Wolfgang Utschick

Technische Universität München, München, Bavaria, Germany

Are you Wolfgang Utschick?

Claim your profile

Publications (323)238 Total impact

  • Thomas Wiese · Maximilian Riemensberger · Wolfgang Utschick

    No preview · Article · Jan 2016 · IEEE Transactions on Signal Processing
  • Source
    Rainer Strobel · Andreas Barthelme · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: Hybrid copper/fiber networks bridge the gap between the fiber link at the distribution point and the customer by using copper wires over the last meters. The G.fast transmission technology has been designed to be used in such a fiber to the distribution point (FTTdp) network. Crosstalk management using MIMO precoding in downlink and MIMO equalization in uplink is a key to the required performance of FTTdp. Currently, there are mostly linear and nonlinear zero-forcing (ZF) methods discussed for precoding in G.fast. This paper presents a spectrum optimization algorithm for both zero-forcing precoding methods. Minimum mean squared error (MMSE) precoding shows significant advantages under certain channel conditions. A performance comparison of MMSE precoding for G.fast with linear and nonlinear zero-forcing methods indicates that MMSE pre-coding outperforms standard linear and nonlinear ZF precoding. Applying the proposed spectrum optimization brings all three methods to a similar performance.
    Full-text · Conference Paper · Dec 2015
  • Quan Kuang · Xiangbin Yu · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: Interference coupling in heterogeneous networks introduces the inherent non-convexity to the network resource optimization problem, hindering the development of effective solutions. A new framework based on multi-pattern formulation has been proposed in this paper to study the energy efficient strategy for joint cell activation, user association and multicell multiuser channel allocation. One key feature of this interference pattern formulation is that the patterns remain fixed and independent of the optimization process. This creates a favorable opportunity for a linear programming formulation while still taking interference coupling into account. A tailored algorithm is developed to solve the formulated network energy saving problem in the dual domain by exploiting the problem structure, which gives a significant complexity saving compared to using standard solvers. Numerical results show a huge improvement in energy saving achieved by the proposed scheme.
    No preview · Article · Nov 2015
  • Source
    Rainer Strobel · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: Hybrid copper/fiber networks bridge the gap between the fiber link and the customer by using copper wires over the last meters. This solution combines energy efficiency and low cost of the copper network with higher fiber data rates. ITU recently finished the G.fast standard for high speed data transmission on copper wires for this application. Coexistence with legacy VDSL2 systems is an important topic for the introduction of the new technology, as the systems share a significant part of the frequency spectrum. This paper investigates the performance of G.fast coexisting with VDSL2. Methods for decentralized spectrum optimization and protection of legacy services are presented.
    Full-text · Conference Paper · Sep 2015
  • A. Krebs · M. Joham · W. Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: We evaluate the performance of a set of low complexity successive interference cancellation (SIC) detection algorithms in comparison to optimal maximum a-posteriori probability (MAP) detection and low complexity linear filter detection in a Turbo multiple-input multiple-output (Turbo-MIMO) system. We show that both linear and SIC soft detection algorithms perform similarly poorly for iterative receivers, even if the channel decoder output is available at the detector. We propose a low complexity combined a-priori/a-posteriori information-based error regularization technique, that improves the performance of the Turbo-MIMO design considerably. With this regularization technique, we show that a decoding gain of 2.2 dB can be achieved in an LTE compliant Turbo-MIMO receiver.
    No preview · Article · Aug 2015
  • Source
    Rainer Strobel · Michael Joham · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: Hybrid copper/fiber networks bridge the gap between the fiber link and the customer by using copper wires over the last meters. This solution combines energy efficiency and low cost of the copper access network with high data rates of a fiber connection. However, the fiber to the distribution point (FTTdp) network must prove its ability to convey data at fiber speed over copper wire bundles under the spectral constraints of the copper access network. This work investigates achievable data rates of the FTTdp network. It provides an analysis of the sources of performance loss in a system implementation due to complexity limitations. Methods to improve achievable rates are shown, that are based on incorporating the limitations in the optimization process. Achievable data rates are analyzed in terms of rate vs. reach curves, based on a statistical channel model and the ITU standard G.fast. The results indicate that optimized linear methods perform well on shorter lines, while nonlinear methods have advantages for long lines.
    Full-text · Conference Paper · Jun 2015
  • Source
    C. Lameiro · W. Utschick · I. Santamaría
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper derives the interference-temperature (IT) limit for a multi-antenna primary user (PU) with a rate constraint. While in the case of a single-antenna PU there is a one-to-one mapping between IT and achievable rate, this correspondence does not hold anymore when a multiple-input multiple-output (MIMO) system is considered. In such cases, the spatial distribution of the interference must be taken into account, since it strongly affects the PU performance. To this end, we derive a closed-form expression for the maximum IT that can be tolerated by identifying the worst-case interference covariance matrix, which results in a multilevel waterfilling problem.
    Full-text · Article · Apr 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: We consider a sum mean square error (SMSE) based transceiver design for the multi-user downlink with multiple linear transmit power constraints. Since the multi-antenna transmitter has only imperfect channel state information (CSI), the average SMSE is minimized via an alternating optimization (AO). For fixed equalizers, the average SMSE minimizing precoders are found via an uplink-downlink SMSE duality. The precoder update therewith transforms to an uplink max-min average SMSE problem, i.e., a minimum mean square error (MMSE) equalizer design and an outer worst-case noise search. This dual problem is optimally solved, e.g., by a gradient projection approach, and strong duality is shown.
    No preview · Article · Apr 2015
  • C. Hellings · W. Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: It is known that parallel relay channels are not separable, i.e., the capacity with joint processing of the subchannels can be higher than the sum of the individual capacities. The same holds for the data rates achievable using partial decode-and-forward in parallel Gaussian MIMO relay channels. However, in this paper, we show that it is sufficient to allow the relay to remap information from one subchannel to another between the decoding and the re-encoding. A carrier-cooperative transmission in the sense of spreading transmit symbols over several subchannels does not bring advantages in terms of achievable rate.
    No preview · Article · Apr 2015
  • A. Grundinger · J. Pickart · M. Joham · W. Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: We study an outage constrained beamformer design in the multi-user vector downlink. The transmitter's statistical information about the fading channels is subject to an additive and a multiplicative random channel error. We split the outage probabilities and the optimization into two parts for the two types of channel errors. An inner problem takes the additive channel errors into account, assuming prior information of the multiplicative errors, and is solved in terms of convex conservative approximations. The outer optimization considers the multiplicative channel errors and adjusts the information for the inner problem. For the outer search, we compare an equal design with a locally optimal iterative search.
    No preview · Article · Apr 2015
  • A. Gründinger · L. Gerdes · M. Joham · W. Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the probabilistically constrained capacity of a Gaussian relay channel in terms of the corresponding outage constrained decode-and-forward (DF) achievable rate and the cut-set bound (CSB). The probabilistic outage constraint is necessary due to Rayleigh fading and the absence of channel state information (CSI) at the transmitting nodes. The DF outage probability can be calculated in closed form, which allows a direct optimization of the joint source and relay transmit strategy. In contrast, the CSB probability can only be calculated in closed form for noncoherent transmission from the source and relay to the destination. For coherent transmission, we propose two upper bounds based on a genie-aided transmission and Markov's inequality. The numerical results verify the quality of the bounds.
    No preview · Article · Apr 2015
  • Christoph Hellings · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: Two main lines of approach can be identified in the recent literature on improper signals and widely linear operations. The augmented complex formulation based on the signal and its complex conjugate is considered as more insightful since it leads to convenient mathematical formulations for many considered problems. Moreover, it allows an easy distinction between proper and improper signals as well as between linear and widely linear operations. On the other hand, the composite real representation using the real and imaginary parts of the signal is closer to the actual implementation, and it allows to readily reuse results that have originally been derived for real-valued signals or proper complex signals. In this work, we aim at getting the best of both worlds by introducing mathematical tools that make the composite real representation more powerful and elegant. The proposed approach relies on a decomposition of real matrices into a block-skew-circulant and a block-Hankel-skew-circulant component. By means of various application examples from the field of signal processing for communications, we demonstrate the usefulness of the proposed framework.
    No preview · Article · Apr 2015 · IEEE Transactions on Signal Processing
  • Source
    David Neumann · Michael Joham · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: We introduce novel blind and semi-blind channel estimation methods for cellular time-division duplexing systems with a large number of antennas at each base station. The methods are based on the maximum a-posteriori principle given a prior for the distribution of the channel vectors and the received signals from the uplink training and data phases. Contrary to the state-of-the-art massive MIMO channel estimators which either perform linear estimation based on the pilot symbols or rely on a blind principle, the proposed semi-blind method efficiently suppresses most of the interference caused by pilot-contamination. The simulative analysis illustrates that the semi-blind estimator outperforms state- of-the-art linear and non-linear approaches to the massive MIMO channel estimation problem.
    Full-text · Article · Mar 2015
  • Source
    Quan Kuang · Wolfgang Utschick · Andreas Dotzler

