Contributions à l’amélioration des systèmes de communication multi-utilisateurs par\ud chaos : synchronisation et analyse des performances

Les radiocommunications constituent actuellement un domaine en plein essor. Depuis quelques années, de nombreux chercheurs étudient la possibilité d'utiliser des signaux chaotiques pour transmettre des données, en particulier dans un contexte multi-utilisateurs. Parmi les différentes techniques d'accès multiple, le CDMA (Code Division Multiple Access) permet à différents utilisateurs de transmettre simultanément sur la même bande de fréquence. Les séquences utilisées actuellement pour l'étalement du spectre sont des séquences dites pseudo-aléatoires binaires à faible intercorrélation générées sur la base d'un registre à décalage (les séquences de Gold) ou bien des séquences binaires orthogonales (les séquences de Walsh).\ud \ud \ud \ud Cette thèse porte sur l’étude d’un système de communication multi-utilisateurs par étalement de spectre utilisant des générateurs de chaos. Les signaux chaotiques peuvent être générés par des systèmes itératifs discrets modélisés par des transformations ponctuelles. \ud \ud \ud Dans un premier temps, nous avons étudié les signaux chaotiques issus de différents systèmes dynamiques, /a priori/ définis par des fonctions classiques continues ou continues par morceaux. En se basant sur les propriétés de corrélation et sur les distributions des énergies des signaux chaotiques, une étude comparative entre les différentes séquences chaotiques a été faite dans le cadre d’une transmission DS-CDMA par séquence chaotique. Le but de cette comparaison est de fournir des éléments permettant de choisir la séquence la mieux adaptée à l’étalement du spectre. Une méthode simple rapide et précise pour prédire le taux d’erreurs binaires pour des systèmes DS-CDMA basé sur le chaos a été proposée en mode mono et multi-utilisateur. Une étude plus poussée sur la distribution de l’énergie a permis d’établir une expression analytique du taux d’erreurs binaires. Ces calculs de performances ont été étendus à un canal de transmission à multi-trajets en mode multi-utilisateurs.\ud \ud Nous avons exploré ensuite le processus de synchronisation des systèmes dynamiques chaotiques. Tout d’abord, nous avons porté notre attention sur l’étude des différentes méthodes d’intégration numérique afin de déterminer une méthode adaptée permettant de réaliser la synchronisation chaotique par couplage avec une faible charge de calcul.\ud \ud Enfin, toujours dans l’idée d’établir la synchronisation du chaos pour les systèmes de transmission de type DS-CDMA. Nous avons proposé des récepteurs intégrant des unités de synchronisation similaires aux unités de synchronisations (/acquisition et poursuite/) utilisées dans les systèmes classiques à étalement du spectre. Ces unités de synchronisations utilisent simultanément une séquence binaire pseudo-aléatoire classique et une séquence chaotique pour établir et maintenir la synchronisation. Ces techniques ont été comparées à une méthode similaire de la littérature, ce qui a permis de montrer l’amélioration de la performance des systèmes proposés, et notamment le fait qu’ils soient opérationnels en mode asynchrone.-------------------------------------------------------------------------------Radiocommunications field is currently in full development. In recent years, many researchers have explored the possibility of using chaotic signals to transmit data, especially in a multi-user case. Among the various multiple access techniques, the CDMA (Code Division Multiple Access) allows different users to transmit simultaneously on the same frequency band. The sequences currently used for classical spread spectrum are the sequences known as pseudo-random binary sequences with low cross-correlation generated on the basis of a shift linear register (Gold sequences) or binary orthogonal sequences (Walsh codes).\ud \ud This thesis has focused on the study of a communication system with multi-user spread spectrum using chaotic generators as spreading sequences. The chaotic signals can be generated by iterative discrete systems modelled by discrete transformations.\ud \ud \ud In a first step, we have studied various chaotic signals from different dynamical systems, / a priori / defined by traditional functions continuous or piece wise linear functions. Relying on the correlation properties and the energy distribution of chaotic signals, a comparative study between different chaotic sequences was made in the framework of chaos-based DS-CDMA systems. The purpose of this comparison is to provide necessary elements to choose the best sequence for a spread spectrum system under an Additive White Gaussian Noise (AWGN) channel. A simple method to rapidly and accurately predict the bit error rate for chaos-based DS-CDMA was proposed in single and multi-user cases. Further study on the energy distribution has resulted in an analytical expression of the bit error rate. These performances have been also been studied and extended to the multi-user case.\ud \ud In a second part, we have explored the synchronization process of chaotic dynamical systems. After reviewing the existing approaches in the literature, we have focused our attention on the study of different methods of digital integration in order to determine an appropriate method to achieve synchronization using coupling with a low a low computing charge.\ud \ud \ud Finally, we have studied the synchronization process for chaos-based DS-CDMA system. We have proposed receivers incorporating synchronization units similar to the synchronization units (/ acquisition and tracking /) used in conventional spread spectrum systems. These synchronization units are using simultaneously a classical binary pseudo-random sequence together with a chaotic sequence in order to achieve and maintain synchronization. These techniques were compared to a similar existing method recently proposed in literature. We have demonstrate the improvement in performance brought by our proposed system, including the fact that this system is also operational in the asynchronous case.\ud \u

