Citizendia

Modulation techniques
Analog modulation
AM · SSB · FM · PM · QAM · SM
Digital modulation
OOK · FSK · ASK · PSK · QAM
MSK · CPM · PPM · TCM · OFDM
Spread spectrum
v • d • e
FHSS · DSSS

Orthogonal frequency-division multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) and Discrete multi-tone modulation (DMT) — is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. In Telecommunications, modulation is the process of varying a periodic Waveform, i In Telecommunications, modulation is the process of varying a periodic Waveform, i Amplitude modulation ( AM) is a technique used in electronic communication most commonly for transmitting information via a Radio Carrier wave Single-sideband modulation ( SSB) is a refinement of Amplitude modulation that more efficiently uses electrical power and bandwidth. Phase modulation (PM is a form of Modulation that represents information as variations in the instantaneous phase of a Carrier wave. Space modulation is a radio Amplitude Modulation technique used in Instrument Landing Systems that incorporates the use of multiple antennas fed with various radio In Telecommunications, modulation is the process of varying a periodic Waveform, i On-off keying ( OOK) is a type of Modulation that represents Digital Data as the presence or absence of a Carrier wave. Frequency-shift keying (FSK is a Frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a Carrier wave Amplitude-shift keying ( ASK) is a form of Modulation that represents Digital Data as variations in the Amplitude of a Carrier Phase-shift keying (PSK is a Digital Modulation scheme that conveys data by changing or modulating the phase of a reference signal In Digital modulation, Minimum-shift keying (MSK is a type of continuous phase Frequency-shift keying that was developed in the late 1960s Continuous phase modulation ( CPM) is a method for modulation of data commonly used in Wireless modems In contrast to other coherent digital Phase Pulse-position modulation is a form of signal Modulation in which M message bits are encoded by transmitting asingle pulse in one of 2^M possible time-shifts In Telecommunication, trellis modulation (also known as trellis coded modulation, or simply TCM) is a modulation scheme which allows highly efficient Spread-spectrum techniques are methods by which Energy generated in a particular bandwidth is deliberately spread in the Frequency domain, resulting Frequency-hopping spread spectrum ( FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using In Telecommunications direct-sequence spread spectrum ( DSSS) is a Modulation technique Frequency-division multiplexing ( FDM) is a form of signal Multiplexing where multiple Baseband signals are Modulated on different frequency In Telecommunications, modulation is the process of varying a periodic Waveform, i A large number of closely-spaced orthogonal sub-carriers are used to carry data. In Mathematics, two Vectors are orthogonal if they are Perpendicular, i Debt AIDS Trade in Africa (or DATA) is a Multinational non-government organization founded in January 2002 in London by U2 's The data are 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. Phase-shift keying (PSK is a Digital Modulation scheme that conveys data by changing or modulating the phase of a reference signal In Digital communications, symbol rate, also known as baud or modulation rate is the number of symbol changes (signalling events made to the transmission medium per second

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, wireless networking and broadband internet access. In communications, wideband is a relative term used to describe a wide range of frequencies in a Spectrum. Digital communications refers to the transmission of a sequence of Digital messages (a Bit stream) or a digitized analog signal Wireless communication is the transfer of information over a distance without the use of electrical conductors or " Wires quot Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29

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, in communications (sometimes called communications channel) refers to the medium used to convey Information from a Attenuation distortion is the Distortion of an Analog signal that occurs during transmission when the Transmission medium does not have a flat In physics interference is the addition ( superposition) of two or more Waves that result in a new wave pattern This is about the phenomenon of loss of signal in telecommunications In Wireless Telecommunications multipath is the propagation phenomenon that results in Radio signals reaching the receiving Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. Equalization (or equalisation, EQ) is the process of changing the frequency envelope of a sound in Audio processing. Narrowband refers to a situation in radio communications where the bandwidth of the message does not significantly exceed the channel's Coherence bandwidth. In communications, wideband is a relative term used to describe a wide range of frequencies in a Spectrum. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). In Telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another In Telecommunication, intersymbol interference ( ISI) is a form of Distortion of a signal in which one symbol interferes with This mechanism also facilitates the design of single-frequency networks, 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. A single-frequency network or SFN is a Broadcast network where several Transmitters simultaneously send the same signal over the same Frequency

Contents

Example of applications

The following list is a summary of existing OFDM based standards and products. For further details, see the Usage section in the end of the article.

Cable

Wireless

Key features

The advantages and disadvantages listed below are further discussed in the Characteristics and principles of operation section.

