Change search
Refine search result
1234 1 - 50 of 176
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the 'Create feeds' function.
  • 1.
    Ainomäe, Ahti
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. Tallinn Univ. of Technol., Tallinn, Estonia.
    Trump, Tõnu
    Tallinn Univ. of Technol., Tallinn, Estonia.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Distributed Diffusion LMS based Energy Detection2014In: Proceedings of 6th International Congress on Ultra Modern Telecommunications and Control Systems and Workshops (ICUMT), 2014, 2014, 176-183 p.Conference paper (Refereed)
    Abstract [en]

    Cognitive radio (CR) is seen as a promising technology to make radio spectrum usage more effective by providing an opportunistic access for secondary users to the licensed spectrum areas. CR systems need to detect the presence of a primary user (PU) signal by continuously sensing the spectrum area of interest. Radiowave propagation effects like fading and shadowing often complicate sensing of spectrum holes because the PU signal can be weak in a particular area. Cooperative spectrum sensing is seen as a prospective solution to enhance the detection of PU signals. This paper studies distributed spectrum sensing in a cognitive radio context. We investigate distributed energy detection schemes without using any fusion center. We propose the usage of distributed, diffusion least mean square (LMS) type of power estimation algorithms. In this paper an Adapt and Combine (ATC) diffusion based power estimation scheme is proposed and the performance is compared with the Combine and Adapt (CTA) and ring-around schemes in a common framework. The PU signal is assumed to be slowly fading. We analyse the resulting energy detection performance and verify the theoretical findings through simulations.

  • 2.
    Ainomäe, Ahti
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. Department of Radio and Telecommunication Engineering, Tallinn University of Technology, Tallinn, Estonia .
    Trump, Tõnu
    Tallin University of Technology.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Distributed Recursive Energy Detection2014In: Proceedings of Wireless Communications and Networking Conference (WCNC), 2014, IEEE Communications Society, 2014, 1242-1247 p.Conference paper (Refereed)
    Abstract [en]

    Recent studies have shown that, while the available licensed radio spectrum becomes more occupied, the assigned spectrum is significantly underutilized. To alleviate the situation, cognitive radio (CR) technology has been proposed to provide an opportunistic access to the licensed spectrum areas. CR systems are able to serve the secondary users for detecting and utilizing so called spectrum holes by sensing and adapting to the environment without causing harmful effects or interference to the licensed primary users (PU). CR systems need to detect the presence of a primary user by continuously sensing the spectrum area of interest. Radiowave propagation effects like fading and shadowing often complicate sensing of spectrum holes because the PU signal can be weak in a particular area. Cooperative spectrum sensing is seen as a prospective solution to enhance the detection of PU signals. This paper studies distributed spectrum sensing in a cognitive radio context. We investigate a distributed energy detection scheme without using any fusion center. Due to reduced communication such a topology is more energy efficient. The PU signal is assumed to be in slow fading. A recursive distributed power estimation and detection scheme is proposed. The theoretical findings are verified through simulations.

  • 3.
    Aldayel, Omar
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. King Saud University, Saudi Arabia.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Alshebeili, Saleh A.
    King Saud University, Saudi Arabia.
    Evaluation of MIMO channel non-stationarity2013In: 2013 Proceedings of the 21st European Signal Processing Conference (EUSIPCO), IEEE , 2013, 6811769- p.Conference paper (Refereed)
    Abstract [en]

    Several MIMO processing algorithms have been proposed that exploit long-term channel statistics, relaying on the critical assumption that this long-term information is valid long enough. In this paper, we consider the Correlation Matrix Distance (CMD) method previously proposed for the evaluation of MIMO channel non-stationarity. We highlight a couple of problems with the CMD measure and propose two new metrics that are more appropriate for non-stationarity evaluation. The performance of the CMD method and new correlation matrix distance metrics is investigated using measured 4×4 MIMO channels. Both Line-of-Sight (LOS) and Non-LOS (NLOS) environments are considered.

  • 4.
    Anton-Haro, Caries
    et al.
    Centre Tecnolo`gic de Telecomunicacions de Catalunya (CTTC, Spain).
    Svedman, Patrick
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Alexiou, Angeliki
    Bell Laboratories, Lucent Technologies (United Kingdom).
    Gameiro, Atilio
    University of Aveiro/Istituto de Telecomunicações (Portugal).
    Cross-layer scheduling for multi-user MIMO systems2006In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 44, no 9, 39-45 p.Article in journal (Refereed)
    Abstract [en]

    The increasing demand of wireless services associated with the scarcity of the radio spectrum and the trend to provide end-to-end QoS in emerging and future applications calls for the design of spectrally efficient systems with QoS support. To fulfill these two requirements of spectral efficiency and QoS provision in the highly dynamic environment of mobile radio requires the collaboration of several layers in the system as well as the use of multiple transmit and receive antennas. In a packet network, one important component to achieve the aforementioned efficiency goals is a properly designed scheduling algorithm. Using an information-theoretic framework, we present an overview of the issues associated with the design of packet scheduling algorithms and review several proposed solutions. The article focuses on the advantages of a cross-layer approach to the resource allocation problem and identifies the trade-offs associated with the increased signaling needs. Also, we thoroughly discuss the additional degrees of freedom that multiple transmit and receive antennas can provide.

