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What is Orthogonal Frequency Division Multiplexing (OFDM)?

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rkarthik1

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what is it, and why is everyone so excited about it?
 

the_penetrator

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what is ofdm?

OFDM = Orthogonal Frequency Division Multiplexing

OFDM is/will be the modulation scheme for future 4G standards, as well as the under development IEEE 802.16a:2004 and IEEE 802.16e:2004 (Wireless Metropolitan Area Networks). It is also present in the DVB-T (Terrestrial Digital Video Broadcasting) and some software radios.

It uses N-point FFT (N can range from 64 to 2048 AFAIK and depending on the necessities of the standard). It can be used for both Line-of-Sight and Non-Line-of-Sight transceivers. These standards regard bands 2-11GHz and 11-66GHz for NLOS.

the_penetrator©
 

flatulent

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what ofdm

This is a modulation format that was public domain idea back in 1917, and was used in commercial products around 1955.

It involves using several transmission frequencies at the same time. The data is split between them. They can be placed close together in frequency by having the spectral nulls of each signal be at the carrier frequency of the other signals.

What is useful about this method is the immunity of multipath problems. Each carrier can be equalized independently of the others. Any selective fading affects only one carrier.
 

ARTMehr

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orthogonal frequency division multiplexing

Short for Orthogonal Frequency Division Multiplexing, an FDM modulation technique for transmitting large amounts of digital data over a radio wave. OFDM works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions. 802.11a WLAN, 802.16 and WiMAX technologies use OFDM.
 

flatulent

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brief ofdm

The 1917 idea could not be done in practice because of the limitations of technology and economics. It was conceived as a secrecy method and not as a spectrum utilization motive. The 1955 products were a classified military system fielded earlier than the date and the Collins Radio Company Kienoplex (possibly spelled wrong) system for HF use sold in 1958.
 

Naveed Alam

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i q modulation dc carrier ghosting

besides all other answers..its important b/c it is a good research field for 4-G Mobile communications..
 

IEEE2006

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ofdm carriers

is OFDM workin in NLOS WiMAX?????????
 

ARTMehr

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Re: what is OFDM?

IEEE2006 said:
is OFDM workin in NLOS WiMAX?????????
h**p://www.srtelecom.com/en/success/downloads/WiMAX_Applications.pdf

h**p://www.gestocomm.cz/data/datasheets/SRtelSymmetry.pdf
 

haseebcu

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Re: what is OFDM?

I have a book for OFDM in wireless communication If u need that I can upload that. MSg me if u want it
 

SkyHigh

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Re: what is OFDM?

Seems like everyone is talking about the history of OFDM and its future.

Why isn't anyone make the concept of OFDM simple for rkarthik1 to understand?

He is asking what is OFDM.

In brief, OFDM is FDM and TDM in one, orthogonal in time and frequency.
In much simpler term, make frequency your x-axis and time be your y-axis.
You have slots of frequency and time.
Therefore OFDM makes the bandwidth more efficient, and the modulation is more robust to use in order to resolve issues in band overlapping and multipath fading.

I don't know if anyone has make it more complicated for u to understand.
 

bauer

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Re: what is OFDM?

Orthogonal frequency-division multiplexing (OFDM), also sometimes called discrete multitone modulation (DMT), is a complex modulation technique for transmission based upon the idea of frequency-division multiplexing (FDM) where each frequency channel is modulated with a simpler modulation. In OFDM the frequencies and modulation of FDM are arranged to be orthogonal with each other which almost eliminates the interference between channels. Although the principles and some of the benefits have been known for 40 years, it is made popular today by the lower cost and availability of digital signal processing components.

The main idea behind OFDM is that since low-rate modulations (i.e modulations with relatively long symbols compared to the channel time characteristics) are less sensitive to multipath, it should be better to send a number of low rate streams in parallel than sending one high rate waveform. This is exactly what OFDM is doing. It divides the frequency spectrum in subbands small enough so that the channel effects are constant (flat) over a given subband. Then a "classical" IQ modulation (BPSK, QPSK, M-QAM, etc) is sent over the subband. If designed correctly, all the fast changing effects of the channel (multipath) disappear as they are now occurring during the transmission of a single symbol and are thus treated as flat fading at the received.