Summary of advantages

Summary of disadvantages

Characteristics and principles of operation

Orthogonality

In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. In Mathematics, two Vectors are orthogonal if they are Perpendicular, i In Electronics, the term crosstalk ( XT) refers to any phenomenon by which a signal transmitted on one circuit or channel of a Transmission system 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. For biologic transmitters see Transmitter substance. A transmitter is an electronic device which usually with the aid of an antenna Frequency-division multiplexing ( FDM) is a form of signal Multiplexing where multiple Baseband signals are Modulated on different frequency

The orthogonality also allows high spectral efficiency, near the Nyquist rate. Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the amount of information that can be transmitted over a given bandwidth in a The Nyquist–Shannon sampling theorem is a fundamental result in the field of Information theory, in particular Telecommunications and Signal processing 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.

The orthogonality allows for efficient modulator and demodulator implementation using the FFT algorithm. Although the principles and some of the benefits have been known since the 1960s, OFDM is popular for wideband communications today by way of low-cost digital signal processing components that can efficiently calculate the FFT. Digital signal processing ( DSP) is concerned with the representation of the signals by a sequence of numbers or symbols and the processing of these signals

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. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Whilst 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. In Wireless Telecommunications multipath is the propagation phenomenon that results in Radio signals reaching the receiving 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.

Guard interval for elimination of inter-symbol 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 channel time characteristics) suffer less from intersymbol interference caused by multipath, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. Channel, in communications (sometimes called communications channel) refers to the medium used to convey Information from a In Telecommunication, intersymbol interference ( ISI) is a form of Distortion of a signal in which one symbol interferes with In Wireless Telecommunications multipath is the propagation phenomenon that results in Radio signals reaching the receiving 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. In Telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another In Telecommunication, intersymbol interference ( ISI) is a form of Distortion of a signal in which one symbol interferes with

The guard interval also eliminates the need for a pulse-shaping filter, and it reduces the sensitivity to time synchronization problems. In digital telecommunication pulse shaping is the process of changing the waveform of transmitted pulses

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 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, 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. In an OFDM symbol the cyclic prefix is a repeat of the end of the symbol at the beginning 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 equalization far simpler at the receiver in OFDM in comparison to conventional single-carrier modulation. Equalization (or equalisation, EQ) is the process of changing the frequency envelope of a sound in Audio processing. The equalizer only has to multiply each sub-carrier by a constant value, or a rarely changed value.

Our example: The OFDM equalization in the above numerical example would require N = 1000 complex multiplications per OFDM symbol, i. e. one million multiplications per second, at the receiver. The FFT algorithm requires Nlog2N = 10000 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 125 multiplications per symbol, i. A finite impulse response (FIR filter is a type of a Digital filter. e. 125 million multiplications per second.

Some of the sub-carriers in some of the OFDM symbols may carry pilot signals for measurement of the channel conditions, i. In Telecommunications a pilot is a signal, usually a single Frequency, transmitted over a Communications system for supervisory control e. the equalizer gain for each sub-carrier. Pilot signals may also be used for synchronization.

If differential modulation such as DPSK is applied to each sub-carrier, equalization can be completely omitted, since these schemes are insensitive to slowly changing amplitude and phase distortion. Phase-shift keying (PSK is a Digital Modulation scheme that conveys data by changing or modulating the phase of a reference signal In signal processing phase distortion or phase-frequency distortion is Distortion that occurs when (a a filter's Phase response is not linear over the

Channel coding and interleaving

OFDM is invariably used in conjunction with channel coding (forward error correction), and almost always uses frequency and/or time interleaving. In Computer science, a channel code is a broadly used term mostly referring to the Forward error correction code and Bit interleaving in communication and In Telecommunication and Information theory, forward error correction (FEC is a System of Error control for Data transmission, whereby Interleaving in Computer science is a way to arrange Data in a non- Contiguous way in order to increase performance