  • 5.
    Asztély, David
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Bengtsson, Mats
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Kristensson, Martin
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Spatio-Temporal Processing for Wireless Communications1998In: First Annual UCSD Conference on Wireless Communications, 1998Conference paper (Refereed)
  • 6.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    A pragmatic approach to multi-user spatial multiplexing2002In: SAM2002: IEEE SENSOR ARRAY AND MULTICHANNEL SIGNAL PROCESSING WORKSHOP PROCEEDINGS, NEW YORK: IEEE , 2002, 130-134 p.Conference paper (Refereed)
    Abstract [en]

    We suggest a new approach for spatial multiplexing in a narrowband system with several access points serving a number of mobile terminals. In contrast to previously suggested algorithms, this approach is formulated based on a systems perspective, generalizing earlier algorithms for jointly optimal beamforming and power control. Since global channel knowledge is required, the algorithm is mainly intended for system simulations where it provides a benchmark for the performance of any spatial multiplexing scheme with equal rate on the spatial channels.

  • 7.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    ALGORITHMIC SOLUTIONS FOR PILOT DESIGN OPTIMIZATION IN ARBITRARILY CORRELATED SCENARIOS2014Conference paper (Refereed)
    Abstract [en]

    We consider optimized design of training sequences, given knowledge of the channel and noise statistics. Recently, pilot designs considering the end performance of the channel estimate, have been proposed, both optimizing the average performance and the performance at a certain outage level. Unfortunately, these problems, as well as previously proposed designs optimizing the channel estimation MSE, are non-convex for arbitrary channel and noise correlations so additional assumptions have been introduced in the literature to be able to find tractable solutions. Here, we show that arbitrarily correlated scenarios can easily be handled by resorting to alternating optimization, for all the previously mentioned problem formulations. Furthermore, we numerically compare the average and outage performance of the proposed algorithms, to alternative solutions adopted from the literature.

  • 8.
    Bengtsson, Mats
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Antenna array signal processing for high rank data models2000Doctoral thesis, monograph (Other scientific)
  • 9.
    Bengtsson, Mats
    KTH, Superseded Departments, Signals, Sensors and Systems.
    From single link MIMO to multi-user MIMO2004In: 2004 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, VOL IV, PROCEEDINGS - AUDIO AND ELECTROACOUSTICS SIGNAL PROCESSING FOR COMMUNICATIONS, NEW YORK: IEEE , 2004, 697-700 p.Conference paper (Refereed)
    Abstract [en]

    For any given single link strategy for transmission over a MIMO channel with channel knowledge at both transmitter and receiver, we propose a general approach to reduce the interference when the strategy is used in an interference limited system. This solution has several interesting interpretations. The structure of the solution is shown to include the global optimum, even though it may be difficult to determine all parameters. However, a suboptimal choice is presented that can be implemented in practical systems using only local information. A numerical example illustrates the performance when trying to minimize the summed mutual information.

  • 10.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Jointly optimal downlink beamforming and base station assignment2001In: 2001 IEEE International Conference on Acoustics, Speech, and Signal Processing, 2001. Proceedings (ICASSP '01)  (Volume:5 ), IEEE conference proceedings, 2001, 2961-2964 p.Conference paper (Refereed)
    Abstract [en]

    We present an algorithm that jointly determines the optimal downlink beamformers and the optimal assignment of each mobile to a base station. The optimality criterion is based on a systems perspective; provide sufficient quality of service for all users, transmitting as little excess power as possible. Since the algorithm is centralized and requires knowledge about all the channels in the system, it may be infeasible in a practical implementation. However, it provides the ultimate benchmark in system evaluations. Numerical examples show substantial gain compared to ordinary base station assignment.

  • 11.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Multi-User Spatial Multiplexing - A Pragmatic Approach2002In: RadioVetenskap och Kommunikation (RVK), 2002, 724-728 p.Conference paper (Other academic)
    Abstract [en]

    We suggest a new approach for spatial multiplexing in a narrowband  system with several access points serving a number of mobile  terminals. In contrast to previously suggested algorithms, this  approach is formulated based on a systems perspective, generalizing  earlier algorithms for jointly optimal beamforming and power  control. Since global channel knowledge is required, the algorithm  is mainly intended for system simulations where it provides a  benchmark for the performance of any spatial multiplexing scheme  with equal rate on the spatial channels.

  • 12.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Optimal Transmission Using Smart Antennas2000In: Proc. IST Mobile Communications Summit, 2000, 359-364 p.Conference paper (Other academic)
    Abstract [en]

    We study the design of beamformers for transmission from a number of base stations to a number of user terminals. Since every transmitted signal will cause interference at all the other user terminals, an optimal beamformer design must be determined jointly for all the beamformers. We review two algorithms that provide an optimal set of downlink beamformers. We also show how to determine not only the beamforming vectors but also the optimal allocation of user terminals to base stations.

