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	Modulation - Wikipedia, the free encyclopedia
		Multiplexing — the transmission of multiple data signals in one frequency band, on different carrier frequencies.
		When OFDM is used in conjunction with channel coding techniques, it is described as Coded orthogonal frequency division multiplexing (COFDM).
		The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.
	en.wikipedia.org /wiki/Modulator (755 words)

	Orthogonal frequency-division multiplexing - Wikipedia, the free encyclopedia
		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.
		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.
		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).
	en.wikipedia.org /wiki/COFDM (2803 words)

	Orthogonal frequency-division multiplexing - Biocrawler (Site not responding. Last check: 2007-10-13)
		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.
		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.
		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.
	www.biocrawler.com /encyclopedia/OFDM (1321 words)

	Coded Orthogonal Frequency Division Multiplexing (COFDM)
		COFDM also allows different groups of bits to be protected with a different strength code rate because some bits are more important for the correct reproduction of the audio than some of the other bits.
		This property is that the discrete frequency spectrum that is calculated by a DFT from a block of data samples has frequency samples that are all equally spaced in frequency, and this spacing equals 1/T, where T is the total duration of the time samples in the block.
		LDPC codes are an old form of FEC code (invented by a famous coding theoretician called Gallagher) that have been "re-discovered" and have attracted a lot of attention from the information theory research community because of their near-optimum performance.
	www.digitalradiotech.co.uk /cofdm.htm (4724 words)

	Explaining some of the magic of COFDM
		When a code having higher rate (less redundancy) is needed then one of these lower-rate 'mother' codes is punctured, that is to say certain of the coded bits are just not transmitted, according to a regular pattern known to the receiver.
		This means that the coded data we transmit should not simply be assigned to the OFDM carriers in order, since at the receiver this would cause the Viterbi soft-decision decoder to be fed with clusters of unreliable bits.
		If two synchronised COFDM transmitters operate on a common frequency there will some- where be locations where the two signals will be received at equal strength (and with a relative delay, depending on the geometry of the situation, which we assume to be within the system limits).
	www.bbc.co.uk /rd/pubs/papers/paper_15/paper_15.html (4025 words)

	Digital TV Group \| Reference \| Tutorial \| Transmission (Site not responding. Last check: 2007-10-13)
		Coded Orthogonal Frequency Division Multiplexing (COFDM) was largely developed into a practical proposition by the CCETT in France in the late 1980s, although the idea of OFDM was originally propounded elsewhere.
		COFDM is a system of modulation well-suited to the propagation and interference environment of terrestrial broadcasting in the VHF (used in UK for DAB) and UHF bands (used in UK for DVB-T).
		The use of coding (the "C" of OFDM) is an important factor in coping with both co-channel analogue TV interferers and the frequency-selective fading caused by multipath.
	www.dtg.org.uk /reference/tutorial_transmission.html (1425 words)

	Orthogonal Frequency Division Multiplexing
		Coded Orthogonal Frequency Division Multiplexing (COFDM) has captured the imagination of the engineers who are defining the current generation of digital broadcast systems.
		In practice each frequency band will have a single carrier that is modulated in amplitude and phase using schemes such as Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK).
		Coded in some way so that the data payload is protected and can be recovered by the receiver using checksum and other algorithms that allows the lost data to be restored.
	www.radioeng.co.uk /ofdm.html (366 words)

	Orthogonal Frequency-Division Multiplexing (OFDM) (Site not responding. Last check: 2007-10-13)
		The demodulation of the received radio signal to baseband, possibly via an intermediate frequency, involves oscillators whose frequencies may not be perfectly aligned with the transmitter frequencies.
		Frequency offsets and symbol clock offsets, however, generally need to be tracked at the receiver.
		From (16), (19) and (20), the frequency estimate is the argument of a sum of complex numbers.
	www.s3.kth.se /signal/grad/OFDM/URSIOFDM9808.htm (4617 words)

	OFDM, VOFDM, COFDM, Orthogonal Frequency Division Multiplexing
		OFDM Receivers for Broadband-Transmission OFDM and the orthogonality principle, The general problem: Data transmission over multipath channels, Single carrier approach, Multi carrier approach, Orthogonal Frequency Division Multiplexing, An OFDM receiver for DVB-T, Tasks of the inner receiver and receiver structure, Channel estimation for OFDM, Performance of the complete receiver...
		OFDM Tutorial Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system.
		Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies.
	www.palowireless.com /ofdm/tutorials.asp (585 words)