Frequency (subcarrier) interleaving increases resistance to frequency-selective channel conditions such as fading. Interleaving in Computer science is a way to arrange Data in a non- Contiguous way in order to increase performance This is about the phenomenon of loss of signal in telecommunications For example, when a part of the channel bandwidth is faded, 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 is faded 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 common type of error correction coding used with OFDM-based systems is convolutional coding, which is often concatenated with Reed-Solomon coding. In Telecommunication, a convolutional code is a type of Error-correcting code in which (a each m - Bit Information symbol (each Reed-Solomon error correction is an Error-correcting code that works by Oversampling a Polynomial constructed from the data Convolutional coding is used as the inner code and Reed-Solomon coding is used for the outer code — usually with additional interleaving (on top of the time and frequency interleaving mentioned above) in between the two layers of coding. The reason why this combination of error correction coding is used is that the Viterbi decoder used for 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 the near-optimal types of error correction coding that use the turbo decoding principle, where the decoder iterates towards the desired solution. Examples of such error correction coding types include turbo codes and LDPC codes. In Electrical engineering and Digital communications, turbo codes (originally in French Turbocodes) are a class of high-performance error correction In Information theory, a low-density parity-check code (LDPC code is an Error correcting code, a method of transmitting a message over a noisy transmission These codes only perform close to the Shannon limit for the Additive White Gaussian Noise (AWGN) channel, however, and some systems that have adopted these codes have concatenated them with either Reed-Solomon (for example on the MediaFLO system) or BCH codes (on the DVB-S2 system) to improve performance further over the wireless channel. In Information theory, the noisy-channel coding theorem establishes that however contaminated with noise interference a communication channel may be it is possible to communicate Explanation In communications, the additive white Gaussian noise ( AWGN) channel model is one in which the only impairment is the linear addition of MediaFLO is Qualcomm 's new technology to broadcast data to portable devices such as cell phones and PDAs. Digital Video Broadcasting - Satellite - Second Generation ( DVB-S2) is an enhanced specification to replace the DVB-S standard developed in 2003 and ratified

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 modulation, channel coding and power allocation may be applied across all sub-carriers, or individually to each sub-carrier. In Telecommunications, modulation is the process of varying a periodic Waveform, i In Computer science, a channel code is a broadly used term mostly referring to the Forward error correction code and Bit interleaving in communication and 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. In Telecommunication and Information theory, forward error correction (FEC is a System of Error control for Data transmission, whereby

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. Asymmetric Digital Subscriber Line ( ADSL) is a form of DSL, a data communications technology that enables faster data transmission over Copper Telephone

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 adopted to the co-channel interference and bandwidth is allocated to whichever subscriber that needs it most. For RE-ADSL2 see ITU G9925 Annex L Rate-adaptive DSL ( RADSL) is a variation of ADSL technology

OFDM extended with multiple access

OFDM in its primary form is considered as a digital modulation technique, and not a multi-user channel access technique, since it is utilized for transferring one bit stream over one communication channel using one sequence of OFDM symbols. In Telecommunications and Computer networks, a channel access method or multiple access method allows several terminals connected to the same Channel, in communications (sometimes called communications channel) refers to the medium used to convey Information from a However, OFDM can be combined with multiple access using time, frequency or coding separation of the users. In Telecommunications and Computer networks, a channel access method or multiple access method allows several terminals connected to the same

In Orthogonal Frequency Division Multiple Access (OFDMA), frequency-division multiple access is achieved by assigning different OFDM sub-channels to different users. Orthogonal Frequency-Division Multiple Access ( OFDMA) is a multi-user version of the popular Orthogonal frequency-division multiplexing (OFDM digital modulation scheme Orthogonal Frequency-Division Multiple Access ( OFDMA) is a multi-user version of the popular Orthogonal frequency-division multiplexing (OFDM digital modulation scheme Frequency Division Multiple Access or FDMA is an Channel access method that is used by radio systems to share a certain Radio spectrum between multiple 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. Code division multiple access ( CDMA) is a Channel access method utilized by various radio communication technologies The Media Access Control (MAC Data communication protocol sub-layer also known as the Medium Access Control is a sublayer of the Data Link Layer specified in the OFDMA is used in the IEEE 802.16 Wireless MAN standard, commonly referred to as WiMAX. The IEEE 80216 Working Group on Broadband Wireless Access Standards, which was established by IEEE Standards Board in 1999 aims to prepare formal specifications

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. Multi-Carrier Code Division Multiple Access ( MC-CDMA) is a Multiple access scheme used in OFDM -based telecommunication systems allowing the system to support Code division multiple access ( CDMA) is a Channel access method utilized by various radio communication technologies Co-channel interference can be mitigated against, meaning that manual fixed channel allocation (FCA) frequency planning is simplified, or complex dynamic channel allocation (DCA) schemes are avoided. In TDMA and FDMA based cellular radio systems and wireless networks channel allocation schemes are required to allocate channels to base stations and access points In TDMA and FDMA based cellular radio systems and wireless networks channel allocation schemes are required to allocate channels to base stations and access points

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 networks (SFNs), where many transmitters send the same signal simultaneously over the same channel frequency. A single-frequency network or SFN is a Broadcast network where several Transmitters simultaneously send the same signal over the same 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. 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 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 single-frequency network or SFN is a Broadcast network where several Transmitters simultaneously send the same signal over the same Frequency 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.

