44 research outputs found
Wireless interference networks with limited feedback
Wir betrachten das Problem der Akquirierung von Kanalzustandsinformationen an den Sendern von drahtlosen Netzwerken und entwickeln Feedbackverfahren und Sendestrategien für verschiedene Netzwerk Architekturen. Die entwickelten Verfahren werden analysiert und die Skalierung der Performance des Gesamtsystems anhand bestimmter Systemparameter bestimmt. Zuerst betrachten wir eine einzelne Zelle eines zellularen Systems und nehmen an, dass die Beamformingvektoren durch ein festes Codebuch vorgegeben sind. Wir entwickeln und analysieren ein neues Feedbackverfahren, dass Flexibilität und Robustheit vereint und dadurch effiziente und zuverlässige Kommunikation mit den Empfängern ermöglicht. Eine Analyse des Verfahrens zeigt, dass die Skalierung des Ratenverlustes durch quantisierte Kanalzustandsinformation besser ist als bei vergleichbaren Verfahren. Für das Feedbackverfahren wird ein spezieller Algorithmus entwickelt der es ermöglicht Codebücher für verschiedene Kanalmodelle zu generieren und zu optimieren. Die analytischen Ergebnisse werden durch Simulationen validiert und bestätigen einen Gewinn gegenüber vergleichbaren Verfahren.
Anschließend betrachten wir zellulare Systeme mit mehreren Zellen. Wir charakterisieren die Freiheitsgrade (degrees of freedom) unter verschiedenen Annahmen über das Kanalmodell. Des weiteren entwickeln wir verschiedene Algorithmen, die die optimalen Freiheitsgrade erreichen können. Anschließend wird ein Feedbackverfahren entwickelt, dass den Feedbackaufwand für die entwickelten Algorithmen signifikant reduziert. Wir analysieren eine breite Klasse von zellularen Systemen die beliebige koordinierte Sendestrategien verwenden. Für diese Klasse von Systemen leiten wir die Skalierung des Ratenverlustes relativ zum Feedbackaufwand her. Abschließend zeigen wir, wie die analytischen Ergebnisse auf das entwickelte Feedbackverfahren angewendet werden können.
Im letzten Kapitel entwickeln wir ein Framework, dass das Potenzial von Compressed Sensing nutzt um den Messaufwand und Feedbackaufwand in zellularen Systemen mit vielen Teilnehmern signifikant zu reduzieren. Das Framework ermöglicht es die Datenraten der Nutzer innerhalb gegebener Fehlerschranken zu schätzen. Grundlage ist neben Compressed Sensing ein neues Messverfahren, dass die Überlagerung von Signalen im Kanal nutzt, um zufällige nicht adaptive Messungen der Kanalkoeffizienten am Empfänger zu ermöglichen. Diese Messungen werden zu einer zentralen Steuereinheit übertragen und dort dekodiert. Wir analysieren die Genauigkeit der Rekonstruktion für einen linearen und einen nicht-linearen Dekodierer und leiten die Skalierung mit der Anzahl der Messungen her. Abschließend zeigen wir, wie der entwickelte Ansatz in zellularen Systemen angewendet werden kann.We consider the problem of acquiring accurate channel state information at the transmitters of a wireless network. We develop different feedback and transmit strategies for different network architectures and analyze their performance. First, we consider a single cell of cellular system and assume that the beamforming vectors are given by a fixed transmit codebook. We develop and analyze a new feedback and transmit strategy which combines flexibility and robustness needed for efficient and reliable communication. We prove that it has better scaling properties compared to classical results on the limited feedback problem in the broadcast channel and that this benefit improves with an increasing number of transmit antennas. We show how feedback codebooks can be designed for different propagation environments. Link level and system level simulations sustain the analytic results showing performance gains of up to 50 % or 70 % compared to zeroforcing when using multiple antennas at the base station and multiple antennas or a single antenna at the terminals, respectively.
