By Richard A. Quinnell, Contributing Editor
After years of wrangling over modulation techniques and wrestling with regulatory issues, ultrawideband communications is finally reaching the market in significant ways. The technology serves as the physical layer for Certified Wireless USB, for ZigBee sensor and control networking, and soon the next-generation Bluetooth medium. Understanding the basis of ultrawideband communications and the issues that still remain will help in selecting the right application of this wireless communications scheme.
What is UWB?
Ultrawideband (UWB) communications exploits the following effect: short bursts of a radio signal result in the signal’s energy being spread over a wide spectrum. Whenever two time-varying signals are multiplied together, the result has the original frequency content of the two signals as well as frequencies that are the sums and the differences of the two original frequency sets. By modulating a gigahertz-range carrier wave with short pulses, on the order of a few carrier cycles, the output frequency spectrum will be spread over many hundreds of megahertz.
This spread spectrum has two attributes that benefit UWB communications. One is that the information bandwidth that the signal can carry is proportional to its modulation bandwidth, so spreading the signal spectrum increases its information-carrying capacity. The second is that spreading the signal’s energy over a wide spectrum results in low energy at any one frequency. This allows UWB communications to take place in occupied frequency bands without generating significant interference to the other signals. To prevent these other signals from interfering with UWB communications, the modulation schemes currently being offered use some form of frequency hopping of the main carrier to "dance around" any interferers.
The fact that UWB signals produce very little power at any given wavelength has been a key to the technology's success in passing regulatory hurdles. Current implementations for most UWB communications target two frequency bands, shown in (Figure 1), where governmental bodies have made spectrum available for UWB communications to share with other users on a not-to-interfere basis. The low-band region occupies 3 to 5 GHz and the high-band region occupies 6 to 9 GHz.
Unfortunately, the choice of carrier band has an impact on system performance. The higher the carrier frequency, the shorter the usable range for UWB communications. In the high-band region, performance is expected to drop by half when compared to low-band. Not all countries have embraced both bands, however. As (Figure 2) shows, the low-band region is fully approved only in the United States. Other world regions are temporarily allowing its use, but that permission is scheduled to expire during the next several years. Only the high-band region is available on a worldwide basis. As a result of these regulatory issues, the current crop of UWB radio devices, all of which utilizes the low-band region, will face a shrinking market.
There are two modulation schemes for UWB signals: direct sequence (DS-UWB) and orthogonal frequency division multiplexing (OFDM-UWB). For a time, the two were contenders for IEEE standardization as the sole UWB method, but supporters of the two approaches were unable to reach a compromise. The IEEE has since abandoned any attempt to create a single unified standard, and both modulation schemes are available in the UWB communications market.
Both approaches have advantages. The DS-UWB approach is simpler to implement. The scheme works on only two frequency bands, which simplifies the transmitter design. A given network will use one band or the other as needed to avoid interference from other networks. DS-UWB also employs a simpler modulation scheme. The data stream directly controls the transmitter, turning it on and off with binary phase shift keying (BPSK). First, however, the data stream is encoded into tokens, which are chosen to prevent repeating data patterns from focusing the signal’s power spectrum.
The OFDM-UWB approach is more elaborate, using frequency hopping over multiple channels to avoid interference as well as allowing the transmitter to eliminate specific channels from the hopping sequence when those channels are too noisy. This enables multiple networks to operate in close proximity with minimal interference, but the penalty is a more complex and costly transmitter. Proponents of DS-UWB also contend that it offers high performance in the presence of multi-path propagation and allows simple control of the signal spectrum through pulse shaping rather than channel manipulation.
Of the two, the OFDM modulation scheme is the best represented in the market. It has been chosen for the UWB radio layer in next-generation Bluetooth, Certified Wireless USB, and WiMedia networks. ZigBee and some proprietary wireless USB implementations use direct sequence.
In order to keep UWB signals below regulatory limits, their total broadcast power must be kept low. As a result, UWB signals typically have a limited range. The WiMedia specification for UWB communications, for instance, indicates an expected performance of 480 Mbits/second over a 3-meter range and 110 Mbits/second for a 10-meter range using low-band carriers. The short range is not a major concern for most intended UWB applications, however. When being used for networking, as in ZigBee, the network structure compensates for the limited range of any given link. The mesh may be small, but the net that it creates can be as large as needed. In other applications, UWB is being used as a cable replacement technology for PCs or consumer media devices. Few PC and consumer installations use cables longer than three meters, so the range of UWB does not compromise its utility.
Only the bandwidth of UWB implementations seems likely to limit the technology’s applicability. While 480 Mbits/second is high compared to current uses for UWB, customer demand for data bandwidth grows continuously. Fortunately, signal processing approaches are helping extend UWB performance. WiQuest Communications, for example, is able to achieve more than 1 Gbit/second using low band UWB. This is enough to support streaming compressed video, and the company is marketing its technology as a cable-replacement scheme for PC graphics and consumer multimedia players.
UWB communications is becoming a widely available technology in the United States, particularly in Certified Wireless USB. However, with other worldwide regulatory bodies forcing UWB to operate in the high-band spectrum, developers still face technology challenges if their products are to have a market outside of the U.S. Radios and antenna design are two areas that still need work. Neither enjoys the technical maturity of low-band radios.
Still, UWB has the potential of becoming a world-wide method for wireless cable replacement once radio units that will operate in the high-band regions essential to Asian and European markets begin appearing, which is expected to occur by 2008. It is only these physical layer issues that need addressing. All the other impediments to such connectivity have been addressed by connectivity standards such as WiMedia and ZigBee.
The right radio and modulation scheme for a given application will still be a choice that designers must make. Issues such as regulatory compliance, interoperability with other equipment, bandwidth, and range will dictate if DS or OFDM modulation, WiMedia or ZigBee networking protocols, or proprietary approaches are required. What is clear, however, is that the UWB approach is becoming a solid basis for short-range, high-bandwidth communications.
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