With the increasing demand for access to the crowded radio frequency spectrum, cognitive radios have been suggested as one solution. Cognitive radios would be frequency agile and able to sense their radio environment and opportunistically use empty spectrum. Spectrum sensing, monitoring a given band of spectrum to see if it is occupied, is an essential part of a cognitive radio. The preferred method of spectrum sensing is the energy detector, which does not require any a priori information about the signal to be detected and is computationally simple to implement. Man-made noise, impulsive in nature, has also become more prevalent with the widespread use of electronic devices. In this thesis, we took measurements of man-made impulsive noise in the broadcast digital television bands to measure its presence, power, and spatial correlation. The effects of impulsive noise on the detection performance of an energy detector were analyzed. Lastly, a wideband RF receiver was designed, built, and tested on the Virginia Tech campus, which would be well suited both to spectrum sensing and taking measurements of impulsive noise. / Master of Science

Nowadays most of the communications systems are designed considering only to work under AWGN (Additive White Gaussian Noise). But the implementation of wireless systems in industrial facilities brings different kind of interference from machines or any other kind of electronic devices. Some of them are sources of randomly and high power noise, which commonly is known as impulsive noise. The objective in this thesis is to study the impact of the impulsive noise on a communication using QAM (Quadrature Amplitude Modulation) and PSK (Phase-Shift Keying) schemes, by observing the BER (Bit Error Rate) and the APD (Amplitude Probability Distribution). For that, it is developed a measurement method that will be used in a real industrial environment in future work.      The content of this thesis is divided in two parts. In the first part is made a program in MATLAB to simulate the communication through a noisy channel. Then is developed a measurement method which is tested in three different ways corresponding to 3 different outputs of an spectrum analyzer, namely, 20,4 MHz IF output, video output and IQ data output.      The relation of impulsive noise is presented in the second part with different statistical properties in the BER and the APD, in the setup with the best performance. At the end of the thesis a concluding section summarizes the results obtained during the work and some lines of future work in a real industrial environment with the developed method.

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This thesis addresses some problems that arise in signal processing when the noise is impulsive and follows a heavy tailed distribution. After reviewing several of the more well known heavy- tailed distributions the common problem of which of these hest models the observations is considered. To this end, a test is proposed for the symmetric alpha stable distribution. The test threshold is found using both asymptotic theory and parametric bootstrap resampling. In doing so, some modifications are proposed for Koutrouvelis' estimator of the symmetric alpha stable distributions parameters that improve performance. In electrical systems impulsive noise is generated externally to the receiver while thermal Gaussian noise is generated internally by the receiver electronics, the resultant noise is an additive combination of these two independent sources. A characteristic function domain estimator for the parameters of the resultant distribution is developed for the case when the impulsive noise is modeled by a symmetric alpha stable distribution. Having concentrated on validation and parameter estimation for the noise model, some problems in signal detection and estimation are considered. Detection of the number of sources impinging on an array is an important first. step in many array processing problems for which the development of optimal methods can be complicated even in the Gaussian case. Here, a multiple hypothesis test for the equality of the eigenvalues of the sample array covariance is proposed. / The nonparametric bootstrap is used to estimate the distributions of the test statistics removing the assumption of Gaussianity and offering improved performance for heavy tailed observations. Finally, some robust estimators are proposed for estimating parametric signals in additive noise. These are based on M-estimators but implicitly incorporate an estimate of the noise distribution. enabling the estimator to adapt to the unknown noise distribution. Two estimators are developed, one uses a nonparametric kernel density estimator while the other models the score function of the noise distribution with a linear combination of basis functions.

Power line communications (PLC) surfers performance degradation due mainly to impulsive noise interference generated by electrical appliances. This thesis studies coded modulation techniques to improve the spectral efficiency and error performance of PLC. Considered in the first part is the application of bit-interleaved coded modulation with iterative decoding (BICM-ID) in class-A impulsive noise environment. In particular, the optimal soft-output demodulator and its suboptimal version are presented for an additive class-A noise (AWAN) channel so that iterative demodulation and decoding can be performed at the receiver. The effect of signal mapping on the error performance of BICM-ID systems in impulsive noise is then investigated, with both computer simulations and a tight error bound on the asymptotic performance. Extrinsic information transfer (EXIT) chart analysis is performed to illustrate the convergence properties of different mappings. The superior performance of BICMID compared to orthogonal frequency-division multiplexing (OFDM) is also clearly demonstrated.<p>Motivated by the successes of both BICM-ID and OFDM in improving the error performance of communications systems in impulsive noise environment, the second part of this thesis introduces a novel scheme of bit-interleaved coded OFDM with iterative decoding (BI-COFDM-ID) over the class-A impulsive noise channel. Here, an iterative receiver composed of outer and inner iteration loops is first described in detail. Error performance improvements of the proposed iterative receiver with different iteration strategies are presented and discussed. Performance comparisons of BI-COFDM-ID, BICM-ID and iteratively decoded OFDM are made to illustrate the superiority of BI-COFDM-ID. The effect of signal mapping on the error performance of BI-COFDM-ID is also studied.