  • 13.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Pragmatic multi-user spatial multiplexing with robustness to channel estimation errors2003In: 2003 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, VOL IV, PROCEEDINGS: SIGNAL PROCESSING FOR COMMUNICATIONS SPECIAL SESSIONS, 2003, 820-823 p.Conference paper (Refereed)
    Abstract [en]

    Using ideas from power control and downlink beamforming, we suggest a pragmatic approach for spatial multiplexing in systems with several access points, each serving one or more mobile terminals. We assume that all channels are known globally, but in the case of fast fading, it is sufficient to know the second order statistics of the channels. Robustness to channel estimation errors is easily incorporated in the algorithm. Since global channel knowledge is required, the algorithm is mainly intended for system simulations where it provides a benchmark for the performance of any spatial multiplexing scheme with equal rate on the spatial channels.

  • 14.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Robust and constrained downlink beamforming2000In: European Signal Processing Conference, 2000, no MarchConference paper (Refereed)
    Abstract [en]

    When antenna arrays are introduced in a cellular system, one critical aspect is the design of beamformers for downlink transmission. We extend a recently proposed strategy for optimal downlink beamforming to give increased robustness to channel uncertainties and to incorporate constraints on the dynamic range. The solution can be efficiently calculated using semidefinite optimization. In a few cases, the optimal solution is given by a time-varying beamformer. We explore the performance of these modified algorithms.

  • 15.
    Bengtsson, Mats
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Sensor array processing for scattered sources1997Licentiate thesis, monograph (Other scientific)
  • 16.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Spatial Interference Suppression for Shared Spectrum2008In: ICT Mobile and Wireless Communications Summit / [ed] Paul Cunningham and Miriam Cunningham, IIMC International Information Management Corporation , 2008Conference paper (Refereed)
    Abstract [en]

    We consider a scenario where a (future generation) cellular system shares the spectrum with a fixed-satellite services (FSS) system or other system that has primary access to the radio spectrum. Normally, the coverage of such a cellular system will have to stay outside an exclusion zone around the FSS ground station, to keep the interference low enough. Here we investigate how spatial interference suppression can be used at a transmitting multi-antenna base station to reduce the exclusion zone and provide downlink coverage closer to the FSS ground station. The study considers both transmit schemes that exploit knowledge about the channel between the base station and mobile station, as well as schemes that only rely on partial or no channel knowledge at all. Numerical examples illustrate that the interference level for the primary system can be reduced by at least 20dB, without a major loss of quality of service within the secondary system.

  • 17.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Astély, David
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Measurements of Spatial Characteristics and Polarization with a Dual Polarized Antenna Array1999In: Proceedings IEEE Vehicular Technology Conference, IEEE , 1999, 366-370 p.Conference paper (Refereed)
    Abstract [en]

    We consider antenna arrays equipped with cross-polarized antenna elements. The multipath propagation will affect both the spatial characteristics and the polarization of the received signals which will influence the performance of the system in terms of, for example, bit error rate or direction estimation. In this paper, we present results from field measurement data recorded at the 1800 MHz band, provided by Ericsson Radio Systems AB. The study is divided into two main parts. In the first part we introduce a diversity gain measure related to the BER of an MRC receiver. The polarization diversity introduced by the radio channel is studied, and compared to the spatial diversity. In the second part we use a parameterized model of the received polarized radio waves and illustrate the typical behavior of the estimated polarization parameters

  • 18.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Bartolomé, Diego
    Vicario, José López
    Centre Tecnològic de Telecom. de Catalunya (CTTC).
    Antón-Haro, Carles
    Centre Tecnològic de Telecom. de Catalunya (CTTC).
    Beamforming and bit-loading strategies for multi-user SDMA with admission control2005In: IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC, 2005, Vol. 2, 842-846 p.Conference paper (Refereed)
    Abstract [en]

    In this paper, we address the problem of transmit beamforming, power allocation, bit loading, and admission control in a multi-user scenario, where several single antenna mobile terminals are served by one base station equipped with an antenna array. In particular, we explore the impact of using either optimal or zero-forcing transmit beamforming schemes in combination with different bit allocation strategies, namely the Maximization of the Sum Rate (MSR), the Maximization of the Minimum Rate (MMR) and a modified version of the latter, for a finite set of modulation schemes. The performance evaluation is conducted by means of system-level simulations, using a realistic channel model for urban micro cells, with an emphasis not only on the aggregated cell throughput but also on the associated fairness issues.

