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We present in this paper indoor measurements at 2.45 GHz of a tripole antenna pair and investigate typical channel characteristics that include the antenna elements. Each of the two antenna arrays consists of three orthogonal co-located dipoles promising high capacity, irrelevant of their orientation. The impact of classic parameters related to polarization, correlation and time dispersion are analyzed...
In this work, we investigate the impact of user clustering on the performance of femtocell-enhanced macrocellular networks. Various degrees of clustering are evaluated: A low degree corresponds to almost uniform user distributions, whereas a high degree relates to densly populated user hot-spots. Clustered user-scenarios are generated by applying a Poisson cluster process. We start out with a plain...
In previous work, we have analyzed the convergence properties of the so-called Projection Approximation Subspace Tracking algorithm (PAST) by means of Singular Value Decomposition methods. Based on this, we extend our analysis and study the global convergence properties of two distributed variants of the PAST algorithm based on average consensus. We provide an insight regarding convergence in the...
We propose a novel distributed QR factorization algorithm for orthogonalizing a set of vectors in a wireless sensor network. The algorithm originates from the classical Gram-Schmidt orthogonalization which we formulate in a distributed way using the dynamic consensus algorithm. In contrast to existing distributed QR factorization algorithms, all elements of matrices Q and R are computed simultaneously...
Identification of microwave power amplifiers including their memory effects, can be done computationally inexpensive with adaptive Wiener models which consist of a transversal filter followed by a static non-linearity that is parametrised using a polynomial basis. For identification of both blocks by simultaneously running gradient type algorithms, the resulting combined adaptive scheme suffers from...
In this work, we discuss the possibility of using precoding as means to mitigate Inter Carrier Interference (ICI) caused by temporal channel variations in Orthogonal Frequency Division Multiplexing (OFDM) systems. Many different precoding schemes have been introduced in the past. However, the promised gains of these techniques are not achievable nor realistic. In this paper, we introduce a practical...
Current Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) based systems for wireless communications enable to adjust power radiated at the pilot symbols. Under time-invariant channels, a power increase at the pilot symbols results in improved quality of the channel estimate. However, under time-variant channels, this is not necessarily the case. Due to the decreased...
In this paper, we focus on a substantial throughput increase of a Radio Frequency Identification (RFID) multiantenna system. We investigate the influence of the channel estimation and the receiver structure on the theoretical throughput of Framed Slotted Aloha (FSA) based RFID systems. We propose an increase of the theoretical throughput by resolving collisions and acknowledging more than one tag...
In this contribution we present a stronger notion of almost sure convergence for a large class of consensus algorithms including also asynchronous updates. We introduce the concept of the so-called relaxed projection algorithms and show that many consensus algorithms can be interpreted as such relaxed projection updates. It is well known that such algorithms converge to a solution lying in the intersection...
Wireless sensor networks possess many challenges. One of the most significant is routing. In this paper, we focus on so-called Self-organizing Coordinate Systems, which are perceived as the current state of the art. Protocols included in this class are intended to create a virtual coordinate system within the network, applying principles of geographical routing. We aim to find a solid starting point...
Many candidate technologies for future wireless communication systems (e.g., multi-user MIMO, interference alignment), rely on accurate Channel State Information (CSI) at the Transmitter (CSIT) to determine the appropriate multi-antenna pre-processing steps. Thus, in frequency division duplexing systems, a dedicated feedback link has to be provided to each attached user for CSI reporting. In order...
In this paper, we present algorithms for the quantization of a correlated unit norm random vector process. Such algorithms are important, e.g., for channel vector quantization in wireless communication systems. Starting from a quantization codebook that uniformly quantizes the unit sphere, we propose to iteratively adapt the codebook to the channel statistics, in order to improve the quantization...
This paper presents an SINR prediction model for LTE systems with the aim of improving the accuracy of physical layer models used for higher-than-physical-level simulations. This physical layer SINR abstraction for zero-forcing receivers takes channel estimation errors into account. It allows for the calculation of the post-equalization receive SINR of an LTE MIMO transmission on a per-layer and subcarrier...
We present a large-scale cell based measurement analysis of the user behavior in a live operational HSDPA network. The motivations are: first, to understand the statistical properties of users in cells for refining network planning procedures; and second, to provide realistic traffic models for simulations of cellular packet-oriented networks. We provide an analysis of mean cell load over daytime,...
The routing possesses a serious challenge in adhoc wireless sensor networks. In this paper, we focus on so-called self-organizing coordinate systems, which are perceived as the current state of the art in this area. More specifically, we focus on providing of a solid starting point by a detection of nodes located on a network's border. Our intention is to limit the number of nodes involved in this...
Multiple tags in Radio Frequency Identification (RFID) systems are scheduled by a medium access control layer using the Framed Slotted Aloha (FSA) or binary tree protocol. The focus of our research is on FSA and passive Ultra High Frequency (UHF) RFID. In current standards, only one tag can be acknowledged per slot. In this work we propose the increase of the theoretical throughput of FSA RFID systems...
In this paper, we evaluate the performance of a standardized OFDM system, namely Long Term Evolution (LTE) downlink, with imperfect symbol timing. A closed form expression of the post-equalization Signal to Interference and Noise Ratio (SINR) is derived and compared with results obtained from a standard compliant simulator. Also, we analyzed the channel estimation performance when Symbol Timing Offset...
The detailed understanding of packet delays in modern wireless networks is crucial to optimize applications and protocols. We conducted high precision latency measurements in operational LTE and HSPA networks, deploying a hybrid approach of active probing and withe-box testing. It allowed us to separately assess the one-way delay contributions of the radio access network and the core network for both...
Discussions of how to measure the performance of computer networks for various applications have been ongoing for over twenty years in the area of network research. The continual increase of data traffic volume has reached a point at which solving any network problem by over-provisioning is not suitable. The quest for alternatives makes it vital to have well-defined metrics for evaluating and sustaining...
In this contribution we analyze the Projection Approximation Subspace Tracking (PAST) algorithm by a novel analysis method based on Singular Value Decomposition (SVD). Based on the gained new insights we propose several robust algorithmic modifications that allow to guarantee stability of the algorithm run for normalized step-sizes. Furthermore we investigate the tracking behavior for such step-size...
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