Wireless designers now have yet another option to add to their bag of tricks. Called ultrawideband (UWB), the basic technology has actually been around as long as wireless. Marconi's original spark transmissions and all early wireless telegraphy were UWB. Such wideband transmissions were banned in the 1920s but rediscovered in the 1960s as new methods of secure radar and radio.
Out of the early days—where UWB was called impulse, baseband, or carrierless radio—has emerged the most modern version. Thanks to semiconductor manufacturing advances and the Federal Communications Commission's (FCC) recent approval, practical UWB circuits and products are becoming available.
Most conventional radio transmissions use the data signal to modulate a sinewave carrier on some specific frequency by the FCC. The usual goal is to select the most spectrally efficient form of modulation to get the maximum data rate through the usually restricted assigned bandwidth channel (see "How UWB Works," below).
In UWB, the serial data is translated into very short pulses of a unique shape, then applied directly to an antenna. Fourier fans know that very short pulses, regardless of their specific shape, produce an extremely wide bandwidth signal. One definition of UWB is that the signal has a bandwidth of at least 25% of the center pulse frequency, or 1.5 GHz, whichever is larger. Figure 1 compares several conventional spectrum uses to UWB. Because of the wide bandwidth needed by UWB and the potential for interference with other services, the FCC has placed UWB in the 3.1- to 10.6-GHz range.
A UWB signal starts as a high-speed rectangular pulse train that is shaped into unique pulses called monocycles (Fig. 2). These aren't one cycle of a sinewave, but instead pulses derived with a Gaussian filter. The desired transmission rate determines the pulse frequency, and the duty cycle is always very low—several percent or less. The pulse width sets the center frequency of the signal bandwidth. This center frequency fC is roughly the reciprocal of the pulse width 1/tW. A 200-pS pulse will produce a center frequency of 1/200 * 1012 = 5 GHz. The general rule of thumb for a 3-dB bandwidth is 1.16 times the center frequency, or in this example, 5.8 GHz.
This technique spreads the signal so that it overlays any other signals in its bandwidth. But the key to the technique is its very low power level, which makes it appear as noise to most other narrowband or spread-spectrum equipment. The low power level also severely limits the range of the signal to the general vicinity of the transmitter.
To transmit data from one point to another, you must modulate the pulses. UWB has two common types of data modulation: pulse-position modulation (PPM) and binary phase-shift keying (BPSK) (Fig. 3). Biphase is the easiest to implement and gives the best spectral efficiency. On-off keying (OOK) and pulse amplitude modulation (PAM), which are simpler for many applications, can be used as well. Multilevel PAM offers the possibility of increasing data speed for a given bandwidth and range.
If the application calls for sharing the spectrum with many users, multiple access methods are used. For instance, the data to be transmitted can be combined with a pseudorandom code as in spread-spectrum communications to permit channelization of the bandwidth. The unique codes allow individual signals to be identified and recovered at the receiver. The key to successful UWB is coding optimized for the application. Then the encoded serial data is sent to the pulse-forming circuit where the monocycles are generated.
Today, CMOS ICs can easily create such pulses very inexpensively at low power. The pulse is usually initiated by a narrow rectangular pulse from a CMOS logic circuit that drives some type of pulse-forming network that shapes the rectangular pulses into the desirable Gaussian monocycles de-scribed earlier.
Because the power levels permitted are so low (50 to 200 µW), no power amplification is needed. As the pulse radiates, it's naturally differentiated on the way to the receiver. The received pulse looks something like Figure 4. At the receiver, a broadband, low-noise amplifier (LNA) increases the signal level, after which a multiplier and integrator autocorrelate to recover the data.
A major challenge in UWB systems is the antenna. A typical wireless antenna is a resonant l/2- or l/4-wavelength device with a relatively narrow bandwidth. By using thicker or physically expanded elements, the bandwidth can be widened. But in UWB, the antenna needs even greater bandwidth.
Wideband antennas have been developed for government and military use. Some are just beginning to reach the commercial market, like antenna maker SkyCross' meanderline antenna (MLA) technology. According to CEO Alan Haase, SkyCross has a new product designed for the forthcoming UWB applications in the 3.1- to 10.6-GHz range. Frank Caimi, the CTO, indicates that this new antenna maintains a VSWR of 2:1 or less and has a linear phase response across the entire band.
Regulatory Issues: UWB has been used in government and military applications for years. It hasn't generally been approved for commercial use. The primary exception is approval given to companies making ground-penetrating radar devices that provide "x-ray vision" to public service entities for detecting flaws in roads or bridges, or for search and rescue. The FCC offered special licenses to companies making these devices, which are still widely used.
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