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Helmersson, Anders
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Publications (10 of 72) Show all publications
Wang Helmersson, K., Frenger, P. & Helmersson, A. (2023). Robust precoding weights for downlink D-MIMO in 6G Communications. In: IEEE (Ed.), : . Paper presented at 2023 IEEE Globecom Workshops (GC Wkshps): 4th Workshop on Emerging Topics in 6G Communications. Presented at Workshop 6G, WS01-1: MIMO..
Open this publication in new window or tab >>Robust precoding weights for downlink D-MIMO in 6G Communications
2023 (English)In: / [ed] IEEE, 2023Conference paper, Oral presentation with published abstract (Other academic)
Abstract [en]

Point-to-point MIMO and massive MIMO techniques have played significantroles in the success of 4G and 5G radio networks, and in 6G we believethat distributed MIMO will play a similar critical role. The performanceof downlink phase coherent distributed MIMO transmission relies on tightphase alignment between the serving access points (APs) in the system.In realistic scenarios, there will always be some level of phasemisalignment between APs due to e.g., differences in the local clocksof the APs, which can severely degrade the performance.

One main contribution of this paper is that we propose the use of aLinear Quadratic Regulator (LQR) based solution for calculating downlinkprecoding weights in D-MIMO systems.  The optimal LQR based precodingsolution is numerically stable and computationally efficient, and it caneasily utilise parallel computing in distributed or centralised hardwareprocessors.  Furthermore, we also show how the LQR based solution can bemodified to include differently sized subsets of serving APs for each UE,which enables a scalable tradeoff between performance and complexity.

Another main contribution of the paper is that we identify a new phasemisalignment problem in D-MIMO. The proposed LQR-based precoding methodis the first solution that takes not only the channel estimation phaseerrors, but also the relative phase errors between serving APs intoaccount when designing the downlink D-MIMO transmission precoder. By this,some of the performance lost due to different causes of phase misalignmentcan be regained. In the scenarios studied in this paper we observe 20-70%performance increase of the proposed method compared to a reference casewhere residual phase errors are ignored when determining the downlinkprecoding weights.

Keywords
Distributed MIMO, downlink robust precoder, phase misalignment, Linear Quadratic Regulator (LQR), Kalman filter, dectriised processing
National Category
Communication Systems Telecommunications
Identifiers
urn:nbn:se:liu:diva-198312 (URN)
Conference
2023 IEEE Globecom Workshops (GC Wkshps): 4th Workshop on Emerging Topics in 6G Communications. Presented at Workshop 6G, WS01-1: MIMO.
Projects
REINDEER project of the European Union's Horizon 2020 research and innovation programme under gran agreement no. 101013425
Available from: 2023-10-04 Created: 2023-10-04 Last updated: 2023-12-21Bibliographically approved
Wang Helmersson, K., Frenger, P. & Helmersson, A. (2022). Uplink D-MIMO Processing and Combining Using Kalman Filter. In: Globecom 2022: . Paper presented at Globecom 2022, Rio de Janeiro, Brazil, Dec 4-8, 2022 (pp. 1703-1708). IEEE
Open this publication in new window or tab >>Uplink D-MIMO Processing and Combining Using Kalman Filter
2022 (English)In: Globecom 2022, IEEE, 2022, p. 1703-1708Conference paper, Published paper (Refereed)
Abstract [en]

We propose a decentralized subset method for optimal processing and combining of uplink signals in a distributed MIMO (D-MIMO) network. We further propose the use of Kalman filters with the square-root implementation to estimate the received uplink signals. This square-root implementation is shown to be numerically stable when inverting the covariance matrix, as it always assures the covariance matrix to be symmetric and positive semi-definite. In the paper we also analyze the computational complexity and cost with different combining methods. We show that the Kalman filter implementation provides the same result as the MMSE method in terms of the spectral efficiencies and equivalent SINR. However, the Kalman filter implementation is shown to be very efficient as it provides the possibility to fully utilize  parallel computing of distributed hardware processors. Moreover, the processing can be decentralized and the estimates can be aggregated from local estimates to as many access points (APs) as needed to reach the desired performance target. A Kalman filter implementation has the flexibility to aggregate signals in different ways, allowing the fronthaul architecture to support connectivity of individual APs in any combination of parallel or serial manners.

