Browsing by Author "Ly, Hung Dinh"
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Item Detection And Opportunistic Spectrum Access In Sensor Networks(Electrical Engineering, 2007-09-17T17:07:28Z)This thesis examines target detection problems in Radar Sensor Networks (RSN) and opportunistic spectrum access problem in Cognitive Sensor Networks (CSN). First, studies on the Space-Time Adaptive Processing (STAP) and radar waveform design are provided. Investigation into the target detection performance gain of RSN when STAP and radar waveform design are combined in RSN is then performed. Studies in this thesis show that detection performance of RSN using our proposal is superior to that of a single radar system using STAP only. To further studies on target detection, the multi-target detection problem in RSN is also examined. Signal, interference, and noise at radar sensors are modeled and analyzed. At the clusterhead of RSN, a Maximum Likelihood Multi-Target Detection algorithm is proposed to estimate the possible number of targets in a surveillance area. Achieved results show that detection performance of RSN is much better than that of a single radar system in terms of the miss-detection probability and the root mean square error. Besides detection in RSN, this thesis studies an opportunistic spectrum access problem and proposes a spectrum access scheme in CSN. The spectrum access scheme is built using Fuzzy Logic System (FLS); and spectrum access decision is based on: (1) spectrum utilization efficiency of the secondary user (SU); (2) its degree of mobility; and (3) its average distance to primary users (PU). The output of the FLS provides the probabilities of accessing spectrum band for SUs and the SU with the highest probability will be assigned the available spectrum. Studies also show that our scheme performs much better than random access approach.Item Information-Theoretically Secure Communication Under Channel Uncertainty(2012-07-16) Ly, Hung DinhSecure communication under channel uncertainty is an important and challenging problem in physical-layer security and cryptography. In this dissertation, we take a fundamental information-theoretic view at three concrete settings and use them to shed insight into efficient secure communication techniques for different scenarios under channel uncertainty. First, a multi-input multi-output (MIMO) Gaussian broadcast channel with two receivers and two messages: a common message intended for both receivers (i.e., channel uncertainty for decoding the common message at the receivers) and a confidential message intended for one of the receivers but needing to be kept asymptotically perfectly secret from the other is considered. A matrix characterization of the secrecy capacity region is established via a channel-enhancement argument and an extremal entropy inequality previously established for characterizing the capacity region of a degraded compound MIMO Gaussian broadcast channel. Second, a multilevel security wiretap channel where there is one possible realization for the legitimate receiver channel but multiple possible realizations for the eavesdropper channel (i.e., channel uncertainty at the eavesdropper) is considered. A coding scheme is designed such that the number of secure bits delivered to the legitimate receiver depends on the actual realization of the eavesdropper channel. More specifically, when the eavesdropper channel realization is weak, all bits delivered to the legitimate receiver need to be secure. In addition, when the eavesdropper channel realization is strong, a prescribed part of the bits needs to remain secure. We call such codes security embedding codes, referring to the fact that high-security bits are now embedded into the low-security ones. We show that the key to achieving efficient security embedding is to jointly encode the low-security and high-security bits. In particular, the low-security bits can be used as (part of) the transmitter randomness to protect the high-security ones. Finally, motivated by the recent interest in building secure, robust and efficient distributed information storage systems, the problem of secure symmetrical multilevel diversity coding (S-SMDC) is considered. This is a setting where there are channel uncertainties at both the legitimate receiver and the eavesdropper. The problem of encoding individual sources is first studied. A precise characterization of the entire admissible rate region is established via a connection to the problem of secure coding over a three-layer wiretap network and utilizing some basic polyhedral structure of the admissible rate region. Building on this result, it is then shown that the simple coding strategy of separately encoding individual sources at the encoders can achieve the minimum sum rate for the general S-SMDC problem.