    Full-text · Conference Paper · Mar 2015
  • Samer Bazzi · Guido Dietl · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: Cooperative precoding techniques in MIMO interference channels (ICs) such as interference alignment (IA) overcome interference, create noise-limited ICs, and achieve a linear rate scaling at high signal-to-noise ratios (SNRs). Alternatively, interference is overcome with the simple deployment of large transmit arrays, as interference becomes negligible by the law of large numbers. In such a case, "selfish" precoding techniques such as maximum ratio transmission (MRT) constitute a good solution. We investigate how many transmit antennas are needed for MRT to emulate the performance of a noiselimited system, i.e., to perform similarly to a system where transmitters have a fixed number of antennas and employ the IA strategy. Assuming a time-division-duplex mode, we calculate the training overheads associated with channel state information acquisition for each precoding type, and include these in the spectral efficiency calculations. We use existing analytical rate expressions for IA and MRT. The rate and overhead analysis allows finding the required number of antennas in closed-form, which varies significantly according to the SNR and coherence interval length. Results for different scenarios and practical SNRs show that transmitters with few dozens of antennas at most using MRT perform similarly to transmitters with four antennas using IA. The main findings of this paper are that the simple MRT strategy does not require massive configurations to emulate the performance of noise-limited systems.
    No preview · Conference Paper · Mar 2015
  • Samer Bazzi · Guido Dietl · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, analytical rate expressions of interference alignment (IA) algorithms applied to the multiple-input multiple-output interference channel are derived in the large system limit. A large system analysis is performed by fixing the number of users and letting the number of transmit and receive antennas go to infinity at a finite fixed ratio. The asymptotic eigenvalue distribution of the direct channel gains resulting from IA algorithms is first obtained. Based on that, large system rate expressions under both water-filling power allocation and equal power allocation are derived in closed form. The obtained expressions are functions of the transmit power and noise power at the receivers and are independent of any other system parameters. Simulation results show that the achievable rates of different IA algorithms converge to the large system rates as the number of transmit and receive antennas increases, thereby showing that the large system expressions are valid for different IA variants. Simulation results also show that large system expressions provide accurate estimates of the average achievable rates for small and finite system parameters.
    No preview · Article · Mar 2015 · IEEE Transactions on Signal Processing
  • Lennart Gerdes · Christoph Hellings · Lorenz Weiland · Wolfgang Utschick

