Ultra-wideband (UWB, ultra-wide band, ultraband, etc.) is a radio technology that can be used for short-range high-bandwidth communications by using a large portion of the radio spectrum in a way that doesn't interfere with other more traditional 'narrow band' uses. It also has applications in radar imaging , precision positioning and tracking technology.
Ultra-Wideband (UWB) may be used to refer to any radio technology having bandwidth exceeding the lesser of 500 MHz or 20% of the arithmetic center frequency, according to Federal Communications Commission (FCC). This article discusses the meaning of Ultra-wideband in the field of radio communications.
Overview
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Ultra-Wideband (UWB) is a technology for transmitting information spread over a large bandwidth (>500 MHz) that should, in theory and under the right circumstances, be able to share spectrum with other users. A February 14, 2002 Report and Order by the FCC authorizes the unlicensed use of UWB in 3.1–10.6 GHz. This is intended to provide an efficient use of scarce radio bandwidth while enabling both high data rate personal-area network (PAN) wireless connectivity and longer-range, low data rate applications as well as radar and imaging systems. More than four dozen devices have been certified under the FCC UWB rules, the vast majority of which are radar, imaging or positioning systems. Deliberations in the International Telecommunication Union Radiocommunication Sector (ITU-R) have resulted in a Report and Recommendation on UWB in November of 2005. National jurisdictions around the globe are expected to act on national regulations for UWB very soon. The UK regulator Ofcom announced a similar decision on 9 August 2007.
Ultra Wideband was traditionally accepted as pulse radio, but the FCC and ITU-R now define UWB in terms of a transmission from an antenna for which the emitted signal bandwidth exceeds the lesser of 500 MHz or 20% of the center frequency. Thus, pulse-based systems—wherein each transmitted pulse instantaneously occupies the UWB bandwidth, or an aggregation of at least 500 MHz worth of narrow band carriers, for example in orthogonal frequency-division multiplexing (OFDM) fashion—can gain access to the UWB spectrum under the rules. Pulse repetition rates may be either low or very high. Pulse-based radars and imaging systems tend to use low repetition rates, typically in the range of 1 to 100 megapulses per second. On the other hand, communications systems favor high repetition rates, typically in the range of 1 to 2 giga-pulses per second, thus enabling short-range gigabit-per-second communications systems. Each pulse in a pulse-based UWB system occupies the entire UWB bandwidth, thus reaping the benefits of relative immunity to multipath fading (but not to intersymbol interference), unlike carrier-based systems that are subject to both deep fades and intersymbol interference.
The FCC power spectral density emission limit for UWB emitters operating in the UWB band is -41.3 dBm/MHz. This is the same limit that applies to unintentional emitters in the UWB band, the so called Part 15 limit. However, the emission limit for UWB emitters can be significantly lower (as low as -75 dBm/MHz) in other segments of the spectrum.
A significant difference between traditional radio transmissions and UWB radio transmissions is that traditional transmissions transmit information by varying the power/frequency/and or phase of a sinusoidal wave. UWB transmissions can transmit information by generating radio energy at specific time instants and occupying large bandwidth thus enabling a pulse-position or time-modulation. But also information can be imparted (modulated) on UWB signals (pulses) by encoding the polarity of the pulse, the amplitude of the pulse, and/or also by using orthogonal pulses. UWB pulses can be sent sporadically at relatively low pulse rates to support time/position modulation, but can also be sent at rates up to the inverse of the UWB pulse bandwidth. Pulse-UWB systems have been demonstrated at channel pulse rates in excess of 1.3 giga-pulses per second using a continuous stream of UWB pulses (Continuous Pulse UWB or "C-UWB"), supporting forward error correction encoded data rates in excess of 675 Mbit/s. Such a pulse-based UWB method using bursts of pulses is the basis of the IEEE 802.15.4a draft standard and working group, which has proposed UWB as an alternative PHY layer.
One of the valuable aspects of UWB radio technology is the ability for a UWB radio system to determine "time of flight" of the direct path of the radio transmission between the transmitter and receiver to a high resolution. With a two-way time transfer technique distances can be measured to high resolution as well as to high accuracy by compensating for local clock drifts and inaccuracies.
Another valuable aspect of pulse-based UWB is that the pulses are very short in space (less than 60 cm for a 500 MHz wide pulse, less than 23 cm for a 1.3 GHz bandwidth pulse), so most signal reflections do not overlap the original pulse, and thus the traditional multipath fading of narrow band signals does not exist. However, there still is inter-pulse interference for fast pulse systems which can be mitigated by coding techniques.
Technical discussion
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One performance measure of a radio in applications like communication, positioning, location, tracking, radar, is the channel capacity for a given bandwidth and signaling format. Channel capacity is the theoretical maximum possible number of bits per second of information that can be conveyed through one or more links in an area. According to the Shannon-Hartley theorem, channel capacity of a properly encoded signal is proportional to the bandwidth of the channel and to the logarithm of signal to noise ratio (SNR)—assuming the noise is additive white Gaussian noise (AWGN). Thus channel capacity increases linearly by increasing bandwidth of the channel to the maximum value available, or equivalently in a fixed channel bandwidth by increasing the signal power exponentially. By virtue of the huge bandwidths inherent to UWB systems, the huge channel capacities can be achieved without invoking higher order modulations that need very high SNR to operate.