Single-frequency networks is a form of transmitter macrodiversity. A single-frequency network or SFN is a Broadcast network where several Transmitters simultaneously send the same signal over the same Frequency In wireless communication Macrodiversity means a situation where several receiver antennas and/or transmitter antennas are used for transferring the same signal The concept can be further utilized in Dynamic single-frequency networks (DSFN), where the SFN grouping is changed from timeslot to timeslot. Dynamic Single Frequency Networks ( DSFN) is a transmitter Macrodiversity technique for OFDM based cellular networks

OFDM may be combined with other forms of space diversity, for example antenna arrays and MIMO channels. Antenna diversity, also known as space diversity, is one in a superset of wireless Diversity schemes that utilizes two or more antennas to improve the quality and reliability In Radio, multiple-input and multiple-output, or MIMO (pronounced mee-moh or my-moh) is the use of multiple antennas at both the transmitter This is done in the IEEE802.11n Wireless LAN standard. IEEE 80211n is a proposed amendment to the IEEE 80211-2007 Wireless networking standard to significantly improve network throughput over previous standards such A wireless LAN or WLAN is a Wireless Local area network, which is the linking of two or more computers or devices without using wires

Linear transmitter power amplifier

An OFDM signal exhibits a high peak-to-average power ratio (PAPR) because the independent phases of the sub-carriers mean that they will often combine constructively. The crest factor or peak-to-average ratio ( PAR) or peak-to-average power ratio ( PAPR) is a measurement of a Waveform, calculated from 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 tradeoff against the above consequences. However, the transmitter output filter which is required to reduce out-of-band spurrii 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. Explanation In communications, the additive white Gaussian noise ( AWGN) channel model is one in which the only impairment is the linear addition of

Transmitter

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). In Signal processing, baseband is an adjective that describes signals and systems whose range of Frequencies is measured from zero to a maximum bandwidth Phase-shift keying (PSK is a Digital Modulation scheme that conveys data by changing or modulating the phase of a reference signal This composite baseband signal is typically used to modulate a main RF carrier.

s[n] is a serial stream of binary digits. By inverse multiplexing, these are first demultiplexed into N parallel streams, and each one mapped to a (possibly complex) symbol stream using some modulation constellation (QAM, PSK, etc. An inverse multiplexer (often abbreviated to " inverse mux " or " imux " allows a data stream to be broken into multiple lower data rate communications Phase-shift keying (PSK is a Digital Modulation scheme that conveys data by changing or modulating the phase of a reference signal ). 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, fc, respectively. In Electronics, a digital-to-analog converter ( DAC or D-to-A) is a device for converting a digital (usually binary code to an Analog signal These signals are then summed to give the transmission signal, s(t).

Receiver

The receiver picks up the signal r(t), which is then quadrature-mixed down to baseband using cosine and sine waves at the carrier frequency. This also creates signals centered on 2fc, 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. An analog-to-digital converter (abbreviated ADC, A/D or A to D) is an electronic integrated circuit which converts continuous signals to

This returns N parallel streams, each of which is converted to a binary stream using an appropriate symbol detector. In general detection is the extraction of information from any clear or clouded ambient or otherwise accessible stream of information without neither support from the sender nor synchronization These streams are then re-combined into a serial stream, {\hat s}[n], which is an estimate of the original binary stream at the transmitter.

Mathematical description

If N sub-carriers are used, and each sub-carrier is modulated using M alternative symbols, the OFDM symbol alphabet consists of MN combined symbols.

The low-pass equivalent OFDM signal is expressed as:

\ \nu(t)=\sum_{k=0}^{N-1}X_k e^{j2\pi kt/T}, \quad 0\le t<T,

where {Xk} are the data symbols, N is the number of sub-carriers, and T is the OFDM symbol time. In Signal processing, baseband is an adjective that describes signals and systems whose range of Frequencies is measured from zero to a maximum bandwidth The sub-carrier spacing of 1 / T makes them orthogonal over each symbol period; this property is expressed as:

\begin{align} &\frac{1}{T}\int_0^{T}\left(e^{j2\pi k_1t/T}\right)^*\left(e^{j2\pi k_2t/T}\right)dt \\ = &\frac{1}{T}\int_0^{T}e^{j2\pi (k_2-k_1)t/T}dt = \delta_{k_1k_2}\end{align}

where (\cdot)^* denotes the complex conjugate operator and \delta\, is the Kronecker delta. In Mathematics, the complex conjugate of a Complex number is given by changing the sign of the Imaginary part. In Mathematics, the Kronecker delta or Kronecker's delta, named after Leopold Kronecker ( 1823 - 1891) is a function of two .