  • 19.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Lilliesköld, Joakim
    KTH, School of Electrical Engineering (EES), Industrial Information and Control Systems.
    Norgren, Martin
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Skog, Isaac
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Sohlström, Hans
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Developing and Implementing a Program Interfacing Project Course in Electrical Engineering2012Conference paper (Refereed)
    Abstract [en]

    In this paper, we describe the ideas behind a second-year Design-Build course in Electrical Engineering. Electrical Engineering is a theoretical subject, and in such it is difficult to maintain the theoretical level in project courses introduced too early in the program, especially when core subjects like electromagnetic field theory are involved. This issue is addressed and we also describe our approach for the assessment of the students. We also discuss the different goals that were set up prior to the course from a program perspective; how we reasoned when designing the course, the assessment structure, and the output once the course was implemented

  • 20.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Martin, Cristoff
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ben Slimane, Slimane
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Zetterberg, Per
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Recent Advances on MIMO Processing in the SATURN Project2002In: Proc. IST Mobile Communications Summit, 2002, 309-313 p.Conference paper (Other academic)
    Abstract [en]

    We highlight some recent results on the theory and algorithms for  MIMO systems, obtained at the Royal Institute of  Technology (KTH) within the SATURN project.   The paper includes a a new approximate expression for the ergodic  channel capacity of a MIMO system with correlated fading, algorithms  for MIMO beamforming under EIRP (equivalent isotropic radiated  power) constraints and a low-complexity algorithm for MIMO channel  estimation in OFDM systems. Also, we show how transmit and  beamformers can be determined in a decentralized fashion without  explicitly estimating the channel response.

  • 21.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Martin, Cristoff
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Slimane, Ben
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Zetterberg, Per
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Recent Advances on MIMO Processing in the SATURN Project2002In: Proc. IST Mobile Communications Summit, 2002Conference paper (Refereed)
    Abstract [en]

    We highlight some recent results on the theory and algorithmsfor MIMO systems, obtained at the Royal Instituteof Technology (KTH) within the SATURN project.The paper includes a a new approximate expression forthe ergodic channel capacity of a MIMO system with correlatedfading, algorithms for MIMO beamforming underEIRP (equivalent isotropic radiated power) constraintsand a low-complexity algorithm for MIMO channel estimationin OFDM systems. Also, we show how transmitand beamformers can be determined in a decentralizedfashion without explicitly estimating the channel response.

  • 22.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing.
    A generalization of weighted subspace fitting to full-rank models2001In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 49, no 5, 1002-1012 p.Article in journal (Refereed)
    Abstract [en]

    The idea of subspace fitting provides a popular framework for different applications of parameter estimation and system identification. Previously, some algorithms have been suggested based on similar ideas, for a sensor array processing problem where the underlying data model is not low rank. We show that two of these algorithms (DSPE and DISPARE) fail to give consistent estimates and introduce a general class of subspace fitting-like algorithms for consistent estimation of parameters from a possibly full-rank data model. The asymptotic performance is analyzed, and an optimally weighted algorithm is derived. The result gives a lower bound on the estimation performance for any estimator based on a low-rank approximation of the linear space spanned by the sample data. We show that in general, for full-rank data models, no subspace-based method can reach the Cramer-Rao lower bound (CRB)

  • 23.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Downlink Beamformer Design Using Semidefinite Optimization1999In: RadioVetenskap och Kommunikation (RVK), 1999, 289-293 p.Conference paper (Other academic)
  • 24.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Low Complexity Estimation of Angular Spread with an Antenna Array1997In: Proceedings of 11th IFAC Symposium on System Identification, 1997, 535-540 p.Conference paper (Refereed)
  • 25.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Low-complexity estimators for distributed sources2000In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 48, no 8, 2185-2194 p.Article in journal (Refereed)
    Abstract [en]

    In antenna array applications, the propagation environment is often more complicated than the ordinarily assumed model of plane wavefronts. Here, a low-complexity algorithm is suggested for estimating both the DOA and the spread angle of a source subject to local scattering, using a uniform linear array. The parameters are calculated from the estimates obtained using a standard algorithm such as root-MUSIC to fit a two-ray model to the data. The algorithm is shown to give consistent estimates, and the statistical performance is studied analytically and through simulations

  • 26.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    On Approximating a Spatially Scattered Source with Two Point Sources1998In: Proceedings Nordic Signal Processing Symposium (NORSIG'98), 1998, 45-48 p.Conference paper (Other academic)
  • 27.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Optimal and Suboptimal Transmit Beamforming2001In: Handbook of Antennas in Wireless Communications / [ed] Lal C. Godara, CRC Press , 2001, 18-1-18-33 p.Chapter in book (Other academic)
  • 28.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Signal Processing.
    Optimal Downlink BeamformingUsing Semidefinite Optimization1999In: Proc. of 37th Annual Allerton Conference on Communication,Control, and Computing, 1999, 987-996 p.Conference paper (Other academic)
    Abstract [en]

    When using antenna arrays at the base station of a cellular system, one critical aspectis the transmission strategy. An optimal choice of beamformers for simultaneous transmissionto several co-channel users must be solved jointly for all users and base stations in anarea. We formulate an optimal transmit strategy and show how the solution can be calculatedefficiently using interior point methods for semidefinite optimization. The algorithmminimizes the total transmitted power under certain constraints to guarantee a specificquality of service. The method provides large flexibility in the choice of constraints andcan be extended to be robust to channel perturbations.

  • 29.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Rooting Techniques for Estimation of Angular Spread with an Antenna Array1997In: Proceedings of IEEE 47th Vehicular Technology Conference. VTC 97, IEEE , 1997, 1158-1162 p.Conference paper (Refereed)
    Abstract [en]

    When antenna arrays are used in mobile communications systems, multipath propagation effects must be taken into account. Local scattering around each signal source causes angular spreading. One important parameter foroptimal transmit and receive algorithms is the width of the angular spread of each scattered source signal. We present a new principle for consistent estimation of direction of arrival as well as angular spread using standardrooting techniques such as root MUSIC, MODE or ESPRIT. The computational complexity is much lower than forpreviously suggested algorithms.

