Browsing by Author "Speidel, Joachim (Prof. Dr.-Ing.)"

Now showing 1 - 11 of 11

Open Access
Auflösungserhöhung von sehr schnellen A/D- und D/A-Umsetzern
(2004) Bittel, Andreas; Speidel, Joachim (Prof. Dr.-Ing.)
Zur digitalen Übertragung von Breitbandkabelsignalen, wie sie in der Kopfstelle eines mit Koaxkabeln aufgebautes TV-Netzes bereit gestellt werden, wurden Untersuchungen angestellt. Dazu wurden A/D- (MAX 104 [1]) und D/A-Umsetzer (RDA 012 [2]) bei 0,5 GHz und 1 GHz Abtastrate mit nomineller Auflösung von 8 bit aufgebaut. Die Qualität der Übertragung mit diesen Bausteinen wurde durch Messung des Videofrequenz- bzw. HF-Störabstands bestimmt. Außerdem wurde die Auflösung in effektiven Bit einer nominellen 8-Bit-A/D-D/A-Umsetzung bestimmt. Um die Qualität der Übertragung zu verbessern, wurden zwei Methoden der Auflösungserhöhung der A/D-Umsetzung untersucht und verglichen: Auflösungserhöhung mit doppeltem Aussteuerbereich und die Methode der Auflösungserhöung mit einfachem Aussteuerbereich. Beide Verfahren benutzen zwei A/D-Umsetzer, wozu analytische Modelle entwickelt und mit Hilfe der Rechnersimulation überprüft wurden. So wurde die Abhängigkeit der Auflösung der beiden Methoden vom Gleichanteil des Eingangssignals der A/D-Umsetzer, ihren Referenzspannungen und ihrer Quantisierungsschwellengenauigkeit ermittelt. Weiterhin wurde für beide Methoden mit Sinussignalen die Frequenzbandbreite, in welcher eine Auflösungserhöhung erfolgt, am realen A/D-Umsetzersystem gemessen. Zur Einstellung des Gleichanteils im Eingangssignal der A/D-Umsetzer, der Referenzspannungen und der Abtastzeitpunkten bei Auflösungserhöhung mit doppeltem Aussteuerbereich wurde ein Maximum-Likelihood-Schätzer entwickelt, der Gleichanteil, Amplitude und Nullphase einer Halb-schwingung bestimmen kann.
Open Access
Channel estimation and precoding in closed-loop distributed multi-antenna systems
(2015) Wild, Thorsten; Speidel, Joachim (Prof. Dr.-Ing.)
Today’s wireless cellular systems are limited by inter-cell interference. Coordinated multi-point transmission and reception (CoMP) is a promising approach to cope with this problem. Here, multiple base stations act as a distributed antenna system, exchanging coordination information and potentially user data via backhaul in order to reduce this interference, increasing spectral efficiency and generating a more homogeneous user experience throughout the entire cell, especially at the currently weak cell edges. This dissertation aims at enabling coordinated multi-point systems by dealing with its current realization challenges. Accurate channel knowledge is required both at transmitter and receiver side in order to realize the CoMP gains. Hence, practical channel estimation algorithms are investigated and developed in order to get as accurate channel knowledge as possible with manageable computational complexity under realistic system operation points. Especially the often neglected obtainment of statistical parameter knowledge is included here, which is challenging under dynamic user scheduling conditions. A multi-user multi-cell channel estimator is provided which, even for larger number of coordinated cells, can get receiver performance fairly close to performance with perfect channel knowledge. Furthermore, different downlink precoding and receive combining strategies are compared against each other under imperfect channel knowledge. As the number of coordinated cells - the cluster size - is limited in practice, mobiles at cluster edges suffer from inter-cluster interference. A novel patented precoding and control signaling scheme is introduced in order to deal with the inter-cluster interference.
