bandwidth of existing copper telephone lines, delivering high-speed data communications at rates up to 8 Mbit/s downstream and 1.3 Mbit/s upstream.DMT...
) standard, in which existing copper wires are used to achieve high-speed data connections.
Long copper wires suffer from attenuation at high frequencies. The fact that OFDM can cope with this frequency selective attenuation and with narrow-band interference are the main reasons it is frequently used in applications such as ADSL modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s. However, DSL cannot be used on every copper pair; interference may become significant if more than 25% of phone lines coming into a central office are used for DSL.
For experimental amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
applications, users have even hooked up commercial off-the-shelf
Commercial off-the-shelf
In the United States, Commercially available Off-The-Shelf is a Federal Acquisition Regulation term defining a nondevelopmental item of supply that is both commercial and sold in substantial quantities in the commercial marketplace, and that can be procured or utilized under government contract...
ADSL equipment to radio transceivers which simply shift the bands used to the radio frequencies the user has licensed.
Powerline Technology
OFDM is used by many powerlinePower line communication
Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...
devices to extend Ethernet connections to other rooms in a home through its power wiring. Adaptive modulation is particularly important with such a noisy channel as electrical wiring.
The IEEE 1901 standards include two incompatible physical layers that both use OFDM.
The ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
G.hn
G.hn
G.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables) is based on a PHY layer that specifies OFDM with adaptive modulation and a Low-Density Parity-Check (LDPC) FEC code.
Wireless local area networks (LAN) and metropolitan area networks (MAN)
OFDM is extensively used in wireless LAN and MAN applications, including IEEE 802.11a/g/nIEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
and WiMAX
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
.
IEEE 802.11a/g/n operating in the 2.4 and 5 GHz bands, specifies a per-stream airside data rates ranging from 6 to 54 Mbit/s. If both devices can utilize "HT mode" added with 802.11n then the top 20 MHz per-stream rate is increased to 72.2 Mbit/s with the option of data rates between 13.5 and 150 Mbit/s using a 40 MHz channel. Four different modulation schemes are used: BPSK, QPSK, 16-QAM, and 64-QAM, along with a set of error correcting rates (1/2–5/6). The multitude of choices allows the system to adapt the optimum data rate for the current signal conditions.
Wireless personal area networks (PAN)
OFDM is also now being used in the WiMedia/Ecma-368 standard for high-speed wireless personal area networkPersonal area network
A personal area network is a computer network used for communication among computer devices, including telephones and personal digital assistants, in proximity to an individual's body. The devices may or may not belong to the person in question. The reach of a PAN is typically a few meters...
s in the 3.1–10.6 GHz ultrawideband spectrum (see MultiBand-OFDM).
Terrestrial digital radio and television broadcasting
Much of Europe and Asia has adopted OFDM for terrestrial broadcasting of digital television (DVB-TDVB-T
DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
, DVB-H
DVB-H
DVB-H is one of three prevalent mobile TV formats. It is a technical specification for bringing broadcast services to mobile handsets. DVB-H was formally adopted as ETSI standard EN 302 304 in November 2004. The DVB-H specification can be downloaded from the official DVB-H website...
and T-DMB) and radio (EUREKA 147 DAB
Digital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
, Digital Radio Mondiale
Digital Radio Mondiale
Digital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly shortwave...
, HD Radio
HD Radio
HD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals...
and T-DMB).
DVB-T
By Directive of the European Commission, all television services transmitted to viewers in the European Community must use a transmission system that has been standardized by a recognized European standardization body, and such a standard has been developed and codified by the DVB Project, Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television. Customarily referred to as DVB-T, the standard calls for the exclusive use of COFDM for modulation. DVB-T is now widely used in Europe and elsewhere for terrestrial digital TV.SDARS
The ground segments of the Digital Audio Radio ServiceDigital Audio Radio Service
Digital Audio Radio Service or DARS refers to any type of digital radio service. In the United States it is the official FCC term for digital radio services....