Place, publisher, year, edition, pages
IEEE, 2022
Series
IEEE Global Communications Conference, ISSN 2334-0983
Keywords
Distributed MIMO, uplink, MMSE, Kalman filter, decentralized processing and combining method
National Category
Telecommunications
Identifiers
urn:nbn:se:liu:diva-190736 (URN)10.1109/GLOBECOM48099.2022.10001527 (DOI)000922633501121 ()9781665435406 (ISBN)9781665435413 (ISBN)
Conference
Globecom 2022, Rio de Janeiro, Brazil, Dec 4-8, 2022
Projects
European Union’s Horizon 2020 research and innovation programme under grant agreements: MINTS MSCA-ITN, No 861222; REINDEER RIA, No 101013425
Funder
EU, Horizon 2020, 861222
Note

Funding: European Union [861222, 101013425]

Available from: 2022-12-24 Created: 2022-12-24 Last updated: 2023-05-09Bibliographically approved
Helmersson, A., Wang Helmersson, K. & Frenger, P. (2022). Uplink D-MIMO with Decentralized Subset Combining. In: ICC 2022 - IEEE International Conference on Communications: . Paper presented at ICC 2022, IEEE International Conference on Communications 2022, Seoul, South Korea, 16–20 May 2022 (pp. 5134-5139). IEEE
Open this publication in new window or tab >>Uplink D-MIMO with Decentralized Subset Combining
2022 (English)In: ICC 2022 - IEEE International Conference on Communications, IEEE, 2022, p. 5134-5139Conference paper, Published paper (Refereed)
Abstract [en]

We propose decentralized processing and combining in the uplink for a distributed MIMO (D-MIMO) network by a subset method. A subset that consists of a partial number of access points (APs) that are connected to a CPU is defined for each UE. A subset that consists of only one AP turns a D-MIMO network into “small cell” network; a subset that consists of all APs connected to the CPU give the same performance as a fully centralized method. The paper also shows that the subset method provides a very scalable trade-off between complexity and performance. It is simpler to realize as the processing can be distributed and parallelized. In the studied simulation scenarios, we show that it is possible to reach 85% of the performance upper bound by including only 20% of total APs in the subset, and to reach 95% of the performance upper bound by including 40% of APs in the subset.

Place, publisher, year, edition, pages
IEEE, 2022
Series
IEEE International Conference on Communications, ISSN 1550-3607, E-ISSN 1938-1883
Keywords
Distributed MIMO, D-MIMO, uplink combining, decentralized sub-set processing
National Category
Telecommunications
Identifiers
urn:nbn:se:liu:diva-186335 (URN)10.1109/ICC45855.2022.9838868 (DOI)000864709905055 ()9781538683484 (ISBN)9781538683477 (ISBN)
Conference
ICC 2022, IEEE International Conference on Communications 2022, Seoul, South Korea, 16–20 May 2022
Projects
MINTS MSC-ITN, No 861222: REINDEER RIA
Note

Funding: European Union [861222, 101013425]

Available from: 2022-06-22 Created: 2022-06-22 Last updated: 2022-12-13Bibliographically approved
Helmersson, A. (2015). Generalized Riccati Equations for Hinfinity Synthesis. Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Generalized Riccati Equations for Hinfinity Synthesis
2015 (English)Report (Other academic)
Abstract [en]

Conditions for the existence of controllers for lineartime-varying (LTV) systems is given. The closed loop performance isspecified in terms of energy gain, which also includes terminalconstraints. The conditions can be formulated either as linear matrixinequalities (LMIs) or as solutions to Riccati differental equationswith algebraic constraints.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. p. 21
Series
LiTH-ISY-R, ISSN 1400-3902 ; 3085
Keywords
Riccati, LTV, DAE, Hamiltonian, synthesis, LMI
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-118015 (URN)
Available from: 2015-05-20 Created: 2015-05-20 Last updated: 2015-05-21Bibliographically approved
Axelsson, P., Helmersson, A. & Norrlöf, M. (2014). H∞-Controller Design Methods Applied to One Joint of a Flexible Industrial Manipulator. In: Boje, Edward and Xia, Xiaohua (Ed.), Proceedings of the 19th IFAC World Congress, 2014: . Paper presented at 19th IFAC World Congress, August 24-29, Cape Town, South Africa (pp. 210-216). International Federation of Automatic Control
Open this publication in new window or tab >>H-Controller Design Methods Applied to One Joint of a Flexible Industrial Manipulator
2014 (English)In: Proceedings of the 19th IFAC World Congress, 2014 / [ed] Boje, Edward and Xia, Xiaohua, International Federation of Automatic Control , 2014, p. 210-216Conference paper, Published paper (Refereed)
Abstract [en]

Control of a flexible joint of an industrial manipulator using H design methods is presented. The considered design methods are i) mixed-H design, and ii) H loop shaping design. Two different controller configurations are examined: one uses only the actuator position, while the other uses the actuator position and the acceleration of end-effector. The four resulting controllers are compared to a standard PID controller where only the actuator position is measured. The choices of the weighting functions are discussed in details. For the loop shaping design method, the acceleration measurement is required to improve the performance compared to the PID controller. For the mixed-H method it is enough to have only the actuator position to get an improved performance. Model order reduction of the controllers is briefly discussed, which is important for implementation of the controllers in the robot control system.

