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  • 1.
    Alizadeh, Mahmoud
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. KTH.
    Rönnow, Daniel
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    KTH.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    A new Block-Structure Modeling Technique for RF Power Amplifiers in a 2x2 MIMO System2017In: 13th International Conference on Advanced Technologies, Systems and Services in Telecommunications (TELSIKS) / [ed] Milovanovic, B. D.; Doncov, N. S.; Stankovic, Z. Z.; Dimitrijevic, T. Z., IEEE , 2017, p. 224-227Conference paper (Refereed)
    Abstract [en]

    A new block-structure behavioral model is proposed for radio frequency power amplifiers in a 2x2 multiple-input multiple-output system including input cross-talk. The proposed model forms kernels of blocks of different nonlinear order that correspond to the significant frequency response of measured frequency domain Volterra kernels. The model can therefore well describe the input-output relationships of the nonlinear dynamic behavior of PAs. The proposed model outperforms conventional models in terms of model errors.

  • 2.
    Amin, Shoaib
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. ACCESS Linnaeus Centre, Department of Signal Processing, KTH Royal Institute of Technology, Stockholm.
    Khan, Zain Ahmed
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. ACCESS Linnaeus Centre, Department of Signal Processing, KTH Royal Institute of Technology, Stockholm.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    ACCESS Linnaeus Centre, Department of Signal Processing, KTH Royal Institute of Technology, Stockholm.
    Rönnow, Daniel
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Concurrent dual-band power amplifier model modification using dual two-tone test2016In: 46th European Microwave Conference (EUMC) 2016, 2016, p. 186-189Conference paper (Refereed)
    Abstract [en]

    A dual two-tone technique for the characterization of memory effects in concurrent dual-band transmitters is revisited to modify a 2D-DPD model for the linearization of concurrent dual-band transmitters. By taking into account the individual nonlinear memory effects of the self- and cross-kernels, a new 2D modified digital pre-distortion (2D-MDPD) model is proposed, which not only supersedes the linearization performance but also reduces the computational complexity compared to the 2D-DP Dmodel in terms of a number of floating point operations (FLOPs). Experimental results show an improvement of 1.7 dBin normalized mean square error (NMSE) and a 58% reduction in the number of FLOPs.

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  • 3.
    Amin, Shoaib
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Zenteno, Efrain
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Landin, Per
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Rönnow, Daniel
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    ACCESS Linnaeus Centre, Signal Processing Lab, KTH Royal Institute of Technology.
    Noise Impact on the Identification of Digital Predistorter Parameters in the Indirect Learning Architecture2012In: 2012 Swedish Communication Technologies Workshop (Swe-CTW), IEEE conference proceedings, 2012, p. 36-39Conference paper (Refereed)
    Abstract [en]

    The indirect learning architecture (ILA) is the mostused methodology for the identification of Digital Pre-distorter (DPD) functions for nonlinear systems, particularly for high power amplifiers. The ILA principle works in black box modeling relying on the inversion of input and output signals of the nonlinear system, such that the inverse is estimated. This paper presents the impact of disturbances, such as noise in the DPD identification. Experiments were performed with a state-of-art Doherty power amplifier intended for base station operation in current telecommunication wireless networks. As expected, a degradation in the performance of the DPD (measured innormalized mean square error (NMSE)) is found in our experiments. However, adjacent channel power ratio (ACPR) can be a misleading figure of merit showing improvement in the performance for wrongly estimated DPD functions.

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  • 4.
    Andersson, Rabé
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Björsell, Niclas
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Robots are a promising investment to fight pandemics2021In: 2021 8th International Conference on Signal Processing and Integrated Networks (SPIN), IEEE , 2021, p. 458-463Conference paper (Refereed)
    Abstract [en]

    Coronavirus caused pandemics as many viruses did through human history. The current pandemic causes overwhelmed healthcare system, locked down cities, and massive fatality among humans. Thus, different robots have been used since the COVID outbreak worldwide to reduce spreading infectious diseases and support frontline healthcare workers. This paper sets out the different robots implemented for hospital, non-hospital use, and possible use that can be deployed amidst the pandemic. A literature survey of versatile robots during COVID-19 is introduced. Roboticists contributed with wheeled and drone robots with various applications to assist medical care systems and society during the ongoing crisis. Pandemics are common throughout human history and difficult to avoid or prevent; thus, we intend to encourage societies, academia, engineers and innovators to invest more in robots that cannot catch the virus and consequently introduce beneficial solutions to fight such pandemic in the future.

