Ultra-wideband

Ultra-wideband (also known as UWB, ultra-wide band and ultraband) is a radio technology pioneered by Robert A. Scholtz and others that can use a very low energy level for short-range, high-bandwidth communications over a large portion of the radio spectrum.^[1] UWB has traditional applications in non-cooperative radar imaging. Most recent applications target sensor data collection, precision locating and tracking applications.^[2]

Unlike spread spectrum, UWB transmits in a manner that does not interfere with conventional narrowband and carrier wave transmission in the same frequency band.

Ultra-wideband is a technology for transmitting information spread over a large bandwidth (>500 MHz); this should, in theory and under the right circumstances, be able to share spectrum with other users. Regulatory settings by the Federal Communications Commission (FCC) in the United States intend to provide an efficient use of radio bandwidth while enabling high-data-rate personal area network (PAN) wireless connectivity; longer-range, low-data-rate applications; and radar and imaging systems.

Ultra wideband was formerly known as pulse radio, but the FCC and the International Telecommunication Union Radiocommunication Sector (ITU-R) currently define UWB as an antenna transmission for which emitted signal bandwidth exceeds the lesser of 500 MHz or 20% of fractional bandwidth. Thus, pulse-based systems—where each transmitted pulse occupies the UWB bandwidth (or an aggregate of at least 500 MHz of narrow-band carrier; for example, orthogonal frequency-division multiplexing (OFDM)—can access the UWB spectrum under the rules. Pulse repetition rates may be either low or very high. Pulse-based UWB 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 one to two gigapulses 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 intersymbol interference), unlike carrier-based systems, which are subject to deep fading and intersymbol interference.

Theory

A significant difference between conventional radio transmissions and UWB is that conventional systems transmit information by varying the power level, frequency, and/or phase of a sinusoidal wave. UWB transmissions transmit information by generating radio energy at specific time intervals and occupying a large bandwidth, thus enabling pulse-position or time modulation. The information can also be modulated on UWB signals (pulses) by encoding the polarity of the pulse, its amplitude and/or by using orthogonal pulses. UWB pulses can be sent sporadically at relatively low pulse rates to support time or 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 gigapulses 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.^[3]

A valuable aspect of UWB technology is the ability for a UWB radio system to determine the "time of flight" of the transmission at various frequencies. This helps overcome multipath propagation, as at least some of the frequencies have a line-of-sight trajectory. With a cooperative symmetric two-way metering technique, distances can be measured to high resolution and accuracy by compensating for local clock drift and stochastic inaccuracy.^[4]

Another feature of pulse-based UWB is that the pulses are very short (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 there is no multipath fading of narrowband signals. However, there is still multipath propagation and inter-pulse interference to fast-pulse systems, which must be mitigated by coding techniques.

Technology

One performance measure of a radio in applications such as communication, locating, tracking and 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 a system can convey through one or more links in an area. According to the Shannon–Hartley theorem, the channel capacity of a properly encoded signal is proportional to the bandwidth of the channel and the logarithm of the signal-to-noise ratio (SNR) (assuming the noise is additive white Gaussian noise). Thus, channel capacity increases linearly by increasing the channel's bandwidth to the maximum value available, or (in a fixed-channel bandwidth) by increasing the signal power exponentially. By virtue of the large bandwidths inherent in UWB systems, large channel capacities could be achieved in principle (given sufficient SNR) without invoking higher-order modulations requiring a very high SNR. Ideally, the receiver signal detector should match the transmitted signal in bandwidth, signal shape and time. A mismatch results in loss of margin for the UWB radio link. Channelization (sharing the channel with other links) is a complex issue, subject to many variables. Two UWB links may 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.

Forward error correction – used in high-data-rate UWB pulse systems – can provide channel performance approaching the Shannon limit.^[5] OFDM receivers typically fix most errors with a low density parity check code inner code followed by some other outer code that fixes the occasional errors (the "error floor") that get past the LDPC correction inner code even at low bit-error rates. For example: The Reed-Solomon code with LDPC Coded Modulation (RS-LCM) adds a Reed–Solomon error correction outer code.^[6] The DVB-T2 standard and the DVB-C2 standard use a BCH code outer code to mop up residual errors after LDPC decoding.^[7] WiMedia over a UWB channel uses a Hybrid automatic repeat request: inner error correction using convolutional and Reed-Solomon coding, outer error correction using a frame check sequence that, when the check fails, triggers automatic repeat-request (ARQ).^[8]

When stealth is required, some UWB formats (mainly pulse-based) may be made to appear like a slight rise in background noise to any receiver unaware of the signal’s complex pattern.

Multipath interference (distortion of a signal because it takes many different paths to the receiver with various phase shift and various polarisation shift) is a problem in narrowband technology. It also affects UWB transmissions, but according to the Shannon-Hartley theorem and the variety of geometries applying to various frequencies the ability to compensate is enhanced. Multipath causes fading, and wave interference is destructive. Some UWB systems use "rake" receiver techniques to recover multipath-generated copies of the original pulse to improve a receiver's performance. Other UWB systems use channel-equalization techniques to achieve the same purpose. Narrowband receivers may use similar techniques, but are limited due to the different resolution capabilities of narrowband systems.

