Abstract Multiuser demodulation algorithms for centralized receivers of asynchronous direct-sequence (DS) spread-spectrum code-division multiple-access (CDMA) systems in frequency-selective fading channels are studied. Both DS-CDMA systems with short (one symbol interval) and long (several symbol intervals) spreading sequences are considered. Linear multiuser receivers process ideally the complete received data block. The approximation of ideal infinite memory-length (IIR) linear multiuser detectors by finite memory-length (FIR) detectors is studied. It is shown that the FIR detectors can be made near-far resistant under a given ratio between maximum and minimum received power of users by selecting an appropriate memory-length. Numerical examples demonstrate the fact that moderate memory-lengths of the FIR detectors are sufficient to achieve the performance of the ideal IIR detectors even under severe near-far conditions. Multiuser demodulation in relatively fast fading channels is analyzed. The optimal maximum likelihood sequence detection receiver and suboptimal receivers are considered. The parallel interference cancellation (PIC) receiver is demonstrated to achieve better performance in known channels than the decorrelating receiver, but it is observed to be more sensitive to channel coefficient estimation errors than the decorrelator. At high channel loads the PIC receiver suffers from bit error rate (BER) saturation, whereas the decorrelating receiver does not. Choice of channel estimation filters is shown to be crucial if low BER is required. Data-aided channel estimation is shown to be more robust than decision-directed channel estimation, which may suffer from BER saturation caused by hang-ups at high signal-to-noise ratios. Multiuser receivers for dynamic CDMA systems are studied. Algorithms for ideal linear detector computation are derived and their complexity is analyzed. The complexity of the linear detector computation is a cubic function of KL, where K and L are the number of users and multipath components, respectively. Iterative steepest descent, conjugate gradient, and preconditioned conjugate gradient algorithms are proposed to reduce the complexity. The computational requirements for one iteration are a quadratic function of KL. The iterative detectors are also shown to be applicable for parallel implementation. Simulation results demonstrate that a moderate number of iterations yields the performance of the corresponding ideal linear detectors. A quantitative analysis shows that the PIC receivers are significantly simpler to implement than the linear receivers and only moderately more complex than the conventional matched filter bank receiver.

The objective of the research is to achieve a systematic combination of acoustic echo reduction components that together achieve a robust performance of the MCAEC system as a whole. Conventional approaches to the acoustic echo reduction system typically assume that individual components would perform ideally. For example, the adaptive algorithm for AEC is often developed in the absence of strong near-end signal, the algorithm for RES is often an added module that is developed as a separate noise reduction component, and the decorrelation procedure for MCAEC is yet another add-on module that simply introduces some form of distortion to the reference signal. The main challenge is in designing a consistent criterion across all modules that can be jointly optimized to form a more consistent framework for acoustic echo reduction. The decorrelation procedure can potentially benefit from the system approach as well if it is designed by taking the near-end listener into account. The MCAEC system should be optimized not only for the echo cancellation and suppression performance, but also for the reference signal quality after the added distortion from the decorrelation procedure. Finally, a tuning strategy is presented to jointly optimize the parameters across all modules using object criteria.

A measurement of the azimuthal decorrelation in di-jet events in proton-proton collisions at $\sqrt{s}= 7\,$TeV is presented. Using $19.6\,$nb$^{-1}$ of data collected in the ATLAS detector this measurement uses the angle ($\Delta\phi$) in the transverse plane between the two leading $p_T$ jets to measure the normalized differential cross section $\frac{1}{\sigma_{tot}}\frac{d\sigma}{d(\Delta\phi)}$. An unfolding correction is a applied to give a jet-level result. The unfolded spectrum is compared to the predictions of two Monte Carlo event generators: Pythia and Herwig++.

A measurement of the azimuthal decorrelation in di-jet events in proton-proton collisions at $\sqrt{s}= 7\,$TeV is presented. Using $19.6\,$nb$^{-1}$ of data collected in the ATLAS detector this measurement uses the angle ($\Delta\phi$) in the transverse plane between the two leading $p_T$ jets to measure the normalized differential cross section $\frac{1}{\sigma_{tot}}\frac{d\sigma}{d(\Delta\phi)}$. An unfolding correction is a applied to give a jet-level result. The unfolded spectrum is compared to the predictions of two Monte Carlo event generators: Pythia and Herwig++.