Place, publisher, year, edition, pages
International Federation of Automatic Control, 2014
Series
World Congress, ISSN 1474-6670 ; Volume# 19, Part# 1
Keywords
Robotics, Flexible, H-infinity control, Accelerometers
National Category
Control Engineering
Identifiers
urn:nbn:se:liu:diva-104785 (URN)10.3182/20140824-6-ZA-1003.00143 (DOI)978-3-902823-62-5 (ISBN)
Conference
19th IFAC World Congress, August 24-29, Cape Town, South Africa
Projects
Vinnova Excellence Center LINK-SICExcellence Center at Linköping-Lund in Information Technology, ELLIIT
Funder
VinnovaeLLIIT - The Linköping‐Lund Initiative on IT and Mobile Communications
Available from: 2014-02-26 Created: 2014-02-26 Last updated: 2021-12-06Bibliographically approved
Axelsson, P., Pipeleers, G., Helmersson, A. & Norrlöf, M. (2014). H∞ Synthesis Method for Control of Non-linear Flexible Joint Models. In: Boje, Edward and Xia, Xiaohua (Ed.), Proceedings of the 19th IFAC World Congress, 2014: . Paper presented at 19th IFAC World Congress, August 24-29, Cape Town, South Africa (pp. 8372-8377). International Federation of Automatic Control
Open this publication in new window or tab >>H Synthesis Method for Control of Non-linear Flexible Joint Models
2014 (English)In: Proceedings of the 19th IFAC World Congress, 2014 / [ed] Boje, Edward and Xia, Xiaohua, International Federation of Automatic Control , 2014, p. 8372-8377Conference paper, Published paper (Refereed)
Abstract [en]

An H synthesis method for control of a flexible joint, with non-linear spring characteristic, is proposed. The first step of the synthesis method is to extend the joint model with an uncertainty description of the stiffness parameter. In the second step, a non-linear optimisation problem, based on nominal performance and robust stability requirements, has to be solved. Using the Lyapunov shaping paradigm and a change of variables, the non-linear optimisation problem can be rewritten as a convex, yet conservative, LMI problem. The method is motivated by the assumption that the joint operates in a specific stiffness region of the non-linear spring most of the time, hence the performance requirements are only valid in that region. However, the controller must stabilise the system in all stiffness regions. The method is validated in simulations on a non-linear flexible joint model originating from an industrial robot.

Place, publisher, year, edition, pages
International Federation of Automatic Control, 2014
Series
World Congress, ISSN 1474-6670 ; Volume# 19, Part# 1
Keywords
Mechanical systems, Flexible, Robust, H-infinity Control
National Category
Control Engineering
Identifiers
urn:nbn:se:liu:diva-104789 (URN)10.3182/20140824-6-ZA-1003.00142 (DOI)978-3-902823-62-5 (ISBN)
Conference
19th IFAC World Congress, August 24-29, Cape Town, South Africa
Projects
Vinnova Excellence Center LINK-SICExcellence Center at Linköping-Lund in Information Technology, ELLIIT
Funder
VinnovaeLLIIT - The Linköping‐Lund Initiative on IT and Mobile Communications
Available from: 2014-02-26 Created: 2014-02-26 Last updated: 2021-12-06Bibliographically approved
Axelsson, P., Pipeleers, G., Helmersson, A. & Norrlöf, M. (2013). H∞ Synthesis Method for Control of Non-linear Flexible Joint Models. Linköping
Open this publication in new window or tab >>H Synthesis Method for Control of Non-linear Flexible Joint Models
2013 (English)Report (Other academic)
Abstract [en]

An H synthesis method for control of a flexible joint, with non-linear spring characteristic, is proposed. The first step of the synthesis method is to extend the joint model with an uncertainty description of the stiffness parameter. In the second step, a non-linear optimisation problem, based on nominal performance and robust stability requirements, has to be solved. Using the Lyapunov shaping paradigm and a change of variables, the non-linear optimisation problem can be rewritten as a convex, yet conservative, LMI problem. The method is motivated by the assumption that the joint operates in a specific stiffness region of the non-linear spring most of the time, hence the performance requirements are only valid in that region. However, the controller must stabilise the system in all stiffness regions. The method is validated in simulations on a non-linear flexible joint model originating from an industrial robot.

