64 research outputs found
Guard Time Optimization for Capacity Maximization of BPSK Impulse UWB Communications
International audienceUsually, for the design of UWB systems, the symbol duration is chosen larger than the delay of the channel impulse response, in order to avoid the ISI (inter symbol interference). However, this approach does not maximize the system capacity. An adaptation of the guard time (GT) is a flexible mean of exploiting system resources efficiently in a multi-path fading environment. The optimal guard time length in BPSK impulse UWB communications is obtained by exhaustively searching for the guard time that maximizes capacity. This approach is complex since it has to be implemented for each channel realization. To reduce this complexity, in this paper we present new optimization methods. The first method assumes the fading channel to be partitioned into classes. Then, a given GT length for each channel class is used. The second optimization method provides a guard time for each channel realization. However, the GT is obtained by looking at simplified metrics which are based on the channel delay spread, the received signal energy, or on an approximation of the capacity formula. Simulation results are performed for UWB communications over WiMedia channels and they show that significant gains are achievable with the proposed guard time adaptation w.r.t. to the use of a constant guard time
Advanced Coding And Modulation For Ultra-wideband And Impulsive Noises
The ever-growing demand for higher quality and faster multimedia content delivery over short distances in home environments drives the quest for higher data rates in wireless personal area networks (WPANs). One of the candidate IEEE 802.15.3a WPAN proposals support data rates up to 480 Mbps by using punctured convolutional codes with quadrature phase shift keying (QPSK) modulation for a multi-band orthogonal frequency-division multiplexing (MB-OFDM) system over ultra wideband (UWB) channels. In the first part of this dissertation, we combine more powerful near-Shannon-limit turbo codes with bandwidth efficient trellis coded modulation, i.e., turbo trellis coded modulation (TTCM), to further improve the data rates up to 1.2 Gbps. A modified iterative decoder for this TTCM coded MB-OFDM system is proposed and its bit error rate performance under various impulsive noises over both Gaussian and UWB channel is extensively investigated, especially in mismatched scenarios. A robust decoder which is immune to noise mismatch is provided based on comparison of impulsive noises in time domain and frequency domain. The accurate estimation of the dynamic noise model could be very difficult or impossible at the receiver, thus a significant performance degradation may occur due to noise mismatch. In the second part of this dissertation, we prove that the minimax decoder in \cite, which instead of minimizing the average bit error probability aims at minimizing the worst bit error probability, is optimal and robust to certain noise model with unknown prior probabilities in two and higher dimensions. Besides turbo codes, another kind of error correcting codes which approach the Shannon capacity is low-density parity-check (LDPC) codes. In the last part of this dissertation, we extend the density evolution method for sum-product decoding using mismatched noises. We will prove that as long as the true noise type and the estimated noise type used in the decoder are both binary-input memoryless output symmetric channels, the output from mismatched log-likelihood ratio (LLR) computation is also symmetric. We will show the Shannon capacity can be evaluated for mismatched LLR computation and it can be reduced if the mismatched LLR computation is not an one-to-one mapping function. We will derive the Shannon capacity, threshold and stable condition of LDPC codes for mismatched BIAWGN and BIL noise types. The results show that the noise variance estimation errors will not affect the Shannon capacity and stable condition, but the errors do reduce the threshold. The mismatch in noise type will only reduce Shannon capacity when LLR computation is based on BIL
Contribution Ă la conception d'un systĂšme de radio impulsionnelle ultra large bande intelligent