  • 5.
    Björsell, Niclas
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Händel, Peter
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Rönnow, Daniel
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Kautz-Volterra modelling of an analogue-to-digital converter using a stepped three-tone excitation2007In: 12th IMEKO TC-4 International Workshop on ADC MODELLING AND TESTING, Iasi, Romania, September 2007, 2007, p. 107-112Conference paper (Refereed)
  • 6.
    Björsell, Niclas
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Royal Inst Technol, ACCESS Linnaeus Ctr, Stockholm, Sweden.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Royal Inst Technol, ACCESS Linnaeus Ctr, Stockholm, Sweden.
    Händel, Peter
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Royal Inst Technol, ACCESS Linnaeus Ctr, Stockholm, Sweden.
    Rönnow, Daniel
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Kautz-Volterra modelling of analogue-to-digital converters2010In: Computer Standards & Interfaces, ISSN 0920-5489, E-ISSN 1872-7018, Vol. 32, no 3, p. 126-129Article in journal (Refereed)
    Abstract [en]

    In many test and measurement applications, the analogue-to-digital converter (ADC) is the limiting component. Using post-correction methods can improve the performance of the component as well as the over all measurement system. In this paper an ADC is characterised by a Kautz-Volterra (KV) model, which utilises a model-based post-correction of the ADC with general properties and a reasonable number of parameters. It is also shown that the inverse model has the same dynamic properties as the direct KV model. Results that are based on measurements on a high-speed 12-bit ADC, shows good results for a third order model.

  • 7.
    Christensen, Simon
    et al.
    Aalborg University, Aalborg, Denmark.
    Bai, Shaoping
    Aalborg University, Aalborg, Denmark.
    Rafique, Sajid
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    O'Sullivan, Leonard
    University of Limrick, Limerick, Ireland.
    Power, Valerie
    University of Limrick, Limerick, Ireland.
    Virk, Gurvinder Singh
    Innovative Technology and Science Limited, Cambridge, United Kingdom.
    AXO-SUIT: A Modular Full-Body Exoskeleton for Physical Assistance2019In: Mechanism Design for Robotics: Proceedings of the 4th IFToMM Symposium on Mechanism Design for Robotics / [ed] Alessandro Gasparetto and Marco Ceccarelli, Cham: Springer Netherlands, 2019, Vol. 66, p. 443-450Chapter in book (Refereed)
    Abstract [en]

    This paper presents the design of a modular full-body assistive exoskeleton (FB-AXO) for older adults which was developed with funding under the AAL funded AXO-SUIT project. Processes used to formulate a prioritized set of functional and design requirements via close-end-user involvement are outlined and used in realizing the exoskeleton. Design of the resulting mechanics and electronics details for the lower and upper-body subsystems (LB-AXO and (UB-AXO)) are described. Innovative designs of shoulder and spine mechanisms are presented. TheFB-AXO system comprises 27 degrees of freedom, of which 17 are passive and 10 active. The exoskeleton assists full-body motions such as walking, standing, bending, as well as performing lifting and carrying tasks to assist older users performing tasks of daily living.

  • 8.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    A Measure to Characterize the Impact of Sets of Scientific Papers2008Report (Other academic)
    Abstract [en]

    A figure-of-merit for quantifying a comparable scientific value ranking the output of a set researchers work is proposed. The method is based on the Garfield Factor (GF) and thus related to the expected number of citations rather than the real ones. The measure allow comparisons of the scientific outputs between different sets of scientists where some papers are joint works between the comparative sets or between constellations with other sets. It has the advantages of having a rather small time-lag and that it is comparable between different subject categories. The measure is very easy to calculate and is complementing the existing indices or measures in an adequate way.

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  • 9.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Behavioural Modelling of Radio Frequency Power Amplifiers: An Evaluation of Some Block Structure and Neural Network Models2005Licentiate thesis, monograph (Other academic)
    Abstract [en]

    This work considers behavioral modelling of radio frequency power amplifiers. Due to the use of modern digital modulation methods power amplifiers are nowadays subjected to signals having a considerable bandwidth and a fast changing envelope. This means that traditional quasi-memoryless amplitude-to-amplitude (AM/AM) and amplitude-to-phase (AM/PM) characteristics are no longer enough to describe and model the behavior of power amplifiers, neither can they be successfully used for linearization.