(SDARS) systems used by XM Satellite Radio
XM Satellite Radio
XM Satellite Radio is one of two satellite radio services in the United States and Canada, operated by Sirius XM Radio. It provides pay-for-service radio, analogous to cable television. Its service includes 73 different music channels, 39 news, sports, talk and entertainment channels, 21 regional...
and Sirius Satellite Radio
Sirius Satellite Radio
Sirius Satellite Radio is a satellite radio service operating in North America, owned by Sirius XM Radio.Headquartered in New York City, with smaller studios in Los Angeles and Memphis, Sirius was officially launched on July 1, 2002 and currently provides 69 streams of music and 65 streams of...
are transmitted using COFDM.
COFDM vs VSB
The question of the relative technical merits of COFDM versus 8VSB8VSB
8VSB is the modulation method used for broadcast in the ATSC digital television standard. ATSC and 8VSB modulation is used primarily in North America; in contrast, the DVB-T standard uses COFDM....
for terrestrial digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
has been a subject of some controversy, especially between Europe and USA. The United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
has rejected several proposals to adopt the COFDM based DVB-T
DVB-T
DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
system for its digital television services, and has instead opted for 8VSB
8VSB
8VSB is the modulation method used for broadcast in the ATSC digital television standard. ATSC and 8VSB modulation is used primarily in North America; in contrast, the DVB-T standard uses COFDM....
(vestigial sideband modulation) operation.
One of the major benefits provided by COFDM is in rendering radio broadcasts relatively immune to multipath
Multipath interference
Multipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two components of the wave interfere...
distortion and signal fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
due to atmospheric conditions or passing aircraft. Proponents of COFDM argue it resists multipath far better than 8VSB. Early 8VSB DTV
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
(digital television) receivers often had difficulty receiving a signal. Also, COFDM allows single-frequency networks, which is not possible with 8VSB.
However, newer 8VSB receivers are far better at dealing with multipath, hence the difference in performance may diminish with advances in equalizer
Equalizer
Equalizer or equaliser may refer to:*Equalization, the process of adjusting the strength of certain frequencies within a signal*An equalization filter for used audio and similar signals...
design. Moreover, 8VSB is nearly a single sideband transmission scheme, while OFDM can be described as a double sideband modulation scheme. This implies that 8VSB (with 3 bit/symbol) modulation offers similar bit rate and require similar bandwidth as 64QAM OFDM (with 6 bit per symbol and sub-carrier), i.e. similar spectral efficiency in (bit/s)/Hz. However, the small 8VSB alphabet of 8 symbols makes it less prone to noise than the 64QAM alphabet of 64 symbols, resulting in lower bit-error rate for the same carrier-to-noise
Carrier-to-noise ratio
In telecommunications, the carrier-to-noise ratio, often written CNR or C/N, is the signal-to-noise ratio of a modulated signal. The term is used to distinguish the CNR of the radio frequency passband signal from the SNR of an analogue base band message signal after demodulation, for example an...
ratio in case of multipath propagation. 8VSB requires less power than 64QAM to transmit a signal the same distance (i.e., the received carrier-to-noise
Carrier-to-noise ratio
In telecommunications, the carrier-to-noise ratio, often written CNR or C/N, is the signal-to-noise ratio of a modulated signal. The term is used to distinguish the CNR of the radio frequency passband signal from the SNR of an analogue base band message signal after demodulation, for example an...
threshold is lower for the same bit error rate).
DIGITAL RADIO
COFDM is also used for other radio standards, for Digital Audio BroadcastingDigital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
(DAB), the standard for digital audio broadcasting at VHF frequencies, for Digital Radio Mondiale
Digital Radio Mondiale
Digital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly shortwave...