Faced with an ever increasing demand of high data-rates and improved adaptability among existing systems, which inturn is resulting in spectrum scarcity, the development of new radio solutions becomes mandatory in order to answer the requirements of these emergent applications. Among the recent innovations in the field of wireless communications,ultra wideband (UWB) has generated significant interest. Impulse based UWB (IR-UWB) is one attractive way of realizing UWB systems, which is characterized by the transmission of sub nanoseconds UWB pulses, occupying a band width up to 7.5 GHz with extremely low power density. This large band width results in several captivating features such as low-complexity low-cost transceiver, ability to overlay existing narrowband systems, ample multipath diversity, and precise ranging at centimeter level due to extremely fine temporal resolution.In this PhD dissertation, we investigate some of the key elements in the realization of an intelligent time-hopping based IR-UWB system. Due to striking resemblance of IR-UWB inherent features with cognitive radio (CR) requirements, acognitive UWB based system is first studied. A CR in its simplest form can be described as a radio, which is aware ofits surroundings and adapts intelligently. As sensing the environment for the availability of resources and then consequently adapting radioâs internal parameters to exploit them opportunistically constitute the major blocks of any CR, we first focus on robust spectrum sensing algorithms and the design of adaptive UWB waveforms for realizing a cognitive UWB radio. The spectrum sensing module needs to function with minimum a-priori knowledge available about the operating characteristics and detect the primary users as quickly as possible. Keeping this in mind, we develop several spectrum sensing algorithms invoking recent results on the random matrix theory, which can provide efficient performance with a few number of samples. Next, we design the UWB waveform using a linear combination of Bsp lines with weight coefficients being optimized by genetic algorithms. This results in a UWB waveform that is spectrally efficient and at the same time adaptable to incorporate the cognitive radio requirements. In the 2nd part of this thesis, some research challenges related to signal processing in UWB systems, namely synchronization and dense multipath channel estimation are addressed. Several low-complexity non-data-aided (NDA) synchronization algorithms are proposed for BPSK and PSM modulations, exploiting either the orthogonality of UWB waveforms or theinherent cyclostationarity of IR-UWB signaling. Finally, we look into the channel estimation problem in UWB, whichis very demanding due to particular nature of UWB channels and at the same time very critical for the coherent Rake receivers. A method based on a joint maximum-likelihood (ML) and orthogonal subspace (OS) approaches is proposed which exhibits improved performance than both of these methods individually.Face Ă une demande sans cesse croissante de haut dĂ©bit et dâadaptabilitĂ© des systĂšmes existants, qui Ă son tour se traduit par lâencombrement du spectre, le dĂ©veloppement de nouvelles solutions dans le domaine des communications sans fil devient nĂ©cessaire afin de rĂ©pondre aux exigences des applications Ă©mergentes. Parmi les innovations rĂ©centes dans ce domaine, lâultra large bande (UWB) a suscitĂ© un vif intĂ©rĂȘt. La radio impulsionnelle UWB (IR-UWB), qui est une solution intĂ©ressante pour rĂ©aliser des systĂšmes UWB, est caractĂ©risĂ©e par la transmission des impulsions de trĂšs courte durĂ©e, occupant une largeur de bande allant jusquâĂ 7,5 GHz, avec une densitĂ© spectrale de puissance extrĂȘmement faible. Cette largeur de bande importante permet de rĂ©aliser plusieurs fonctionnalitĂ©s intĂ©ressantes, telles que lâimplĂ©mentation Ă faible complexitĂ© et Ă coĂ»t rĂ©duit, la possibilitĂ© de se superposer aux systĂšmes Ă bande Ă©troite, la diversitĂ© spatiale et la localisation trĂšs prĂ©cise de lâordre centimĂ©trique, en raison de la rĂ©solution temporelle trĂšs fine.Dans cette thĂšse, nous examinons certains Ă©lĂ©ments clĂ©s dans la rĂ©alisation d'un systĂšme IR-UWB intelligent. Nous avons tout dâabord proposĂ© le concept de radio UWB cognitive Ă partir des similaritĂ©s existantes entre l'IR-UWB et la radio cognitive. Dans sa dĂ©finition la plus simple, un tel systĂšme est conscient de son environnement et s'y adapte intelligemment. Ainsi, nous avons tout dâabord focalisĂ© notre recherchĂ© sur lâanalyse de la disponibilitĂ© des ressources spectrales (spectrum sensing) et la conception dâune forme dâonde UWB adaptative, considĂ©rĂ©es comme deux Ă©tapes importantes dans la rĂ©alisation d'une radio cognitive UWB. Les algorithmes de spectrum sensing devraient fonctionner avec un minimum de connaissances a priori et dĂ©tecter rapidement les utilisateurs primaires. Nous avons donc dĂ©veloppĂ© de tels algorithmes utilisant des rĂ©sultats rĂ©cents sur la thĂ©orie des