Impact of Finger Placement on the Correlation Properties of Rake Combined Signals

3G mobile devices and base stations employ rake receivers. An important issue in the design of such receivers is finger allocation. This paper explores the relationship between finger placement and the correlation properties of rake combined signals. The dependence of correlation coefficients on system parameters such as the multipath characteristics of the propagation channel, the number of users, the processing gain and the thermal noise power is also discussed. Several conclusions useful in the analysis and design of rake receivers are drawn. A low complexity finger placement algorithm is finally suggested. In the proposed receiver, finger allocation is based on the correlation properties of the desired signal component only. The receiver performs close to complex structures in the literature

Advanced index modulation techniques for future wireless networks

In the research study proposed in this Ph.D Thesis, we consider Index Modulation as a novel tool to enhance energy and spectral efficiencies for upcoming 5G networks, including wireless sensor networks and internet of things. In this vein, spatial modulation was proposed to enhance the capacity of wireless systems to partially achieve the capacity of MIMO systems but at lower cost, making it a technique that has attracted significant attention over the past few years. As such, SM schemes have been regarded as possible candidates for spectrum- and energy-efficient next generation MIMO systems. However, the implementation of the SM is also challenging because of its heavy dependence on channel characteristics, channel correlation, corrupted CSI and the need to have adequate spacing between antennas. Moreover, the SM requires multiple antennas at the transmitter which adds cost to the hardware implementation. In addition, the number of mapped bits in SM is limited by the physical size of the wireless device where only small number of antennas can be used. The switching time wasted by RF antenna switches adds to the complexity of the issue. In this Thesis, we study the drawbacks of SM in the articles indicated, namely Performance Comparison of Spatial Modulation Detectors Under Channel Impairments that is placed in the Appendix at the end of Thesis as it is a conference paper, and The Impact of Antenna Switching Time on Spatial Modulation that is put in Chapter 1. In the first article, we have shown that channel impairments have serious impacts on the BER performance and on the capacity of the SM system and that the SM is too sensitive to both imperfect and correlated channels. In the second article, we have demonstrated that the switching time defined as the time needed by the system to turn off an antenna and turn on another one, which is an inherent property of RF industrial switches used in SM systems, is in the order of nanoseconds and naturally influences the transmission rate of SM systems because of introducing systematic transmission gaps or pauses. Given the speed limitation of practical RF switches in performing transitions, antenna transition-based technologies like SM schemes are capped in terms of data rate performance. In fact, the effective data rate of SM will remain hostage to developments in industrial RF switches. This brings restrictions to the implementation and operation issues when extremely high data rates become a necessity. It is shown by the assemblage of our results that the switching time Tsw which is a requirement for transitions between antennas to happen, dictates restrictions on data rate, capacity and spectral efficiency of SM systems. Furthermore, we propose baseband non-hardware-based indexing modulation schemes based on frequency-index modulation, coherent chaotic modulation and non-coherent differential chaotic modulation schemes as potential alternatives to SM, that would also fit wireless sensor networks and internet of things applications. In this regard, we have proposed three articles. The first article which represents frequency index modulation is called Frequency Index Modulation for Low Complexity Low Energy Communication Networks and is placed in Chapter 2 of this Thesis. In this article, we explore a low complexity multi-user communication system based on frequency index modulation that suits Internet of Things (IoT) applications and we show that such a system would constitute an excellent candidate for wireless sensor applications, where it represents a simpler substitution for frequency-hopping (FH) based architectures, in which the hops carry extra bits. The third article which concerns coherent chaotic modulation is called Design of an Initial-Condition Index Chaos Shift Keying Modulation and is located in Chapter 3. In this article, an initial condition index chaos shift keying modulation is proposed. This design aims to increase the spectral and energy efficiencies to unprecedented levels. The proposed scheme exploits the initial conditions to generate different chaotic sequences to convey extra bits per transmission. In comparison to rival modulation schemes, the results obtained in the proposed work show a promising data rate boost and a competitive performance. The last article employs a non-coherent differential chaotic shift-key system named Permutation Index DCSK Modulation Technique for Secure Multi-User High-Data-Rate Communication Systems that is found in the Appendix. In this original design, where each data frame is divided into two time slots in which the reference chaotic signal is sent in the first time slot and a permuted replica of the reference signal multiplied by the modulating bit is sent in the second time slot, we target enhancing data security, energy and spectral efficiencies. Overall, in light of the high demands for bandwidth and energy efficiencies of futuristic systems, the suggested soft indexing mechanisms are successful candidates with promising results