(DRM), the standard for digital broadcasting at shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
and medium wave frequencies (below 30 MHz) and for DRM+ a more recently introduced standard for digital audio broadcasting at VHF frequencies. (30 to 174 MHz)
The USA again uses an alternate standard, a proprietary system developed by iBiquity
IBiquity
iBiquity Digital Corporation is a company formed by the merger of USA Digital Radio and Lucent Digital Radio, with the goal of creating an in-band on-channel digital radio system for the United States and around the world...
dubbed HD Radio
HD Radio
HD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals...
. However, it uses COFDM as the underlying broadcast technology to add digital audio to AM (medium wave) and FM broadcasts.
Both Digital Radio Mondiale and HD Radio are classified as in-band on-channel
In-band on-channel
In-band on-channel is a hybrid method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency....
systems, unlike Eureka 147 (DAB: Digital Audio Broadcasting
Digital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
) which uses separate VHF or UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
frequency bands instead.
BST-OFDM used in ISDB
The band-segmented transmission orthogonal frequency division multiplexing (BST-OFDM) system proposed for Japan (in the ISDB-T, ISDB-TSB, and ISDB-C broadcasting systems) improves upon COFDM by exploiting the fact that some OFDM carriers may be modulated differently from others within the same multiplex. Some forms of COFDM already offer this kind of hierarchical modulationHierarchical modulation
Hierarchical modulation, also called layered modulation, is one of the signal processing techniques for multiplexing and modulating multiple data streams into one single symbol stream, where base-layer symbols and enhancement-layer symbols are synchronously overplayed before...
, though BST-OFDM is intended to make it more flexible. The 6 MHz television channel may therefore be "segmented", with different segments being modulated differently and used for different services.
It is possible, for example, to send an audio service on a segment that includes a segment composed of a number of carriers, a data service on another segment and a television service on yet another segment—all within the same 6 MHz television channel. Furthermore, these may be modulated with different parameters so that, for example, the audio and data services could be optimized for mobile reception, while the television service is optimized for stationary reception in a high-multipath environment.
Ultra-wideband
Ultra-widebandUltra-wideband
Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...
(UWB) wireless personal area network technology may also utilise OFDM, such as in Multiband OFDM (MB-OFDM). This UWB specification is advocated by the WiMedia Alliance
WiMedia Alliance
The WiMedia Alliance is a non-profit open industry association that promotes and enables the rapid adoption, regulation, standardization and multi-vendor interoperability of ultra-wideband worldwide....
(formerly by both the Multiband OFDM Alliance [MBOA] and the WiMedia Alliance, but the two have now merged), and is one of the competing UWB radio interfaces.
FLASH-OFDM
Fast low-latency access with seamless handoff orthogonal frequency division multiplexing (Flash-OFDM), also referred to as F-OFDM, was based on OFDM and also specified higher protocol layersOpen systems architecture
Open systems architecture, in telecommunication, is a standard that describes the layered hierarchical structure, configuration, or model of a communications or distributed data processing system that:...
. It was developed by Flarion, and purchased by Qualcomm
Qualcomm
Qualcomm is an American global telecommunication corporation that designs, manufactures and markets digital wireless telecommunications products and services based on its code division multiple access technology and other technologies. Headquartered in San Diego, CA, USA...
in January 2006. Flash-OFDM was marketed as a packet-switched cellular bearer, to compete with GSM and 3G
3G
3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
networks. As an example, 450 MHz frequency bands previously used by NMT-450
Nordic Mobile Telephone
NMT is the first fully automatic cellular phone system...
and C-Net C450
C-Netz
The Radio Telephone Network C , was a first generation analog cellular phone system deployed and operated in Germany by DeTeMobil...
(both 1G analogue networks, now mostly decommissioned) in Europe are being licensed to Flash-OFDM operators.
In Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
, the license holder Digita began deployment of a nationwide "@450" wireless network in parts of the country since April 2007. It was purchased by Datame in 2011.