matrices alĂ©atoires, qui sont capables de fournir de bonnes performances, avec un petit nombre d'Ă©chantillons. Ensuite, nous avons proposĂ© une mĂ©thode de conception de la forme d'onde UWB, vue comme une superposition de fonctions B-splines, dont les coefficients de pondĂ©ration sont optimisĂ©s par des algorithmes gĂ©nĂ©tiques. Il en rĂ©sulte une forme d'onde UWB qui est spectralement efficace et peut sâadapter pour intĂ©grer les contraintes liĂ©es Ă la radio cognitive. Dans la 2Ăšme partie de cette thĂšse, nous nous sommes attaquĂ©s Ă deux autres problĂ©matiques importantes pour le fonctionnement des systĂšmes UWB, Ă savoir la synchronisation et lâestimation du canal UWB, qui est trĂšs dense en trajets multiples. Ainsi, nous avons proposĂ© plusieurs algorithmes de synchronisation, de faible complexitĂ© et sans sĂ©quence dâapprentissage, pour les modulations BPSK et PSM, en exploitant l'orthogonalitĂ© des formes d'onde UWB ou la cyclostationnaritĂ© inhĂ©rente Ă la signalisation IR-UWB. Enfin, nous avons travaillĂ© sur l'estimation du canal UWB, qui est un Ă©lĂ©ment critique pour les rĂ©cepteurs Rake cohĂ©rents. Ainsi, nous avons proposĂ© une mĂ©thode dâestimation du canal basĂ©e sur une combinaison de deux approches complĂ©mentaires, le maximum de vraisemblance et la dĂ©composition en sous-espaces orthogonaux,dâamĂ©liorer globalement les performances
Intelligent Processing in Wireless Communications Using Particle Swarm Based Methods
There are a lot of optimization needs in the research and design of wireless communica- tion systems. Many of these optimization problems are Nondeterministic Polynomial (NP) hard problems and could not be solved well. Many of other non-NP-hard optimization problems are combinatorial and do not have satisfying solutions either. This dissertation presents a series of Particle Swarm Optimization (PSO) based search and optimization algorithms that solve open research and design problems in wireless communications. These problems are either avoided or solved approximately before.
PSO is a bottom-up approach for optimization problems. It imposes no conditions on the underlying problem. Its simple formulation makes it easy to implement, apply, extend and hybridize. The algorithm uses simple operators like adders, and multipliers to travel through the search space and the process requires just five simple steps. PSO is also easy to control because it has limited number of parameters and is less sensitive to parameters than other swarm intelligence algorithms. It is not dependent on initial points and converges very fast.
Four types of PSO based approaches are proposed targeting four different kinds of problems in wireless communications. First, we use binary PSO and continuous PSO together to find optimal compositions of Gaussian derivative pulses to form several UWB pulses that not only comply with the FCC spectrum mask, but also best exploit the avail- able spectrum and power. Second, three different PSO based algorithms are developed to solve the NLOS/LOS channel differentiation, NLOS range error mitigation and multilateration problems respectively. Third, a PSO based search method is proposed to find optimal orthogonal code sets to reduce the inter carrier interference effects in an frequency redundant OFDM system. Fourth, a PSO based phase optimization technique is proposed in reducing the PAPR of an frequency redundant OFDM system. The PSO based approaches are compared with other canonical solutions for these communication problems and showed superior performance in many aspects. which are confirmed by analysis and simulation results provided respectively. Open questions and future
ï»żOpen questions and future works for the dissertation are proposed to serve as a guide for the future research efforts
Ultra Wideband Communications: from Analog to Digital
ï»żUltrabreitband-Signale (Ultra Wideband [UWB]) können einen
signifikanten Nutzen im Bereich drahtloser Kommunikationssysteme haben. Es
sind jedoch noch einige Probleme offen, die durch Systemdesigner und
Wissenschaftler gelöst werden mĂŒssen. Ein Funknetzsystem mit einer derart
groĂen Bandbreite ist normalerweise auch durch eine groĂe Anzahl an
Mehrwegekomponenten mit jeweils verschiedenen Pfadamplituden
gekennzeichnet. Daher ist es schwierig, die zeitlich verteilte Energie
effektiv zu erfassen. AuĂerdem ist in vielen FĂ€llen der naheliegende
Ansatz, ein kohÀrenter EmpfÀnger im Sinne eines signalangepassten Filters
oder eines Korrelators, nicht unbedingt die beste Wahl. In der vorliegenden
Arbeit wird dabei auf die bestehende Problematik und weitere
Lösungsmöglichkeiten eingegangen.
