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See detailExploiting Jamming Attacks for Energy Harvesting in Massive MIMO Systems
Al-Hraishawi, Hayder UL; Chatzinotas, Symeon UL; Ottersten, Björn UL

Scientific Conference (2021, June)

In this paper, the performance of an RF energy harvesting scheme for multi-user massive multiple-input multiple-output (MIMO) is investigated in the presence of multiple active jammers. The key idea is to ... [more ▼]

In this paper, the performance of an RF energy harvesting scheme for multi-user massive multiple-input multiple-output (MIMO) is investigated in the presence of multiple active jammers. The key idea is to exploit the jamming transmissions as an energy source to be harvested at the legitimate users. To this end, the achievable uplink sum rate expressions are derived in closed-form for two different antenna configurations. An optimal time-switching policy is also proposed to ensure user-fairness in terms of both harvested energy and achievable rate. Besides, the essential trade-off between the harvested energy and achievable sum rate are quantified in closed-form. Our analysis reveals that the massive MIMO systems can make use of RF signals of the jamming attacks for boosting the amount of harvested energy at the served users. Numerical results illustrate the effectiveness of the derived closed-form expressions over Monte-Carlo simulations. [less ▲]

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See detailExploiting linguistic resources for self-employment: Workplace practices and language use of Thai immigrant entrepreneurs in the German periphery
Serwe, Stefan Karl UL

Doctoral thesis (2015)

This thesis is about immigrants in self-employment in Germany. More specifically, it is about immigrants who make a culturally-endowed practice or knowledge economically available in their new home. These ... [more ▼]

This thesis is about immigrants in self-employment in Germany. More specifically, it is about immigrants who make a culturally-endowed practice or knowledge economically available in their new home. These immigrant business owners are generally referred to as ethnic entrepreneurs. Yet, according to research and widely-held public opinions in Germany, a problem that hampers the success of these businesses is the owner's lack of proficiency in the German language, which apparently impedes them from accessing institutional support and from offering their services to the majority population. However, the ways that language competence actually affects the entry of immigrants into self-employment and the execution of their daily work as business owners has received only insufficient attention, including from a sociolinguistic perspective. Hence, this thesis aspires to examine the pathways of immigrants into self-employment and how these pathways are shaped by the owners' language knowledge. Secondly, by analysing the workplace practices in close empirical detail, it aims to understand how the owners' language knowledge impacts the execution of their work and whether these practices constitute sites of language learning for the owners. Thirdly, it intends to document the challenges that the business owners face at their workplaces, in order to understand to what extent they are due to insufficient levels of language knowledge. In order to answer these research questions, this study focuses on three businesses owned and operated by first-generation female immigrants from Thailand in the federal state of Saarland in southwest Germany. Businesses by Thai immigrants are particularly interesting for the purposes of this investigation, because they have flourished in recent years and because they offer their services in markets that target the majority, primarily German-speaking population. The ventures are typical examples of ethnic small businesses created by Thai immigrants in Germany: Thai massage salons and food retails stores. The first is a large Thai massage salon run by Kanita, the second a small Thai massage salon managed by Patcharin, and the third business is a food retail store owned and operated by Wipa. The three owners (and their staff) differ in terms of their competence in German. Kanita has only minimal competence in German, Patcharin has partial competence, while Wipa has maximum competence in the language. The analysis of the pathways into self-employment of these three owners exhibits a number of similarities. In all three cases, their migration to Germany was triggered by marriage to a German national. Previously, they all had professional careers which were directly connected to their appropriation of English, a language that later enabled communication with their German husbands. While their move to Germany was generally motivated by prospects of a better future, they were unable to find work due to a lack of German proficiency. German was primarily learned informally. Informal learning was fostered by their prior experience of language learning, their individual engagement in the form of self-study and reflexivity, a deliberate exposure to German through media, and their engagement with native speakers in practices towards which that they had developed an affinity. Interestingly, these practices were either their previous professional practices or activities that they took up after migration and later on developed into their own businesses. Realising the demand for their services in the open market and among the majority population kindled Kanita's, Patcharin's and Wipa's flame to enter self-employment. They deliberately adapted their services to the requirements of their customers, but also in reaction to prejudices about their professions. The support of their husbands and other associates was also significant, as they handled tasks that were difficult to perform linguistically, as, for example, the registration of the businesses and the preparation of administrative paperwork. In sum, these findings suggest that an advanced standard level of German was no prerequisite for these owners to enter self-employment, but that the prospects of engaging in self-employed work acted as an incentive to improve their German language skills. A detailed analysis of the workplace practices at Kanita's Massage Salon shows that working with minimal competence in German is possible, but that its success depends on several factors. To complete their workplace actions, Kanita and her staff draw on both resources in their linguistic repertoires, Thai and German. In addition, all workplace actions are designed as routine actions with discursive routines that all staff members are able to master quickly. Their successful accomplishment also depends on the customers' familiarity with these actions. Therefore, formal inconsistencies in German do generally not impede the performance of work, but problems are primarily due to the staff's or the customer's inexperience with the routines. If problems occur, they tend to be solved in cooperation with colleagues or by drawing on the material resources available to the staff members. In comparison to Kanita, Patcharin and her staff have partial competence in German, which is instrumental for the performance of the key practices at their workplace. The analysis shows that their restricted competence in German is important for their work, as it provides the tools for Patcharin and her staff to perform discursive practices during the massage treatment, such as finding out about their clients' health problems, building rapport with clients, giving instructions or clarifications about the treatment, or providing assessments of their customers' health issues. Talk is an important part of the massage treatment at Patcharin's salon and it aids to construct the professional identity that Patcharin claims for herself, namely to provide a high-quality and personalised service to her customers. Thai is less relevant, but in interactions between Patcharin and her staff it serves to exchange information and coordinate work. Wipa has maximum competence in German and Thai, which allows her to manage her store and serve her customers independently and competently in line with her professional aspirations. The key practices at her store of explaining and ordering stock illustrate how Wipa relies on both the use of Thai and German to effectively perform these actions. Her maximum competence in German and in Thai permits her to make 'rational' choices about the suitability of her suppliers and to provide her customers with advice that is tuned to their linguistic and cultural background. The conclusions drawn from these findings are that an advanced competence in German in not a prerequisite for immigrant entrepreneurs to start their businesses. The owners attune their workplace actions to the level of competence in their linguistic repertoires and operate effectively. Moreover, self-employed work provides the owners with the necessary motivation and the need to appropriate German. On the other hand, the data suggest that a greater proficiency in German becomes important, if the immigrant entrepreneur wants to differentiate her business from direct competitors, as it allows them to move beyond the concrete performance of routine actions. [less ▲]

