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Topic: Orthogonal frequency division modulation

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	Orthogonal frequency-division multiplexing - Wikipedia, the free encyclopedia
		Orthogonal frequency-division multiplexing (OFDM), also sometimes called discrete multitone modulation (DMT), is a modulation technique for transmission based upon the idea of frequency-division multiplexing (FDM), where each frequency channel carries a separate stream of data.
		In OFDM the frequencies are chosen so that the modulated data streams are orthogonal to each other, which greatly simplifies the design of both the transmitter and the receiver, and also allows high spectral efficiency.
		OFDM modulation and demodulation are typically implemented using digital filter banks generally using the Fast Fourier Transform (FFT).
	en.wikipedia.org /wiki/Discrete_multitone_modulation (2794 words)

	Modulation
		Modulation describes a range of techniques for encoding information on a carrier signal, typically a sine-wave signal.
		When OFDM is used in conjunction with channel coding techniques, it is described as 'Coded orthogonal frequency division modulation' (COFDM).
		Modulation is frequently used in conjunction with various channel access methods.
	www.ebroadcast.com.au /lookup/encyclopedia/mo/Modulation.html (157 words)

	Orthogonal frequency division modulation
		Orthogonal frequency division modulation (OFDM) is a modulation technique for encoding information into an analog signal.
		An OFDM signal may be regarded as the sum of a number of individual sub-carrier signals, each modulated (typically using QAM) by its own modulating signal.
		OFDM modulation and demodulation are typically (as of 2001) implemented using digital filter banks generally using the Fast Fourier Transform.
	www.ebroadcast.com.au /lookup/encyclopedia/co/COFDM.html (467 words)

	Orthogonal frequency-division multiplexing - Biocrawler (Site not responding. Last check: 2007-10-31)
		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.
		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).
		OFDM modulation and demodulation are typically (as of 2001) implemented using digital filter banks generally using the Fast Fourier Transform (FFT).
	www.biocrawler.com /encyclopedia/OFDM (1331 words)

	Coded Orthogonal Frequency Division Multiplexing (COFDM)
		OFDM, as used on DAB and Freeview actually uses narrowband channels (subcarriers), but there are many of these narrowband channels transmitted in parallel, so the overall spectrum is wide (but this doesn't mean that it uses wideband transmission principles).
		The OFDM modulator consists of the block in the diagram that is labelled 'IDFT', which stands for inverse discrete Fourier transform.
		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.
	www.digitalradiotech.co.uk /cofdm.htm (4724 words)

	Frequency-division multiplexing - Wikipedia, the free encyclopedia
		In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal.
		Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA).
		The concept corresponding to frequency division multiplexing in the optical domain is known as wavelength division multiplexing.
	en.wikipedia.org /wiki/Frequency-division_multiplexing (302 words)

	Orthogonal Frequency Division Multiplexing
		Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream.
		OFDM is similar to FDMA in that the multiple user access is achieved by subdividing the available bandwidth into multiple channels that are then allocated to users.
		OFDM can be implemented equally well, with coherent (non-differential) modulation and demodulation to maximize the signal-to-noise ratio performance of the system.
	www.infomatek.com /OFDM.htm (508 words)

	Orthogonal Frequency-Division Multiplexing (OFDM)
		OFDM is a type of multichannel modulation that divides a given channel into many parallel subchannels or subcarriers, so that multiple symbols are sent in parallel.
		OFDM is often motivated by two of its many attractive features: it is considered to be spectrally efficient and it offers an elegant way to deal with equalization of dispersive slowly fading channels.
		As the OFDM signal is the sum of a large number of independent, identically distributed components its amplitude distribution becomes approximately Gaussian due to the central limit theorem.
	www.s3.kth.se /signal/grad/OFDM/URSIOFDM9808.htm (4617 words)

	Orthogonal Frequency Division Multiplexing (OFDM)
		OFDM signals typically have a time guard of 800 ns, however, which provides very good performance on channels having delay spreads up to 250 ns.
		With 802.11a OFDM modulation, the binary serial signal is divided into groups (symbols) of 1, 2, 4, or 6 bits, depending on data rate chosen, and converted into complex numbers representing applicable constellation points.
		OFDM’s high degree of spectral efficiency, resiliency to interference and multi-path distortion, and existing inclusions in the leading higher rate wireless LAN standards, provides a strong based for the development of newer broadband wireless networks.
	www.wireless-nets.com /papers/wireless_lan_ofdm.htm (1654 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)