Im ersten Abschnitt geht es um âImpulse Radio UWBâ-Systeme mit
niedriger Datenrate. Bei diesen Systemen kommt ein inkohÀrenter EmpfÀnger
zum Einsatz. InkohÀrente Signaldetektion stellt insofern einen
vielversprechenden Ansatz dar, als das damit aufwandsgĂŒnstige und robuste
Implementierungen möglich sind. Dies trifft vor allem in AnwendungsfÀllen
wie den von drahtlosen Sensornetzen zu, wo preiswerte GerÀte mit langer
Batterielaufzeit nötigsind. Dies verringert den fĂŒr die KanalschĂ€tzung
und die Synchronisation nötigen Aufwand, was jedoch auf Kosten der
Leistungseffizienz geht und eine erhöhte Störempfindlichkeit gegenĂŒber
Interferenz (z.B. Interferenz durch mehrere Nutzer oder schmalbandige
Interferenz) zur Folge hat.
Um die Bitfehlerrate der oben genannten Verfahren zu bestimmen, wurde
zunÀchst ein inkohÀrenter Combining-Verlust spezifiziert, welcher
auftritt im Gegensatz zu kohÀrenter Detektion mit Maximum Ratio Multipath
Combining. Dieser Verlust hÀngt von dem Produkt aus der LÀnge des
Integrationsfensters und der Signalbandbreite ab.
Um den Verlust durch inkohÀrentes Combining zu reduzieren und somit die
Leistungseffizienz des EmpfÀngers zu steigern, werden verbesserte
Combining-Methoden fĂŒr Mehrwegeempfang vorgeschlagen. Ein analoger
EmpfÀnger, bei dem der Hauptteil des Mehrwege-Combinings durch einen
âIntegrate and Dumpâ-Filter implementiert ist, wird fĂŒr UWB-Systeme
mit Zeit-Hopping gezeigt. Dabei wurde die Einsatzmöglichkeit von dĂŒnn
besetzten Codes in solchen System diskutiert und bewertet. Des Weiteren
wird eine Regel fĂŒr die Code-Auswahl vorgestellt, welche die StabilitĂ€t
des Systems gegen Mehrnutzer-Störungen sicherstellt und gleichzeitig den
Verlust durch inkohÀrentes Combining verringert.
Danach liegt der Fokus auf digitalen Lösungen bei inkohÀrenter
Demodulation. Im Vergleich zum AnalogempfÀnger besitzt ein
DigitalempfÀnger einen Analog-Digital-Wandler im Zeitbereich gefolgt von
einem digitalen Optimalfilter. Der digitale Optimalfilter dekodiert den
Mehrfachzugriffscode kohÀrent und beschrÀnkt das inkohÀrente Combining
auf die empfangenen Mehrwegekomponenten im Digitalbereich. Es kommt ein
schneller Analog-Digital-Wandler mit geringer Auflösung zum Einsatz, um
einen vertretbaren Energieverbrauch zu gewÀhrleisten. Diese Digitaltechnik
macht den Einsatz langer Analogverzögerungen bei differentieller
Demodulation unnötig und ermöglicht viele Arten der digitalen
Signalverarbeitung. Im Vergleich zur Analogtechnik reduziert sie nicht nur
den inkohÀrenten Combining-Verlust, sonder zeigt auch eine stÀrkere
Resistenz gegenĂŒber Störungen. Dabei werden die Auswirkungen der
Auflösung und der Abtastrate der Analog-Digital-Umsetzung analysiert. Die
Resultate zeigen, dass die verminderte Effizienz solcher
Analog-Digital-Wandler gering ausfÀllt. Weiterhin zeigt sich, dass im
Falle starker Mehrnutzerinterferenz sogar eine Verbesserung der Ergebnisse
zu beobachten ist. Die vorgeschlagenen Design-Regeln spezifizieren die
Anwendung der Analog-Digital-Wandler und die Auswahl der Systemparameter in
AbhÀngigkeit der verwendeten Mehrfachzugriffscodes und der Modulationsart.
