The Promise of Ultra-WidebandJune 1, 2005 By: Eric Holland Sensors
The recent buzz about ultra-wideband (UWB) has spurred reactions ranging from fear and uncertainty about interference problems, to excitement about the possibility of high-speed wireless connectivity for the desktop. For most sensor applications, the gigabit promises of UWB offer a questionable benefit for devices that typically generate small amounts of data. However, UWB offers other, less obvious advantages, such as precise ranging and operation in hostile RF environments. Although this technology looks promising, hurdles remain for quelling interference and developing a communications standard for UWB devices.
The Current State of UWB
Ultra-wideband involves modulation techniques to spread a signal over a bandwidth >500 MHz. This is in contrast to conventional modulation formats that typically have modulated bandwidths of no more than a few megahertz. Since spectrum congestion is a problem, where could such a wide signal be used without interfering with existing systems? In 2002 the FCC allocated limited use of spectrum between 3.1 GHz and 10.6 GHz for UWB applications. To address interference issues UWB devices are limited to very low peak transmit power levels. Between 3.1 GHz and 10.6 GHz, UWB device limits are the same as those for devices classified as unintentional radiators, a term used to describe devices that weren't designed to be transmitting radios but may yet radiate EMI.
Concern still exists: Will the proliferation of UWB devices interfere with existing communication systems such as GPS, PCS, and Nexrad, as well as sensitive aviation guidance systems? The ability of UWB to coexist with these systems, GPS in particular, has been the topic of several recent studies (www.darpa.mil/ato/programs/netex.htm). Meanwhile, competing standards proposals and manufacturers using different approaches to UWB signal generation are fighting it out in studies of interference characteristics and how different UWB methods perform. It's also uncertain what the FCC's final ruling will be with regard to UWB.
Current UWB radios typically use one of two methods to generate the signal: pulse-based and modulated carrier. In pulse-based radios, the transmitter generates small-duration, high-energy pulses which are then modulated with data by varying either the amplitude or the positioning of the pulses. Modulated carrier-based systems use conventional modulation techniques and radio architectures, albeit with much larger modulated bandwidth. The two proposed techniques in this category are direct-sequence spread-spectrum (DS-UWB) and multiband orthogonal frequency division multiplexing (MB-OFDM).
Pulse-based UWB radios could be considered the classical approach to this technology, which was once termed "impulse" or "carrier-free" radio. A primary advantage to pulse-based systems is the simplicity of transmitter design; it's easy to design pulse transmitters to consume very little power and to use very little circuitry. A challenge of the design is to efficiently use the spectrum while obeying FCC limits. The spectral content of pulse waveforms is highly dependent on the shape of the pulse generated. In a simple pulse generation circuit, the output power is concentrated in the center of the desired band, rolling slowly off at the edges, as shown in Figure 1. In order to fit the signal within the FCC spectral mask (shown in black) the edges of the band contain little power, leaving much of the available spectrum unused. Using more advanced pulse shaping can overcome this, but then the advantage of a simple transmitter is lost.
Figure 1. This figure shows the spectral mask of a simple pulse-based UWB transmitter (shown in red) compared to the spectral mask allowed by the FCC (shown in black). Most of the energy is concentrated in the center of the band, and this means the available spectrum at the edges of the band is not efficiently used.
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