    In this thesis, sampled input and output data are used for identification and validation of some block structure models with memory. The time-discrete Volterra model, the Wiener model, the Hammerstein model, and the radial-basis function neural network are all identified and compared with respect to in-band and out-of-band errors. Two different signal types, i.e. multi tones and noise, with different powers, peak-to-average ratios, and bandwidths have been used as input to the amplifier. Two different power amplifiers were investigated, one designed for the third generation mobile telecommunication systems and one for the second generation.

    A stepped three-tone measurement technique based on digitally modulated baseband signals is presented. The third-order Volterra kernel were determined from identified inter-modulation products. The properties of the Volterra kernel along certain parts in the three dimensional frequency space were analysed and compared to the Wiener and Hammerstein models.

  • 10.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Parameter-reduction for Volterra models in radio frequency power amplifier applications2007In: RF Measurement Technology Conference, Gävle 11-12 September, 2007Conference paper (Other academic)
  • 11.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Radio Frequency Power Amplifiers: Behavioral Modeling, Parameter-Reduction, and Digital Predistortion2009Book (Other academic)
  • 12.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Radio Frequency Power Amplifiers: Behavioral Modeling, Parameter-Reduction, and Digital Predistortion2007Doctoral thesis, comprehensive summary (Other academic)
  • 13.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Rönnow, Daniel
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    A Kautz-Volterra Behavioral Model for RF Power Amplifiers2006In: IEEE MTT-S International Microwave Symposium Digest, 2006, 2006, p. 485-488Conference paper (Refereed)
    Abstract [en]

    A new type of behavioral power amplifier (PA) model, a discrete-time Kautz-Volterra (KV) model, is presented. In the model a priori knowledge of the system properties in terms of different poles for different nonlinear orders is used, which is needed for modeling nonlinear and linear memory effects in PAs. An accurate model can thus be achieved with a small number of parameters. Simulated results of parallel Hammerstein and Wiener structures and from modeling the behavior of a PA are presented

  • 14.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Rönnow, Daniel
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    A Parameter-Reduced Volterra Model for Dynamic RF Power Amplifier Modeling based on Orthonormal Basis Functions2007In: International Journal of RF and Microwave Computer-Aided Engineering, ISSN 1096-4290, E-ISSN 1099-047X, Vol. 17, no 6, p. 542-551Article in journal (Refereed)
    Abstract [en]

    A nonlinear dynamic behavioral model for radio frequency power amplifiers is presented. It uses orthonormal basis functions, Kautz functions, with complex poles that are different for each nonlinear order. It has the same general properties as Volterra models, but the number of parameters is significantly smaller. Using frequency weighting the out-of-band model error can be reduced. Using experimental data it was found that the optimal poles were the same for different input powers and for the different nonlinear orders. The optimal poles were also the same for direct and inverse models, which could be explained theoretically to be a general property of nonlinear systems with negligible linear memory effects. The model can be used as either a direct or inverse model with the same model error for power amplifiers with negligible linear memory effects.

  • 15.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Wisell, David
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Extension of the Hammerstein Model for Power Amplifier Applications2004In: 63rd ARFTG Conference Digest - Spring 2004, 2004, p. 131-137Conference paper (Refereed)
  • 16.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik. Royal Institute of Technology, Signal Processing Lab, Stockholm, Sweden.
    Wisell, David
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik. Royal Institute of Technology, Signal Processing Lab, Stockholm, Sweden; Ericsson AB, Gävle, Sweden.
    Eng, Anders
    Syntronic AB, Gävle, Sweden.
    Rönnow, Daniel
    WestemGeco AS, Asker, Norway.
    A Study of a Variable-Capacitance Drain Network's Influence on Dynamic Behavioral Modeling of an RF PA2007In: 69th ARFTG Microwave measurement conference, Honolulu, HI, USA, 2007, p. 112-116Conference paper (Refereed)
    Abstract [en]

    An investigation of the complex-valued parameters of two different dynamic behavioral models due to changes in the drain decoupling network of a PA has been performed. The study is based on complex-envelope measurements on a real PA. The complex coefficients of the parallel Hammerstein (PH) and the optimal pole placements of the Kautz-Volterra (KV) models are analyzed. It is concluded that the parameters follow smooth, predictable functions and that the KV poles give a robust description of the PA's memory effects compared with the PH model.