To avoid intersymbol interference in multipath fading channels, a guard interval of length Tg is inserted prior to the OFDM block. During this interval, a cyclic prefix is transmitted such that the signal in the interval -T_\mathrm{g}\le t<0 equals the signal in the interval (T-T_\mathrm{g})\le t<T. The OFDM signal with cyclic prefix is thus:

\ \nu(t)=\sum_{k=0}^{N-1}X_ke^{j2\pi kt/T}, \quad -T_\mathrm{g}\le t<T

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 fc. In general, the transmitted signal can be represented as:

\begin{align}s(t) & = \frac{1}{2} \Re\left\{\nu(t) e^{j2\pi f_c t}\right\} \\ & = \sum_{k=0}^{N-1}|X_k|\cos\left(2\pi [f_c + k/T]t + \arg[X_k]\right)\end{align}

Usage

OFDM system comparison table

Key features of some common OFDM based systems are presented in the following table.

Standard nameDAB Eureka 147DVB-TDVB-HDMB-T/HIEEE 802.11a
Ratified year19951997200420061999
Frequency range of
today's equipment
(MHz)		174 - 240
1452 - 1492		470 - 862470 - 862470 – 8624915 - 5825
Channel spacing B
(MHz)		1. Digital Audio Broadcasting ( DAB) also known as Eureka 147, is a Digital radio technology for Broadcasting Radio stations used in Digital Audio Broadcasting ( DAB) also known as Eureka 147, is a Digital radio technology for Broadcasting Radio stations used in DVB-T is an abbreviation for Digital Video Broadcasting – Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of DVB-H ( Digital Video Broadcasting - Handheld) is one of three prevalent mobile TV formats DMB-T/H or DTMB ( GB 20600-2006 is the Digital terrestrial television standard applied in the People's Republic of China (PRC including IEEE 80211a-1999 or 80211a, is an amendment to the IEEE 80211 specification that added a higher throughput of up to 54 Mbit/s by using the 5 GHz 71288820
Number of subcarriers N192, 384, 768 or 15362K mode: 1705
8K mode: 6817		1705, 3409, 68171 (single-carrier)
3780 (multi-carrier)		52
Subcarrier modulation schemeDQPSKQPSK, 16QAM or 64QAMQPSK, 16QAM or 64QAM4QAM, 4QAM-NR,[1]
16QAM, 32QAM and 64QAM. 		BPSK, QPSK, 16QAM or 64QAM
Useful symbol length TU
(μs)		2K mode: 224
8K mode: 896		224, 448, 896500 (multi-carrier)3. 2
Additional guard interval TG
(Fraction of TU)		1/4, 1/8, 1/16, 1/321/4, 1/8, 1/16, 1/321/4, 1/6, 1/91/4
Subcarrier spacing
Δf = 1/(TU) ≈ B/N
(Hz)		2K mode: 4464
8K mode: 1116		4464, 2232, 11168 M (single-carrier)
2000 (multi-carrier)		312. 5K
Net bit rate R
(Mbit/s)		0. A megabit is a unit of Information or computer storage abbreviated Mbit (or Mb) 576 - 1. 1524. 98 - 31. 67
(typically 24)		3. 7 - 23. 84. 81 - 32. 496 - 54
Link spectral efficiency R/B
(bit/s/Hz)		0. Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the amount of information that can be transmitted over a given bandwidth in a 34 - 0. 670. 62 - 4. 00. 62 - 4. 00. 60 - 4. 10. 30 - 2. 7
Inner FECConv coding with code rates

1/4, 3/8 or 1/2

Conv coding with code rates

1/2, 2/3, 3/4, 5/6 or 7/8

Conv coding with code rates

1/2, 2/3, 3/4, 5/6 or 7/8

LDPC with code rates

0. In Telecommunication and Information theory, forward error correction (FEC is a System of Error control for Data transmission, whereby In Telecommunication, a convolutional code is a type of Error-correcting code in which (a each m - Bit Information symbol (each In Telecommunication, a convolutional code is a type of Error-correcting code in which (a each m - Bit Information symbol (each In Telecommunication, a convolutional code is a type of Error-correcting code in which (a each m - Bit Information symbol (each In Information theory, a low-density parity-check code (LDPC code is an Error correcting code, a method of transmitting a message over a noisy transmission 4, 0. 6 or 0. 8