Classical signal processing such as channel coding, power allocation, adaptive modulation and coding can be applied for a given subband or over the subbands. Multiuser allocation is also possible, either using time, coding or frequency separation of the users.
Contents
[hide]

* 1 Comparison to FDM
* 2 Coupling with "channel coding"
* 3 Characteristics
o 3.1 Benefits
o 3.2 Disadvantages of OFDM
* 4 OFDM feature abstract
* 5 Usage
o 5.1 ADSL
o 5.2 HomePlug powerline alliance
o 5.3 Wireless local area networks (LAN) and metropolitan area networks (MAN)
o 5.4 Terrestrial digital radio and television broadcasting
+ 5.4.1 DVB-T's implementation of COFDM for digital television
+ 5.4.2 DRM and Eureka-147's (DAB) implementation of COFDM for digital audio broadcasting
o 5.5 Ultra wideband
o 5.6 Flash-OFDM
o 5.7 BST-OFDM
* 6 Ideal encoder
* 7 Mathematical description
* 8 OFDM history
* 9 See also
* 10 References
* 11 External links

[edit]

Comparison to FDM

In FDM, multiple signals are sent out at the same time, but on different frequencies. Most people are familiar with FDM from radio and television: normally, each station broadcasts on a particular frequency band (range of frequencies) or channel.

* OFDM takes this concept further: In OFDM, a single transmitter transmits on many (typically dozens to thousands) different orthogonal frequencies (i.e. frequencies that are independent with respect to the relative phase relationship between the frequencies). Also, because the frequencies are so closely spaced, each one only has room for a Narrowband signal.
* This modulation technique coupled with the use of advanced modulation techniques on each component, results in a signal with high resistance to interference.

[edit]

Coupling with "channel coding"

OFDM is almost always used in conjunction with channel coding—an error correction technique—to create coded orthogonal FDM or COFDM. It is a complex technology to implement, but it is now widely used in digital telecommunications systems to make it easier to encode and decode such signals. The system has been used in broadcasting as well as certain types of computer networking technology. This is particularly due to the fact that such signals show good resistance to multipath fading, best known as the source of "ghosting" on analog television broadcasts.

According to Stott, 1997 [1], "The 'COFDM magic' is achieved by the use of channel-state information (CSI). In the presence of CW interferers and/or a selective channel, some OFDM carriers will be worse affected than others." The channel coding thus allows the receiver to integrate information about the physical S/N ratios of the subchannels into the error correction of its Viterbi decoder, yielding significantly better performance than uncoded OFDM can attain with similar channel characteristics.
[edit]

Characteristics

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.
[edit]

Benefits

The benefits of using OFDM are many, including high spectrum efficiency, resistance against multipath interference (particularly in wireless communications), and ease of filtering out noise (if a particular range of frequencies suffers from interference, the carriers within that range can be disabled or made to run slower). Also, 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 bandwidth is allocated to whichever stream needs it most.

An extremely important benefit from using multiple sub-carriers is that because each carrier operates at a relatively low bitrate, the duration of each symbol is relatively long. If one sends, say, a million bits per second over a single baseband channel, then the duration of each bit must be under a microsecond. This imposes severe constraints on sychronization and removal of multipath interference. If the same million bits per second are spread among N subcarriers, the duration of each bit can be longer by a factor of N, and the constraints of timing and multipath sensitivity are greatly relaxed. For moving vehicles, the doppler effect on signal timing is another constraint that causes difficulties for some other modulation schemes.

OFDM modulation and demodulation are typically (as of 2001) implemented using digital filter banks generally using the Fast Fourier Transform (FFT).

Although highly complex, COFDM has high performance under even very challenging channel conditions.

By combining the OFDM technique with error-correcting codes, adaptive equalization and reconfigurable modulation, COFDM has the following properties:

* resistance against link dispersion
* resistance against slowly changing phase distortion and fading
* resistance against multipath using guard interval and cyclic prefix
* resistance against frequency response nulls and constant frequency interference
* resistance against burst noise

COFDM also generally has a nearly 'white' spectrum, giving it benign electromagnetic interference properties with respect to other signals.

Some COFDM systems use some of the sub-carriers to carry pilot signals, which are used for frequency synchronization. (Loss of synchronization causes errors in the decoded data).

In 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 subcarriers, whereas in general they will actually reinforce coverage over a wide area. This is very beneficial in many countries, as it permits the operation of national single frequency networks, and avoids the replication of program content on different carrier frequencies which is necessary with FM or other forms of radio broadcasting. Also, because effectively the bit rate is slowed down on each sub-carrier, the effects of "ghosting" are much reduced. Such single frequency networks utilise the available spectrum more effectively than existing analogue radio networks.
[edit]

Disadvantages of OFDM

However, OFDM suffers from time-variations in the channel, or presence of a carrier frequency offset. This is due to the fact that the OFDM subcarriers are spaced closely in frequency. Imperfect frequency synchronization causes a loss in subcarrier orthogonality which severely degrades performance.