Place, publisher, year, edition, pages
Linköping: , 2013. p. 6
Series
LiTH-ISY-R, ISSN 1400-3902 ; 3068
Keywords
Mechanical systems, Flexible, Robust, H-infinity Control
National Category
Control Engineering
Identifiers
urn:nbn:se:liu:diva-100891 (URN)LiTH-ISY-R-3068 (ISRN)
Projects
Vinnova Excellence Center LINK-SIC
Funder
Vinnova
Available from: 2013-11-14 Created: 2013-11-14 Last updated: 2021-12-06Bibliographically approved
Axelsson, P., Helmersson, A. & Norrlöf, M. (2013). H∞-Controller Design Methods Applied to One Joint of a Flexible Industrial Manipulator. Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>H-Controller Design Methods Applied to One Joint of a Flexible Industrial Manipulator
2013 (English)Report (Other academic)
Abstract [en]

Control of a flexible joint of an industrial manipulator using H design methods is presented. The considered design methods are i) mixed-H design, and ii) H loop shaping design. Two different controller configurations are examined: one uses only the actuator position, while the other uses the actuator position and the acceleration of end-effector. The four resulting controllers are compared to a standard PID controller where only the actuator position is measured. The choices of the weighting functions are discussed in details. For the loop shaping design method, the acceleration measurement is required to improve the performance compared to the PID controller. For the mixed-H method it is enough to have only the actuator position to get an improved performance. Model order reduction of the controllers is briefly discussed, which is important for implementation of the controllers in the robot control system.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. p. 7
Series
LiTH-ISY-R, ISSN 1400-3902 ; 3067
Keywords
Robotics, Flexible, H-infinity control, Accelerometers
National Category
Control Engineering
Identifiers
urn:nbn:se:liu:diva-100898 (URN)LiTH-ISY-R-3067 (ISRN)
Projects
Vinnova Excellence Center LINK-SIC
Funder
Vinnova
Available from: 2013-11-14 Created: 2013-11-14 Last updated: 2021-12-06Bibliographically approved
Ankelhed, D., Helmersson, A. & Hansson, A. (2012). A Partially Augmented Lagrangian Method for Low Order H-Infinity Controller Synthesis Using Rational Constraints. IEEE Transactions on Automatic Control, 57(11), 2901-2905
Open this publication in new window or tab >>A Partially Augmented Lagrangian Method for Low Order H-Infinity Controller Synthesis Using Rational Constraints
2012 (English)In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 57, no 11, p. 2901-2905Article in journal (Refereed) Published
Abstract [en]

This technical note proposes a method for low order H-infinity synthesis where the constraint on the order of the controller is formulated as a rational equation. The resulting nonconvex optimization problem is then solved by applying a partially augmented Lagrangian method. The proposed method is evaluated together with two well-known methods from the literature. The results indicate that the proposed method has comparable performance and speed.

Keywords
Augmented Lagrangian, H-infinity synthesis, Linear matrix inequalities (LMIs), Rational constraints
National Category
Control Engineering
Identifiers
urn:nbn:se:liu:diva-86652 (URN)10.1109/TAC.2012.2191333 (DOI)000310890900019 ()
Funder
Swedish Research Council, 60519401
Available from: 2012-12-20 Created: 2012-12-20 Last updated: 2017-12-06
Helmersson, A. (2012). Employing Kronecker Canonical Form for LMI-Based H-infinity Synthesis Problems. IEEE Transactions on Automatic Control, 57(8), 2062-2067
Open this publication in new window or tab >>Employing Kronecker Canonical Form for LMI-Based H-infinity Synthesis Problems
2012 (English)In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 57, no 8, p. 2062-2067Article in journal (Refereed) Published
Abstract [en]

The Kronecker canonical form (KCF) can be employed when solving H-infinity synthesis problem. The KCF structure reveals variables that can be eliminated in the semidefinite program that defines the controller. The structure can also be used to remove states in the controller without sacrificing performance. In order to find the KCF structure, we can transform the relevant matrices to a generalized upper triangular (Guptri) form using orthogonal transformations. Thus, we can avoid finding the KCF structure explicitly, which is a badly conditioned problem.

Keywords
Controller synthesis, Kronecker canonical form (KCF), Linear matrix inequality (LMI), Model reduction, Semidefinite programming, Singular value decomposition (SVD)
National Category
Control Engineering
Identifiers
urn:nbn:se:liu:diva-84748 (URN)10.1109/TAC.2011.2179846 (DOI)000306916900016 ()
Available from: 2012-10-19 Created: 2012-10-19 Last updated: 2017-12-07
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