PUBLIKATIONEN in 2011
International Symposium on Wireless Communication Systems, ISWCS 2011: Homepage
H. Degenhardt, T. Unger & A. Klein, „Self-interference aware MMSE filter design for a cellular multi-antenna two-way relaying scenario“, in Proc. Proceedings of the Eighth International Symposium on Wireless Communication Systems ISWCS, Aachen, Germany, Nov 2011.
A single cell scenario with different multi-antenna nodes, i.e., a base station and several mobile stations, is considered. An intermediate multi-antenna relay station is used to support the bidirectional communications. In such a scenario, the required data rates for up- and downlink are typically different which is considered by introducing asymmetric rate requirements. Non-regenerative two-way relaying is applied and the nodes can subtract the back-propagated self-interference. To fully utilize this capability of the nodes for the transceive filter design at the relay station, an MMSE based transceive filter is derived which does not suppress self-interference. Thus, the required number of antennas at the relay station is reduced. Additionally, the transceive filter is optimized for asymmetric rate requirements by introducing a weighting parameter. Furthermore, an alternating optimization between the transceive filter at the relay station and the transmit and receive filters at the nodes is introduced to fulfill the asymmetric rate requirements and to increase the achievable sum rates. Simulation results show that the proposed filter design at the nodes and at the relay station requires less antennas and achieves higher sum rates compared to conventional approaches.
International Symposium on Wireless Communication Systems, ISWCS 2011: Homepage
R. SivaSiva Ganesan & A. Klein, „Projection based Space-Frequency Interference Alignment in a Multi-Carrier Multi-User Two-Way Relay Network“, in Proc. IEEE International Symposium on Wireless Communication Systems, pp. 566-570, Nov 2011
We consider a multi-carrier multi-user two-way relay network with multiple single antenna node pairs and a single nonregenerative relay with two antennas. In a conventional multi-user two-way relaying scheme where different node pairs transmit their data streams on different subcarriers, only one data stream per channel use can be achieved. Taking into account that the nodes can perform self interference cancellation, one antenna at the relay is sufficient to achieve one data stream per channel use. Hence, the two antennas at the relay are not fully utilized. In this paper, using the interference alignment technique in spacefrequency dimensions, it is shown that when the number of users in the system is large, almost two data streams per channel use are achievable. The gain comes from the joint consideration of the spatial and frequency dimensions at the relay. A new projection based interference alignment algorithm is proposed. The ideas of signal alignment and channel alignment  are used to achieve the interference alignment solution. Furthermore, for the performance improvement of the proposed interference alignment scheme at moderate Signal to Noise Ratios (SNRs), a new gradient based method for optimization of the projection space at the relay is proposed.
International OFDM-Workshop, InOWo'11: Homepage
A. Kühne & A. Klein, „Consideration of inaccurate outdating parameters in adaptive OFDMA“, in Proc. 16th International OFDM-Workshop InOWo'11, Hamburg, Germany, Sep 2011
In this paper, an adaptive multi-user OFDMA scheme in the downlink is considered which is aware of outdated channel knowledge. Assuming that the base station has knowledge about the outdating parameters in terms of correlation coefficients between the outdated channels and the actual channels of the different users, it is possible to adjust the Signal-to-Noise Ratio (SNR) thresholds for the applied modulation schemes to guarantee a certain target Bit Error Rate (BER) in the presence of outdated Channel Quality Information (CQI). As these outdating parameters cannot be assumed to be perfectly known in a realistic scenario, the impact of inaccurate outdating parameters on the system performance is investigated. Furthermore, a safety margin approach is presented to reduce the loss in system performance.
IEEE Vehicular Technology Conference, VTC2011: Homepage
R. SivaSiva Ganesan, T. Weber & A. Klein, „Interference Alignment in Multi-User Two-Way Relay Networks“, in Proc. IEEE Vehicular Technology Conference, May 2011.
In this paper, we consider a bidirectional communication between K node pairs, where each node is equipped with multiple antennas and has a message to be transmitted to its communication partner. There is no direct link between the 2K nodes and a relay is employed to enable the communication. It is assumed that the relay does not have enough antennas to perform receive and transmit zero forcing. As interferences are unavoidable at each receiver, we call this channel K user pair symmetric relay interference channel. We show that the Degrees of Freedom (DoF) achievable by the multiple input multiple output (MIMO) Interference Alignment in such a K user pair symmetric relay interference channel are the same as in a K user symmetric interference channel  without a relay. Besides this, the presence of a relay simplifies the algorithm for computing the alignment solution. We propose a two step closed form solution for the alignment problem based on the two way relaying protocol.
