Narrow- and ultra-wideband (UWB) signals exhibit different characteristics when propagating through matter. UWB signals are characterized by a wide bandwidth and a relatively low center frequency. Narrow-band signals maintain a sinusoidal shape during propagation. UWB signals do not always behave likewise, which (combined with other special propagation and penetration characteristics) could be exploited for a number of challenging applications. However, few applications have been developed because of strict regulations by the Federal Communications Commission. We have carried out a series of experiments to determine the behavior of UWB signals and their properties.1
We used a commercial pulser (AVTECH Electrosystems Ltd.) to generate test signals, employing two identical transverse-electromagnetic (TEM) horn antennas (transmitter and receiver). We used a Tektronix digital-storage oscilloscope for signal acquisition. The propagation medium selected for testing was placed between the TEM antennas: see Figure 2(A). Propagation environments can place fundamental limitations on the performance of wireless-communications systems.2 This can be caused by either natural or man-made materials.3 Moreover, the signal may reach the receiver via multiple paths with different signal delays, producing a complex time-varying signal.2,4 To understand these characteristics, our initial experiments involved common materials such as a thick book, a wooden board, styrofoam, a thick brick, and a plexiglas® sheet. When excited with a nanopulse, the UWB antenna differentiates the input pulse several times. The received signals showed differing amplitudes, pulse widths, and rise-time dependence according to the medium's composition.
Figure 1. (A) Experimental framework for UWB propagation measurements. (B) The propagating solution is placed between two plexiglas boundary plates.
After this initial development phase, we performed experiments to test the effects of aqueous solutions of varying salt concentrations using nanopulses with 7V amplitude, 1ns pulse width, and 1kHz repetition rate. We placed the propagating solution within a set of plexiglas boundary plates: see Figure 2(B). We obtained measurements in a radio-frequency (RF)-shielded room (85dB attenuation at 10GHz) as well as in an open laboratory area. The RF-shielded room is highly reflective, causing many paths of propagation. We controlled the composition of the medium between the antennas by adjusting the concentration of saline solution. It had been shown previously that the relative position of the medium affects the shape of the received signal.5 If the medium is placed very close to the transmitting antenna, the signal reflects from the medium's surface and is superimposed on the radiated signal. On the other hand, if it is placed far from the transmitting antenna the amount of scattering is very small. Our setup considered these effects. To characterize changes in the signal, we adopted a new approach for pulse-width measurements in which PW-1, PW-2, and PW-3 refer to three different measurable characteristics (subpulsewidth definitions: see Figure 2). Our results show that an increase in the concentration of salt increases the signal's attenuation. Thus, the amplitude of the propagated signal and the concentration of the solution through which the signal propagates are inversely related. Further analysis reveals that PW-1, PW-2, and PW-3 increase with a corresponding increase in the salt concentration, similar to the pulse rise time. However, PW-1 does not show a significant increase compared to PW-2 and PW-3. The latter define the rise time of the propagated signal. In addition, the pulse width does not show significant sensitivity to different salt concentrations for molality of less than 0.9m, but increases above 1.0m. Propagation delay is measured as the time between 10 and 15% of the signal amplitude between the transmitting and receiving sides. Our measurements indicate that as the pulse travels through media of higher salt concentrations the propagation delay increases almost linearly.
Figure 2. The nanopulse is divided into three new pulse-width definitions to characterize propagation through media of differing molalities.
These results provide a basis for studying UWB-signal propagation in biological matter. This investigation also provides an initial framework for many potential followup lines of research. For instance, the new parameters enable analysis of environments that can place fundamental limitations on the performance of wireless-communication systems. Based on the observation that PW1 remains relatively constant for varying degrees of salt concentration while PW3 increases, it is instructive to consider the development of a generalization of this concept to see if PW-1 data can be used to detect and identify a particular type of material, i.e., a liquid with high solute concentration versus tissue type, bone materials, clothing, etc. The potential for system miniaturization can be pursued separately. For example, the design of compact microstrip-UWB antennas is presently under consideration at the Air Force Research Laboratory, while miniaturization of the other subsystems is also worth considering.
Louisiana Tech University
Rastko Selmic is an associate professor of electrical engineering. His current research interests include UWB-signal propagation, sensor networks, smart sensors and actuators, intelligent control, and fault detection in nonlinear systems.
Air Force Research Laboratory (AFRL)
Atindra Mitra is a radio-frequency (RF) systems engineer and the founding director of the AFRL RF Systems Integration Laboratory at the Institute for Development Commercialization of Advanced Sensor Technology. His current research interests include unmanned-aerial-vehicle applications and distributed radar systems, radar waveforms, signal/data-processor hardware design and architectures, and radar and sensor networks with adaptive and intelligent processing.