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See detailExploiting locality of interaction in factored Dec-POMDPs
Oliehoek, Frans A.; Spaan, Matthijs T. J.; Vlassis, Nikos UL et al

in Int. Joint Conf. on Autonomous Agents and Multi-Agent Systems (2008)

Decentralized partially observable Markov decision processes (Dec-POMDPs) constitute an expressive framework for multiagent planning under uncertainty, but solving them is provably intractable. We ... [more ▼]

Decentralized partially observable Markov decision processes (Dec-POMDPs) constitute an expressive framework for multiagent planning under uncertainty, but solving them is provably intractable. We demonstrate how their scalability can be improved by exploiting locality of interaction between agents in a factored representation. Factored Dec-POMDP representations have been proposed before, but only for Dec-POMDPs whose transition and observation models are fully independent. Such strong assumptions simplify the planning problem, but result in models with limited applicability. By contrast, we consider general factored Dec-POMDPs for which we analyze the model dependencies over space (locality of interaction) and time (horizon of the problem). We also present a formulation of decomposable value functions. Together, our results allow us to exploit the problem structure as well as heuristics in a single framework that is based on collaborative graphical Bayesian games (CGBGs). A preliminary experiment shows a speedup of two orders of magnitude. [less ▲]

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See detailExploiting Long-Term Channel Correlation in Limited Feddback SDMA through Channel Phase Codebook
Huang, Yongming; Yang, Luxi; Bengtsson, Mats et al

in IEEE Transactions on Signal Processing (2011), 59(3), 1217-1228

Improving channel information quality at the base station (BS) is crucial for the optimization of frequency division duplexed (FDD) multi-antenna multiuser downlink systems with limited feedback. To this ... [more ▼]