Wir zeigen, wie unter Anwendung erweiterter Modulationsverfahren die
Leistungseffizienz verbessert werden kann und schlagen ein Verfahren zur
UnterdrĂŒckung schmalbandiger Störer vor, welches auf Soft Limiting
aufbaut. Durch die Untersuchungen und Ergebnissen zeigt sich, dass
inkohÀrente EmpfÀnger in UWB-Kommunikationssystemen mit niedriger
Datenrate ein groĂes Potential aufweisen.
AuĂerdem wird die Auswahl der benutzbaren Bandbreite untersucht, um einen
Kompromiss zwischen inkohÀrentem Combining-Verlust und StabilitÀt
gegenĂŒber langsamen Schwund zu erreichen. Dadurch wurde ein neues Konzept
fĂŒr UWB-Systeme erarbeitet: wahlweise kohĂ€rente oder inkohĂ€rente
EmpfÀnger, welche als UWB-Systeme Frequenz-Hopping nutzen. Der wesentliche
Vorteil hiervon liegt darin, dass die Bandbreite im Basisband sich deutlich
verringert. Mithin ermöglicht dies einfach zu realisierende digitale
Signalverarbeitungstechnik mit kostengĂŒnstigen Analog-Digital-Wandlern.
Dies stellt eine neue Epoche in der Forschung im Bereich drahtloser
Sensorfunknetze dar.
Der Schwerpunkt des zweiten Abschnitts stellt adaptiven Signalverarbeitung
fĂŒr hohe Datenraten mit âDirect Sequenceâ-UWB-Systemen in den
Vordergrund. In solchen Systemen entstehen, wegen der groĂen Anzahl der
empfangenen Mehrwegekomponenten, starke Inter- bzw.
Intrasymbolinterferenzen. AuĂerdem kann die FunktionalitĂ€t des Systems
durch Mehrnutzerinterferenz und Schmalbandstörungen deutlich beeinflusst
werden. Um sie zu eliminieren, wird die âWidely Linearâ-Rangreduzierung
benutzt. Dabei verbessert die Rangreduzierungsmethode das
Konvergenzverhalten, besonders wenn der gegebene Vektor eine sehr groĂe
Anzahl an Abtastwerten beinhaltet (in Folge hoher einer Abtastrate).
ZusÀtzlich kann das System durch die Anwendung der R-linearen Verarbeitung
die Statistik zweiter Ordnung des nicht-zirkularen Signals vollstÀndig
ausnutzen, was sich in verbesserten SchÀtzergebnissen widerspiegelt.
Allgemeine kann die Methode der âWidely Linearâ-Rangreduzierung auch in
andern Bereichen angewendet werden, z.B. in âDirect
Sequenceâ-Codemultiplexverfahren (DS-CDMA), im MIMO-Bereich, im Global
System for Mobile Communications (GSM) und beim Beamforming.The aim of this thesis is to investigate key issues encountered in the
design of transmission schemes and receiving techniques for Ultra Wideband
(UWB) communication systems. Based on different data rate applications,
this work is divided into two parts, where energy efficient and robust
physical layer solutions are proposed, respectively.
Due to a huge bandwidth of UWB signals, a considerable amount of multipath
arrivals with various path gains is resolvable at the receiver. For low
data rate impulse radio UWB systems, suboptimal non-coherent detection is a
simple way to effectively capture the multipath energy. Feasible techniques
that increase the power efficiency and the interference robustness of
non-coherent detection need to be investigated. For high data rate direct
sequence UWB systems, a large number of multipath arrivals results in
severe inter-/intra-symbol interference. Additionally, the system
performance may also be deteriorated by multi-user interference and
narrowband interference. It is necessary to develop advanced signal
processing techniques at the receiver to suppress these interferences.
Part I of this thesis deals with the co-design of signaling schemes and
receiver architectures in low data rate impulse radio UWB systems based on
non-coherent detection.â We analyze the bit error rate performance of
non-coherent detection and characterize a non-coherent combining loss,
i.e., a performance penalty with respect to coherent detection with maximum
ratio multipath combining. The thorough analysis of this loss is very
helpful for the design of transmission schemes and receive techniques
innon-coherent UWB communication systems.â We propose to use optical
orthogonal codes in a time hopping impulse radio UWB system based on an
analog non-coherent receiver. The âanalogâ means that the major part of
the multipath combining is implemented by an integrate and dump filter. The
introduced semi-analytical method can help us to easily select the time
hopping codes to ensure the robustness against the multi-user interference
and meanwhile to alleviate the non-coherent combining loss.â The main
contribution of Part I is the proposal of applying fully digital solutions
in non-coherent detection. The proposed digital non-coherent receiver is
based on a time domain analog-to-digital converter, which has a high speed
but a very low resolution to maintain a reasonable power consumption.
Compared to its analog counterpart, itnot only significantly reduces the
non-coherent combining loss but also offers a higher interference
robustness. In particular, the one-bit receiver can effectively suppress
strong multi-user interference and is thus advantageous in separating
simultaneously operating piconets.The fully digital solutions overcome the
difficulty of implementing long analog delay lines and make differential
UWB detection possible. They also facilitate the development of various
digital signal processing techniques such as multi-user detection and
non-coherent multipath combining methods as well as the use of advanced
modulationschemes (e.g., M-ary Walsh modulation).â Furthermore, we
present a novel impulse radio UWB system based on frequency hopping, where
both coherent and non-coherent receivers can be adopted. The key advantage
is that the baseband bandwidth can be considerably reduced (e.g., lower
than 500 MHz), which enables low-complexity implementation of the fully
digital solutions. It opens up various research activities in the
application field of wireless sensor networks.
Part II of this thesis proposes adaptive widely linear reduced-rank
techniques to suppress interferences for high data rate direct sequence UWB
systems, where second-order non-circular signals are used. The reduced-rank
techniques are designed to improve the convergence performance and the
interference robustness especially when the received vector contains a
large number of samples (due to a high sampling rate in UWB systems). The
widely linear processing takes full advantage of the second-order
statistics of the non-circular signals and enhances the estimation
performance. The generic widely linear reduced-rank concept also has a
great potential in the applications of other systems such as Direct
Sequence Code Division Multiple Access (DS-CDMA), Multiple Input Multiple
Output (MIMO) system, and Global System for Mobile Communications (GSM), or
in other areas such as beamforming
Peak-to-Average-Power-Ratio (PAPR) Reduction Techniques for Orthogonal-Frequency-Division- Multiplexing (OFDM) Transmission
Wireless communication has experienced an incredible growth in the last decade. Two decades ago,the number of mobile subscribers was less than 1% of the world\u27s population. As of 2011, the number of mobile subscribers has increased tremendously to 79.86% of the world\u27s population.
Robust and high-rate data transmission in mobile environments faces severe problems due to the time-variant channel conditions, multipath fading and shadow fading. Fading is the main limitation on wireless communication channels. Frequency selective interference and fading, such as multipath fading, is a bandwidth bottleneck in the last mile which runs from the access point to the user. The last mile problem in wireless communication networks is caused by the environment of free space channels through which the signal propagates. Orthogonal Frequency Division Multiplexing (OFDM) is a promising modulation and multiplexing technique due to its robustness against multipath fading. Nevertheless, OFDM suffers from high Peak-to-Average- Power-Ratio (PAPR), which results in a complex OFDM signal.
In this research, reduction of PAPR considering the out-of-band radiation and the regeneration of the time-domain signal peaks caused by filtering has been studied and is presented. Our PAPR reduction was 30% of the Discrete Fourier Transform (DFT) with Interleaved Frequency Division Multiple Access (IFDMA) utilizing Quadrature Phase Shift Keying (QPSK) and varying the roll-off factor. We show that pulse shaping does not affect the PAPR of Localized Frequency Division Multiple Access (LFDMA) as much as it affects the PAPR of IFDMA. Therefore, IFDMA has an important trade-off relationship between excess bandwidth and PAPR performance, since excess bandwidth increases as the roll-off factor increases. In addition, we studied a low complexity clipping scheme, applicable to IFDMA uplink and OFDM downlink systems for PAPR reduction. We show that the performance of the PAPR of the Interleaved-FDMA scheme is better than traditional OFDMA for the uplink
transmission system. Our reduction of PAPR is 53% when IFDMA is used instead of OFDMA in the uplink direction. Furthermore, we also examined an important trade-off relationship between clipping distortion and quantization noise when the clipping scheme is used for OFDM downlink systems. Our results show a significant reduction in the PAPR and the out-of-band radiation caused by clipping for OFDM downlink transmission system
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