Open Access
Coding and modulation for spectral efficient transmission
(2010) Muhammad, Nabil Sven; Speidel, Joachim (Prof. Dr.-Ing.)
We investigate channel coding and modulation for bandwidth-efficient transmission. For the bit-interleaved coded modulation (BICM) system, we optimize mappings for quadrature amplitude modulation with respect to the resulting symbol and bit error rates at the demapper, and to the achievable BICM and signal set capacities. Further, we design mappings that are most suited for receivers, which exchange information between demapper and decoder in an iterative manner. The mappings are not restricted to certain symbol alphabets, and thus, the optimized mappings outperform similar state-of-the-art schemes. For multi-antenna systems, we examine multidimensional mappings as a means to minimize bit error rates at an iterative receiver. Finally, we derive for block codes over the binary erasure channel closed-form expressions for their transfer characteristics to be used in the extrinsic information transfer chart. Based on that, capacity approaching irregular, i.e., time-varying, codes are suggested.
Open Access
Decentralized interference coordination for the downlink of fully loaded heterogeneous wireless networks
(2019) Bleicher, Zarah M. L.; Speidel, Joachim (Prof. Dr.-Ing.)
The relentless evolution towards an overwhelming increase of mobile data traffic, where mobile phone subscribers demand the highest data rates and comprehensive coverage presents current and future mobile communication networks with demanding requirements. Previous homogeneous networks were mostly designed to optimize the sum capacity and peak data rates rather than take the individual user experience into account, and therefore failed to meet these requirements. Multi-layer networks, also known as heterogeneous networks, can improve the coverage and capacity of the cellular network and bring the network closer to the user. Moreover, the introduction of the smaller cells into the macro cellular network can improve the performance, especially in hotspots and indoors, which results in a better user experience. Frequency spectrum is rare and valuable, thus solely adding further bandwidth does not meet the demand. However, when reusing the bandwidth, inter-cell interference from neighboring cells leads to performance degradations, in particular for users located at the cell edges. Within a multi-layer network, additional and even more dynamic interference is present, caused by different kinds of cells, like macro-, pico-, femtocells, and relays. Therefore, addressing the interference issue is essential. This thesis examines suitable interference coordination algorithms and introduces an advanced interference coordination technique for heterogeneous networks. Whereas current techniques require significant communication between base stations, reduce the available bandwidth notably or do not consider interference between the small cells, the advanced technique is located in the small cells with only marginal information exchange. Its performance is investigated by means of computer simulations for fully loaded heterogeneous networks on the system layer. As a result, the proposed technique reduces the impact on the surrounding cells significantly, making in-home communication services attractive, leading to a tremendous advantage for service providers as well as the end-user.
Open Access
Equalization and blind signal combining algorithms for mobile television broadcast reception
(2016) Ahmed, Rana; Speidel, Joachim (Prof. Dr.-Ing.)
Open Access
Investigations of the error resilient rate distortion optimization for H.264 video transport over packet lossy channels
(2010) Zeng, Hao; Speidel, Joachim (Prof. Dr.-Ing.)
This dissertation first presents a rigorous analysis of the rate-distortion optimization (RDO) framework in H.264 and other similar video coding standards. The analysis is based on the rate-distortion theory and provides a deep theoretical insight into the RDO practice. In the analysis, the crucial assumptions and models on which the RDO is based, are formulated and discussed. Then the validity of deploying the RDO in error resilient video encoding is analyzed. Finally, the dissertation proposes and investigates two modification schemes to the error resilient RDO (ER-RDO) framework in the H.264 reference codec. The schemes are inspired by the aforementioned analyses and seek to find better complexity-performance balance points. The simulation results suggest that the performance of the existing H.264 ER-RDO framework has already been highly optimized. On the other side, algorithm complexity reduction is still possible using one of the proposed schemes.
Open Access
Low-complexity adaptive digital equalizers for electronic dispersion compensation in optical fiber links
(2022) Efinger, Daniel; Speidel, Joachim (Prof. Dr.-Ing.)