Improving channel information quality at the base station (BS) is crucial for the optimization of frequency division duplexed (FDD) multi-antenna multiuser downlink systems with limited feedback. To this end, this paper proposes to estimate a particular representation of channel state information (CSI) at the BS through channel norm feedback and a newly developed channel phase codebook, where the long-term channel correlation is efficiently exploited to improve performance. In particular, the channel representation is decomposed into a gain-related part and a phase-related part, with each of them estimated separately. More specifically, the gain-related part is estimated from the channel norm and channel correlation matrix, while the phase-related part is determined using a channel phase codebook, constructed with the generalized Lloyd algorithm. Using the estimated channel representation, joint optimization of multiuser precoding and opportunistic scheduling is performed to obtain an SDMA transmit scheme. Computer simulation results confirm the advantage of the proposed scheme over state of the art limited feedback SDMA schemes under correlated channel environment. [less ▲]

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See detailExploiting Long-Term Redundancies in Reconstructed Video
Stojanovic, Aleksandar UL; Ohm, Jens-Rainer

in IEEE Journal on Selected Topics in Signal Processing (2013)

Detailed reference viewed: 108 (5 UL)
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See detailExploiting Long-term Statistics in Spatially Correlated Multi-user MIMO Systems with Quantized Channel Norm Feedback
Björnson, Emil; Ottersten, Björn UL

in Proceedings IEEE International Conference on Acoustics, Speech, and Signal (ICASSP) (2008)

In wireless multiple antenna and multi-user systems, the spatial dimensions may be exploited to increase the performance by means of antenna gain, spatial diversity, and multi-user diversity. A limiting ... [more ▼]

In wireless multiple antenna and multi-user systems, the spatial dimensions may be exploited to increase the performance by means of antenna gain, spatial diversity, and multi-user diversity. A limiting factor in such systems is the channel information required by the transmitter to control the intra-cell interference. Herein, the properties of spatially correlated channels with longterm statistical information at the transmitter and fixed-rate feedback of the quantized Euclidean channel norm are analyzed using a spectral subspace decomposition framework. A spatial division multiple access scheme is proposed with interference suppression at the receiver and joint scheduling and zero-forcing beamforming at the transmitter. Closed-form expressions for first and second moments of the feedback conditional channel statistics are derived. It is shown that only a few bits of feedback are required to achieve reliable rate estimation and weighted sum-rate maximization. [less ▲]

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See detailExploiting orthogonality in DVB-S2X through time pre-compensation
Andrenacci, Stefano UL; Chatzinotas, Symeon UL; Vanelli-Coralli, Alessandro et al

in Proceedings of 2016 8th Advanced Satellite Multimedia Systems Conference and the 14th Signal Processing for Space Communications Workshop (2016)

The superframing option of the recent DVB-S2X standard specifies, for the so-called SF-Pilot fields, the use of the orthogonal set of Walsh-Hadamard (WH) sequences. In order to exploit this orthogonality ... [more ▼]

The superframing option of the recent DVB-S2X standard specifies, for the so-called SF-Pilot fields, the use of the orthogonal set of Walsh-Hadamard (WH) sequences. In order to exploit this orthogonality, waveforms coming from different beams to the k-th User Terminal (UT) should be quasi perfectly aligned in time. While in the downlink part of a terrestrial system this is quite straightforward, in satellite system, having a satellite as a relay, this is not the case, especially when large baudrates are considered in the transmission. A procedure to compensate for timing misalignment amongst waveforms is here presented and the advantages are quantified through numerical simulations.In particular, Channel State Information (CSI) estimation errors, which are fundamental for precoding techniques, are evaluated. While the focus of the work is on systems which enable precoding techniques, the procedure can be applied in each scenario which uses the superframing structure of DVB-S2X in an interference limited scenario. [less ▲]

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See detailExploiting polarization for spectrum awareness in cognitive satellite communications
Sharma, S. K.; Chatzinotas, Symeon UL; Ottersten, Björn UL

in Exploiting polarization for spectrum awareness in cognitive satellite communications (2014)

The continuously increasing demand of spectrum and current static spectrum allocation policies are rendering the available radio spectrum scarce. To address the problem of spectrum scarcity in the ... [more ▼]