	Welcome to THE COFDM Vs. 8-VSB SERIES (Site not responding. Last check: 2007-10-13)
		COFDM avoids interference from multipath echoes by increasing the length of the signal samples, so that it is greater than the temporal spread of the multipath, and by applying a ìguard intervalî between data symbols where the receiver does not look for information.
		The concept of using parallel data transmission and frequency division multiplexing was first published in the mid-1960s, and a U.S. patent was filed and issued for this approach in 1970.
		In COFDM, the spectrum of the transmitted signal is sharply confined in frequency because it is the sum of equally spaced subchannel signals, each of which is sharply confined.
	www.hdtvmagazine.com /archives/cofdmvs8vsbfcc2.html (9754 words)

	Multiplexing
		The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code.
		FDMA is the division of the frequency band allocated for wireless cellular telephone communication into 30 channels, each of which can carry a voice conversation or, carry data of a digital service.
		Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system.
	www.andreas-schwope.de /ASIC_s/Schnittstellen/Data_Lines/body_multiplexing.html (1113 words)

	3 (Site not responding. Last check: 2007-10-13)
		In this study OFDM and space-time coding is used as a modulation technique in order to cope with mobile communication channel constraints.
		Space-time coding techniques have been proposed for transmitter diversity and then employed in the OFDM system to further reduce fading and obtain the better signal quality from the diversity gain.
		Orthogonal Frequency Division Multiplexing is a multi-carrier modulation technique in which a single high rate data-stream is divided into multiple low rate data-streams that are modulated using subcarriers which are orthogonal to each other.
	www.dho.edu.tr /ENSTITUNet/thesis/abstractECom_eng3.htm (317 words)

	What is COFDM? - a definition from Whatis.com - see also: coded orthogonal frequency division multiplexing
		COFDM is used predominately in Europe and is supported by the Digital Video Broadcasting (DVB) set of standards.
		COFDM is resistant to multipath effects because it uses multiple carriers to transmit the same signal.
		A single frequency network is a network of several stations that broadcast the same signal simultaneously using multiple transmitters.
	searchnetworking.techtarget.com /sDefinition/0,,sid7_gci331325,00.html (461 words)

	modulation, in communications: Frequency and Phase Modulation
		In frequency modulation (FM), the frequency of the carrier wave is varied in such a way that the change in frequency at any instant is proportional to another signal that varies with time.
		A further extension of the technology, coded orthogonal frequency-division multiplexing (COFDM), is widely used in Europe and elsewhere where the digital audio broadcast (DAB) standard has been adopted.
		Phase modulation, like frequency modulation, is a form of angle modulation (so called because the angle of the sinewave carrier is changed by the modulating wave).
	www.factmonster.com /ce6/sci/A0859708.html (269 words)

	OFDM as a modulation technique for wireless communications, with a CDMA comparison
		The amount the frequency changes due to the Doppler effect depends on the relative motion between the source and receiver and on the speed of propagation of the wave.
		Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream.
		Coded Orthogonal Frequency Division Multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission.
	www.skydsp.com /publications/4thyrthesis/chapter1.htm (6503 words)

	Technology Update: 802.11a speeds wireless LANs
		COFDM breaks the ceiling of the data bit rate by 1) sending data in a massively parallel fashion, and 2) slowing the symbol rate down so each symbol transmission is much longer than the typical delay spread.
		COFDM slows the symbol rate while packing many bits in each symbol transmission, making the symbol rate substantially slower than the data bit rate.
		COFDM also uses coding to allow for recovery of errors and to add more interference rejection by spreading information across all carriers.
	www.networkworld.com /news/tech/2001/0129tech.html (928 words)

	VLSI for OFDM
		Orthogonal frequency-division multiplexing (OFDM), coded OFDM (COFDM), and discrete multitone (DMT) forms of modulation are increasingly finding their way into wired and wireless communications in applications including digital television, high-speed wired data connections, and wireless local area networks.
		Thus, subcarriers in frequency segments of the channel with a high signal-to-noise ratio (SNR) may use high-rate modulation, while those with degraded SNR use low-rate modulation or are not modulated.
		The common method of obtaining orthogonality of subcarriers is to choose their frequency spacing equal to the inverse of the subcarrier symbol duration.
	www.comsoc.org /~ci/private/1998/oct/Weste.html (4061 words)

	smartBridges :: Wireless Network Products
		COFDM is a modulation scheme that divides a single digital signal across 1,000 or more signal carriers simultaneously.
		COFDM has the ability to overcome multi-path effects by using multiple carriers to transmit the same signal.
		COFDM is commonly used in IEEE 802.11a and 802.11g standard.
	www.smartbridges.com /education/articles.asp?id=395 (1971 words)

	ISP Planet - Technology - Broadband Mobile Wireless using COFDM (Site not responding. Last check: 2007-10-13)
		COFDM is UBS's proprietary version of multicarrier OFDM technology, which breaks a digital signal into chunks—IP packets in this case—and sends them out over multiple low-bandwidth channels instead of one broadband channel.
		COFDM is designed to operate in the licensed 2.3 to 2.4MHz band reserved for digital broadcast and multimedia applications.
		Dolgonos' idea is that COFDM will deliver the bandwidth to a vehicle—with the modem possibly mounted outside where reception will be better anyway—and some other wireless medium (Bluetooth?) will deliver the signal to the laptop or PDA inside.
	www.isp-planet.com /technology/mobile_broadband.html (844 words)