In this paper, we explore a cooperative code-division-multiple-access(CDMA) network. Users cooperate by forwarding each other¡¦s messages toward the destination. For simplicity, we assume that signal reception at the destination is well-synchronized. Due to practical design issues of CDMA systems, spreading waveforms allocated to users are not perfectly orthogonal in general. This results in multiple-access interference(MAI) at relays and destination. In CDMA uplink networks one common approach is to adopt decorrelating multi-user detection, but it will lead to noise amplification[16,17]. Therefore, we employ relay-assisted decorrelating multiuser detector(RAD-MUD) to mitigate MAI[1] by performing half of decorrelation at the relay and destination respectively. Based on the availability of CSI at relays, we can further adopt cooperative strategies to improve performance, e.g., transmit beamforming and selective relaying. The destination side will use minimum mean-square error(MMSE) detector to demodulate source symbols. In the existing literatures, channel state information(CSI) is assumed to be perfectly known at relay and destination. Actually, CSI is obtained from channel estimation, which usually contains estimation errors. In order to alleviate effects of channel estimation, one goal of this thesis is to design a robust system. Using estimated CSI and statistical property channel estimation errors, we design robust precoder and detector for the relay and destination. It shows that, even with distortion on channel estimations, the system still achieve excellent transmission efficiency. From the simulation results, it shows that the robust design is better than the system without consider channel estimation errors. Finally, we can see that the stable robust design can effectively mitigate effects of imperfect CSI.

In current mammography practice, radiologists typically view mammograms in a symmetric, side-by-side, configuration in the belief that abnormalities will be made salient because they break the perceived symmetry. The literature on the use of symmetry as an aid to signal detection is limited and this thesis has taken a psychophysical approach to investigate the radiologist’s task of detecting a small mass (a blob) in paired mammogram backgrounds. Initial experiments used Gaussian white noise and synthetic mammogram backgrounds to test observer performance for the radiologist’s task using symmetric (side-by-side) displays and animated (the two images of a pair alternated sequentially in the same location) displays. The use of animated displays was then tested using real mammogram backgrounds in the subsequent experiments. The results showed that side-by-side presentation of paired images does not provide any benefit for the detection of a blob, whereas, alternated presentation enabled the observer to use the correlation present between the paired images to improve detection performance. The effect of alternation was not evident when applied to the task of detecting a small mass in real mammogram pairs and subsequent investigation suggested that the loss of effect resulted from the lack of scale invariance of real images. This meant that, regardless of the level of global correlation between two images, the localised correlation, at a region size reflecting the visual angle subtended by the fovea, was much lower. Thus, decorrelation by the visual system was ineffective and performance for the detection of a blob in the paired images was also ineffective. This thesis suggests that, whilst animated displays can be a powerful tool for the identification of differences between paired images, the underpinning mechanism of decorrelation makes them unsuited for mammograms where scale invariance means that correlation at local levels is a fraction of the global correlation level.

The major application of active noise control (ANC) has been focused on using a single reference signal; the work on multiple reference ANC is very scarce. Here, the behavior of multiple reference ANC is analyzed in both the frequency and time domain, and the coherence functions are provided to evaluate the effectiveness of multiple reference ANC. When there are multiple noise sources, multiple reference sensors are needed to generate complete reference signals. A simplified method combines those signals from multiple reference sensors into a single reference signal. Although this method could result in satisfactory noise control effects under special circumstances, the performance is generally compromised. A widely adopted method feeds each reference signal into a different control filter. This approach suffers from the problem of ill-conditioning when the reference signals are correlated. The problem of ill-conditioning results in slow convergence rate and high sensitivity to measurement error especially when the FXLMS algorithm is applied. To handle this particular problem, the decorrelated Filtered-X LMS (DFXLMS) algorithm is developed and studied in this thesis. Both simulations and experiments have been conducted to verify the DFXLMS algorithm and other issues associated with multiple reference ANC. The results presented herein are consistent with the theoretical analysis, and favorably indicate that the DFXLMS algorithm is effective in improving the convergence speed. To take the maximum advantage of the TMS320C30 DSP board used to implement the controller, several DSP programming issues are discussed, and assembly routines are given in the appendix. Furthermore, a graphical user interface (GUI) running under Windows' environment is introduced. The main purpose of the GUI is to facilitate parameters modification, real time data monitoring and DSP process control. / Master of Science