The continuously increasing demand of spectrum and current static spectrum allocation policies are rendering the available radio spectrum scarce. To address the problem of spectrum scarcity in the satellite paradigm, cognitive satellite communications has been considered as a promising technique. In addition to the existing spectrum sharing dimensions such as frequency, time, and space, polarization can be exploited as an additional degree of freedom in order to explore the spectral gaps in the underutilized licensed spectrum. In this context, this chapter firstly provides an overview of the existing works in polarization-based spectrum sharing. Secondly, it presents the theoretical analysis of energy detection technique for dual polarized Additive White Gaussian Noise and Rayleigh fading channels considering the spectral coexistence scenarios of dual and hybrid satellite systems. Thirdly, it provides the comparison of different combining techniques in terms of the sensing performance. Finally, it provides interesting future research directions in this domain. © 2015 by IGI Global. All rights reserved. [less ▲]

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See detailExploiting Polarization for Spectrum Awareness in Cognitive Satellite Communications
Sharma, Shree Krishna UL; Chatzinotas, Symeon UL; Ottersten, Björn UL

in Software-Defined and Cognitive Radio Technologies for Dynamic Spectrum Access and Management (2015)

The continuously increasing demand of spectrum and current static spectrum allocation policies are rendering the available radio spectrum scarce. To address the problem of spectrum scarcity in the ... [more ▼]

The continuously increasing demand of spectrum and current static spectrum allocation policies are rendering the available radio spectrum scarce. To address the problem of spectrum scarcity in the satellite paradigm, cognitive Satellite Communications (SatComs) has been considered as a promising technique. In addition to the existing spectrum sharing dimensions such as frequency, time and space, polarization domain can be exploited as an additional degree of freedom in order to explore the spectral gaps in the under-utilized licensed spectrum. In this context, this chapter firstly provides an overview of the existing works in polarization-based spectrum sharing. Secondly, it presents the theoretical analysis of Energy Detection technique for dual polarized Additive White Gaussian Noise (AWGN) and Rayleigh fading channels considering the spectral coexistence scenarios of dual and hybrid satellite systems. Thirdly, it provides the comparison of different combining techniques in terms of the sensing performance in the considered dual polarized channels with the help of theoretical analysis and numerical results. Finally, it provides interesting future research directions in this domain. [less ▲]

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See detailExploiting Polarization for Spectrum Sensing in Cognitive SatComs
Sharma, Shree Krishna UL; Chatzinotas, Symeon UL; Ottersten, Björn UL

in Proceedings of 7th International Conference on Cognitive Radio Oriented Wireless Networks (2012, June)

Exploring new techniques for spectrum sensing (SS), which can detect the weak primary signals instantly, has been an important research challenge. In this paper, the problem of enhancing SS efficiency in ... [more ▼]

Exploring new techniques for spectrum sensing (SS), which can detect the weak primary signals instantly, has been an important research challenge. In this paper, the problem of enhancing SS efficiency in cognitive SatComs has been considered. The analysis of different combining techniques has been carried out for SS using dual polarized antenna. Furthermore, polarization states of received signals are exploited and based on obtained polarization states, Optimum Polarization Based Combining (OPBC) technique has been used for carrying out SS in the satellite terminal. The sensing performance of OPBC technique has been compared to selection combining (SC), equal gain combining (EGC) and maximum ratio combining (MRC) techniques. The simulation results show that OPBC technique achieves a great improvement in sensing efficiency over other considered techniques at the expense of complexity in a dual polarized AWGN channel. [less ▲]

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See detailExploiting Quantized Channel Norm Feedback Through Conditional Statistics in Arbitrarily Correlated MIMO Systems
Björnson, Emil; Hammarwall, David; Ottersten, Björn UL

in IEEE Transactions on Signal Processing (2009), 57(10), 4027-4041

In the design of narrowband multi-antenna systems, a limiting factor is the amount of channel state information (CSI) available at the transmitter. This is especially evident in multi-user systems, where ... [more ▼]