  • 17.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Wisell, David
    Rönnow, Daniel
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    A Comparative Analysis of Behavioral Models for RF Power Amplifiers2006In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 54, no 1, p. 348-359Article in journal (Refereed)
    Abstract [en]

    A comparative study of nonlinear behavioral models with memory for radio-frequency power amplifier (PAs) is presented. The models are static polynomial, parallel Hammerstein (PH), Volterra, and radial basis-function neural network (RBFNN). Two PAs were investigated: one was designed for the third-generation (3G) mobile telecommunication systems and one was designed for the second-generation (2G). The RBFNN reduced the total model error slightly more than the PH, but the error out of band was significantly lower for the PH. The Volterra was found to give a lower model error than did a PH of the same nonlinear order and memory depth. The PH could give a lower model error than the best Volterra, since the former could be identified with a higher nonlinear order and memory depth. The qualitative conclusions are the same for the 2G and 3G PAs, but the model errors are smaller for the latter. For the 3G PA, a static polynomial gave a low model error as low as the best PH and lower than the RBFNN for the hardest cross validation. The models with memory, PH, and RBFNN, showed better cross-validation performance, in terms of lower model errors, than a static polynomial for the hardest cross validation of the 2G PA.

  • 18.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Wisell, David
    Rönnow, Daniel
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Behavioural Amplifier Modelling Using Sampled Complex Envelope Measurement Data2005In: Proc. Gigahertz 2005, 2005, p. 213-216Conference paper (Refereed)
  • 19.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Wisell, David
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik. Ericsson AB, Gävle, Sweden.
    Rönnow, Daniel
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Nonlinear Behavioral Modeling of Power Amplifiers Using Radial-Basis Function Neural Networks2005In: 2005 IEEE MTT-S International Microwave Symposium Digest, 2005, p. 1967-1970Conference paper (Refereed)
    Abstract [en]

    A radial-basis function neural network (RBFNN) is proposed for modeling the dynamic nonlinear behavior of RF power amplifiers. In the model the signal's envelope is used. The model requires less training than a model using both IQ-data. Sampled input and output signals from a power amplifier for 3G were used in the identification and validation. The RBFNN is compared with a parallel Hammerstein model. For a memory depth of one sample the RBFNN gives a better model, in- and out-of-band; for three samples the RBFNN reduces the in-band error more while the Hammerstein model reduces the error out-of-band more.

  • 20.
    Isaksson, Magnus
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Wisell, David
    Rönnow, Daniel
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Wide-Band Dynamic Modeling of Power Amplifiers Using Radial-Basis Function Neural Networks2005In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 53, no 11, p. 3422-3428Article in journal (Refereed)
  • 21.
    Isaksson, Magnus
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Zenteno, Efrain
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    A synthetic vector network analyzing measurement system2011In: IEEE Transactions on Instrumentation and Measurement, ISSN 0018-9456, E-ISSN 1557-9662, Vol. 60, no 6, p. 2154-2161Article in journal (Refereed)
    Abstract [en]

    In this paper a synthetic vector network analyzing measurement system is presented. The system is based on a hardware set-up, including a signal generator and a vector signal analyzer, with the vector network analyzing functionality implemented in the software. The measurements of the proposed system demonstrated comparable performance in terms of accuracy and speed compared with a modern traditional vector network analyzer, but it is more flexible due to its inherent software implementation. The proposed system’s ability to measure nonlinear phenomena is addressed and discussed, and some preliminary results are given.