This thesis addresses electronic equalization of intersymbol interference caused by chromatic and polarization mode dispersion in intensity-modulated optical communication links with direct detection. The simple and cost-efficient system setup is, even at high bit rates of 40 Gbit/s and beyond, of interest for short-haul optical links in metropolitan, aggregation or local area networks. Therefore, this thesis investigates preferably simple and low-complexity equalizer structures, which are able to compensate well for the nonlinear characteristics and influences of the intensity-modulated optical communication link with direct detection. Starting with system modeling and the introduction to different equalization methods, we identify low-complexity feed-forward and decision-feedback equalizers in the first part of this thesis. We further put their chromatic and polarization mode dispersion compensation performance to the broader context by comparison to maximum likelihood sequence estimation. Finally, we come to the investigation of adaptation algorithms for equalizer coefficient adjustment, which accounts for the time-variant nature of polarization mode dispersion, while still targeting preferably simple and efficient realization.
Open Access
Mess- und Diagnoseverfahren für ein neuartiges Feldbus-System mit Mehrträger-Datenübertragung
(2018) Knaub, Alexander; Speidel, Joachim (Prof. Dr.-Ing.)
Die zunehmende Automatisierung der industriellen Prozesse stellt hinsichtlich Datenraten und Fehlersicherheit immer höhere Anforderungen an die eingesetzten Feldbusnetzwerke. Aus diesem Grund wurde in früheren Arbeiten ein neuartiges Feldbussystem erarbeitet, welches auf dem Mehrträger-Modulationsverfahren discrete multitone (DMT) basiert. Diese Arbeit befasst sich mit den Verfahren zur Systemdiagnose des neuartigen Feldbussystems auf der Modulationsebene. Hierfür werden verschiedene Konzepte der Diagnosewerkzeuge ausgearbeitet und umfassend diskutiert. Anhand von Simulationsergebnissen werden Anforderungen an die Eingangsimpedanz der Diagnosewerkzeuge ermittelt, sodass das Anschließen der Diagnosewerkzeuge an die Busleitung eine möglichst geringe Änderung des Systemzustands hervorruft. Es wird das Potenzial der Systemdiagnose mittels Messgeräten wie z.B. schnellen Speicheroszilloskopen aufgezeigt und der erforderliche Algorithmus zur Abtastratenwandlung beschrieben. Bei der hierfür notwendigen Interpolationsfilterung kann das Quantisierungsrauschen des Analog-Digital-Umsetzers verringert werden. Der Einfluss der Dimensionierung des Interpolationsfilters auf die Signalqualität wird analytisch und mit Simulationen untersucht. Zudem werden verschiedene Verfahren zur Schätzung und Korrektur der Abtasttaktabweichung beschrieben und hinsichtlich der Schätzgenauigkeit miteinander verglichen. Für eine lückenlose Verfolgung der über das Netzwerk übertragenen binären Daten müssen Diagnosewerkzeuge die DMT-Demodulation sehr schnell in Hardware vornehmen. Ein für diesen Zweck entwickeltes Diagnosewerkzeug auf Basis eines FPGA mit einer flexiblen Datenschnittstelle zu einem Diagnoseprogramm wird vorgestellt. Abschließend werden die hierdurch ermöglichten Diagnosealgorithmen beschrieben, einschließlich eines Ausblicks auf die Rekonstruktion der Netzwerktopologie anhand der Kanalmessungen.
Open Access
Performance analysis of rate adaptive wireless communication system with regenerative relaying using low density parity check codes
(2019) Chaoudhry, Bushra Bashir; Speidel, Joachim (Prof. Dr.-Ing.)
Open Access
Un-coordinated multi-user and inter-cell interference alignment based on partial and outdated information for large cellular networks
(2016) Aziz, Danish; Speidel, Joachim (Prof. Dr.-Ing.)