Conv coding with code rates

1/2, 2/3 or 3/4

Outer FEC (if any)NoneRS(204,188,t=8)RS(204,188,t=8) + MPE-FECBCH code (762,752)
Maximum travelling speed
(km/h)		200 - 60053 - 185
Time interleaving depth
(ms)		3850. In Telecommunication, a convolutional code is a type of Error-correcting code in which (a each m - Bit Information symbol (each In Telecommunication and Information theory, forward error correction (FEC is a System of Error control for Data transmission, whereby Reed-Solomon error correction is an Error-correcting code that works by Oversampling a Polynomial constructed from the data Reed-Solomon error correction is an Error-correcting code that works by Oversampling a Polynomial constructed from the data Interleaving in Computer science is a way to arrange Data in a non- Contiguous way in order to increase performance 6 - 3. 50. 6 - 3. 5200 - 500
Adaptive transmission
(if any)		NoneNone
Multiple access method
(if any)		NoneNone
Typical source coding192 kbit/s
MPEG2 Audio
layer 2		4 Mbit/s
MPEG2		Not defined
(MPEG-2 or H. In Telecommunications and Computer networks, a channel access method or multiple access method allows several terminals connected to the same A kilobit is a unit of information abbreviated kbit (or kb) The standard definition is 1 kilobit = 103 bit = 1000 Bit. MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information" MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information" 264 w/MP2)

ADSL

OFDM is used in ADSL connections that follow the G.DMT (ITU G.992.1) standard, in which existing copper wires are used to achieve high-speed data connections. MPEG-1 Audio Layer II ( MP2, sometimes incorrectly called Musicam) is an Audio codec defined by ISO/IEC 11172-3 Asymmetric Digital Subscriber Line ( ADSL) is a form of DSL, a data communications technology that enables faster data transmission over Copper Telephone In Telecommunications, ITU G9921 (better known as GDMT) is an ITU standard for ADSL using Discrete multitone modulation. In Telecommunications, ITU G9921 (better known as GDMT) is an ITU standard for ADSL using Discrete multitone modulation.

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 modems. Modem (from mo dulator- dem odulator is a device that modulates an analog carrier signal to encode Digital information 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. In the field of Telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls

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. Amateur radio, often called ham radio, is both a Hobby and a service in which participants called "hams" use various types of Radio communications Commercial off-the-shelf ( COTS) is a term for Software or Hardware, generally technology or computer products that are ready-made and available for sale

Powerline Technology

OFDM is used by powerline devices to extend Ethernet connections to other rooms in a home through its power wiring. Power line communication ( PLC) also known as power line carrier, mains communication, power line telecom ( PLT) or power line Adaptive modulation is particularly important with such a noisy channel as electrical wiring. Link adaptation, or adaptive modulation and coding (AMC, is a term used in wireless communications to denote the matching of the Modulation, Coding and

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

OFDM is also now being used in some wireless LAN and MAN applications, including IEEE 802.11a/g (and the defunct European alternative HIPERLAN/2) and WiMAX. IEEE 80211 is a set of standards for wireless local area network (WLAN computer communication developed by the IEEE LAN/MAN Standards Committee ( IEEE 802 HiperLAN (HIgh PErformance Radio LAN is a Wireless LAN standard WiMAX, an approximate acronym of Worldwide Interoperability for Microwave Access, is a Telecommunications technology that provides for the wireless transmission

IEEE 802.11a, operating in the 5 GHz band, specifies airside data rates ranging from 6 to 54 Mbit/s. IEEE 80211a-1999 or 80211a, is an amendment to the IEEE 80211 specification that added a higher throughput of up to 54 Mbit/s by using the 5 GHz Four different modulation schemes are used: BPSK, 4-QAM, 16-QAM, and 64-QAM, along with a number of convolutional encoding schemes. Phase-shift keying (PSK is a Digital Modulation scheme that conveys data by changing or modulating the phase of a reference signal This allows the system to adapt to the optimum data rate vs. error rate for the current conditions.

Clearwire, a wireless Internet Service Provider who provides access to metropolitan areas across the United States, utilizes OFDM in both their current 2. Clearwire Corporation (stylized as "clearw˙re" in the logo is a Wireless broadband Internet service provider (ISP serving markets in the An Internet service provider ( ISP, also called Internet access provider or IAP) is a company which primarily offers their customers access to the Internet 5 GHz network and the planned expansion of their WiMax network.

Wireless personal area networks (PAN)

OFDM is also now being used in the WiMedia / Ecma-368 standard for high-speed wireless personal area networks in the 3. 1-10. 6 GHz ultrawideband spectrum. See www. wimedia. com.