  • 22.
    Jain, Prakhar
    et al.
    Thapar University of Technology.
    Bera, Tarun Kumar
    Thapar University of Technology.
    Rafique, Sajid
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Singla, Ashish
    Thapar University of Technology.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Comparative study of knee joint torque estimations for linear and rotary actuators using bond graph approach for stand–sit–stand motions2020In: International Journal of Advanced Robotic Systems, ISSN 1729-8806, E-ISSN 1729-8814, Vol. 17, no 5, p. 1-13Article in journal (Refereed)
    Abstract [en]

    Stand–sit–stand (STS) motions are the most frequently performed activities of everyday life and require extensive movement of knee joint. People suffering from knee joint disorders face difficulties in performing this motion. The compact knee exoskeleton (KE) has proven to be a viable, less complex, and cheaper alternative to the available entire lower-, upper-, and full-body exoskeletons. With growing number of technical glitches and finite battery life problems, there exist risks of sudden failure of the actuator of KE that could be detrimental for the vulnerable users. To overcome this problem, there is a need to accommodate a backup actuator in KE which can continue providing assistance during movement if the primary actuator ceases to function. This article provides a performance comparison of a four-bar mechanism-driven KE that can accommodate both the linear and the rotary actuators. The modelling and simulation of the system are performed using the bond graph (BG) technique. The results successfully showed that both actuators offered desired ranges of motions needed for STS motion. Furthermore, the knee joint torques developed by the linear and rotary actuators were found to be 40 Nm and 57 Nm, respectively, which corresponds to 60% and 85% of the total torque required by the knee joint to perform STS motions, thereby reducing the user effort to 40% and 15%, respectively. Thus, both actuators are self-capable to provide necessary assistance at the knee joint even if the primary actuator ceases to work due to a sudden fault, the secondary actuator will provide the required rotation of the thigh link and will continue to deliver the assistive torque. The article also effectively shows the application of BG approach to model the multidisciplinary systems like KE as it conveniently models the system containing various elements in different energy domains.

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  • 23.
    Jain, Prakhar
    et al.
    Department of Mechanical Engineering, Thapar Institute of Engineering & Technology (TIET) (Deemed University), India.
    Bera, Tarun Kumar
    Department of Mechanical Engineering, Thapar Institute of Engineering & Technology (TIET) (Deemed University), India.
    Singla, Ashish
    Department of Mechanical Engineering, Thapar Institute of Engineering & Technology (TIET) (Deemed University), India.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics. Faculty of Health and Occupational Studies, University of Gävle, Sweden.
    Linear actuator–based knee exoskeleton for stand–sit–stand motions: a bond graph approach2022In: Simulation (San Diego, Calif.), ISSN 0037-5497, E-ISSN 1741-3133, Vol. 98, no 8, p. 627-644Article in journal (Refereed)
    Abstract [en]

    People with knee disorders often find it difficult to perform common mobility tasks, such as stand–sit–stand motions. High knee torque is required to complete such transitions, as the chances of toppling increase during these motions. Most of the existing conventional approaches, such as wheelchairs and crutches, have failed to provide complete independence to the users. Conversely, contemporary systems like lower body exoskeletons which are bulky, complex, and expensive do not specifically target the knee joint instead of assisting other joints. Hence, there is a need to aid the knee joint using a robotic knee exoskeleton capable of accurately providing the desired knee torque. In the present work, to assist the user in performing the stand–sit–stand motions, an electromyography sensor-based four-bar knee exoskeleton actuated by a linear actuator is proposed. The modeling of the complete exoskeleton is developed using bond graph technique, as the components exist in different energy domains and it is possible to frame a dynamic bond graph model using only kinematic equations. The prototype is fabricated, and experiments are carried out on an artificial limb to prove the efficacy of the design of the current knee exoskeleton. The assistive torque developed by the actuator at the knee joint of the exoskeleton is found to be suitable to assist the wearer. As a result, little effort is required by the wearer for performing the stand–sit–stand motions. The rotation of the thigh link of the developed exoskeleton was found to be suitable for performing the stand–sit–stand activity.

  • 24.
    Johansson, Anders
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Mathematics.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Rönnow, Daniel
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    An analysis of Kautz-Volterra models for modeling block structure nonlinear systems2010Report (Other academic)
    Abstract [en]

    Kautz-Volterra (KV) models of some nonlinear systems were analyzed. The relationsbetween the true pole of the analyzed systems and the optimal pole of the KV modelswere analyzed. The properties of nonlinear systems depend on the order of thesubsystems, since nonlinear operators do not commute. Wiener (H-N) andHammerstein (N-H) systems were analyzed.