We characterize the degrees of freedom (i.e. the multiplexing gain) of multi-cellular systems under different assumptions on the channel model and for different system setups. We propose different algorithms that possibly achieve the optimal degrees of freedom. The first algorithm aims on aligning the interference at each receiver in a subspace of the available receive space. Our second algorithm aims on directly maximizing the signal-to-interference-plus-noise ratio (SINR) of all receivers. By allowing symbol extensions over time or frequency and including a user selection we are able to achieve the alignment of interference for many system setups and exploit multi-user diversity. For coordinated transmit strategies we find the scaling of the performance loss with the feedback load. A distributed interference alignment algorithm is introduced. The algorithm makes efficient use of quantized channel state information and significantly reduces the feedback overhead. We develop a framework that we call compressive rate estimation. To this end, we assume that the composite channel gain matrix (i.e. the matrix of all channel gains between all network nodes) is compressible which means it can be approximated by a sparse or low rank representation. We develop a sensing protocol that exploits the superposition principle of the wireless channel and enables the receiving nodes to obtain non-adaptive random measurements of columns of the composite channel matrix. The random measurements are fed back to a central controller who decodes the composite channel gain matrix (or parts of it) and estimates individual user rates. We analyze the rate loss for a linear and a non-linear decoder and find the scaling laws according to the number of non-adaptive measurements
Distributed precoding systems in multi-gateway multibeam satellites: regularization and coarse beamforming
This paper deals with the problem of beamforming design in a multibeam satellite, which is shared by different groups of terminals -clusters-, each served by an Earth station or gateway. Each gateway precodes the symbols addressed to its respective users; the design follows an MMSE criterion, and a regularization factor judiciously chosen allows to account for the presence of mutually interfering clusters, extending more classical results applicable to one centralized station. More importantly, channel statistics can be used instead of instantaneous channel state information, avoiding the exchange of information among gateways through backhaul links. The on-board satellite beamforming weights are designed to exploit the degrees of freedom of the satellite antennas to minimize the noise impact and the interference to some specific users. On-ground beamforming results are provided as a reference to compare the joint performance of MMSE precoders and on-board beamforming network.Agencia Estatal de Investigación | Ref. TEC2016-76409-C2-2-RAgencia Estatal de Investigación | Ref. TEC2016-75103-C2-2-RXunta de Galici
Advances in Multi-User Scheduling and Turbo Equalization for Wireless MIMO Systems
Nach einer Einleitung behandelt Teil 2 Mehrbenutzer-Scheduling für die
Abwärtsstrecke von drahtlosen MIMO Systemen mit einer Sendestation und
kanaladaptivem precoding: In jeder Zeit- oder Frequenzressource kann eine
andere Nutzergruppe gleichzeitig bedient werden, räumlich getrennt durch
unterschiedliche Antennengewichte. Nutzer mit korrelierten Kanälen sollten
nicht gleichzeitig bedient werden, da dies die räumliche Trennbarkeit
erschwert. Die Summenrate einer Nutzermenge hängt von den Antennengewichten
ab, die wiederum von der Nutzerauswahl abhängen. Zur Entkopplung des
Problems schlägt diese Arbeit Metriken vor basierend auf einer geschätzten
Rate mit ZF precoding. Diese lässt sich mit Hilfe von wiederholten
orthogonalen Projektionen abschätzen, wodurch die Berechnung von
Antennengewichten beim Scheduling entfällt. Die Ratenschätzung kann
basierend auf momentanen Kanalmessungen oder auf gemittelter Kanalkenntnis
berechnet werden und es können Datenraten- und Fairness-Kriterien
berücksichtig werden. Effiziente Suchalgorithmen werden vorgestellt, die
die gesamte Systembandbreite auf einmal bearbeiten können und zur
Komplexitätsreduktion die Lösung in Zeit- und Frequenz nachführen können.
Teil 3 zeigt wie mehrere Sendestationen koordiniertes Scheduling und
kooperative Signalverarbeitung einsetzen können. Mittels orthogonalen
Projektionen ist es möglich, Inter-Site Interferenz zu schätzen, ohne
Antennengewichte berechnen zu müssen. Durch ein Konzept virtueller Nutzer
kann der obige Scheduling-Ansatz auf mehrere Sendestationen und sogar
Relays mit SDMA erweitert werden. Auf den benötigten Signalisierungsaufwand
wird kurz eingegangen und eine Methode zur Schätzung der Summenrate eines
Systems ohne Koordination besprochen. Teil4 entwickelt Optimierungen für
Turbo Entzerrer. Diese Nutzen Signalkorrelation als Quelle von Redundanz.