T-Mobile Slovensko in Slovakia
Slovakia
The Slovak Republic is a landlocked state in Central Europe. It has a population of over five million and an area of about . Slovakia is bordered by the Czech Republic and Austria to the west, Poland to the north, Ukraine to the east and Hungary to the south...
offers Flash-OFDM connections with a maximum downstream speed of 5.3 Mbit/s, and a maximum upstream speed of 1.8 Mbit/s, with a coverage of over 70 percent of Slovak population.
T-Mobile Germany uses Flash-OFDM to backhaul Wi-Fi HotSpots on the Deutsche Bahn's ICE high speed trains.
American wireless carrier Nextel Communications
Nextel Communications
Nextel Communications, commonly styled NEXTEL and formerly traded on the NASDAQ as NXTL, now a part of the Sprint Nextel Corporation, was a United States telecommunications firm operating a nationwide push to talk mobile communications system. Unlike other mobile networks, the Nextel network...
field tested wireless broadband network technologies including Flash-OFDM in 2005. Sprint
Sprint Nextel
Sprint Nextel Corporation is an American telecommunications company based in Overland Park, Kansas. The company owns and operates Sprint, the third largest wireless telecommunications network in the United States, with 53.4 million customers, behind Verizon Wireless and AT&T Mobility...
purchased the carrier in 2006 and decided to deploy the mobile version of WiMAX
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
, which is based on Scalable Orthogonal Frequency Division Multiple Access (SOFDMA) technology.
Citizens Telephone Cooperative launched a mobile broadband service based on Flash-OFDM technology to subscribers in parts of Virginia
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...
in March 2006. The maximum speed available was 1.5 Mbit/s. The service was discontinued on April 30, 2009.
Digiweb Ltd.
Digiweb
Digiweb is a telecommunications company in Ireland, supplying business and consumer broadband and web hosting.Digiweb is 100% Irish owned and run, and is headquartered in Dundalk, County Louth with its technical, installation and sales offices in Dublin....
launched a mobile broadband network using Flash-OFDM technology at 872 MHz in July 2007 in Ireland and Digiweb also owns a national 872MHz license in Norway. Voice handsets are not yet available as of November 2007. The deployment is live in a small area north of Dublin only.
Butler Networks operates a Flash-OFDM network in Denmark at 872 MHz.
In The Netherlands, KPN-telecom will start a pilot around July 2007.
History
- 1957: Kineplex, multi-carrier HF modem (R.R. Mosier & R.G. Clabaugh)
- 1966: Chang, Bell Labs: OFDM paper and patent
- 1971: Weinstein & Ebert proposed use of FFT and guard interval
- 1985: Cimini described use of OFDM for mobile communications
- 1985: TelebitTelebitTelebit was a US-based modem manufacturer, most notable for their TrailBlazer series of high-speed modems. One of the first modems to routinely exceed 9600 bit/s speeds, the TrailBlazer used a proprietary modulation scheme that proved highly resilient to interference, earning the product an almost...
Trailblazer Modem introduced incorporating a 512 carrier Packet Ensemble Protocol - 1987: Alard & Lasalle: COFDM for digital broadcasting
- September 1988: TH-CSF LER, first experimental Digital TV link in OFDM, Paris area
- 1989: OFDM international patent application PCT/FR 89/00546, filed in the name of THOMSON-CSF, Fouche, de Couasnon, Travert, Monnier and all
- October 1990: TH-CSF LER, first OFDM equipment field test, 34 Mbit/s in an 8 MHz channel, experiments in Paris area
- December 1990: TH-CSF LER, first OFDM test bed comparison with VSB in Princeton USA
- September 1992: TH-CSF LER, second generation equipment field test, 70 Mbit/s in an 8 MHz channel, twin polarisations. Wuppertal, Germany
- October 1992: TH-CSF LER, second generation field test and test bed with BBC, near London, UK
- 1993: TH-CSF show in Montreux SW, 4 TV channel and one HDTV channel in a single 8 MHz channel
- 1993: Morris: Experimental 150Mbit/s OFDM wireless LAN
- 1994: , Method and apparatus for multiple access between transceivers in wireless communications using OFDM spread spectrum
- 1995: ETSI Digital Audio BroadcastingDigital audio broadcastingDigital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
standard EUreka: first OFDM based standard - 1997: ETSI DVB-TDVB-TDVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
standard - 1998: Magic WAND project demonstrates OFDM modems for wireless LAN
- 1999: IEEE 802.11a wireless LAN standard (Wi-Fi)
- 2000: Proprietary fixed wireless access (V-OFDM, FLASH-OFDM, etc.)
