Ultra-wideband Wireless
UWB specifics

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.