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  • 25.
    Khan, Zain Ahmed
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. KTH, Royal Institute of Technology.
    Händel, Peter
    KTH, Royal Institute of Technology, Stockholm, Sweden.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    A Comparative Analysis of the Complexity/Accuracy Tradeoff in the Mitigation of RF MIMO Transmitter Impairments2017In: 2017 89th ARFTG Microwave Measurement Conference (ARFTG), IEEE, 2017, article id 8000827Conference paper (Refereed)
    Abstract [en]

    This paper presents a comparative analysis of the complexity accuracy tradeoff in state-of-the-art RF MIMO transmitter mitigation models. The complexity and accuracy of the candidate models depends on the basis functions considered in these models. Therefore, a brief description of the mitigation models is presented accompanied by derivations of the model complexities in terms of the number of FLOPs. Consequently, the complexity accuracy tradeoff in the candidate models is evaluated for a 2 × 2 RF MIMO transmitter. Furthermore, the model complexities are analyzed for increasing nonlinear orders and number of antennas.

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  • 26.
    Khan, Zain Ahmed
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Royal Institute of Technology, KTH.
    Zenteno, Efrain
    Universidad Católica San Pablo.
    Händel, Peter
    Royal Institute of Technology (KTH).
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Digital Predistortion for Joint Mitigation of I/Q Imbalance and MIMO Power Amplifier Distortion2017In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 65, no 1, p. 322-333, article id 7600411Article in journal (Refereed)
    Abstract [en]

    This paper analyzes the joint effects of in-phase and quadrature (I/Q) imbalance and power amplifier (PA) distortion for RF multiple input multiple output (MIMO) transmitters in the presence of crosstalk. This paper proposes candidate models for the digital predistortion of static I/Q imbalanced sources exciting a dynamic MIMO Volterra system. The proposed models are enhanced using a novel technique based on subsample resolution to account for dynamic I/Q imbalance distortions. Finally, the computational complexity of the proposed models is analyzed for implementation suitability in digital platforms. It is shown that the error spectrum for the proposed models in subsample resolution reaches the noise floor of the measurements. The proposed models achieve a normalized mean squared error of -50 dB and an adjacent channel power ratio of -57 dB for signal bandwidths upto 65 MHz and crosstalk levels ranging to -10 dB. These results demonstrate the effectiveness of the proposed techniques in the joint mitigation of I/Q imbalance and PA distortion with crosstalk for a typical 2x2 MIMO telecommunication setup.

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  • 27.
    Khan, Zain Ahmed
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Department of Information Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden.
    Zenteno, Efrain
    Universidad Católica San Pablo, Arequipa, Peru.
    Händel, Peter
    Department of Information Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Extraction of the Third-Order 3x3 MIMO Volterra Kernel Outputs Using Multitone Signals2018In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 66, no 11, p. 4985-4999Article in journal (Refereed)
    Abstract [en]

    This paper uses multitone signals to simplify the analysis of 3×3 multiple-input multiple-output (MIMO) Volterra systems by isolating the third-order kernel outputs from each other. Multitone signals fed to an MIMO Volterra system yield a spectrum that is a permutation of the sums of the input signal tones. This a priori knowledge is used to design multitone signals such that the third-order kernel outputs are isolated in the frequency domain. The signals are designed by deriving the conditions for the offset and spacing of the input frequency grids. The proposed technique is then validated for the six possible configurations of a 3x3 RF MIMO transmitter impaired by crosstalk effects. The proposed multitone signal design is used to extract the third-order kernel outputs, and their relative contributions are analyzed to determine the dominant crosstalk effects for each configuration.

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  • 28.
    Khan, Zain Ahmed
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. KTH Royal Institute of Technology.
    Zenteno, Efrain
    Universidad Católica San Pablo.
    Händel, Peter
    KTH Royal Institute of Technology.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. University of Gävle.
    Identification of Third Order 3x3 MIMO Volterra Kernels using Multitone Excitation SignalsManuscript (preprint) (Other academic)
  • 29.
    Khan, Zain Ahmed
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. KTH Royal Institute of Technology, Stockholm, Sweden.
    Zenteno, Efrain
    Department of Electronics and Telecommunications, Universidad Catolica San Pablo, Arequipa, Peru.
    Händel, Peter
    KTH Royal Institute of Technology, Stockholm, Sweden.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Multitone design for third order MIMO volterra kernels2017In: 2017 IEEE MTT-S International Microwave Symposium (IMS), IEEE conference proceedings, 2017, p. 1553-1556Conference paper (Refereed)
    Abstract [en]