Because the signal is the sum of a large number of subcarriers, it tends to have a high peak-to-average power ratio (PAPR). Also, it is necessary to minimise intermodulation between the subcarriers, which would effectively raise the noise floor both in-channel and out of channel. For this reason circuitry must be very linear. This is demanding, especially in relation to high power RF circuitry, which also needs to be efficient in order to minimise power consumption.
[edit]

OFDM feature abstract

* No intercarrier guard bands.
* Maximum spectral efficiency (Nyquist rate)
* Easy implementation by FFTs
* Controlled overlapping of bands
* Very sensitive time-freq. synchronization

[edit]

Usage

OFDM is used in many communications systems such as: ADSL, the 802.11a and 802.11g Wireless LAN standards, Digital audio broadcasting including EUREKA 147 and Digital Radio Mondiale, DVB, some UWB systems, WiMax, and PLC.

OFDM technology is additionally used in point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications.
[edit]

ADSL

OFDM is used in ADSL connections that follow the G.DMT (ITU G.992.1) standard.

The fact that COFDM does not interfere easily with other signals is the main reason it is frequently used in applications such as ADSL modems in which existing copper wires are used to achieve high-speed data connections. The lack of interference means no wires need to be replaced (otherwise it would be cheaper to replace them with fiber). 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.
[edit]

HomePlug powerline alliance

OFDM is used by HomePlug 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.
[edit]

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. For amateur radio applications, experimental 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.

IEEE 802.11a, operating in the 5 GHz band, specifies airside data rates ranging from 6 to 54 Mbit/s. Below contains a listing of the eight specified PHY data rates. Four different modulation schemes are used: BPSK, 4-QAM, 16-QAM, and 64-QAM. Each higher performing modulation scheme requires better channel condition for accurate transmission. These modulation schemes are coupled with the various forward error correction convolutional encoding schemes to give a multitude of Number of data bits per symbol (Ndbps) performance.
Data rate (Mbit/s) Modulation Coding rate Ndbps 1472 byte transfer duration (μs)
6 BPSK 1/2 23 2012
9 BPSK 3/4 36 1344
12 4-QAM 1/2 48 1008
18 4-QAM 3/4 72 672
24 16-QAM 1/2 96 504
36 16-QAM 3/4 144 336
48 64-QAM 2/3 192 252
54 64-QAM 3/4 216 224
[edit]

Terrestrial digital radio and television broadcasting

Much of Europe and Asia has adopted COFDM for terrestrial broadcasting of television and radio. The television standard is called DVB-T and the radio standard is called DAB.
[edit]

DVB-T's implementation of COFDM for digital television

COFDM is also now widely used in Europe and elsewhere for terrestrial digital TV using the DVB-T standard. 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. 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 question of the relative technical merits of COFDM versus 8VSB has been a subject of some controversy between Europe and USA.

The debate over 8VSB vs COFDM modulation is still ongoing. 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. Moreover, 8VSB modulation requires less power to transmit a signal the same distance. In less-populated areas, 8VSB often pulls ahead of COFDM because of this. In urban areas, however, COFDM still offers better reception than 8VSB.
[edit]

DRM and Eureka-147's (DAB) implementation of COFDM for digital audio broadcasting

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).

* 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 (mediumwave) 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 VHF or UHF broadcasts instead.

[edit]

Ultra wideband

UWB (ultra wideband) 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.
[edit]

Flash-OFDM

Flash-OFDM (Fast Low-latency Access with Seamless Handoff Orthogonal Frequency Division Multiplexing) is a system that is based on OFDM and specifies also higher protocol layers. It has been developed and is marketed by Flarion. Flash-OFDM has generated interest as a packet-switched cellular bearer, on which area it would compete with GSM and 3G networks. As an example, old 450 MHz frequency bands that were used by NMT (an 1G analog network, now decommissioned) in Europe are being considered to be licenced to Flash-OFDM operators.

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 licences they own nationwide in the 2.5GHz frequency range. Sprint subsequently has decided to deploy the mobile version of WiMAX which is based on SOFDMA, scalable orthogonal frequency domain multiple access technology.

OFDM
[edit]

BST-OFDM

The BST-OFDM (Band Segmented Transmission - Orthogonal Frequency Division Multiplexing) system proposed for Japan improves upon COFDM by exploiting the fact that some OFDM carriers may be modulated differently from others within the same multiplex. 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.






 
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