	OFDM Orthogonal Frequency Division Multiplexing
		Orthogonal Frequency Division Multiplexing (OFDM) is special form of multi-carrier modulation, initially proposed in the 1970s.
		In a multipath channel, most conventional modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel.
		OFDM is significantly less sensitive to intersymbol interference, because a special set of signals is used to build the composite transmitted signal.
	www.wireless.per.nl /wireless/telelearn/ofdm/index.htm (291 words)

	Modulation scheme for orthogonal frequency division multiplexing systems or the like patent invention (Site not responding. Last check: 2007-10-31)
		A method as claimed in claim 1, wherein said adjusting includes selecting a modulation for a subcarrier when a signal-to-noise ratio per subcarrier of the communication channel is greater than a predetermined value, and selecting another modulation when the signal-to-noise ratio per subcarrier of the communication channel is less than a predetermined value.
		An apparatus as claimed in claim 1, wherein said modulation encoder selects a modulation on a subcarrier when a signal-to-noise ratio per subcarrier of the communication channel is greater than a predetermined value, and selects another modulation when the signal-to-noise ratio per subcarrier of the communication channel is less than a predetermined value.
		An apparatus as claimed in claim 16, wherein said modulation encoder selects a modulation on a subcarrier when a signal-to-noise ratio per subcarrier of the communication channel is greater than a predetermined value, and selects another modulation when the signal-to-noise ratio per subcarrier of the communication channel is less than a predetermined value.
	www.freshpatents.com /Modulation-scheme-for-orthogonal-frequency-division-multiplexing-systems-or-the-like-dt20050317ptan20050058057.php?type=claims (745 words)

	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.
		OFDM is sometimes called multi-carrier or discrete multi-tone modulation.
		Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing is a technology developed by Iospan Wireless that uses multiple antennas to transmit and receive radio signals.
	www.wave-report.com /tutorials/OFDM.htm (526 words)

	IEC: OFDM for Mobile Data Communications
		OFDM is a modulation and multiple-access technique that has been explored for more than 20 years.
		OFDM, or multitone modulation as it is sometimes called, is presently used in a number of commercial wired and wireless applications.
		OFDM enables the creation of a very flexible system architecture that can be used efficiently for a wide range of services, including voice and data.
	www.iec.org /online/tutorials/ofdm (373 words)

	Home
		The major advantage offered by this modulation scheme is that it provides an increase of system capacity through a system processing gain that is linearly proportional to the ratio of the RF bandwidth to the information rate.
		This waveform-based OFDM symbol increases the receiver immunity to channel interference, noise, and multi-path effects.
		HSD employs a special set of linear orthogonal constraints to minimize the quadratic objective function and accomplish the signal recovery.
	www.moriahtechnologies.com (353 words)

	Orthogonal Frequency Division Multiplexing (Site not responding. Last check: 2007-10-31)
		Popular for wireless modulation, orthogonal frequency division multiplexing (OFDM) is used in broadcast applications and "last-mile" connectivity applications such as multichannel multipoint distribution systems (MMDS) and wireless local area networks.
		Conventional single-carrier modulation schemes such as amplitude modulation (AM) or frequency modulation (FM) send one signal at a time, using one radio frequency.
		OFDM is a multi-carrier modulation scheme that sends multiple high-speed signals concurrently on different frequencies.
	www.altera.com /products/ip/altera/s-alt-ofdm.html (318 words)

	MP Digest (Site not responding. Last check: 2007-10-31)
		In this type of transmitter, the output signal is amplified from the output of the modulator using conventional linear amplifier devices.
		This approach provides a lower modulator wideband noise floor, determined by the VCO phase noise, compared to what is available from a linear design using a quadrature modulator.
		Polar modulation technology is experiencing increased interest as the need grows to simultaneously provide high quality signal modulation of bandwidth efficient signals while achieving high DC to RF efficiency.
	www.mpdigest.com /Articles/2006/Feb2006/Tropian/Default.htm (1449 words)

	orthogonal frequency-division multiplexing - a Whatis.com definition
		Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies.
		In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM).
		OFDM is used in European digital audio broadcast services.
	whatis.techtarget.com /gDefinition/0,294236,sid44_gci523666,00.html (225 words)

	COFDM for Fixed Outdoor Wireless
		Orthogonal Frequency Division Multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands.
		OFDM is a multi-carrier modulation technique that is unlike other modulation techniques.
		The sub-carriers are orthogonal with each other and hence do not interfere with each other.
	www.smartbridges.com /education/print-ready.asp?id=407 (644 words)