Terrestrial digital radio and television broadcasting

Much of Europe and Asia has adopted OFDM for terrestrial broadcasting of digital television (DVB-T, DVB-H and T-DMB) and radio (EUREKA 147 DAB, Digital Radio Mondiale, HD Radio and T-DMB). DVB-T is an abbreviation for Digital Video Broadcasting – Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of DVB-H ( Digital Video Broadcasting - Handheld) is one of three prevalent mobile TV formats Digital Multimedia Broadcasting ( DMB) is a Digital Radio transmission system for sending Multimedia ( Radio, TV Digital Audio Broadcasting ( DAB) also known as Eureka 147, is a Digital radio technology for Broadcasting Radio stations used in Digital Audio Broadcasting ( DAB) also known as Eureka 147, is a Digital radio technology for Broadcasting Radio stations used in Digital Radio Mondiale ( DRM) is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly HD Radio is the name used by iBiquity for a system of digital transmission for audio broadcast stations Digital Multimedia Broadcasting ( DMB) is a Digital Radio transmission system for sending Multimedia ( Radio, TV

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,[2] 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. [3] 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.

COFDM vs. VSB

The question of the relative technical merits of COFDM versus 8VSB has been a subject of some controversy, especially between Europe and USA. The United States has rejected several proposals to adopt COFDM for its digital television services, and has instead opted for 8VSB (vestigial sideband modulation) operation. The United States of America —commonly referred to as the 8VSB is the 8-level Vestigial sideband modulation method adopted for terrestrial broadcast of the ATSC digital television standard in the United States, Single-sideband modulation ( SSB) is a refinement of Amplitude modulation that more efficiently uses electrical power and bandwidth.

One of the major benefits provided by COFDM is that it renders radio broadcasts relatively immune to multipath distortion and signal fading due to atmospheric conditions or passing aircraft. In Wireless Telecommunications multipath is the propagation phenomenon that results in Radio signals reaching the receiving This is about the phenomenon of loss of signal in telecommunications Proponents of COFDM argue that it resists multipath far better than 8VSB. Early 8VSB DTV (digital television) receivers often had difficulty receiving a signal in urban environments.

However, newer 8VSB receivers are far better at dealing with multipath, hence the difference in performance may diminish with advances in demodulator design. Moreover, 8VSB modulation requires less power to transmit a signal the same distance, i. e. , the received carrier-to-noise threshold is lower for the same bit error rate. In Telecommunications, the carrier-to-noise ratio, often written CNR or C/N, is the Signal-to-noise ratio (SNR of a modulated signal In Telecommunication, an error ratio is the Ratio of the number of Bits elements, characters, or blocks incorrectly received In less-populated areas, 8VSB may have an advantage because of this. In urban areas, however, COFDM is believed to offer better reception than 8VSB.

In practice, it may be impossible to settle this debate without empirical history. One difficulty in fully assessing the two systems' relative performance in multipath environments is that the spatial distribution of multipath cannot be modeled well. Due to the chaotic nature of multipath, the process is non-stationary, both temporally and spatially, in the stochastic sense. In Mathematics, chaos theory describes the behavior of certain dynamical systems – that is systems whose state evolves with time – that may exhibit dynamics that Stochastic (from the Greek "Στόχος" for "aim" or "guess" means Random. Thus, the probability distribution of impaired receiving locations is not tractable. In Probability theory and Statistics, a probability distribution identifies either the probability of each value of an unidentified Random variable

Digital radio

COFDM is also used for other radio standards, for digital audio broadcasting (DAB), the standard for digital audio broadcasting at VHF frequencies, and also for Digital Radio Mondiale (DRM), the standard for digital broadcasting at shortwave and mediumwave frequencies (below 30 MHz). Digital Audio Broadcasting ( DAB) also known as Eureka 147, is a Digital radio technology for Broadcasting Radio stations used in Very high frequency (VHF is the Radio frequency range from 30 MHz to 300 MHz. Digital Radio Mondiale ( DRM) is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly Shortwave Radio operates between the frequencies of 3000 KHz (3 Medium Wave (MW is a part of the Medium frequency (MF radio band used mainly for AM broadcasting.

The USA again uses an alternate standard, a proprietary system developed by iBiquity dubbed "HD Radio". iBiquity Digital Corporation is a company formed by the Merger of USA Digital Radio and Lucent Digital Radio, with the goal of creating HD Radio is the name used by iBiquity for a system of digital transmission for audio broadcast stations 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. In-band on-channel ( IBOC) is a method of transmitting Digital radio and Analog radio broadcast signals simultaneously on Digital Audio Broadcasting ( DAB) also known as Eureka 147, is a Digital radio technology for Broadcasting Radio stations used in Very high frequency (VHF is the Radio frequency range from 30 MHz to 300 MHz.

