Science Fair Project Encyclopedia
Orthogonal frequency-division multiplexing
Orthogonal frequency-division multiplexing (OFDM), also sometimes called discrete multitone modulation (DMT), is a transmission technique based upon the idea of frequency-division multiplexing (FDM) where multiple signals are sent out at different frequencies. Most people are familiar with FDM from the use of radio and television: normally, each station is designated to broadcast at a particular frequency or channel. OFDM takes this concept further, breaking an individual transmission down into multiple low-frequency signals (typically dozens to thousands). This, coupled with the use of advanced modulation techniques on each component, results in a signal with high "orthogonality"—resistance to interference.
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 telecommunications where digital systems make it easier to encode and decode such signals. The system has found use 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, best known as the source of "ghosting" on analog television broadcasts.
An OFDM baseband signal is the sum of a number of orthogonal sub-carriers, with 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.
The benefits of using OFDM are many, including high spectrum efficiency, resistant against multipath interference (particularly in wireless communications), and easy to filter out noise (if a particular range of frequencies suffers from interference, the carriers within that range can be disabled or made to run slower), and that 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.
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:
- resistant against link dispersion
- resistant against slowly changing phase distortion and fading
- resistant against multipath using guard interval
- resistant against frequency response nulls and constant frequency interference
- resistant 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 synchronizations, as frequency shifts during the transmission using the main modulation/demodulation process transform into bit 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.
However, OFDM suffers from time-variations in the channel, or presence of a carrier frequency offset. Moreover, due to the mathematical FFT operation applied at the transmitter, the signal also tends to have a high peak-to-average ratio. All mentioned effects are emphasised when several users are sending data to the same base station.
The fact that COFDM is not wont to 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.
OFDM is also now being used in some wireless LAN applications, including IEEE 802.11a/g and the European alternative HIPERLAN/2. 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.
Digital radio and television
COFDM is also now widely used in Europe and elsewhere where the Eureka 147 Digital Audio Broadcast (DAB) standard has been adopted for digital radio broadcasting, and also for digital TV in the DVB digital TV 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. In 2001, a technical report compiled by the COFDM Technical Group concluded that COFDM did not offer any significant advantages over 8VSB.
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.
COFDM is also used for other radio standards, most prominently for Digital Radio Mondiale (DRM), the standard for digital broadcasting at 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, unline Eureka 147 which uses UHF broadcasts instead.
- Chang, R. W. (1966). Synthesis of band-limited orthogonal signals for multi-channel data transmission, Bell Systems Technical Journal (46), 1775-1796.
- Chang, R. W. & and Gibbey, R. A. (1968). A theoretical study of performance of an orthogonal multiplexing data transmission scheme, IEEE Transactions on Communications Technology (16) (4), 529-540.
- Saltzberg, B. R. (1967). Performance of an efficient parallel data transmission system, IEEE Transactions on Communications Technology (15) (6), 805-811.
- Page on Orthogonal Frequency Division Multiplexing at http://www.iss.rwth-aachen.de/Projekte/Theo/OFDM/node6.html accessed on 13th Nov. 2002.
- Siemens demos 360 Mbit/s wireless
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