Browsing by Author "Dr. Alexandra Duel-Hallen, Committee Member"

Now showing 1 - 3 of 3

On Locally Invertible Encoders and Multidimensional Convolutional Codes
(2006-08-10) Lobo, Ruben Gerald; Dr. Mladen A. Vouk, Committee Co-Chair; Dr. Donald L. Bitzer, Committee Co-Chair; Dr. Brian L. Hughes, Committee Member; Dr. Alexandra Duel-Hallen, Committee Member; Dr. Ernest Stitzinger, Committee Member
Multidimensional (m-D) convolutional codes generalize the well known notion of a 1-D convolutional code defined over a univariate polynomial ring with coefficients in a finite field to multivariate polynomial rings. The more complicated structure of a multivariate polynomial ring when compared to a univariate one, however, makes the generalization nontrivial. While 1-D convolutional codes have been thoroughly understood and have wide applications in communication systems, the theory of m-D convolutional codes is still in its infancy, and these codes lack unified notation and practical implementation. This dissertation develops a sequence space approach for realizing m-D convolutional codes. While most of the existing research is focused on algebraic aspects, fundamental issues regarding practical implementation that are well developed and fairly straightforward in the 1-D case have remained undefined for m-D convolutional codes. In this dissertation we address some of these issues. We define a new notion of sequence space ordering and show that certain multivariate polynomial matrices which we call as locally invertible encoders, when transformed to the sequence space domain, have an invertible subsequence map between their input and output sequences. This subsequence map has a well defined structure that allows for the explicit construction of locally invertible encoders by performing elementary operations on the ground field without the use of any polynomial operations. We use the invertible subsequence map to introduce a novel method to encode and invert multidimensional sequences. We show that locally invertible encoders have good structural properties which make them a natural choice to generate multidimensional convolutional codes.
On Quantifying Covertness of Ultra-Wideband Impulse Radio
(2002-08-26) Bharadwaj, Arjun; Dr. Keith Townsend, Committee Chair; Dr. Brian hughes, Committee Member; Dr. Alexandra Duel-Hallen, Committee Member
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.
Space-Time Coding for Large Antenna Arrays
(2006-03-14) Yu, Xinying; Dr. Brian L. Hughes, Committee Chair; Dr. Alexandra Duel-Hallen, Committee Member; Dr. Hamid Krim, Committee Member; Dr. Carl Meyer, Committee Member
Multiple-input multiple-output (MIMO) systems can greatly improve the capacity and performance of wireless communications. In particular, space-time coding techniques have received much attention in recent years as an efficient approach to achieving the performance gains offered by MIMO channels. Thus far, most work on space-time coding has focused on systems with small antenna arrays or high signal-to-noise ratios (SNRs), for which it has been shown that codes should be designed according to the rank and determinant criteria. For such scenarios, coherent space-time coding and differential space-time modulation (DSTM) schemes have been designed, for systems with or without channel knowledge at the receiver, respectively. In recent years, there has been some work on coherent space-time coding for large arrays, which indicates that the code design metric should be chosen diffently from that for small arrays. In this dissertation, we study the design of space-time coding for large arrays. We focus on three aspects: performance analysis, code construction and decoding algorithms. We first analyze the asymptotic performance of differential space-time modulation. A new upper bound on the pairwise-error probability is derived for large arrays. This bound suggests that Euclidean distance is an appropriate design criterion for DSTM with large numbers of antennas, which is similar to the design of coherent space-time coding for the large-array regime. For two transmit antennas and four or more receive antennas, we use the new design criterion to obtain several new unitary codes with large minimum Euclidean distance. The proposed codes outperform some existing codes, for example, the well-known Alamouti code, for large receive arrays. Although the codes designed according to the new design criterion achieve good performance, most of them require maximum-likelihood (ML) decoding, which is undesirable for high-rate codes. On the other hand, the Alamouti code, which is designed for high-SNR regime, enables simple linear ML decoding. It is of interest to design codes that perform well for large arrays, but which also allow simple decoding at the receiver. We first consider the design of unitary codes, for use with and without channel knowledge at the receiver. For two transmit antennas, we consider a structure which is a modification of the Alamouti code. We optimize the new code with respect to the Euclidean distance criterion. We then show that the new code allows us to use two suboptimal decoders that have complexity comparable to the Alamouti decoder. The analytical bit-error performance and the constellation-constrained capacity are derived for the suboptimal decoders. For coherent detection, the coding structure is extended to non-unitary constellations. We also extend the new code to more than two transmit antennas. Conventional DSTM assumes that the channel remains constant for two adjacent transmission blocks, which is questionable for some time-varying channels. In this dissertation, we investigate the performance of the new code when fast-fading is encountered. We show that multiple-symbol decision-feedback differential detection (DFDD) can be used to reduce the performance degradation of the new code in fast-fading channels. We also consider the use of suboptimal decoders in DFDD to further reduce the decoding complexity.