Multiuser detection employing recurrent neural networks for DS-CDMA systems.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2006.Over the last decade, access to personal wireless communication networks has evolved to a point of necessity. Attached to the phenomenal growth of the telecommunications industry in recent times is an escalating demand for higher data rates and efficient spectrum utilization. This demand is fuelling the advancement of third generation (3G), as well as future, wireless networks. Current 3G technologies are adding a dimension of mobility to services that have become an integral part of modem everyday life. Wideband code division multiple access (WCDMA) is the standardized multiple access scheme for 3G Universal Mobile Telecommunication System (UMTS). As an air interface solution, CDMA has received considerable interest over the past two decades and a great deal of current research is concerned with improving the application of CDMA in 3G systems. A factoring component of CDMA is multiuser detection (MUD), which is aimed at enhancing system capacity and performance, by optimally demodulating multiple interfering signals that overlap in time and frequency. This is a major research problem in multipoint-to-point communications. Due to the complexity associated with optimal maximum likelihood detection, many different sub-optimal solutions have been proposed. This focus of this dissertation is the application of neural networks for MUD, in a direct sequence CDMA (DS-CDMA) system. Specifically, it explores how the Hopfield recurrent neural network (RNN) can be employed to give yet another suboptimal solution to the optimization problem of MUD. There is great scope for neural networks in fields encompassing communications. This is primarily attributed to their non-linearity, adaptivity and key function as data classifiers. In the context of optimum multiuser detection, neural networks have been successfully employed to solve similar combinatorial optimization problems. The concepts of CDMA and MUD are discussed. The use of a vector-valued transmission model for DS-CDMA is illustrated, and common linear sub-optimal MUD schemes, as well as the maximum likelihood criterion, are reviewed. The performance of these sub-optimal MUD schemes is demonstrated. The Hopfield neural network (HNN) for combinatorial optimization is discussed. Basic concepts and techniques related to the field of statistical mechanics are introduced and it is shown how they may be employed to analyze neural classification. Stochastic techniques are considered in the context of improving the performance of the HNN. A neural-based receiver, which employs a stochastic HNN and a simulated annealing technique, is proposed. Its performance is analyzed in a communication channel that is affected by additive white Gaussian noise (AWGN) by way of simulation. The performance of the proposed scheme is compared to that of the single-user matched filter, linear decorrelating and minimum mean-square error detectors, as well as the classical HNN and the stochastic Hopfield network (SHN) detectors. Concluding, the feasibility of neural networks (in this case the HNN) for MUD in a DS-CDMA system is explored by quantifying the relative performance of the proposed model using simulation results and in view of implementation issues

A nonlinear dynamic system for spread spectrum code acquisition

Thesis (S.M.)--Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences, 1999.Includes bibliographical references (leaves 88-89).Nonlinear differential equations and iterated maps can perform any computation. Sometimes, the most difficult part of performing a useful computation, however, is writing the program. Furthermore, in practice, we often need to build special purpose computing hardware suited to run a particular program. Nonlinear dynamics provides a novel and useful language for constructing "algorithms" and "computer architectures." We apply the language of nonlinear dynamics to solve a fast coding problem which has previously been implemented by a Digital Signal Processor chip in digital wireless receivers. We eventually hope to produce a novel physical system which exhibits the nonlinear dynamics we require, thereby creating one of the first nonlinear dynamic systems engineered to perform a practical computation. This system, called an Analog Feedback Shift Register (AFSR), should be a faster, more reliable, less expensive, and lower power Spread Spectrum (SS) code acquisition system for wireless receivers. A prohibitive factor in creating ubiquitous short range, digital radio transceivers is the difficulty and expense of creating a mechanism for locking onto the incoming Spread Spectrum code sequence. AFSR is also potentially useful in other applications where low cost, low power channel sharing or addressing is required, for example in wireless auto-identification tags.by Benjamin William Vigoda.S.M