Trotzdem kann eine Kombination mit MIMO precoding sinnvoll sein, da bei
Annahme realistischer Fehler in der Kanalkenntnis am Sender keine optimale
Interferenzunterdrückung möglich ist. Mit Hilfe von EXIT Charts wird eine
neuartige Methode zur adaptiven Nutzung von a-priori-Information zwischen
Iterationen entwickelt, die die Konvergenz verbessert. Dabei wird gezeigt,
wie man semi-blinde Kanalschätzung im EXIT chart berücksichtigen kann.
In Computersimulationen werden alle Verfahren basierend auf
4G-Systemparametern überprüft.After an introduction, part 2 of this thesis deals with downlink multi-user
scheduling for wireless MIMO systems with one transmitting station
performing channel adaptive precoding:Different user subsets can be served
in each time or frequency resource by separating them in space with
different antenna weight vectors. Users with correlated channel matrices
should not be served jointly since correlation impairs the spatial
separability.The resulting sum rate for each user subset depends on the
precoding weights, which in turn depend on the user subset. This thesis
manages to decouple this problem by proposing a scheduling metric based on
the rate with ZF precoding such as BD, written with the help of orthogonal
projection matrices. It allows estimating rates without computing any
antenna weights by using a repeated projection approximation.This rate
estimate allows considering user rate requirements and fairness criteria
and can work with either instantaneous or long term averaged channel
knowledge.Search algorithms are presented to efficiently solve user
grouping or selection problems jointly for the entire system bandwidth
while being able to track the solution in time and frequency for complexity
reduction.
Part 3 shows how multiple transmitting stations can benefit from
cooperative scheduling or joint signal processing. An orthogonal projection
based estimate of the inter-site interference power, again without
computing any antenna weights, and a virtual user concept extends the
scheduling approach to cooperative base stations and finally included SDMA
half-duplex relays in the scheduling.Signalling overhead is discussed and a
method to estimate the sum rate without coordination.
Part 4 presents optimizations for Turbo Equalizers. There, correlation
between user signals can be exploited as a source of redundancy.
Nevertheless a combination with transmit precoding which aims at reducing
correlation can be beneficial when the channel knowledge at the transmitter
contains a realistic error, leading to increased correlation. A novel
method for adaptive re-use of a-priori information between is developed to
increase convergence by tracking the iterations online with EXIT charts.A
method is proposed to model semi-blind channel estimation updates in an
EXIT chart.
Computer simulations with 4G system parameters illustrate the methods using realistic channel models.Im Buchhandel erhältlich:
Advances in Multi-User Scheduling and Turbo Equalization for Wireless MIMO Systems / Fuchs-Lautensack,Martin
Ilmenau: ISLE, 2009,116 S.
ISBN 978-3-938843-43-
Recommended from our members
Scheduling heuristic for reduced complexity of coordinated beamforming in large multi-carrier heterogeneous wireless networks
The research and development of wireless communication systems is often based on relatively simple models of the network topology, the radio channel and the radio propagation. This is considered to be mostly appropriate, as only under these conditions the complex technical problems in this field can be fully solved to their theoretical boundaries. However, it can also be the case that algorithms or concepts created under simplified assumptions perform in a significantly different way, when they are applied in more realistic scenarios.
This Thesis presents research work which can be seen as a step towards extending the existing research on Coordinated Beamforming to a complex network scenario, i.e. to a large-scale heterogeneous multi-carrier network. For this purpose, a complex simulation framework has been developed. This is used to analyse the significant implications the conditions in a complex network can have on the achievable performance gains. In more detail, the out of cluster interference and the number of mobile stations are identified as factors which heavily influence the performance. This knowledge is then used to design a novel scheduling heuristic, designed to be able to adapt to the particular network scenarios and to estimate the extent of the achievable performance gains. Our simulation results show that the new heuristic achieves significant performance gains for a low number of mobile stations (by applying zero forcing precoding) as well as for a high number of mobile stations (by a coordinated resource assignment that intelligently pairs mobile stations when applying maximum ratio transmission). The Thesis also demonstrates that the effect of the out of cluster interference can cause the reduction of the achievable gains. Due to the knowledge of performance limiting factors, the scheduling heuristic is in addition able to realize a trade-off between complexity and performance by excluding transmission parameters from the scheduling process which are not expected to be beneficial
Técnicas de equalização para MIMO massivo com amplificação não linear
The dawn of the new generation of mobile communications and the trafic
explosion that derives from its implementation pose great challenge. The
milimeter wave band and the use of massive number of antennas are technologies
which, when combined, allow the transmission of high data rate,
functioning in zones of the electromagnetic spectrum that are less explored
and with capability of allocation of dozens of GHz of bandwidth.
