UWB allows a system to operate across a range of frequency bands, without
interfering with existing communication systems. This is because UWB uses very
low transmit power. UWB pulses are often measured in picoseconds. A picosecond
represents one trillionth of a second. UWB can still maintain a high data rate
because it operates in the time domain rather than in the frequency domain. UWB
signals consist of high-speed electromagnetic pulses, rather than sine waves.
This enables the waves to traverse many frequencies unimpeded and unnoticed.
Because of their extremely short duration, these UWB pulses function in
a continuous band of frequencies, which can span several gigahertz. As shown in
Figure
, the shorter the
pulse in time, the higher its center frequency, and the broader the spread of
its frequency spectrum.
Spatial Capacity
UWB is superior to
other short-range wireless schemes in other ways. The growing demand for
greater wireless data capacity and the crowding of RF spectra both favor
systems that offer high bit rates concentrated in smaller physical areas. This
metric is referred to as spatial capacity. Measured in kilobits per second per
square meter (Kbps/m2), spatial capacity is a gauge of data intensity, in much
the same way that lumens per square meter can be used to determine the
illumination intensity of a light fixture. As increasing numbers of broadband
users gather in crowded spaces such as airports, hotels, convention centers,
and workplaces, the most critical parameter of a wireless system will be its
spatial capacity. UWB technology excels in spatial capacity, as Figure
illustrates.
UWB Modulation – Radio with No Carrier
UWB
wireless is unlike familiar forms of radio communications, such as AM/FM,
police/fire radio, and television. These narrowband services, which avoid
interfering with each other by staying within the confines of their allocated
frequency bands, all use a carrier wave. Information is impressed on the
underlying carrier signal by somehow modulating its amplitude, frequency, or
phase. The information is extracted, or de-modulated, upon reception. This is
shown in Figure
.
UWB
technology is very unique. Rather than employing a carrier signal, UWB
emissions are composed of a series of intermittent pulses. By varying the
amplitude, polarity, timing, or other characteristics of individual pulses,
information is coded into the data stream. In a bipolar modulation scheme, a
digital one represents a positive, or rising pulse, while a zero represents an
inverted, or falling pulse. In amplitude modulation, full-amplitude pulses
represent ones, and half-amplitude pulses represent zeros. Pulse-position
modulation sends identical pulses but alters the transmission timing. Delayed
pulses indicate zeros. These modulation techniques are shown in Figure
.
Various
other terms have been used for the UWB transmission mode in the past, including
carrierless, baseband, and impulse-based.
Go Low and
Short
There is a recent trend toward sending lower-power signals over
shorter ranges. Before 1980, during the early days of radio telephony, a single
tower with a high-powered transmitter might cover an entire city. However,
because of limited spectrum availability, the single tower could not serve many
customers. As recently as 1976, radio telephone providers in New York City
could handle only 545 mobile telephone customers at a time. This is an
extremely small number by current standards. Cellular telephony was able to
accommodate a greater number of customers by drastically reducing both power
and distance. This allows the same spectrum to be reused many times within a
geographic area. Now, UWB is expected to do the same thing for WLANs.