Diskrečiosioms bangelių transformacijoms galima panaudoti daugybę „motininių“ bangelių, tokių kaip Daubechies, Morlet‘o, Le Gall‘o arba Haar‘o. Daubechies 9/7 (CDF 9/7) bangelės istoriškai yra pirmoji bangelių šeima, kuri buvo išpopuliarinta Ingridos Daubechies (1987 m. Ingrid Daubechies suformavo vieną iš pagrindinių bangelių bazių). CDF 9/7 bangelės yra ypač veiksmingos ortogonaliosios bangelės, plačiai naudojamos praktikoje (FTB pirštų atspaudų glaudinimas, vaizdų kodavimo standartas JPEG2000 ir pan.). Šiame darbe yra aptariamos ir algoritmizuojamos dvi CDF 9/7 transformacijos versijos, būtent: bazinė CDF 9/7 ir CDF 9/7 su daline blokų dekoreliacija. Darbo tikslas – atlikti palyginamąją bazinės CDF 9/7 ir CDF 9/7 su daline blokų dekoreliacija analizę vaizde sukauptos energijos „pakavimo“ spektrinėje bangelių srityje savybės kontekste. Palyginimo kriterijumi yra pasirenkamas dvimatis hiperbolinis skaitmeninių vaizdų filtras, t.y. apdorojamas vaizdas pervedamas į spektrų bangelių sritį, taikant abi CDF 9/7 transformacijos versijas. Gautieji spektrai apdorojami fiksuoto lygmens hiperboliniais filtrais. Po filtravimo įvertinama atkurtų vaizdų kokybė. / Many kernels (“mother” wavelets) can be used for the discrete wavelet transform DWT, like those of Daubechies, Morlet, discrete Le Gall transform (DLGT) or the discrete Haar transform (HT). Cohen-Daubechies-Feauveau (CDF 9/7) wavelet are the historically first family of biorthogonal wavelets, which was made popular by Ingrid Daubechies. These are not the same as the orthogonal Daubechies wavelets, and also not very similar in shape and properties. However their construction idea is the same. The JPEG 2000 compression standard uses the biorthogonal CDF 5/3 wavelet (also called the LeGall 5/3 wavelet) for lossless compression and a CDF 9/7 wavelet for lossy compression. In this paper, two distinct DWT (CDF 9/7 and CDF 9/7 with decorrelation) as well as their computational algorithms are discussed, analyzed and compared. Comparison criteria are chosen to be one – dimensional hyperbolic filters and smoothness level of the digital signal under processing.