BST-OFDM used in ISDB

The BST-OFDM (Band Segmented Transmission Orthogonal Frequency Division Multiplexing) 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. Integrated Services Digital Broadcasting (ISDB is a Japanese standard for Digital television (DTV and Digital radio used by the country's radio Integrated Services Digital Broadcasting (ISDB is a Japanese standard for Digital television (DTV and Digital radio used by the country's radio Integrated Services Digital Broadcasting (ISDB is a Japanese standard for Digital television (DTV and Digital radio used by the country's radio Some forms of COFDM already offer this kind of hierarchical modulation, though BST-OFDM is intended to make it more flexible. Link adaptation, or adaptive modulation and coding (AMC, is a term used in wireless communications to denote the matching of the Modulation, Coding and 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 comprised 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

UWB (ultra-wideband) wireless personal area network technology may also utilise OFDM, such as in Multiband OFDM (MB-OFDM). Ultra-wideband (aka UWB, ultra-wide band, ultraband, etc is a radio technology that can be used at very low energy levels for short-range high-bandwidth 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. 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

Flash-OFDM

Flash-OFDM (Fast Low-latency Access with Seamless Handoff Orthogonal Frequency Division Multiplexing), which is also referred to as F-OFDM, is a system that is based on OFDM and specifies also higher protocol layers. Open systems architecture, in Telecommunication, is a standard that describes the layered hierarchical structure configuration, or model of a communications It has been developed and is marketed by Flarion. Flarion Technologies is a Telecommunications company specializing in Broadband access to the Internet and Wireless technology Flash-OFDM has generated interest as a packet-switched cellular bearer, on which area it would compete with GSM and 3G networks. GSM ( Global System for Mobile communications: originally from Groupe Spécial Mobile) is the most popular standard for Mobile phones in the 3G is the third generation of mobile phone standards and Technology, superseding 2 As an example, old 450 MHz frequency bands that were used by NMT-450 and C-Net C450 (both 1G analog networks, now mostly decommissioned) in Europe are already being licensed to Flash-OFDM operators. NMT ( Nordisk MobilTelefoni or Nordiska MobilTelefoni-gruppen, Nordic Mobile Telephony in English is the first fully-automatic Cellular phone The Radio Telephone Network C (German Funktelefonnetz-C, abbreviated as C-Netz) was a first generation analog Cellular phone In Finland the license holder Digita has begun deployment of its nationwide "@450" wireless network, operational in parts of the country since April 2007 and planned coverage of all of Finland in 2009.

American wireless carrier Sprint Nextel had stated plans for field testing Flash-OFDM (along with other wireless broadband network technologies) for their 4G offering, which will be deployed using the licenses they own nationwide in the 2. Sprint Nextel Corporation ( is a Telecommunications company based in Overland Park, Kansas. 5 GHz frequency range. Sprint subsequently has decided to deploy the mobile version of WiMAX, which is based on SOFDMA, scalable orthogonal frequency division multiple access technology.

T-Mobile already offers Flash-OFDM connection to its subscribers in Slovakia. The maximum download speed is 5. 3 Mbit/s, whereas upload speed is limited to 1. 8 Mbit/s.

T-Mobile Germany uses FLASH-OFDM to backhaul Wi-Fi connections on Deutsche Bahn's ICE high speed trains. This deployment showcases the excellent mobility support of FLASH-OFDM as the ICE trains reach speeds up to 300 km/h.

Citizens Telephone Cooperative launched a Flash-OFDM service to subscribers in parts of Virginia in March, 2006. The Commonwealth of Virginia ( is an American state The maximum speed available is 1. 5 Mbit/s. [4]

Digiweb Ltd. launched a mobile broadband network using FLASH-OFDM technology at 872 MHz in July 2007 in Ireland and also will be launching in Norway. Digiweb is an telecommunications company in Ireland, supplying business and consumer Broadband and web hosting Voice handsets are not yet available at the time of writing (November 2007). The deployment is live in a small area north of Dublin only.

Butler Networks is currently trialing FLASH-OFDM technology in Denmark at 872 MHz as well.

In The Netherlands, KPN-telecom will start a pilot around July 2007.

History

See also

References

Notes

  1. ^ NR refers to Nordstrom-Robinson code
  2. ^ DIRECTIVE 95/47/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL on the use of standards for the transmission of television signals
  3. ^ ETSI Standard: EN 300 744 V1. 5. 1 (2004-11).
  4. ^ http://www.citizens.coop/aboutus/newsreleases/TrulyMobileWireless.pdf
  5. ^ Nortel 3G World Congress Press Release

External links

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