Power line communications (PLC) surfers performance degradation due mainly to impulsive noise interference generated by electrical appliances. This thesis studies coded modulation techniques to improve the spectral efficiency and error performance of PLC. Considered in the first part is the application of bit-interleaved coded modulation with iterative decoding (BICM-ID) in class-A impulsive noise environment. In particular, the optimal soft-output demodulator and its suboptimal version are presented for an additive class-A noise (AWAN) channel so that iterative demodulation and decoding can be performed at the receiver. The effect of signal mapping on the error performance of BICM-ID systems in impulsive noise is then investigated, with both computer simulations and a tight error bound on the asymptotic performance. Extrinsic information transfer (EXIT) chart analysis is performed to illustrate the convergence properties of different mappings. The superior performance of BICMID compared to orthogonal frequency-division multiplexing (OFDM) is also clearly demonstrated.<p>Motivated by the successes of both BICM-ID and OFDM in improving the error performance of communications systems in impulsive noise environment, the second part of this thesis introduces a novel scheme of bit-interleaved coded OFDM with iterative decoding (BI-COFDM-ID) over the class-A impulsive noise channel. Here, an iterative receiver composed of outer and inner iteration loops is first described in detail. Error performance improvements of the proposed iterative receiver with different iteration strategies are presented and discussed. Performance comparisons of BI-COFDM-ID, BICM-ID and iteratively decoded OFDM are made to illustrate the superiority of BI-COFDM-ID. The effect of signal mapping on the error performance of BI-COFDM-ID is also studied.

Recent wireless communication research faces the challenge of meeting a predicted 1000x increase in demand for wireless Internet data over the next decade. Among the key reasons for such explosive increase in demand include the evolution of Internet as a provider of high-definition video entertainment and two-way video communication, accessed via mobile wireless devices. One way to meet some of this demand is by using multiple antennae at the transmitter and receiver in a wireless device. For example, a system with 4 transmit and 4 receive antennae can provide up to a 4x increase in data throughput. Another key aspect of the overall solution would require sharing radio frequency spectral resources among users, causing severe amounts of interference to wireless systems. Consequently, wireless receivers with multiple antennae would be deployed in network environments that are rife with interference primarily due to wireless resource sharing among users. Other significant sources of interference include computational platform subsystems, signal leakage, and external electronics. Interference causes severe degradation in communication performance of wireless receivers. Having accurate statistical models of interference is a key requirement to designing, and analyzing the communication performance of, multi-antenna wireless receivers in the presence of interference. Prior work on statistical modeling of interference in multi-antenna receivers utilizes either the Gaussian distribution, or non-Gaussian distributions exhibiting either statistical independence or spherical isotropy. This dissertation proposes a framework, based on underlying statistical-physical mechanism of interference generation and propagation, for modeling multi-antenna interference in various network topologies. This framework can model interference which is spherically isotropic, or statistically independent, or somewhere on a continuum between these two extremes. The dissertation then utilizes the derived statistical models to analyze communication performance of multi-antenna receivers in interference-limited wireless networks. Accurate communication performance analysis can highlight the tradeoffs between communication performance and computational complexity of various multi-antenna receiver designs. Finally, using interference statistics, this dissertation proposes receiver algorithms that best mitigate the impact of interference on communication performance. The proposed algorithms include multi-antenna combining strategies, as well as, antenna selection algorithms for cooperative communications. / text

Future smart grid systems will intelligently monitor and control energy flows in order to improve the efficiency and reliability of power delivery. This monitoring and control requires low-power, low-cost and highly reliable two-way communications between customers and utilities. To enable these two-way communication links, powerline communication (PLC) systems are attractive because they can be deployed over existing outdoor and indoor power lines. Power lines, however, have traditionally been designed for one-directional power delivery and remain hostile environments for communication signal propagation. In particular, non-Gaussian noise that is dominated by asynchronous impulsive noise and periodic impulsive noise, is one of the primary factors that limit the communication performance of PLC systems. For my PhD dissertation, I propose transmitter and receiver methods to mitigate the impact of asynchronous impulsive noise and periodic impulsive noise, respectively, on PLC systems. The methods exploit sparsity and/or cyclostationarity of the noise in both time and frequency domains, and require no or minor training overhead prior to data transmission. Compared to conventional PLC systems, the proposed transceivers achieve dramatic improvement (up to 1000x) in coded bit error rates in simulations, while maintaining similar throughput. / text

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L’objectif de cette thèse est d’étudier le comportement des codes LDPC dans un environnement où l’interférence générée par un réseau n’est pas de nature gaussienne mais présente un caractère impulsif. Un premier constat rapide montre que sans précaution, les performances de ces codes se dégradent très significativement. Nous étudions tout d’abord les différentes solutions possibles pour modéliser les bruits impulsifs. Dans le cas des interférences d’accès multiples qui apparaissent dans les réseaux ad hoc et les réseaux de capteurs, il nous semble approprié de choisir les distributions alpha-stables. Généralisation de la gaussienne, stables par convolution, elles peuvent être validées théoriquement dans plusieurs situations.Nous déterminons alors la capacité de l’environnement α-stable et montrons par une approche asymptotique que les codes LDPC dans cet environnement sont bons mais qu’une simple opération linéaire à l’entrée du décodeur ne permet pas d’obtenir de bonnes performances. Nous avons donc proposé différentes façons de calculer la vraisemblance en entrée du décodeur. L’approche optimale est très complexe à mettre en oeuvre. Nous avons étudié plusieurs approches différentes et en particulier le clipping dont nous avons cherché les paramètres optimaux. / The goal of this PhD is to study the performance of LDPC codes in an environment where interference, generated by the network, has not a Gaussian nature but presents an impulsive behavior.A rapid study shows that, if we do not take care, the codes’ performance significantly degrades.In a first step, we study different approaches for impulsive noise modeling. In the case of multiple access interference that disturb communications in ad hoc or sensor networks, the choice of alpha-stable distributions is appropriate. They generalize Gaussian distributions, are stable by convolution and can be theoretically justified in several contexts.We then determine the capacity if the α-stable environment and show using an asymptotic method that LDPC codes in such an environment are efficient but that a simple linear operation on the received samples at the decoder input does not allow to obtain the expected good performance. Consequently we propose several methods to obtain the likelihood ratio necessary at the decoder input. The optimal solution is highly complex to implement. We have studied several other approaches and especially the clipping for which we proposed several approaches to determine the optimal parameters.