Com o aumento das ameaças na Internet, firewalls tornaram-se mecanismos de defesa cada vez mais utilizados. No entanto, sua configuração é notadamente complexa, podendo resultar em erros. Vários estudos foram realizados com o intuito de resolver tais problemas, mas a grande maioria deles se concentrou em trabalhar diretamente no nível de configuração, o que possui limitações. O presente trabalho investiga maneiras de extrair políticas em mais alto nível a partir de regras de firewall em baixo nível, o que é mais intuitivo. A fim de extrair as políticas reais a partir de regras de firewall, o problema do descorrelacionamento é estudado e algoritmos anteriormente propostos para resolvê-lo são apresentados e discutidos. É apresentado, também, um tipo de grafo para a melhor visualização e análise de correlacionamento entre regras. Além disso, é pesquisado o agrupamento de regras descorrelacionadas, que tem o objetivo de elevar o nível das mesmas. São apresentados dois algoritmos para realizar o agrupamento, sendo um deles novo. A seguir, é proposta uma nova metodologia de extração de políticas de firewall. A primeira parte desta consiste na utilização de um novo tipo de descorrelacionamento, o descorrelacionamento hierárquico. Este é acompanhado por uma nova maneira de agrupar regras descorrelacionadas hierarquicamente, o agrupamento hierárquico. A segunda parte é uma nova modelagem de regras de firewall que fazem parte de blacklist ou whitelist, separando-as das demais regras na extração de políticas. Algumas maneiras de realizar esta separação também são discutidas. Por fim, são relatadas as conclusões e possibilidades de trabalhos futuros. / As the number of threats in the Internet grows, firewalls have become a very important defense mechanism. However, configuring a firewall is not an easy task and is prone to errors. Several investigations have been made towards solving these issue. However, most of them have focused on working directly at the configuration level and have a number of limitations. This work investigates methods to extract higher level policies from low level firewall rules. Aiming at extracting real policies from firewall rules, we analyse the firewall decorrelation problem and previously proposed algoritmhs to solve it. In addition, a new type of graph is presented aiming at better visualising and analysing rules’ correlation. We also investigate the merging of decorrelated rules, with the goal of defining more abstract rules. Two algorithms are then presented and a new methodology for the extraction of firewall policies is proposed. This methodology is twofold. The first part consists of the use a new type of decorrelation: the hierachical decorrelation, which is introduced along with a new way of hierarchically merging decorrelated rules. The second part is a new model for blacklist or whitelist firewall rules, separating them from the other rules in the policy extraction. We also present alternatives for accomplishing this separation. Finally, we conclpude and point out directions for future work.

Com o aumento das ameaças na Internet, firewalls tornaram-se mecanismos de defesa cada vez mais utilizados. No entanto, sua configuração é notadamente complexa, podendo resultar em erros. Vários estudos foram realizados com o intuito de resolver tais problemas, mas a grande maioria deles se concentrou em trabalhar diretamente no nível de configuração, o que possui limitações. O presente trabalho investiga maneiras de extrair políticas em mais alto nível a partir de regras de firewall em baixo nível, o que é mais intuitivo. A fim de extrair as políticas reais a partir de regras de firewall, o problema do descorrelacionamento é estudado e algoritmos anteriormente propostos para resolvê-lo são apresentados e discutidos. É apresentado, também, um tipo de grafo para a melhor visualização e análise de correlacionamento entre regras. Além disso, é pesquisado o agrupamento de regras descorrelacionadas, que tem o objetivo de elevar o nível das mesmas. São apresentados dois algoritmos para realizar o agrupamento, sendo um deles novo. A seguir, é proposta uma nova metodologia de extração de políticas de firewall. A primeira parte desta consiste na utilização de um novo tipo de descorrelacionamento, o descorrelacionamento hierárquico. Este é acompanhado por uma nova maneira de agrupar regras descorrelacionadas hierarquicamente, o agrupamento hierárquico. A segunda parte é uma nova modelagem de regras de firewall que fazem parte de blacklist ou whitelist, separando-as das demais regras na extração de políticas. Algumas maneiras de realizar esta separação também são discutidas. Por fim, são relatadas as conclusões e possibilidades de trabalhos futuros. / As the number of threats in the Internet grows, firewalls have become a very important defense mechanism. However, configuring a firewall is not an easy task and is prone to errors. Several investigations have been made towards solving these issue. However, most of them have focused on working directly at the configuration level and have a number of limitations. This work investigates methods to extract higher level policies from low level firewall rules. Aiming at extracting real policies from firewall rules, we analyse the firewall decorrelation problem and previously proposed algoritmhs to solve it. In addition, a new type of graph is presented aiming at better visualising and analysing rules’ correlation. We also investigate the merging of decorrelated rules, with the goal of defining more abstract rules. Two algorithms are then presented and a new methodology for the extraction of firewall policies is proposed. This methodology is twofold. The first part consists of the use a new type of decorrelation: the hierachical decorrelation, which is introduced along with a new way of hierarchically merging decorrelated rules. The second part is a new model for blacklist or whitelist firewall rules, separating them from the other rules in the policy extraction. We also present alternatives for accomplishing this separation. Finally, we conclpude and point out directions for future work.