	[No title]
		The amplitudes and spacings of the carriers are configured such that the spectral energy of each carrier is significantly attenuated at the frequencies of each of the two adjacent carriers.
		The modulating QAM signals can be coded in order to be more robust in the presence of multipath propagation and/or interference, and Clarity proposes the use of its proprietary coding schema, designated Vector OFDM ("VOFDM").
		Specifically, based upon the Hardin laboratory test results, we will permit the use of OFDM modulation, coded or uncoded, so long as a minimum of 256 QAM-modulated carriers are used and the transmitted signal complies with the spectral mask and power limitations set out in our MDS and ITFS rules for digital emissions.
	www.fcc.gov /Bureaus/Mass_Media/Orders/1999/da990554.txt (1079 words)

	Orthogonal Frequency Division Multiplexing
		In OFDM the subcarrier pulse used for transmission is chosen to be rectangular.
		By using an IFFT for modulation we implicitly chose the spacing of the subcarriers in such a way that at the frequency where we evaluate the received signal (indicated as arrows) all other signals are zero.
		Fortunately there's an easy solution for this problem: The OFDM symbols are artificially prolonged by periodically repeating the 'tail' of the symbol and precede the symbol with it (see figure 0.5).
	www.ert.rwth-aachen.de /Projekte/Theo/OFDM/node6.html (482 words)

	622-Mbps Orthogonal Frequency Division Multiplexing Modulator (Site not responding. Last check: 2007-10-31)
		The Communications Technology Division at the NASA Lewis Research Center is developing advanced electronic technologies for the space communications and remote sensing systems of tomorrow.
		The OFDM modulator is particularly suited to high-data-rate downlinks to ground terminals or direct data downlinks from near-Earth science platforms.
		The OFDM modulator architecture can also be precompensated for channel disturbances and alleviate amplitude degradations caused by nonlinear transponder characteristics.
	www.grc.nasa.gov /WWW/RT1998/5000/5650nguyen.html (208 words)

	Digital TV.com.au :: How it Works
		As a modulation scheme, it has to be considered as an additional tool given to RF network planners, to introduce new possibilities in the use of the scarce radio frequency spectrum.
		As shown in figure 7, hierarchical modulation can be viewed as a separation of the RF channel in two virtual circuits, each having a specific bitrate capacity, a specific roughness and accordingly, covering two slightly different areas.
		One primary usage of the hierarchical modulation is to slightly modify the modulation parameters to facilitate the indoor reception for portable receivers.
	www.digital-tv.com.au /howitworks.html (5791 words)

	DATV
		The second is DVB-T. This uses COFDM (Coded Orthogonal Frequency Division Multiplexing) modulation designed for UHF television channels with 8 MHz bandwidth.
		I & Q Modulator for HF Transmitting DRM is quite strait ford by contracting an I & Q modulator for HF (80 meter band).
		The BC549 and the BC559 transistors are a low-level RF amplifier stage to feed in to 50 ohms.
	www.qsl.net /zl1wtt/datv.htm (916 words)

	[No title]
		Specifically, Cisco requests the Commission declare that a minimum of 256 QAM-modulated carriers (or tones) is not necessary in order to use OFDM modulation in the MDS and ITFS.
		Based on a study commissioned by Clarity, the Commission declared that OFDM could be used in the MDS and ITFS and required that a minimum of 256 QAM-modulated carriers (or "tones") be utilized.
		Cisco notes that the Commission has previously indicated that it would approve certain digital modulation types without test data if it were determined to be a subset of an already approved modulation type.
	www.fcc.gov /ftp/Bureaus/Mass_Media/Public_Notices/BLS_Notices/da011582.txt (595 words)

	What is OFDM? - A Word Definition From the Webopedia Computer Dictionary
		(pronounced as separate letters) Short for Orthogonal Frequency Division Multiplexing, an FDM modulation technique for transmitting large amounts of digital data over a radio wave.
		OFDM reduces the amount of crosstalk in signal transmissions.
		This thesis investigates the effectiveness of Orthogonal Frequency Division Multiplexing (OFDM) as a modulation technique for wireless radio applications.
	www.webopedia.com /TERM/8/OFDM.html (140 words)

	Wisair â“ Ultrawideband (UWB) and Wireless USB (W-USB) Chipset Solutions (Site not responding. Last check: 2007-10-31)
		Multiband OFDM presents unique advantages for personal area networks, as it addresses performance requirements (such as data throughput and range), while maintaining low cost, and low power consumption.
		Multiband OFDM divides the UWB spectrum (3.1 to 10.6 GHz) into 528-MHz-wide sub bands and uses OFDM modulation to transmit the information in each sub band.
		The OFDM symbols are then interleaved over three contiguous sub bands across both time and frequency to provide a robust link and maximum range to support multiple access between piconets.
	www.wisair.com /technology/multiband-ofdm (190 words)

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