2.5G and 3G Wireless Networks
Ultra Wideband Wireless
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Ultra Wideband Wireless Networks
- by Steve Steinke
A handful of press accounts describing a novel kind of wireless technology began appearing in the latter part of 2000. The most extreme claims about Ultra Wideband (UWB) technology were: that it could deliver hundreds of Mbits/sec of throughput; that its power requirements to link to destinations hundreds of feet away were as little as 1/1,000th that of competing technologies such as Bluetooth or 802.11b; that transceivers could be small enough to tag grocery items and small packages; and that traffic interception or even detecting operation of the devices would be practically impossible. A slightly different way to look at the difficulty of detection and interception would be to claim that UWB devices wouldn't interfere with other electromagnetic spectrum users.
While we're certainly years away from any significant deployment of UWB devices, each of the stupendous claims made for the technology has at least a modicum of supporting evidence.
UWB devices operate by modulating extremely short-duration pulses-pulses on the order of 0.5 nanoseconds. Though a system might employ millions of pulses each second, the short duration keeps the duty cycle low-perhaps 0.5 percent-compared to the near-100-percent duty cycle of spread spectrum devices. The low duty cycle of UWB devices is the key to their low power consumption. Intel Architecture Labs has calculated the comparative spatial throughput capacity of various technologies (see Figure 1); UWB clearly has by far the highest potential for this particular metric.
In principle, pulse-based transmission is much like the original spark-gap radio that Marconi demonstrated
across the Atlantic in 1901. Unlike most modern radio equipment, pulse-based signals don't modulate a fixed-frequency carrier. If you examine a carrier-based signal with a spectrum analyzer, you'll generally see a large component at the carrier frequency and smaller components at frequencies above and below the carrier frequency based on the modulation scheme (see Figure 2). Pulse-based systems show more or less evenly distributed energy across a broad range of frequencies-perhaps a range 2GHz or 3GHz wide for existing UWB gear. With low levels of energy across a broad frequency range, UWB signals are extremely difficult to distinguish from noise, particularly for ordinary narrowband receivers.
One significant additional advantage of short-duration pulses is that multipath distortion can be nearly eliminated. Multipath effects result from reflected signals that arrive at the receiver slightly out of phase with a direct signal, canceling or otherwise interfering with clean reception. (If you try to receive broadcast TV where there are tall buildings or hills for signals to bounce from, you've likely seen "ghost" images on your screen-the video version of multipath distortion.) The extremely short pulses of UWB systems can be filtered or ignored-they can readily be distinguished from unwanted multipath reflections.
Alternatively, detecting reflections of short pulses can serve as the foundation of a high-precision radar system. In fact, UWB technology has been deployed for 20 years or more in classified military and "spook" applications. The duration of a 0.5 nano- second pulse corresponds to a resolution of 15 centimeters, or about 6 inches; UWB-based radar has been used to detect collisions, "image" targets on the other side of walls, and search for land mines.
ULTRA WIDEBAND VS. SPREAD SPECTRUM
Spread spectrum technologies do artificially what UWB does naturally: array signals across a wide spectrum so that the power concentrated in any particular band is below the threshold where it would interfere with other users of that band, or even be
detectable by a narrowband receiver. Spread spectrum signals begin life as
ordinary narrowband modulated waveforms. Then a spreading function-direct sequence and frequency hopping are the two most common methods-rapidly cycles the original signal through multiple individual narrowband slots. Because spread spectrum signals are "artificially" spread, their duty cycles are close to 100 percent. UWB technology has it all over spread spectrum where transmission power consumption is concerned. Furthermore, spread spectrum devices require more complex electronics than UWB, first because UWB circuits can be fundamentally simpler than narrowband circuits, but also because spread spectrum requires additional components and processing operations with pseudo-random noise generators and the associated task of synchronization. This added complexity has power-usage, device-size, and cost repercussions, too.
As with any other technology, UWB technology's strong points determine its shortcomings. UWB emissions can potentially interfere with many other consumers of the electromagnetic spectrum. Users of Global Positioning Systems (GPSs), particularly those in the aviation industry who use GPS data for navigation and landing, have serious reservations about widespread mobile devices that introduce even very low levels of interference into the 1.2GHz and 1.5GHz bands. Sprint and Qualcomm, whose Code Division Multiple Access (CDMA) technology underlies Sprint's PCS systems, conducted studies that showed degradations of cell phone service in the presence of UWB devices. The National Association of Broadcasters opposed FCC approval for UWB out of concern for spectrum used by remote camera crews and for satellite-based content distribution.
While it's hardly surprising that current owners would resist even minuscule incursions into their spectrum, UWB technology doesn't provide a free-lunch windfall of hitherto unused spectrum. Its ready observability in the time domain doesn't carry over to the frequency domain, but that doesn't mean there's no impact on existing services whose frequency domain signatures are easier to observe. It's also possible for UWB vendors to filter specific frequencies that are important for public safety and other overriding concerns, though if the owners of all the relevant spectrum had to be filtered, there wouldn't be any left for UWB to operate in.
The FCC has issued a Notice of Proposed Rule Making with respect to UWB technology, with a decision expected before the end of 2001. UWB proponents have requested that their devices be governed by Part 15.209 rules, which set emission limits for such things as hair dryers and laptop computers. FCC approval would only be the beginning of a process of regulatory activity, however. Once UWB is on track with regulatory approval, it seems likely that there would also be a process of standardization aimed at minimizing interference with other technologies, among other concerns.
In many respects, the excitement over UWB technology turns out not to be unrealistic at all. It seems unlikely that unreasonable regulatory obstacles will impede further development, though it's dangerous to underestimate the amount of delay a standards committee can add to a technology introduction. And no matter what, Bluetooth and 802.11b devices will never be able to find studs in your walls, detect your cat's presence several rooms away, or prevent your SUV from colliding with a cement truck.