Ideally, the receiver signal detector should be matched to the transmitted signal in bandwidth, signal shape and time. Any mismatch results in loss of margin for the UWB radio link.
Channelization (sharing the channel with other links) is a complex problem subject to many practical variables. Typically two UWB links can share the same spectrum by using orthogonal time-hopping codes for pulse-position (time-modulated) systems, or orthogonal pulses and orthogonal codes for fast-pulse based systems.
Current forward error correction (FEC) technology; as demonstrated recently in some very high data rate UWB pulsed systems, like LDPC (Low Density Parity Coding), perhaps in combination with Reed-Solomon codes, can provide channel performance approaching the Shannon limit. When stealth is required, some UWB formats (mainly pulse-based) can fairly easily be made to look like nothing more than a slight rise in background noise to any receiver that is unaware of the signal’s complex pattern.
Multipath (distortion of a signal because it takes many different paths to the receiver) is an enemy of narrow-band radio--it causes fading where wave interference is destructive. Some UWB systems use "rake" receiver techniques to recover multipath generated copies of the original pulse to improve performance on receiver. Other UWB systems use channel equalization techniques to achieve the same purpose. Narrowband receivers can use similar techniques, but are limited due to the poorer resolution capabilities of narrowband systems.
There has been much concern over the interference of narrow band signals and UWB signals that share the same spectrum; traditionally the only radio technology that operated using pulses was spark gap transmitters; which were banned due to excessive interference. However, UWB is much lower power. The subject was extensively covered in the proceedings that led to the adoption of the FCC rules in the US, and also in the 6 meetings relating to UWB of the ITU-R that led to the ITU-R Report and Recommendations on UWB technology. In particular, many common pieces of equipment emit impulsive noise (notably hair dryers) and the argument was successfully made that the noise floor would not be raised excessively by wider deployment of wideband transmitters of low power.
Applications
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Due to the extremely low emission levels currently allowed by regulatory agencies, UWB systems tend to be short-range and indoors. However, due to the short duration of the UWB pulses, it is easier to engineer extremely high data rates, and data rate can be readily traded for range by simply aggregating pulse energy per data bit using either simple integration or by coding techniques. Conventional OFDM technology can also be used subject to the minimum bandwidth requirement of the regulations. High data rate UWB can enable wireless monitors, the efficient transfer of data from digital camcorders, wireless printing of digital pictures from a camera without the need for an intervening personal computer, and the transfer of files among cell phone handsets and other handheld devices like personal digital audio and video players.
UWB is used as a part of location systems and real time location systems. The precision capabilities combined with the very low power makes it ideal for certain radio frequency sensitive environments such as hospitals and healthcare. Another benefit of UWB is the short broadcast time which enables implementers of the technology to install orders of magnitude more transmitter tags in an environment relative to competitive technologies. USA based Parco Merged Media Corporation was the first systems developer to deploy a commercial version of this system in a Washington, DC hospital.
UWB is also used in "see-through-the-wall" precision radar imaging technology, precision positioning and tracking (using distance measurements between radios), and precision time-of-arrival-based localization approaches. It exhibits excellent efficiency with a spatial capacity of approximately 10,000,000,000,000 bit/s/m².
UWB is a possible technology for use in personal area networks and appears in IEEE 802.15.3a draft PAN standard.
From Wikipedia, the free encyclopedia
Tuesday, August 28, 2007
Ultra-wideband
Organic light-emitting diode
An organic light-emitting diode (OLED) is any light-emitting diode (LED) whose emissive electroluminescent layer comprises a film of organic compounds. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited. They are deposited in rows and columns onto a flat carrier by a simple "printing" process. The resulting matrix of pixels can emit light of different colors.
Such systems can be used in television screens, computer displays, portable system screens, advertising, information and indication. OLEDs can also be used in light sources for general space illumination, and large-area light-emitting elements. OLEDs typically emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point-light sources.
A great benefit of OLED displays over traditional liquid crystal displays (LCDs) is that OLEDs do not require a backlight to function. Thus they draw far less power and, when powered from a battery, can operate longer on the same charge. OLED-based display devices also can be more effectively manufactured than LCDs and plasma displays. But degradation of OLED materials has limited the use of these materials. See Drawbacks.
OLED technology was also called Organic Electro-Luminescence (OEL), before the term "OLED" became standard.
History
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Bernanose and co-workers first produced electroluminescence in organic materials by applying a high-voltage alternating current (AC) field to crystalline thin films of acridine orange and quinacrine. In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doped anthracene.
The low electrical conductivity of such materials limited light output until more conductive organic materials became available, especially the polyacetylene, polypyrrole, and polyaniline "Blacks". In a 1963 series of papers, Weiss et al. first reported high conductivity in iodine-"doped" oxidized polypyrrole.They achieved a conductivity of 1 S/cm. Unfortunately, this discovery was "lost", as was a 1974 report of a melanin-based bistable switch with a high conductivity "ON" state. This material emitted a flash of light when it switched.
