03-12-2012, 06:40 PM
Ultra-wideband
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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.[4] Such a pulse-based UWB method (using bursts of pulses) is the basis of the IEEE 802.15.3a draft standard and working group, which has proposed UWB as an alternative PHY layer.[5]
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.[citation needed]
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 the multipath fading of narrowband signals does not exist. However, there is still multipath propagation and inter-pulse interference to fast-pulse systems which must be mitigated by coding techniques.[citation needed]
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 which may be conveyed 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 technology (as demonstrated in high-data-rate UWB pulse systems such as low density parity check code) can—perhaps in combination with Reed–Solomon error correction—provide channel performance approaching the Shannon limit. 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.[3]
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.[citation needed] 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.