In the design of narrowband multi-antenna systems, a limiting factor is the amount of channel state information (CSI) available at the transmitter. This is especially evident in multi-user systems, where the spatial user separability determines the multiplexing gain, but it is also important for transmission-rate adaptation in single-user systems. To limit the feedback load, the unknown and multi-dimensional channel needs to be represented by a limited number of bits. When combined with long-term channel statistics, the norm of the channel matrix has been shown to provide substantial CSI that permits efficient user selection, linear precoder design, and rate adaptation. Herein, we consider quantized feedback of the squared Frobenius norm in a Rayleigh fading environment with arbitrary spatial correlation. The conditional channel statistics are characterized and their moments are derived for both identical, distinct, and sets of repeated eigenvalues. These results are applied for minimum mean square error (MMSE) estimation of signal and interference powers in single- and multi-user systems, for the purpose of reliable rate adaptation and resource allocation. The problem of efficient feedback quantization is discussed and an entropy-maximizing framework is developed where the post-user-selection distribution can be taken into account in the design of the quantization levels. The analytic results of this paper are directly applicable in many widely used communication techniques, such as space-time block codes, linear precoding, space division multiple access (SDMA), and scheduling. [less ▲]

Detailed reference viewed: 145 (0 UL)
See detailExploiting Resource Heterogeneity in DTN
Sandulescu, Gabriel UL; Schaffer, Peter UL; Nadjm-Tehrani, Simin UL

in Wireless Communications & Mobile Computing (2012), 13(3), 230-243

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See detailExploiting the Hard-wired Vulnerabilities of Newscast via Connectivity-splitting Attack
Muszynski, Jakub UL; Varrette, Sébastien UL; Jimenez Laredo, Juan Luis UL et al

in Proc. of the IEEE Intl. Conf. on Network and System Security (NSS 2014) (2014, October)

Newscast is a model for information dissemination and mem- bership management in large-scale, agent-based distributed systems. It deploys a simple, peer-to-peer data exchange protocol. The Newscast pro ... [more ▼]

Newscast is a model for information dissemination and mem- bership management in large-scale, agent-based distributed systems. It deploys a simple, peer-to-peer data exchange protocol. The Newscast pro- tocol forms an overlay network and keeps it connected by means of an epidemic algorithm, thus featuring a complex, spatially structured, and dynamically changing environment. It has recently become very popu- lar due to its inherent resilience to node volatility as it exhibits strong self-healing properties. In this paper, we analyze the robustness of the Newscast model when executed in a distributed environment subjected to malicious acts. More precisely, we evaluate the resilience of Newscast against cheating faults and demonstrate that even a few naive cheaters are able to defeat the protocol by breaking the network connectivity. Concrete experiments are performed using a framework that implements both the protocol and the cheating model considered in this work. [less ▲]

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See detailExploiting the Potential of Process Data in Complex Problem Solving Assessment
Greiff, Samuel UL; Müller, Jonas UL

Scientific Conference (2014, March)

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See detailExploiting Transistor-Level Reconfiguration to Optimize Combinational Circuits on the Example of a Conditional Sum Adder
Raitza, Michael; Kumar, Akash; Volp, Marcus UL et al

Scientific Conference (2017, March)

Silicon nanowire reconfigurable field effect transistors (SiNW RFETs) abolish the physical separation of n-type and p-type transistors by taking up both roles in a configurable way within a doping-free ... [more ▼]

Silicon nanowire reconfigurable field effect transistors (SiNW RFETs) abolish the physical separation of n-type and p-type transistors by taking up both roles in a configurable way within a doping-free technology. However, the potential of transistor-level reconfigurability has not been demonstrated in larger circuits, so far. In this paper, we present first steps to a new compact and efficient design of combinational circuits by employing transistor-level reconfiguration. We contribute new basic gates realized with silicon nanowires, such as 2/3-XOR and MUX gates. Exemplifying our approach with 4-bit, 8-bit and 16-bit conditional carry adders, we were able to reduce the number of transistors to almost one half. With our current case study we show that SiNW technology can reduce the required chip area by 16 %, despite larger size of the individual transistor, and improve circuit speed by 26 %. [less ▲]

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See detailExploration du territoire et déambulation de la mémoire. Constructions d’identités dans l’œuvre d’Anne-Lise Grobéty
Barthelmebs-Raguin, Hélène UL

in CRIN : Cahiers de Recherches des Instituts Néerlandais de Langue et Littérature Françaises (2016)

Detailed reference viewed: 37 (0 UL)
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See detailExploration of Different School Value-Added Models in a Highly Heterogeneous Educational Context
Levy, Jessica UL; Brunner, Martin; Keller, Ulrich UL et al

Scientific Conference (2019, April)

Detailed reference viewed: 107 (16 UL)