In a subsequent 1977 paper, Shirakawa et al. reported high conductivity in similarly oxidized and iodine-doped polyacetylene. Heeger, MacDiarmid & Shirakawa received the 2000 Nobel Prize in Chemistry for "The discovery and development of conductive organic polymers". The Nobel citation made no reference to the earlier discoveries.
Modern work with electroluminescence in such polymers culminated with Burroughs et al. 1990 paper in the journal Nature reporting a very-high-efficiency green-light-emitting polymer. The OLED timeline since 1996 is well documented on oled-info.com site.
Related technologies
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Small molecules
OLED technology was first developed at Eastman Kodak Company by Dr. Ching Tang using Small-molecules. The production of small-molecule displays requires vacuum deposition, which makes the production process more expensive than other processing techniques (see below). Since this is typically carried out on glass substrates, these displays are also not flexible, though this limitation is not inherent to small-molecule organic materials. The term OLED traditionally refers to this type of device, though some are using the term SM-OLED.
Molecules commonly used in OLEDs include organo-metallic chelates (for example Alq3, used in the first organic light-emitting device) and conjugated dendrimers.
Recently a hybrid light-emitting layer has been developed that uses nonconductive polymers doped with light-emitting, conductive molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longevity that they have in the SM-OLEDs.
PLED
Polymer light-emitting diodes (PLED) involve an electroluminescent conductive polymer that emits light when subjected to an electric current. Developed by Cambridge Display Technology, they are also known as Light-Emitting Polymers (LEP). They are used as a thin film for full-spectrum color displays and require a relatively small amount of power for the light produced. No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial inkjet printing. The substrate used can be flexible, such as PET. Thus, flexible PLED displays may be produced inexpensively.
Typical polymers used in PLED displays include derivatives of poly(p-phenylene vinylene) and poly(fluorene). Substitution of side chains onto the polymer backbone may determine the color of emitted light or the stability and solubility of the polymer for performance and ease of processing.
TOLED
Transparent organic light-emitting device (TOLED) uses a proprietary transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent). TOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight.
SOLED
Stacked OLED (SOLED) uses a novel pixel architecture that is based on stacking the red, green, and blue subpixels on top of one another instead of next to one another as is commonly done in CRTs and LCDs. This improves display resolution up to threefold and enhances full-color quality. 1439
Working principle
An OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are made of special organic polymer molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors.
A voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode. Thus, the cathode gives electrons to the emissive layer and the anode withdraws electrons from the conductive layer; in other words, the anode gives electron holes to the conductive layer.
Soon, the emissive layer becomes negatively charged, while the conductive layer becomes rich in positively charged holes. Electrostatic forces bring the electrons and the holes towards each other and recombine. This happens closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons, (unlike in inorganic semiconductors). The recombination causes a drop in the energy levels of electrons, accompanied by an emission of radiation whose frequency is in the visible region. That is why this layer is called emissive.
The device does not work when the anode is put at a negative potential with respect to the cathode. In this condition, holes move to the anode and electrons to the cathode, so they are moving away from each other and do not recombine.
Indium tin oxide is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer. Metals such as aluminium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the polymer layer.
Advantages
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The radically different manufacturing process of OLEDs lends itself to many advantages over flat-panel displays made with LCD technology. Since OLEDs can be printed onto any suitable substrate using inkjet printer or even screen printing technologies, they can theoretically have a significantly lower cost than LCDs or plasma displays. Printing OLEDs onto flexible substrates opens the door to new applications such as roll-up displays and displays embedded in clothing.
OLEDs enable a greater range of colors, brightness, and viewing angle than LCDs, because OLED pixels directly emit light. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an "off" OLED element produces no light and consumes no power. Energy is also wasted in LCDs because they require polarizers which filter out about half of the light emitted by the backlight. Additionally, color filters in color LCDs filter out two-thirds of the light.
OLEDs also have a faster response time than standard LCD screens. Whereas a standard LCD currently has around 8 millisecond response time(though can be much lower such as 2 miliseconds), an OLED can have less than 0.01ms response time.
Commercial uses
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OLED technology is used in commercial applications such as small screens for mobile phones and portable digital audio players (MP3 players), car radios, digital cameras and high-resolution microdisplays for head-mounted displays. Such portable applications favor the high light output of OLEDs for readability in sunlight, and their low power drain. Portable displays are also used intermittently, so the lower lifespan of OLEDs is less important here. Prototypes have been made of flexible and rollable displays which use OLED's unique characteristics. OLEDs have been used in most Motorola and Samsung color cell phones, as well as some Sony Ericsson phones, notably the Z610i, and some models of the Sony Walkman.
eMagin Corporation is the only manufacturer of active matrix OLED-on-silicon displays. These are currently being developed for the US military, the medical field and the future of entertainment where an individual can immerse themselves in a movie or a video game.