Constrained Linear and Non-Linear Adaptive Equalization Techniques for MIMO-CDMA Systems

Researchers have shown that by combining multiple input multiple output (MIMO) techniques with CDMA then higher gains in capacity, reliability and data transmission speed can be attained. But a major drawback of MIMO-CDMA systems is multiple access interference (MAI) which can reduce the capacity and increase the bit error rate (BER), so statistical analysis of MAI becomes a very important factor in the performance analysis of these systems. In this thesis, a detailed analysis of MAI is performed for binary phase-shift keying (BPSK) signals with random signature sequence in Raleigh fading environment and closed from expressions for the probability density function of MAI and MAI with noise are derived. Further, probability of error is derived for the maximum Likelihood receiver. These derivations are verified through simulations and are found to reinforce the theoretical results. Since the performance of MIMO suffers significantly from MAI and inter-symbol interference (ISI), equalization is needed to mitigate these effects. It is well known from the theory of constrained optimization that the learning speed of any adaptive filtering algorithm can be increased by adding a constraint to it, as in the case of the normalized least mean squared (NLMS) algorithm. Thus, in this work both linear and non-linear decision feedback (DFE) equalizers for MIMO systems with least mean square (LMS) based constrained stochastic gradient algorithm have been designed. More specifically, an LMS algorithm has been developed , which was equipped with the knowledge of number of users, spreading sequence (SS) length, additive noise variance as well as MAI with noise (new constraint) and is named MIMO-CDMA MAI with noise constrained (MNCLMS) algorithm. Convergence and tracking analysis of the proposed algorithm are carried out in the scenario of interference and noise limited systems, and simulation results are presented to compare the performance of MIMO-CDMA MNCLMS algorithm with other adaptive algorithms

Survey of FPGA applications in the period 2000 – 2015 (Technical Report)

Romoth J, Porrmann M, Rückert U. Survey of FPGA applications in the period 2000 – 2015 (Technical Report).; 2017.Since their introduction, FPGAs can be seen in more and more different fields of applications. The key advantage is the combination of software-like flexibility with the performance otherwise common to hardware. Nevertheless, every application field introduces special requirements to the used computational architecture. This paper provides an overview of the different topics FPGAs have been used for in the last 15 years of research and why they have been chosen over other processing units like e.g. CPUs

Efficient complementary sequences-based architectures and their application to ranging measurements

Premio Extraordinario de Doctorado de la UAH en 2015En las últimas décadas, los sistemas de medición de distancias se han beneficiado de los avances en el área de las comunicaciones inalámbricas. En los sistemas basados en CDMA (Code-Division Multiple-Access), las propiedades de correlación de las secuencias empleadas juegan un papel fundamental en el desarrollo de dispositivos de medición de altas prestaciones. Debido a las sumas ideales de correlaciones aperiódicas, los conjuntos de secuencias complementarias, CSS (Complementary Sets of Sequences), son ampliamente utilizados en sistemas CDMA. En ellos, es deseable el uso de arquitecturas eficientes que permitan generar y correlar CSS del mayor número de secuencias y longitudes posibles. Por el término eficiente se hace referencia a aquellas arquitecturas que requieren menos operaciones por muestra de entrada que con una arquitectura directa. Esta tesis contribuye al desarrollo de arquitecturas eficientes de generación/correlación de CSS y derivadas, como son las secuencias LS (Loosely Synchronized) y GPC (Generalized Pairwise Complementary), que permitan aumentar el número de longitudes y/o de secuencias disponibles. Las contribuciones de la tesis pueden dividirse en dos bloques: En primer lugar, las arquitecturas eficientes de generación/correlación para CSS binarios, derivadas en trabajos previos, son generalizadas al alfabeto multinivel (secuencias con valores reales) mediante el uso de matrices de Hadamard multinivel. Este planteamiento tiene dos ventajas: por un lado el aumento del número de longitudes que pueden generarse/correlarse y la eliminación de las limitaciones de las arquitecturas previas en el número de secuencias en el conjunto. Por otro lado, bajo ciertas condiciones, los parámetros de las arquitecturas generalizadas pueden ajustarse para generar/correlar eficientemente CSS binarios de mayor número de longitudes que con las arquitecturas eficientes previas. En segundo lugar, las arquitecturas propuestas son usadas para el desarrollo de nuevos algoritmos de generación/correlación de secuencias derivadas de CSS que reducen el número de operaciones por muestra de entrada. Finalmente, se presenta la aplicación de las secuencias estudiadas en un nuevo sistema de posicionamiento local basado en Ultra-Wideband y en un sistema de posicionamiento local basado en ultrasonidos