    No preview · Article · Jan 2015 · IEEE Transactions on Information Theory
  • M. Riemensberger · L. Gerdes · W. Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: We discuss noisy network coding bounds on the capacity region of Gaussian multiple access relay networks. These networks feature a single destination node and multiple nodes that can be either sources or relays or both. We show that simple inner and outer bounds on the capacity region obtained from noisy network coding exhibit a submodular structure and differ only in a constant, which is independent of the channel parameters and grows linearly only in the number of dedicated relay nodes. This tightens previous results where the gap grows linear in the total network size. Furthermore, the combination of submodularity with convexity of the bound expressions with respect to the optimal quantization noise parameters leads to an efficient characterization of a noisy network coding achievable rate region for multiple access relay networks via Lagrangian duality.
    No preview · Article · Oct 2014
  • L. Gerdes · L. Weiland · W. Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we consider the partial decode-and-forward (PDF) strategy for the Gaussian multiple-input multiple-output (MIMO) relay channel. The input distribution that maximizes the achievable PDF rate for this channel is still unknown in general. Therefore, it has so far only been possible to determine the maximum PDF rate if the best PDF strategy is equivalent to the decode-and-forward (DF) strategy, point-to-point (P2P) transmission from source to destination, or if PDF achieves the cut-set bound (CSB), i.e., for special cases where Gaussian channel inputs are known to be optimal. In this work, we exploit the properties of the generalized singular value decomposition (GSVD) to show that the maximum PDF rate for the Gaussian MIMO relay channel is also achieved by Gaussian inputs if the row spaces of the source-relay and the source-destination channel gain matrices are disjoint. Furthermore, we show that the optimal PDF rate can be determined as the solution of a convex optimization problem in that case.
    No preview · Article · Oct 2014
  • Source
    Lennart Gerdes · Christoph Hellings · Lorenz Weiland · Wolfgang Utschick
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper considers the partial decode-and-forward (PDF) strategy for the Gaussian multiple-input multiple-output (MIMO) relay channel. Unlike for the decode-and-forward (DF) strategy or point-to-point (P2P) transmission, for which Gaussian channel inputs are known to be optimal, the input distribution that maximizes the achievable PDF rate for the Gaussian MIMO relay channel has remained unknown so far. For some special cases, e.g., for relay channels where the optimal PDF strategy reduces to DF or P2P transmission, it could be deduced that Gaussian inputs maximize the PDF rate. For the general case, however, the problem has remained open until now. In this work, we solve this problem by proving that the maximum achievable PDF rate for the Gaussian MIMO relay channel is always attained by Gaussian channel inputs. Our proof relies on the channel enhancement technique, which was originally introduced by Weingarten et al. to derive the (private message) capacity region of the Gaussian MIMO broadcast channel. By combining this technique with a primal decomposition approach, we first establish that jointly Gaussian source and relay inputs maximize the achievable PDF rate for the aligned Gaussian MIMO relay channel. Subsequently, we use a limiting argument to extend this result from the aligned to the general Gaussian MIMO relay channel.
    Preview · Article · Sep 2014