The cellular networks have gone through rapid evolution during the past decade. However, their performance is still limited due to the problem of interference. Therefore, interference management in current and future cellular networks is still an ongoing research topic. Interference Alignment is one of the techniques to manage the interference efficiently by using "align" and "suppression" strategy. In the first part of this thesis we focus on Coordinated inter cell interference alignment in a large cellular network. We assess the performance of interference alignment based transmit precoding under specific receiver strategies and coordination scenarios by comparing with different state of the art precoding schemes. We continue our assessment by considering imperfect channel state information at the transmitter. The results show that the gains of coordinated alignment based transmission are very sensitive to the receiver strategies and imperfections as compared to the other precoding schemes. However, in case of the availability of good channel conditions with very slow moving users, coordinated interference alignment outperforms the other baselines even with imperfect channel state information. In addition to that, we propose efficient user selection methods to enhance the performance of coordinated alignment. The results of our assessment draws important conclusions about the application of coordinated interference alignment in practical systems. In the second part of the thesis we consider a cellular system where each cell is serving multiple users simultaneously using the same radio resource. In this scenario, we have to manage not only the inter cell interference but also the multi user interference. For this purpose, we propose a novel Uncoordinated transmit precoding scheme for multi user cellular networks which is based on the alignment of multi user interference with partial and outdated inter cell interference. We show analytically that our scheme approaches the performance optimal transmission scheme. With the help of simulations we show that our proposal outperforms the state of the art non-alignment based multi user transmit precoding schemes We further propose user selection methods which exploit the diversity gains and improve the system spectral efficiency. In order to assess the feasibility of our proposal in a real system, we evaluate our scheme with practical constraints like imperfect information at the transmitter and limited feedback in uplink channel. For the proof of concept we also evaluate the performance of our scheme with measured channels using a software defined measurement platform. Finally, we also assess the application of our proposal in future heterogeneous networks. The outcome of our efforts states that as an interference alignment based transmission scheme, our scheme is a good candidate to manage the two dimensional interference in multi user cellular networks. It outperforms the non-alignment baselines in many scenarios even with practical constraints.
Open Access
Verfahren zur Verringerung nichtlinearer Effekte in Sender und Empfänger optisch kohärenter Mehrträgersysteme
(2016) Rörich, David; Speidel, Joachim (Prof. Dr.-Ing.)
Aufgrund des weltweit zunehmenden Datenverkehrs werden die Anforderungen an optische Kommunikationssysteme hinsichtlich Bitrate, spektraler Effizienz und Bandbreitenflexibilität in den kommenden Jahren steigen. Um diesen gerecht zu werden, ist für zukünftige Systeme unter anderem das Mehrträgerverfahren orthogonaler Frequenzmultiplex (OFDM) in Kombination mit kohärentem Empfang in der Diskussion. Da OFDM-Systeme jedoch empfindlich gegenüber nichtlinearen Störeinflüssen sind, stellt diese Arbeit Verfahren vor, die solche Einflüsse verringern. So werden Beeinträchtigungen der Kanalschätzung durch senderseitige Quantisierungsfehler untersucht und Möglichkeiten aufgezeigt, diese zu reduzieren oder sogar gänzlich zu eliminieren. Darüber hinaus wird der Einfluss der nichtlinearen Kennlinie des elektro-optischen Modulators studiert und demonstriert, dass sich nichtlineare Verzerrungen durch digitale Vorverarbeitung des Sendesignals deutlich verringern lassen. Empfängerseitig beschäftigt sich diese Arbeit mit Synchronisationsverfahren zur Rahmenerkennung und Schätzung der Frequenzabweichung zwischen Träger und Lokaloszillator. Hierfür werden digitale Verfahren entwickelt, die unter Einfluss von chromatischer Dispersion und Verstärker- sowie Phasenrauschen auch für Modulationsformate mit hoher spektraler Effizienz eine hinreichend genaue Schätzung liefern.