  • 30.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Signal Waveform Estimation from Array Data in Angular Spread Environment1996In: 30:th Asilomar Conference on Signals, Systems & Computers, IEEE , 1996, 355-359 p.Conference paper (Refereed)
    Abstract [en]

    We analyze different direction based signal copy algorithms for a model of angular spread caused by local scattering around each signal source. Optimal algorithms, in terms of signal to interference and noise ratio, are derived for both rapidly and slowly time varying angular spread and a low complexity an-hoc algorithm is suggested. Simulations show that these algorithms outperform traditional signal copy algorithms in terms of signalto interference and noise ratio as well as outage probability.

  • 31.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Uplink and Downlink Beamforming for Fading Channels1999In: Proceedings of SPAWC’99, Signal Processing Advances in Wireless Communications, IEEE , 1999, 350-353 p.Conference paper (Refereed)
    Abstract [en]

    We highlight some issues in the design of beamformers for transmission and reception in communications systems with antenna arrays at the base stations. We assume a radio channel characterized by frequency flat Rayleighfading which is correlated (but not coherent) from antenna element to antenna element. Different design criteria are considered and we show the relationship between downlink and uplink formulations and solutions. The resulting beamformers are typically given by quadratically constrained optimization problems which could be expected to give increased robustness to signal cancellation. Unfortunately, signal cancellation is still present, but diagonal loading can be introduced to minimize the problem.

  • 32.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Svedman, Patrick
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Comparison and Cost Analysis of Smart and Opportunistic MIMO OFDM Schemes2006In: IST Mobile & Wireless Communications Summit 2006, 2006Conference paper (Refereed)
    Abstract [en]

    This paper attempts to compare the system downlinkperformance of a smart antenna scheme for OFDMA thatutilizes channel covariance information with an opportunisticmulti-antenna scheme for OFDMA that uses fast limited feedbackof the instantaneous channel condition. The schemes areevaluated by means of simulations in a low load scenario andalso compared to a simple HSDPA-like scheme. The smartantenna method manages to utilize the multiple antennas betterthan the opportunistic scheme. Furthermore, the performanceof the schemes is inserted into a cost model that assumes thatthe receivers are relays that use short-range technologies tocommunicate with user equipments. Also in terms of cost peruser equipment, the smart antenna scheme proves to be moresuitable for the studied scenario.

  • 33.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Svedman, Patrick
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Zhang, Xi
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing.
    System comparison of smart and dumb antennas2005In: VTC2005-SPRING: 2005 IEEE 61ST VEHICULAR TECHNOLOGY CONFERENCE, VOLS 1-5, PROCEEDINGS, 2005, 230-233 p.Conference paper (Refereed)
    Abstract [en]

    Different approaches have been proposed to utilize multiple antennas at both transmitters and receivers of wireless systems. Herein, we provide a system comparison between covariance feedback schemes where a large amount of information is fed back at a low rate and opportunistic beamforming schemes where a small amount of information is fed back at a high rate.

  • 34.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Völcker, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    On the estimation of Azimuth distributions and Azimuth Spectra2001In: IEEE 54TH VEHICULAR TECHNOLOGY CONFERENCE, VTC FALL 2001, 2001, 1612-1615 p.Conference paper (Refereed)
    Abstract [en]

    The increased popularity of array antennas for wireless communication, has inspired investigations of the spatio-temporal properties of the radio channel, among others the azimuthal shape of the scattering clusters. Based on a statistical analysis, we show that trying to estimate the shape of the scattering Is an ill-posed problem and that the heavy-tailed Power Azimuth Spectra reported in the literature may be an artifact stemming only from the measurement procedure.

  • 35.
    Bengtsson, Mats
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Yu, Kai
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Ottersten, Björn
    KTH, Superseded Departments, Signals, Sensors and Systems.
    D523, Part 1. Single and Dual Multi-Sensor Channel Characterisation - Analysis and Models: Stochastic Models2001Report (Other academic)
  • 36.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. Dept. of Electrical Engineering, Linkoping University, Sweden.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Optimal Multiuser Transmit Beamforming: A Difficult Problem with a Simple Solution Structure2014In: IEEE signal processing magazine (Print), ISSN 1053-5888, E-ISSN 1558-0792, Vol. 31, no 4, 142-148 p.Article in journal (Refereed)
    Abstract [en]

    Transmit beamforming is a versatile technique for signal transmission from an array of antennas to one or multiple users [1]. In wireless communications, the goal is to increase the signal power at the intended user and reduce interference to nonintended users. A high signal power is achieved by transmitting the same data signal from all antennas but with different amplitudes and phases, such that the signal components add coherently at the user. Low interference is accomplished by making the signal components add destructively at nonintended users. This corresponds mathematically to designing beamforming vectors (that describe the amplitudes and phases) to have large inner products with the vectors describing the intended channels and small inner products with nonintended user channels.

