On Quantifying Covertness of Ultra-Wideband Impulse Radio
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Date
2002-08-26
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Abstract
Modern tactical communication networks require robust, rapidly deployable and covert systems. These networks should be inherently adaptive and easily reconfigured. Impulse Radio (IR) is a time-hopping ultra wideband CDMA communication system that possess unique characteristics which make it a promising candidate for future tactical military radio networks. IR makes a good candidate because of its covertness, low power spectral density and relative immunity to multipath fading.
Beyond qualitative assertions about the performance and covertness of impulse radio, there has not been a thorough quantitative evaluation of the covertness of IR. Thus there is a need for an unclassified quantitative method for defining Low Probability of Detection (LPD) characteristics of a system.
In this thesis, we compare the performance of impulse radio with DS CDMA with and without severe local interferers. Cellular systems are typically narrowband and thus the DS CDMA system was modified so that the spectral characteristics of the chip sequences match the monopulses of impulse radio. Results showed that the performance of impulse radio was better than that of DS CDMA for single and multiple users with and without severe local interferers.
We developed a sophisticated multi-radiometer system ideal for detecting the information carrying pulses of impulse radio. A covertness metric (Signal power-to-Noise spectral density) was defined and the detector was used to quantify the covertness of impulse radio. Since the system parameters of impulse radio may be unknown to an interceptor, the covertness was calculated for the ideal case as well as varying amounts of prior knowledge. Single and multiple user cases with non-overlapping pulses were considered and average covertness was determined for the multiple user overlapping case. The covertness of impulse radio was compared with COTS systems. Results showed that impulse radio demonstrated good covertness even when an ideal multi-radiometer detector was used. Covertness of impulse radio was much better than conventional communication systems like IS-95 and wideband CDMA.
An effective network design is essential for taking advantage of the gains of impulse radio. We evaluate the performance and covertness of a peer-to-peer topology with distributed closed loop power control. Saturated links which do not converge to a steady state even at maximum transit powers were eliminated by link level monitoring. Random topologies were generated and capacity bounds were determined for networks in a simulated area. An acceptable covertness measure was defined and covertness was calculated by network simulations. We found that covertness degrades when link lengths increase due to user mobility. Optimal link lengths for a given number of users and simulated area were specified. The peer-to-peer topology was found to have limitations because of inadequate network monitoring. Hence it is essential to develop a flexible hierarchical topology with network monitoring to maintain covertness in impulse radio networks.
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Keywords
radiometer, covertness, impulse radio, UWB
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Degree
MS
Discipline
Electrical Engineering