In this dissertation we consider a massive MIMO millimeter wave system
employing a hybrid architecture, i.e., the number of transmit and receive
antennas are lower than the number of radio frequency chains. As consequence,
the precoder and equalizers should be designed in both digital and
analog domains. In the literature, most of the proposed hybrid beamforming
schemes were evaluated without considering the effects of nonlinear amplifications. However, these systems face non-avoidable nonlinear effects due
to power amplifiers functioning in nonlinear regions. The strong nonlinear
effects throughout the transmission chain will have a negative impact on the
overall system performance and thus its study and the design of equalizers
that take into account these effects are of paramount importance.
This dissertation proposes a hybrid iterative equalizer for massive MIMO millimeter
wave SC-FDMA systems. The user terminals have low complexity,
just equipped with analog precoders based on average angle of departure,
each with a single radio frequency chain. At the base station it is designed
an hybrid analog-digital iterative equalizer with fully connected architecture
in order to eliminate both the multi-user interference and the nonlinear distortion
caused by signal amplification during the transmission. The equalizer
is optimized by minimizing the bit error rate, which is equivalent to minimize
the mean square error rate. The impact of the saturation threshold of the
amplifiers in the system performance is analysed, and it is demonstrated that
the iterative process can efficiently remove the multi-user interference and
the distortion, improving the overall system performance.O surgimento de uma nova geração de comunicações móveis e a explosão
de tráfego que advém da sua implementação apresenta grandes desafios. A
banda de ondas milimétricas e o uso massivo de antenas são tecnologias que,
combinadas, permitem atingir elevadas taxas de transmissão, funcionando
em zonas do espectro electromagnético menos exploradas e com capacidade
de alocação de dezenas de GHz para largura de banda.
Nesta dissertação foi considerado um sistema de MIMO massivo de ondas
milimétricas usando uma arquitectura híbrida, i.e., o número de antenas para
transmissão e recepção é menor que o número de cadeias de radiofrequência.
Consequentemente, o pré-codificador e equalizadores devem ser projectados
nos domínios digital e analógico. Na literatura, a maioria dos esquemas
híbridos de beamforming são avaliados sem ter em conta os efeitos de não linearidade
da amplificação do sinal. No entanto, estes sistemas sofrem
inevitavelmente de efeitos não lineares devido aos amplificadores de potência
operarem em regiões não lineares. Os fortes efeitos das não-linearidades ao
longo da cadeia de transmissão têm um efeito nefasto no desempenho do
sistema e portanto o seu estudo e projecto de equalizadores que tenham em
conta estes efeitos são de extrema importância.
Esta dissertação propõe um equalizador híbrido para sistemas baseados em
ondas milimétricas para MIMO massivo com modulação SC-FDMA. Os terminais
de utilizador possuem baixa complexidade, equipados apenas com
pré-codificadores analógicos baseados no ângulo médio de partida, cada um
com uma única cadeia de radiofrequência. Na estação base é projectado
um equalizador iterativo híbrido analógico-digital com arquitectura completamente
conectada de modo a eliminar a interferencia multi-utilizador e a
distorção causada pela amplificação do sinal aquando da transmissão. O
equalizador é optimizado minimizando a taxa de erro de bit, o que é equivalente
a minimizar a taxa de erro quadrático médio. O impacto do limiar
de saturação dos amplificadores no desempenho do sistema é analisado, e é
demonstrado que o processo iterativo consegue eliminar de modo eficiente
a interferência multi-utilizador e a distorção, melhorando o desempenho do
sistema.Mestrado em Engenharia Eletrónica e Telecomunicaçõe