  • 37.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Optimality Properties and Low-Complexity Solutions to Coordinated Multicell Transmission2010In: 2010 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE GLOBECOM 2010, IEEE , 2010, 1-6 p.Conference paper (Refereed)
    Abstract [en]

    Base station cooperation can theoretically improve the throughput of multicell systems by coordinating interference and serving cell edge terminals through multiple base stations. In practice, the extent of cooperation is limited by the increase in backhaul signaling and computational demands. To address these concerns, we propose a novel distributed cooperation structure where each base station has responsibility for the interference towards a set of terminals, while only serving a subset of them with data. Weighted sum rate maximization is considered, and conditions for beamforming optimality and the optimal transmission structure are derived using Lagrange duality theory. This leads to distributed low-complexity transmission strategies, which are evaluated on measured multiantenna channels in a typical urban multicell environment.

  • 38.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Pareto characterization of the multicell MIMO performance region with simple receivers2012In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 60, no 8, 4464-4469 p.Article in journal (Refereed)
    Abstract [en]

    We study the performance region of a general multicell downlink scenario with multiantenna transmitters, hardware impairments, and low-complexity receivers that treat interference as noise. The Pareto boundary of this region describes all efficient resource allocations, but is generally hard to compute. We propose a novel explicit characterization that gives Pareto optimal transmit strategies using a set of positive parameters-fewer than in prior work. We also propose an implicit characterization that requires even fewer parameters and guarantees to find the Pareto boundary for every choice of parameters, but at the expense of solving quasi-convex optimization problems. The merits of the two characterizations are illustrated for interference channels and ideal network multiple-input multiple-output (MIMO).

  • 39.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Signal Processing.
    Receive combining vs. multistream multiplexing in multiuser MIMO systems2011In: Communication Technologies Workshop (Swe-CTW), 2011 IEEE Swedish, IEEE Communications Society, 2011, 103-108 p.Conference paper (Refereed)
    Abstract [en]

    In single-user transmission, the receive antennas should preferably be used to enable multiplexing. The situation is different under multiuser transmission, where only the number of transmit antennas limits the multiplexing gain. The system therefore has the choice between sending one stream per scheduled user (i.e., combining receive antennas for diversity) or selecting a smaller number of users and multiplex multiple streams to each of them. This tradeoff is investigated herein, based on zero-forcing (with receive antenna combining) and block-diagonalization precoding which represents the two extremes. Based on asymptotic analysis and numerical examples, the unexpected conclusion is that each user only should receive one stream and use its antennas to achieve a receive combining gain. This is explained by zero-forcing having a stronger resilience towards spatial correlation and larger benefit from multiuser diversity. This fundamental result has positive implications for the design of multiuser systems as it reduces the hardware constraints at the user devices.

  • 40.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Zheng, Gan
    Interdisciplinary Centre for Security, Reliability and Trust, University of Luxembourg.
    Computational Framework for Optimal Robust Beamforming in Coordinated Multicell Systems2011In: Computational Advances in Multi-Sensor Adaptive Processing (CAMSAP), IEEE Signal Processing Society, 2011, 245-248 p.Conference paper (Refereed)
    Abstract [en]

    Coordinated beamforming can significantly improvethe performance of cellular systems through joint interferencemanagement. Unfortunately, such beamforming optimization problems are typically NP-hard in multicell scenarios, making heuristic beamforming the only feasible choice in practice. Thispaper proposes a new branch-reduce-and-bound algorithm thatsolves such optimization problems globally, with a complexitysuitable for benchmarking and analysis. Compared to priorwork, the framework handles robustness to uncertain intercell interference and numerical analysis shows higher efficiency.

  • 41.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hammarwall, David
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Zakhour, Randa
    Institut Eurécom, 2229 route des crêtes, BP 193, F-06560, Sophia Antipolis, France.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Gesbert, David
    Institut Eurécom, 2229 route des crêtes, BP 193, F-06560, Sophia Antipolis, France.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Feedback design in multiuser MIMO systems using quantization splitting and hybrid instantaneous/statistical channel information2008In: ICT-MobileSummit 2008 Conference Proceedings / [ed] Paul Cunningham and Miriam Cunningham, IIMC International Information Management Corporation , 2008, , 8 p.Conference paper (Refereed)
    Abstract [en]

    In the design of next generation multiuser communication systems, multiple antenna transmission is an essential part providing spatial multiplexing gain and allowing efficient use of resources. A major limiting factor in the resource allocation is the amount of channel state information (CSI) available at the transmitter, particularly in multiuser systems where the feedback from each user terminal must be limited. To this effect we propose two independent approaches for an efficient representation of the channel in multiuser MIMO systems. In the first approach, channel quantization is considered where the total number of feedback bits is limited. A resource allocation scheme is proposed where the available rate is split between the scheduling phase, where all users feed back a coarse CSI quantization, and the precoding phase where the selected receivers refine their CSI. The optimum splitting of the available feedback rate provides a large increase in performance and even simple heuristic splitting gives a noticeable advantage. In the second approach, we exploit a combination of instantaneous and statistical channel information. For spatially correlated Rayleigh and Ricean channels, it is shown that the CSI to large extent can be represented by the channel norm when the long-term channel statistics are known. Within a minimum mean square error (MMSE) estimation framework, feedback of a few bits of the quantized channel norm is sufficient to perform efficient resource allocation and achieve performance close to that of full CSI.