IEEE Wireless Communications and Networking Conference, WCNC: Homepage
H. Degenhardt & A. Klein, „Non-regenerative multi-antenna two-hop relaying under an asymmetric rate constraint“, in Proc. IEEE Wireless Communications and Networking Conference (WCNC 2011), Cancun, Mexico, Mar 2011.
In this work, the combination of one-way and two-way relaying is examined in a scenario where two multi-antenna nodes exchange information under an asymmetric rate constraint via a half-duplex non-regenerative multi-antenna relay. Two-way relaying overcomes the multiplexing loss of one-way relaying, but additional optimizations are required to fulfill the asymmetric rate constraint. To enable the consideration of suboptimal lowcomplexity approaches, a hybrid one-way / two-way scheme is suggested which ensures that the asymmetric rate constraint can always be fulfilled. The optimization problem of maximizing the overall sum rate for the considered scenario is formulated and an approach for transceive filter and power optimization considering the asymmetric rate constraint is derived. Simulation results for different link qualities and rate constraints confirm the theoretical considerations and show that the proposed algorithm performs close to the upper bound in case of high asymmetric rate constraints or highly asymmetric channels.
Internat. Conference on Information, Communications and Signal Processing, ICICS: Homepage
T. Hunziker, U. Ur Rehman, D. Dahlhaus, „Spectrum sensing in cognitive radios: design of DFT filter banks achieving maximal time-frequency resolution“, Proc. 8th Int. Conf. on Information, Communications and Signal Processing, Singapore, Dec. 2011
Filter banks facilitate an estimation of the power spectral density of broad-band non-stationary signals, an operation required in many cognitive radio systems. The samples at the filter bank output may serve as a basis for an estimation of the input signal energy within any time-frequency (TF) region of interest. In order to achieve a high resolution in both time and frequency, the prototype window underlying a Discrete Fourier Transform (DFT) filter bank needs to exhibit high TF concentration. Moreover, in order to provide uncorrelated samples in case of white input processes, the TF-translated versions of the prototype window that underlie the elementary filtering operations need to constitute an orthogonal set. In this paper we present a technique to design DFT filter banks that possess these two required properties in an optimal manner. The numerical optimization procedure takes advantage of a parametrization of paraunitary filter banks and relies on semidefinite programming. We analyze the residual leakage of our optimized filter banks and draw a comparison against Thomson’s multitaper method.
General Assembly and Scientific Symposium, URSIGASS: Homepage
Kynthia Stavrakakis (1), Tilmann Wittig (2), Wolfgang Ackermann(1), and Thomas Weiland(1)
(1) Institut für Theorie Elektromagnetischer Felder, Technische Universität Darmstadt, Schlossgartenstr. 8, 64289 Darmstadt, Germany; firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
(2) Computer Simulation Technology AG, Bad Nauheimer Str. 19, 64289 Darmstadt, Germany; email@example.com
„Model Order Reduction Methods for Multivariate Parameterized Dynamical Systems obtained by the Finite Integration Theory“, at General Assembly and Scientific Symposium, 2011 XXXth URSI Digital Object Identifier: 10.1109/URSIGASS.2011.6050259, Publication Year: 2011 , Page(s): 1 – 4
In electrodynamic field computations the continuousMaxwell equations are typically discretized in the space variables, i. e the continuous space is mapped onto a finite set of discrete elements leading to a system of differential equations constituting the Maxwell grid equations. These dynamical systems can be very large. Due to limited computational, accuracy and storage capabilities, simplified models, obtained by means of model order reduction (MOR) methods, which capture the main features of the original model are then successfully used instead of the original models. Most commonly MOR via projection is used. Variation of model parameters like geometrical or material parameters give rise to multivariate dynamical systems. It is aimed that also the simplified models keep this parameter dependence. In this work, MOR methods are presented for multivariate systems based on the finite integration technique (FIT). The methods are applied to numerical examples with both geometrical and material variations.
Books, Chapters in Books
A. Kühne & A. Klein, „Combination of Adaptive and Non-Adaptive Multi-User OFDMA Schemes in the Presence of User-Dependent Imperfekt CSI“, in OFDM – Concepts for Future Communications Systems, H. Rohling (Eds.), pp. 142-145; Springer Verlag, no. ISBN: 978-3-642-17495-7, 2011.