	WinDRM Docs (Site not responding. Last check: 2007-10-13)
		COFDM spreads these carriers (WinDRM has up to 51) spaced apart at precise frequencies (thus FDM) across a bandwidth of 2.3 or 2.5 KHz. It is this spacing that makes them unique (i.e.
		Orthogonal, the O in OFDM) which keeps the demodulator (WinDRM’s receiver) from seeing frequencies other than its own. COFDM has close to a white noise spectrum which gives it “benign electromagnetic interference properties with respect to other signals.” WinDRM can not be used to decode commercial DRM transmissions.
		The accuracy of the receiving station’s tuning for the COFDM signal is dependent upon this factor which may allow up to 50Hz tuning error without degrading the decoding process. It is important to note however, that once sync is obtained, no further “tuning” of the signal should be attempted.
	n1su.com /windrm/new/WinDRM_Docs.htm (3831 words)

	COFDM (Site not responding. Last check: 2007-10-13)
		Coded Orthogonal Frequency Division Multiplexing (COFDM) [1, 2] has been specified for digital broadcasting systems for both audio -- Digital Audio Broadcasting (DAB) [3] and (terrestrial) television -- Digital Video Broadcasting (DVB-T) [4, 5, 6].
		COFDM is particularly well matched to these applications, since it is very tolerant of the effects of multipath (provided a suitable guard interval is used).
		Provided the range of delays of the multipath (natural or 'unnatural') does not exceed the designed tolerance of the system (slightly greater than the guard interval) all the received-signal components contribute usefully.
	www.real-time-vision.com /background/cofdm.shtml (145 words)

	La Trobe University White Papers at silicon.com (Site not responding. Last check: 2007-10-13)
		Orthogonal Frequency Division Multiplexing (OFDM) is one of the modulation techniques used in digital video broadcasting systems.
		Polynomial cancellation coded orthogonal frequency division multiplexing with overlapping symbol periods (Overlap PCC-OFDM) is a modulation technique that is less sensitive to frequency errors and more robust against multipath distortion...
		Polynomial cancellation coded orthogonal frequency division multiplexing (PCC-OFDM) with overlapping symbol periods is a modulation technique which overcomes many of the disadvantages of OFDM.
	whitepapers.silicon.com /0,3800002488,39026420c,00.htm (508 words)

	Digital Radio Mondiale
		Just how many of these can be carried effectively in the multiplex depends on the data capacity available.
		Each of these permutations has a theoretical data capacity as well as a practical one when all the overheads of the encoding processes are added.
		Typically broadcasters use 11.6, 14.6, 17.4 and 20.9 kilobits per second (kbps) on Short-wave and the handful of Medium Wave broadcasts have used bit rates about 18.4 kbps in Mode B and 19.6 kbps in Mode A.
	www.radioeng.co.uk /drm.html (240 words)

	Thesis abstracts of Communication Systems Group (Site not responding. Last check: 2007-10-13)
		On such channels, orthogonal frequency division multiplexing (OFDM) is a very powerful technique and, recently, several proposals of CDMA for OFDM systems have been made.
		However, assuming a single-cell scenario and a channel code of a sufficiently large free distance, it is at high system loads concluded that coded orthogonal frequency division multiplexing access (OFDMA) outperforms all the considered direct-sequence schemes.
		It is also concluded that coded OFDMA achieves essentially the same robustness against inter-cell interference as schemes that use direct-sequence spreading.
	www.s2.chalmers.se /research/comsys/thesis/andersphd.html (322 words)

	WinDRM Docs
		COFDM uses many parallel narrow band sub-carriers instead of just one single wide band carrier for transporting the data.
		The accuracy of the receiving station’s tuning for the COFDM signal is dependent upon this factor which may allow up to 50Hz tuning error without affecting the decoding process.
		The X axis measures the PSD of 0 to 50dB while the Y axis is frequency from 0 to 12 KHz.
	n1su.com /windrm/docs/1.1 (6066 words)

	Orthogonal Frequency Division Multiplexing (OFDM)
		This standard specifies a 2.4 GHz operating frequency with data rates of 1 and 2 Mbps using either direct sequence or frequency hopping spread spectrum.
		Four of the sub-carriers are pilot sub-carriers that the system uses as a reference to disregard frequency or phase shifts of the signal during transmission.
		The PLCP Preamble field is present for the receiver to acquire an incoming OFDM signal and synchronize the demodulator.
	www.wireless-nets.com /papers/wireless_lan_ofdm.htm (1654 words)

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