  • 42.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Jaldén, Niklas
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Optimality Properties, Distributed Strategies, and Measurement-Based Evaluation of Coordinated Multicell OFDMA Transmission2011In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 59, no 12, 6086-6101 p.Article in journal (Refereed)
    Abstract [en]

    The throughput of multicell systems is inherently limited by interference andthe available communication resources. Coordinated resource allocation is the key to efficient performance, but the demand on backhaul signaling andcomputational resources grows rapidly with number of cells, terminals, andsubcarriers. To handle this, we propose a novel multicell framework with dynamic cooperation clusters where each terminal is jointly served by a small set of base stations. Each base station coordinates interference to neighboring terminals only, thus limiting backhaul signalling and making the framework scalable. This framework can describe anything from interference channels to ideal joint multicell transmission. The resource allocation (i.e., precoding and scheduling) is formulated as an optimization problem (P1) with performance described by arbitrary monotonic functions of the signal-to-interference-and-noise ratios (SINRs) and arbitrary linear power constraints. Although (P1) is nonconvex and difficult to solve optimally, we are able to prove: 1) optimalityof single-stream beamforming; 2) conditions for full power usage; and 3) a precoding parametrization based on a few parameters between zero and one. These optimality properties are used to propose low-complexity strategies: both a centralized scheme and a distributed version that only requires local channel knowledge and processing. We evaluate the performance on measuredmulticell channels and observe that the proposed strategies achieve close-to-optimal performance among centralized and distributed solutions, respectively. In addition, we show that multicell interference coordination can give substantial improvements in sum performance, but that joint transmission is very sensitive to synchronization errors and that some terminals can experience performance degradations.

  • 43.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kountouris, Marios
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Receive Combining vs. Multi-Stream Multiplexing in Downlink Systems With Multi-Antenna Users2013In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 61, no 13, 3431-3446 p.Article in journal (Refereed)
    Abstract [en]

    In downlink multi-antenna systems with many users, the multiplexing gain is strictly limited by the number of transmit antennas and the use of these antennas. Assuming that the total number of receive antennas at the multi-antenna users is much larger than, the maximal multiplexing gain can be achieved with many different transmission/reception strategies. For example, the excess number of receive antennas can be utilized to schedule users with effective channels that are near-orthogonal, for multi-stream multiplexing to users with well-conditioned channels, and/or to enable interference-aware receive combining. In this paper, we try to answer the question if the data streams should be divided among few users (many streams per user) or many users (few streams per user, enabling receive combining). Analytic results are derived to show how user selection, spatial correlation, heterogeneous user conditions, and imperfect channel acquisition (quantization or estimation errors) affect the performance when sending the maximal number of streams or one stream per scheduled user-the two extremes in data stream allocation. While contradicting observations on this topic have been reported in prior works, we show that selecting many users and allocating one stream per user (i.e., exploiting receive combining) is the best candidate under realistic conditions. This is explained by the provably stronger resilience towards spatial correlation and the larger benefit from multi-user diversity. This fundamental result has positive implications for the design of downlink systems as it reduces the hardware requirements at the user devices and simplifies the throughput optimization.

  • 44.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Optimal Coordinated Beamforming in the Multicell Downlink with Transceiver Impairments2012In: 2012 IEEE Global Telecommunications Conference (GLOBECOM), New York: IEEE conference proceedings, 2012, 4775-4780 p.Conference paper (Refereed)
    Abstract [en]

    Physical wireless transceivers suffer from a variety of impairments that distort the transmitted and received signals. Their degrading impact is particularly evident in modern systems with multiuser transmission, high transmit power, and low-cost devices, but their existence is routinely ignored in the optimization literature for multicell transmission. This paper provides a detailed analysis of coordinated beamforming in the multicell downlink. We solve two optimization problems under a transceiver impairment model and derive the structure of the optimal solutions. We show numerically that these solutions greatly reduce the impact of impairments, compared with beamforming developed for ideal transceivers. Although the so-called multiplexing gain is zero under transceiver impairments, we show that the gain of multiplexing can be large at practical SNRs.

  • 45.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Capacity Limits and Multiplexing Gains of MIMO Channels with Transceiver Impairments2013In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 17, no 1, 91-94 p.Article in journal (Refereed)
    Abstract [en]

    The capacity of ideal MIMO channels has a high-SNR slope that equals the minimum of the number of transmit and receive antennas. This letter analyzes if this result holds when there are distortions from physical transceiver impairments. We prove analytically that such physical MIMO channels have a finite upper capacity limit, for any channel distribution and SNR. The high-SNR slope thus collapses to zero. This appears discouraging, but we prove the encouraging result that the relative capacity gain of employing MIMO is at least as large as with ideal transceivers.