- 2002: IEEE 802.11g standard for wireless LAN
- 2004: IEEE 802.16IEEE 802.16IEEE 802.16 is a series of Wireless Broadband standards authored by the Institute of Electrical and Electronics Engineers . The IEEE Standards Board in established a working group in 1999 to develop standards for broadband Wireless Metropolitan Area Networks...
standard for wireless MAN (WiMAX) - 2004: ETSI DVB-HDVB-HDVB-H is one of three prevalent mobile TV formats. It is a technical specification for bringing broadcast services to mobile handsets. DVB-H was formally adopted as ETSI standard EN 302 304 in November 2004. The DVB-H specification can be downloaded from the official DVB-H website...
standard - 2004: Candidate for IEEE 802.15.3a standard for wireless PAN (MB-OFDM)
- 2004: Candidate for IEEE 802.11nIEEE 802.11nIEEE 802.11n-2009 is an amendment to the IEEE 802.11-2007 wireless networking standard to improve network throughput over the two previous standards—802.11a and 802.11g—with a significant increase in the maximum net data rate from 54 Mbit/s to 600 Mbit/s with the use of four...
standard for next generation wireless LAN - 2005: OFDMA is candidate for the 3GPP Long Term Evolution3GPP Long Term Evolution3GPP Long Term Evolution, usually referred to as LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals. It is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using new modulation techniques...
(LTE) air interface E-UTRA downlink. - 2007: The first complete LTE air interface implementation was demonstrated, including OFDM-MIMO, SC-FDMA and multi-user MIMO uplink
See also
- Single-carrier frequency-domain-equalization (SC-FDE)
- Single-carrier FDMASingle-carrier FDMASingle-carrier FDMA is a frequency-division multiple access scheme. Like other multiple access schemes , it deals with the assignment of multiple users to a shared communication resource...
(SC-FDMA) - ATSC Standards
- Paul BaranPaul BaranPaul Baran was a Polish American engineer who was a pioneer in the development of computer networks.He invented packet switching techniques, and went on to start several companies and develop other technologies that are an essential part of the Internet and other modern digital...
External links
- Numerous useful links and resources for OFDM - WCSP Group - University of South Florida (USF)
- WiMAX Forum, WiMAX, the framework standard for 4G mobile personal broadband
- Stott, 1997 http://www.bbc.co.uk/rd/pubs/papers/paper_15/paper_15.shtml Technical presentation by J H Stott of the BBC's R&D division, delivered at the 20 International Television Symposium in 1997; this URL accessed 24 January 2006.
- Page on Orthogonal Frequency Division Multiplexing at http://www.iss.rwth-aachen.de/Projekte/Theo/OFDM/node6.html accessed on 24 September 2007.
- A tutorial on the significance of Cyclic Prefix (CP) in OFDM Systems.
- Siemens demos 360 Mbit/s wireless
- 1994 US Patent 5,282,222 for wireless data transmission - The patent "tree" rooted on this patent has upwards of 20,000 nodes and leaves references.
- An Introduction to Orthogonal Frequency Division Multiplex Technology
- Short Introduction to OFDM - Tutorial written by Prof. Debbah, head of the Alcatel-Lucent Chair on flexible radio.
- Short free tutorial on COFDM by Mark Massel formerly at STMicroelectronics and in the digital TV industry for many years.
- A popular book on both COFDM and US ATSC by Mark Massel
- A research report by Ali Imran(NUCES)