Introduction into UWB technology
The vast majority of people even "advanced" in technique practically heard nothing about such field of radio-engineering as Ultra-Wideband (UWB) technology. And no wonder - such systems you will not buy neither in a shop no in radio markets, it is difficult to find them even in the Internet. Much of it is resulted from the fact that only in recent years the hardware components have appeared which able to demonstrate all its possibilities. And also their cardinal difference in traditional radio-engineering systems is of great importance, and all new always hardly fight way in life. Therefore all those who are interested to know about new properties of UWB systems, should be aquatinted with bases of this technology.
The majority of traditional radio-engineering systems works in rather narrow band and used as carrier wave for data transmitting harmonic (sine wave) signals. However the frequency bandwidth determinate the information content of radio-engineering systems, and to increase the information ability of the system it is necessary to widen its waveband.
In connection with fast information system development and constant increase in information streams, this problem becomes more and more actual both for a radio communication, and for radiolocation. The relevance of the problem was the reason of fast development in recent years the technologies using ultra-wideband (UWB) signals.
According to definition of UWB signals, introduced by DARPA experts of the United States Department of Defense and specified by the Federal commission on communication (FCC) of the USA, signal bandwidth should be more than 25 % from the central frequency. And though this definition doesn't cover all variety of ultra-wideband systems and signals, it is used now by the majority of developers.
The problem of changing over to UWB signals is especially relevant for radiolocation systems. Usual radars have used a band of frequencies that does not exceed 10 percent of the carrier frequency, allow only to develop a target (with relatively low accuracy), but don't allow to receive its image. To increase the radar information content, the target recognition mode is used, which allows to receive additional information about the target using after appropriate processing some its indications ("radioportrait"). This mode requires significant increase in the bandwidth of the radar, and as consequence, new approaches both in methods and technologies.
Increasing of information content of ultra-wideband radar, takes place because of smaller pulse volume signal for range detection. For example, when the length of a sounding pulse changes from 1 µs to 1 ns, the depth of the pulse volume decreases from 300 m to 30 cm. Thus, radar instrument probing the surveillance space becomes finer and much more sensitive. And bandwidth of UWB signal reaches a few gigahertz.
As a result of reducing of the above impulse volume, UWB radars gain a number of new properties:
• improved detection target range measurement accuracy as well as target range resolution;
• decreasing the radar “dead zone”;
• identification of target class and type takes place and as a result we have a "radioportrait" of the target, because the reflected signal carries information not only about the target as a whole, but also about its separate elements;
• increased radar immunity to passive interference such as rain, mist, underlying surface, aerosols, metal-plated strips etc. This is because scattering cross section. (SCS) of interference source within f small pulse volume becomes comparable with SCS of the target;
• Improve the radar’s immunity to external narrowband electromagnetic radiation effects and noise;
• increased the probability of target detection and improved stability observing a target at the expense of elimination of the lobe structure of the secondary-radiation pattern of irradiated targets, since oscillations reflected from the individual parts of the target do not interfere and cancel, which provides a more uniform radar cross section;
• improved stability when observing targets at low elevation angles at the expense of eliminating the interference gaps in the antenna pattern. This is because the main signal, and any ground return signal, arrive at the antenna at different times, which thus enables their selection;
• changing of the radiated signal characteristics (the width and shape of antenna patter) by changing its settings, that resulted an opportunity to get ultra-narrow antenna pattern;
• increased the radar’s secretiveness by using a signal that will be hard to detect.
Broad and widespread of UWB signals also requires a radically new approach to the functional design, technical, technological and methodological solutions for critical subsystems.
The revolutionary achievements of recent years in technological complex for creation devices for signal-generating, signal-emittance, receiving and processing of signals based on advanced methods and solutions create the possibility to implement systems with UWB signals that previously was attainable only in certain laboratory and experimental designs. The following technologies have appeared which are basic for UWB radio-electronics:
• technology of generation ultrashort pulse (duration 1 ns and shorter) with almost unlimited resource with high stability and great recurrent frequency;
• technology of emitting of such impulses directly into space (UWB antenna technique);
• technology of high-speed digital processing of mass data (computer engineering);
In this regard, usage of UWB signals in radiolocation resulted in independent research and development field, using their own methods of theoretical analysis and nonconventional hard-ware solutions.
Interest in UWB radars began to grow rapidly in the 80’s. The first field of their application should be detection of low-observable targets, where as it was expected they have remarkable advantage over conventional narrowband radars. However, final clarification in solving this problem hasn’t been found yet. Never the less usage of UWB radars in this field remains relevant.
The second field of UWB radars usage is detecting and tracking objects over short distances such as several meters or dozens of meters. Those were radars that detect objects in solid mediums (soil, ice) and radars that detect objects in the air. The practical need for this class of radar is very big. So, large number of acting military and industrial radars was created, solving a wide variety of tasks. Radars working on short distance are simple in design and also small in dimensions and weight. This allows to execute their development and improvement in a very short time and quickly to respond to market needs.
The third field of UWB radars usage is receiving of radio images of objects as a result of remarkable increasing in getting information about the target both in volume and in its quality. Such radars are also will be widely used, but in the long-term. Today UWB signals are used to receive radio images only in Synthetic Aperture Radar (SAR) airborne mounted. These radars as a rule are used for mapping terrain and to search various objects on the ground, hidden by vegetation or disguised in some other way.
The fourth field of UWB radars usage is control of water areas, airports, forests, territories of different purpose. Such radars occupy an intermediate position between short-range radars and long-range ones. They provide not only high resolution of targets, but also greater stability against passive and active interference.
Not less relevant usage of UWB signals in radio communication that makes it possible to eliminate disadvantages of the communications systems of multipath wave propagation.
Over the last years all over the world one can see fast growing interest in UWB technology in general and in UWB radars in particular. Today in a number of leading countries of the world accumulated extensive resources in the field related to UWB technology. It may be said without exaggeration that an invisible race in the field of UWB technologies has began. And those who will reach success, has an opportunity to acquire a significant share of this promising market of high technologies.