Antenna systems

• Distributed MIMO: To increase the transmission range, this system exploits distributed antennas among different nodes.
• Multiple-antenna: Multiple-antenna systems (such as MIMO) have been used to increase system throughput and reception reliability. Since UWB has almost impulse-like channel response, a combination of multiple antenna techniques is preferable as well. Coupling MIMO spatial multiplexing with UWB's high throughput gives the possibility of short-range networks with multi-gigabit rates.

Applications

Ultra-wideband characteristics are well-suited to short-distance applications, such as PC peripherals. Due to low emission levels permitted by regulatory agencies, UWB systems tend to be short-range indoor applications. Due to the short duration of UWB pulses, it is easier to engineer high data rates; data rate may be exchanged for range by aggregating pulse energy per data bit (with integration or coding techniques). Conventional orthogonal frequency-division multiplexing (OFDM) technology may also be used, subject to minimum-bandwidth requirements. High-data-rate UWB may enable wireless monitors, the efficient transfer of data from digital camcorders, wireless printing of digital pictures from a camera without the need for a personal computer and file transfers between cell-phone handsets and handheld devices such as portable media players.^[9] UWB is used for real-time location systems; its precision capabilities and low power make it well-suited for radio-frequency-sensitive environments, such as hospitals. Another feature of UWB is its short broadcast time.

Ultra-wideband is also used in "see-through-the-wall" precision radar-imaging technology,^[10][11][12] precision locating and tracking (using distance measurements between radios), and precision time-of-arrival-based localization approaches.^[13] It is efficient, with a spatial capacity of approximately 10¹³ bit/s/m². UWB radar has been proposed as the active sensor component in an Automatic Target Recognition application, designed to detect humans or objects that have fallen onto subway tracks.^[14]

UWB has been a proposed technology for use in personal area networks, and appeared in the IEEE 802.15.3a draft PAN standard. However, after several years of deadlock, the IEEE 802.15.3a task group^[15] was dissolved^[16] in 2006. The work was completed by the WiMedia Alliance and the USB Implementer Forum. Slow progress in UWB standards development, the cost of initial implementation, and performance significantly lower than initially expected are several reasons for the limited use of UWB in consumer products (which caused several UWB vendors to cease operations in 2008 and 2009).^[17]

Regulation

Ultra-wideband refers to radio technology with a bandwidth exceeding the lesser of 500 MHz or 20% of the arithmetic center frequency, according to the U.S. Federal Communications Commission (FCC). A February 14, 2002 FCC Report and Order^[18] authorized the unlicensed use of UWB in the frequency range from 3.1 to 10.6 GHz. The FCC power spectral density emission limit for UWB transmitters is −41.3 dBm/MHz. This limit also applies to unintentional emitters in the UWB band (the "Part 15" limit). However, the emission limit for UWB emitters may be significantly lower (as low as −75 dBm/MHz) in other segments of the spectrum.

Deliberations in the International Telecommunication Union Radiocommunication Sector (ITU-R) resulted in a Report and Recommendation on UWB in November 2005. UK regulator Ofcom announced a similar decision^[19] on 9 August 2007. More than four dozen devices have been certified under the FCC UWB rules, the vast majority of which are radar, imaging or locating systems.

There has been concern over interference between narrowband and UWB signals that share the same spectrum. Earlier, the only radio technology that used pulses were spark-gap transmitters, which international treaties banned because they interfere with medium-wave receivers. UWB, however, uses lower power. The subject was extensively covered in the proceedings that led to the adoption of the FCC rules in the U.S. and in the meetings relating to UWB of the ITU-R leading to its Report and Recommendations on UWB technology. Commonly used electrical appliances emit impulsive noise (for example, hair dryers) and proponents successfully argued that the noise floor would not be raised excessively by wider deployment of low power wideband transmitters.

China allowed 24 GHz UWB Automotive Short Range Radar in Nov 2012.^[20]

Technology groups

See also

References

External links

• IEEE 802.15.4a Includes a C-UWB physical layer, may be obtained from
• Standard ECMA-368 High Rate Ultra Wideband PHY and MAC Standard
• Standard ECMA-369 MAC-PHY Interface for ECMA-368
• Standard ISO/IEC 26907:2007
• Standard ISO/IEC 26908:2007
• ITU-R Recommendations – SM series See: RECOMMENDATION ITU R SM.1757 Impact of devices using ultra-wideband technology on systems operating within radiocommunication services.
• FCC (GPO) Title 47, Section 15 of the Code of Federal Regulations SubPart F: Ultra-wideband
• Use of MIMO techniques for UWB
• Numerous useful links and resources regarding Ultra-Wideband and UWB testbeds – WCSP Group – University of South Florida (USF)
• The Ultra-Wideband Radio Laboratory at the University of Southern California