  • 46.
    Björnson, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Zheng, Gan
    Interdisciplinary Centre for Security, Reliability and Trust, University of Luxembourg.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Robust Monotonic Optimization Framework for Multicell MISO Systems2012In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 60, no 5, 2508-2523 p.Article in journal (Refereed)
    Abstract [en]

    The performance of multiuser systems is both difficult to measure fairly and to optimize. Most resource allocation problems are nonconvex and NP-hard, even under simplifying assumptions such as perfect channel knowledge, homogeneous channel properties among users, and simple power constraints. We establish a general optimization framework that systematically solves these problems to global optimality. The proposed branch-reduce-and-bound (BRB) algorithm handles general multicell downlink systems with single-antenna users, multiantenna transmitters, arbitrary quadratic power constraints, and robustness to channel uncertainty. A robust fairness-profile optimization (RFO) problem is solved at each iteration, which is a quasiconvex problem and a novel generalization of max-min fairness. The BRB algorithm is computationally costly, but it shows better convergence than the previously proposed outer polyblock approximation algorithm. Our framework is suitable for computing benchmarks in general multicell systems with or without channel uncertainty. We illustrate this by deriving and evaluating a zero-forcing solution to the general problem.

  • 47.
    Björsell, Niclas
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics..
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Information Science and Engineering.
    Large Antenna Array for Low-Latency and Ultra-Reliable Communication2017Conference paper (Other academic)
  • 48.
    Brandt, Rasmus
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Asplund, Henrik
    Ericsson Research.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Interference Alignment in Frequency — a Measurement Based Performance Analysis2012In: 2012 19th International Conference on Systems, Signals and Image Processing, IWSSIP 2012, IEEE conference proceedings, 2012, 227-230 p.Conference paper (Refereed)
    Abstract [en]

    The idea of interference alignment (IA) has shown great promise in many theoretical studies. Yet it is not clear under what operating conditions it will perform better than traditional multiple access schemes under realistic network conditions. Here, we use measured channels to evaluate a number of different IA schemes and related methods, focusing on wideband single-antenna transmission, using frequency extensions for the IA. The radio channels were measured jointly from three base station sites in an urban macrocell scenario, over a 20 MHz bandwidth, allowing to emulate a scenario with 3 interfering cells (i.e. 3 interfering transmit/receive pairs). The numerical results show clear gains using IA related methods, compared to frequency planning with frequency reuse 3 and to uncoordinated transmission, even at low to moderate SNR.

  • 49.
    Brandt, Rasmus
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Distributed CSI Acquisition and Coordinated Precoding for TDD Multicell MIMO Systems2016In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 65, no 5, 2890-2906 p.Article in journal (Refereed)
    Abstract [en]

    Several distributed coordinated precoding methods exist in the downlink multicell MIMO literature, many of which assume perfect knowledge of received signal covariance and local effective channels. In this work, we let the notion of channel state information (CSI) encompass this knowledge of covariances and effective channels. We analyze what local CSI is required in the WMMSE algorithm for distributed coordinated precoding, and study how this required CSI can be obtained in a distributed fashion. Based on pilot-assisted channel estimation, we propose three CSI acquisition methods with different tradeoffs between feedback and signaling, backhaul use, and computational complexity. One of the proposed methods is fully distributed, meaning that it only depends on over-the-air signaling but requires no backhaul, and results in a fully distributed joint system when coupled with the WMMSE algorithm. Naively applying the WMMSE algorithm together with the fully distributed CSI acquisition results in catastrophic performance however, and therefore we propose a robustified WMMSE algorithm based on the well known diagonal loading framework. By enforcing properties of the WMMSE solutions with perfect CSI onto the problem with imperfect CSI, the resulting diagonally loaded spatial filters are shown to perform significantly better than the naive filters. The proposed robust and distributed system is evaluated using numerical simulations, and shown to perform well compared with benchmarks. Under centralized CSI acquisition, the proposed algorithm performs on par with other existing centralized robust WMMSE algorithms. When evaluated in a large scale fading environment, the performance of the proposed system is promising.

  • 50.
    Brandt, Rasmus
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fast-Convergent Distributed Coordinated Precoding for TDD Multicell MIMO Systems2015In: Proc. IEEE Int. Workshop Computational Advances in Multi-Sensor Adaptive Process. (CAMSAP'15), 2015Conference paper (Refereed)
    Abstract [en]

    Several distributed coordinated precoding methods relying on over-the-air (OTA) iterations in time-division duplex (TDD) networks have recently been proposed. Each OTA iteration incurs overhead, which reduces the time available for data transmission. In this work, we therefore propose an algorithm which reaches good sum rate performance within just a few number of OTA iterations, partially due to non-overhead-incurring local iterations at the receivers. We formulate a scalarized multi-objective optimization problem where a linear combination of the weighted sum rate and the multiplexing gain is maximized. Using a well-known heuristic for smoothing the optimization problem together with a linearization step, the distributed algorithm is derived. When numerically compared to the state-of-the-art in a scenario with 1 to 3 OTA iterations allowed, the algorithm shows significant sum rate gains at high signal-to-noise ratios.

1234 1 - 50 of 176
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf