207 research outputs found
Spectrum control and iterative coding for high capacity multiband OFDM
The emergence of Multiband Orthogonal Frequency Division Modulation (MB-OFDM) as an ultra-wideband (UWB) technology injected new optimism in the market through realistic commercial implementation, while keeping promise of high data rates intact. However, it has also brought with it host of issues, some of which are addressed in this thesis.
The thesis primarily focuses on the two issues of spectrum control and user capacity for the system currently proposed by the Multiband OFDM Alliance (MBOA). By showing that line spectra are still an issue for new modulation scheme (MB-OFDM), it proposes a mechanism of scrambling the data with an increased length linear feedback shift register (compared to the current proposal), a new set of seeds, and random phase reversion for the removal of line spectra. Following this, the thesis considers a technique for increasing the user capacity of the current MB-OFDM system to meet the needs of future wireless systems, through an adaptive multiuser synchronous coded transmission scheme. This involves real time iterative generation of user codes, which are generated over time and frequency leading to increased capacity. With the assumption of complete channel state information (CSI) at the receiver, an iterative MMSE algorithm is used which involves replacement of each users s signature with its normalized MMSE filter function allowing the overall Total Squared Correlation (TSC) of the system to decrease until the algorithm converges to a fixed set of signature vectors. This allows the system to be overloaded and user\u27s codes to be quasi-orthogonal. Simulation results show that for code of length nine (spread over three frequency bands and three time slots), ten users can be accommodated for a given QoS and with addition of single frequency sub-band which allows the code length to increase from nine to twelve (four frequency sub-bands and three time slots), fourteen users with nearly same QoS can be accommodated in the system. This communication is overlooked by a central controller with necessary functionalities to facilitate the process. The thesis essentially considers the uplink from transmitting devices to this central controller. Furthermore, analysis of this coded transmission in presence of interference is carried to display the robustness of this scheme through its adaptation by incorporating knowledge of existing Narrowband (NB) Interference for computing the codes. This allows operation of sub-band coexisting with NB interference without substantial degradation given reasonable interference energy (SIR=-l0dB and -5dB considered). Finally, the thesis looks at design implementation and convergence issues related to code vector generation whereby, use of Lanczos algorithm is considered for simpler design and faster convergence. The algorithm can be either used to simplify design implementation by providing simplified solution to Weiner Hopf equation (without requiring inverse of correlation matrix) over Krylov subspace or can be used to expedite convergence by updating the signature sequence with eigenvector corresponding to the least eigenvalue of the signature correlation matrix through reduced rank eigen subspace search
Ultra Wideband Communications: from Analog to Digital
Ultrabreitband-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 müssen. Ein Funknetzsystem mit einer derart
großen Bandbreite ist normalerweise auch durch eine große Anzahl an
Mehrwegekomponenten mit jeweils verschiedenen Pfadamplituden
gekennzeichnet. Daher ist es schwierig, die zeitlich verteilte Energie
effektiv zu erfassen. Außerdem ist in vielen Fällen der naheliegende
Ansatz, ein kohärenter Empfänger 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 „Impulse Radio UWB”-Systeme mit
niedriger Datenrate. Bei diesen Systemen kommt ein inkohärenter Empfänger
zum Einsatz. Inkohärente Signaldetektion stellt insofern einen
vielversprechenden Ansatz dar, als das damit aufwandsgünstige und robuste
Implementierungen möglich sind. Dies trifft vor allem in Anwendungsfällen
wie den von drahtlosen Sensornetzen zu, wo preiswerte Geräte mit langer
Batterielaufzeit nötigsind. Dies verringert den für die Kanalschätzung
und die Synchronisation nötigen Aufwand, was jedoch auf Kosten der
Leistungseffizienz geht und eine erhöhte Störempfindlichkeit gegenüber
Interferenz (z.B. Interferenz durch mehrere Nutzer oder schmalbandige
Interferenz) zur Folge hat.
Um die Bitfehlerrate der oben genannten Verfahren zu bestimmen, wurde
zunächst ein inkohärenter Combining-Verlust spezifiziert, welcher
auftritt im Gegensatz zu kohärenter Detektion mit Maximum Ratio Multipath
Combining. Dieser Verlust hängt von dem Produkt aus der Länge des
Integrationsfensters und der Signalbandbreite ab.
Um den Verlust durch inkohärentes Combining zu reduzieren und somit die
Leistungseffizienz des Empfängers zu steigern, werden verbesserte
Combining-Methoden für Mehrwegeempfang vorgeschlagen. Ein analoger
Empfänger, bei dem der Hauptteil des Mehrwege-Combinings durch einen
„Integrate and Dump”-Filter implementiert ist, wird für UWB-Systeme
mit Zeit-Hopping gezeigt. Dabei wurde die Einsatzmöglichkeit von dünn
besetzten Codes in solchen System diskutiert und bewertet. Des Weiteren
wird eine Regel für die Code-Auswahl vorgestellt, welche die Stabilität
des Systems gegen Mehrnutzer-Störungen sicherstellt und gleichzeitig den
Verlust durch inkohärentes Combining verringert.
Danach liegt der Fokus auf digitalen Lösungen bei inkohärenter
Demodulation. Im Vergleich zum Analogempfänger besitzt ein
Digitalempfänger einen Analog-Digital-Wandler im Zeitbereich gefolgt von
einem digitalen Optimalfilter. Der digitale Optimalfilter dekodiert den
Mehrfachzugriffscode kohärent und beschränkt das inkohärente Combining
auf die empfangenen Mehrwegekomponenten im Digitalbereich. Es kommt ein
schneller Analog-Digital-Wandler mit geringer Auflösung zum Einsatz, um
einen vertretbaren Energieverbrauch zu gewährleisten. 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 inkohärenten Combining-Verlust, sonder zeigt auch eine stärkere
Resistenz gegenüber 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 ausfällt. 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
Abhängigkeit der verwendeten Mehrfachzugriffscodes und der Modulationsart.
Wir zeigen, wie unter Anwendung erweiterter Modulationsverfahren die
Leistungseffizienz verbessert werden kann und schlagen ein Verfahren zur
Unterdrückung schmalbandiger Störer vor, welches auf Soft Limiting
aufbaut. Durch die Untersuchungen und Ergebnissen zeigt sich, dass
inkohärente Empfänger in UWB-Kommunikationssystemen mit niedriger
Datenrate ein großes Potential aufweisen.
Außerdem wird die Auswahl der benutzbaren Bandbreite untersucht, um einen
Kompromiss zwischen inkohärentem Combining-Verlust und Stabilität
gegenüber langsamen Schwund zu erreichen. Dadurch wurde ein neues Konzept
für UWB-Systeme erarbeitet: wahlweise kohärente oder inkohärente
Empfänger, 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 kostengünstigen Analog-Digital-Wandlern.
Dies stellt eine neue Epoche in der Forschung im Bereich drahtloser
Sensorfunknetze dar.
Der Schwerpunkt des zweiten Abschnitts stellt adaptiven Signalverarbeitung
für hohe Datenraten mit „Direct Sequence”-UWB-Systemen in den
Vordergrund. In solchen Systemen entstehen, wegen der großen Anzahl der
empfangenen Mehrwegekomponenten, starke Inter- bzw.
Intrasymbolinterferenzen. Außerdem kann die Funktionalität des Systems
durch Mehrnutzerinterferenz und Schmalbandstörungen deutlich beeinflusst
werden. Um sie zu eliminieren, wird die „Widely Linear”-Rangreduzierung
benutzt. Dabei verbessert die Rangreduzierungsmethode das
Konvergenzverhalten, besonders wenn der gegebene Vektor eine sehr große
Anzahl an Abtastwerten beinhaltet (in Folge hoher einer Abtastrate).
Zusätzlich kann das System durch die Anwendung der R-linearen Verarbeitung
die Statistik zweiter Ordnung des nicht-zirkularen Signals vollständig
ausnutzen, was sich in verbesserten Schätzergebnissen widerspiegelt.
Allgemeine kann die Methode der „Widely Linear”-Rangreduzierung auch in
andern Bereichen angewendet werden, z.B. in „Direct
Sequence”-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.● 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.● We propose to use optical
orthogonal codes in a time hopping impulse radio UWB system based on an
analog non-coherent receiver. The “analog” 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.● 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).● 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
Characterization of Ultra Wideband Multiple Access Performance Using Time Hopped-Biorthogonal Pulse Position Modulation
The FCC\u27s release of its UWB First Report and Order in April 2002 spawned renewed interest in impulse signaling research. This work combines Time Hopped (TH) multiple access coding with 4-ary UWB Biorthogonal Pulse Position Modulation (TH-BPPM). Multiple access performance is evaluated in a multipath environment for both synchronous and asynchronous networks. Fast time hopping is implemented by replicating and hopping each TH-BPPM symbol NH times. Bit error expressions are derived for biorthogonal TH-BPPM signaling and results compared with previous orthogonal TH-PPM work. Without fast time hopping (NH = 1), the biorthogonal TH-BPPM technique provided gains equivalent to Gray-coded QPSK; improved BER at a given Eb/No and an effective doubling of the data rate. A synchronized network containing up to NT = 15 transmitters yields an average BER improvement (relative to an asynchronous network) of approximately -6.30 dB with orthogonal TH-PPM and approximately 5.9 dB with biorthogonal TH-BPPM. Simulation results indicate that doubling the number of multipath replications (NMP) reduces BER by approximately 3.6 dB. Network performance degrades as NT and NMP increase and synchronized network advantages apparent in the NMP = 0 case diminish with multipath interference present. With fast time hopping (NH \u3e 1) improves BER performance whenever NMP \u3c NH while reducing effective data rate by 1/NH. Compared to the NH = 1 synchronized network, TH-BPPM modulation using NH = 10 provides approximately 5.9 dB improvement at NMP = 0 and approximately 3.6 dB improvement at NMP = 5. At NMP = 10, the BER for the hopped and NH = 1 cases are not statistically different; with NH = 10 hops, BER improvement varies from approximately 0.57 to 0.14 dB (minimal variation between synchronous and asynchronous network performance)
Performance Comparison of TR and FSRUWB System Using Particle Filter: Effects of Frequency, Data Rate, Multi-Path and Multi-Channel Communication
In this study, we introduced a novel scheme based on Transmitted References (TR) and Frequency Shifted Reference (FSR) for ultra-wideband (UWB) system. By taking into account tracking loop-based particle filtering together with a data collecting approach for single and multi-path channel situations, the suggested method is an enhanced model. Each particle's location is determined using this filtering technique, which is then utilised to calculate the timing inaccuracy and regulate the UWB system's timing pulse. Also, it can tackle the multimodal distribution of errors then effectively approximate the optimal solution. The data distribution is discretised via a number of particles that are weighted samples evolving concerning time duration. The simulation results show that, in terms of error rate, number of particles, and delay response, the recommended model of FSR-UWB with particle filter performs better than the TR-UWB with and without considering particle filter
Performance Enhancement of Ultra Wideband WPAN using Narrowband Interference Mitigation Techniques
A new promising technique adopted by 4G community is ultra-wideband technology, which offers a solution for high bandwidth, high data rate, low cost, low power consumption, position location capability etc. A conventional type of UWB communication is impulse radio, where very short transient pulses are transmitted rather than a modulated carrier. Consequently, the spectrum is spread over several GHz, complying with the definition of UWB. Currently, the Rake receiver used for spread spectrum is considered a very promising candidate for UWB reception, due to its capability of collecting multipath components. Since UWB signals occupy such a large bandwidth, they operate as an overlay system with other existing narrowband (NB) radio systems overlapping with their bands. In order to ensure a robust communication link, the issue of coexistence and interference of UWB systems with current indoor wireless systems must be considered. Ultra Wideband technology with its application, advantages and disadvantages are discussed in detail. Design of UWB short pulse and a detail study IEEE 802.15.3a UWB channel models statistical characteristics have been analyzed through simulation. Simulation studies are performed and improved techniques are suggested for interference reduction in both Impulse Radio based UWB and Transmitted Reference type of UWB system. Modified TR-UWB receiver with UWB pulse design at transmitter end and notch filtering at receiver’s front end proved to be more efficient in single NBI, multiple NBI and WBI suppression. Extensive simulation studies to support the efficacy of the proposed schemes are carried out in the MATLAB. Bit error rate (BER) performance study for different data rates over different UWB channel models are also analyzed using proposed receiver models. Performance improvement of TR-UWB system is noticed using the proposed techniques
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