    This paper proposes a technique for designing multitone signals that can separate the third order multiple input multiple output (MIMO) Volterra kernels. Multitone signals fed to a MIMO Volterra system yield a spectrum that is a permutation of the sums of the input signal tones. This a priori knowledge is used to design multitone signals such that the output from the MIMO Volterra kernels does not overlap in the frequency domain, hence making it possible to separate these kernels from the output of the MIMO Volterra system. The proposed technique is applied to a 2×2 RF MIMO transmitter to determine its dominant hardware impairments. For input crosstalk, the proposed method reveals the dominant self and cross kernels whereas for output crosstalk, the proposed method reveals that only the self kernels are dominant.

  • 30.
    Khan, Zain Ahmed
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Department of Signal Processing, KTH Royal Institute of Technology, Stockholm, Sweden .
    Zenteno, Efrain
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Department of Signal Processing, KTH Royal Institute of Technology, Stockholm, Sweden .
    Händel, Peter
    Department of Signal Processing, KTH Royal Institute of Technology, Stockholm.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Study of the Power Amplifier transfer function symmetry and its relation to dynamic effects2015In: 2015 IEEE 16th Annual Wireless and Microwave Technology Conference (WAMICON), IEEE conference proceedings, 2015, p. 1-6Conference paper (Refereed)
    Abstract [en]

    Future wireless networks will employ unprecedentedly high number of transmitters along-with larger bandwidth signals. Therefore, complexity efficient Power Amplifier (PA) modelling methods with the ability to quantify memory effects are required. This paper presents a low complexity method to quantify memory effects by studying the symmetry in the PA transfer functions estimated with the density estimation method. It is shown that the symmetry helps in reducing the method complexity by more than two orders of magnitude. Further, it is verified that the symmetry can be used as a non-model based metric for quantifying memory effects in PAs. 

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  • 31.
    Khan, Zain Ahmed
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. KTH.
    Zenteno, Efrain
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. KTH.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    KTH.
    Density estimation models for strong nonlinearities in RF power amplifiers2014In: Proceedings of the Asia-Pacific Microwave Conference 2014, 2014, p. 116-118Conference paper (Refereed)
    Abstract [en]

    The paper presents a method for modeling strongnonlinear effects in power amplifiers based on the principlesof density estimation. The static nonlinear transfer function isobtained by averaging measured data. The performance obtainedwith density estimation is similar to the one using high ordernonlinear static polynomial models. The benefit of consideredmethod over the ones using polynomial models is that the formerestimates blindly the structure of the transfer function and doesnot suffer from numerical instabilities.

  • 32.
    Landin, Per
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Barbé, Kurt
    Vrije Universiteit Brussel.
    Van Moer, Wendy
    Vrije Universiteit Brussel.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    KTH Royal Institute of Technology.
    Proofs of Derivations in Memory Polynomial Baseband Modeling of RF Power Amplifiers2011Report (Other academic)
    Abstract [en]

    This paper contains supporting derivations for the paperMemory Polynomial Baseband Modeling of RF Power Am-plifiers. All references to numbered equations, propositionsand assumptions are to the corresponding number inMemoryPolynomial Baseband Modeling of RF Power Amplifiers.

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  • 33.
    Landin, Per
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Keskitalo, Niklas
    University of Gävle, Center for RF Measurement Technologies.
    Tornblad, Olof
    Infineon Technologies.
    Sideband asymmetries in RF Power LDMOS Before and After Digital Predistortion2009Conference paper (Other academic)
    Abstract [en]

     The merging of the fields of RF engineering and signal processing has introduced concepts such as behavioral modeling and enabled digital linearization schemes for wireless devices, such as power amplifiers (PAs). Despite that this process has been going on for a number of years much work remains to be done. The links between physical behavior and mathematical models are far from well-understood as are the optimum strategies for device design. This study focus on digital predistortion properties of a one-stage PA consisting of a power transistor mounted in a test fixture. The device under test (DUT) is an Infineon PTF210451E, a 45W transistor intended for usage in the frequency bands 2010-2025 MHz and 2110-2170 MHz. The test fixture is also designed by Infineon Technologies.  The signal types used in the measurements are single and double carrier wideband code division multiple access (WCDMA) signals. The double carrier WCDMA signals have tone-spacings of 5, 10 and 15 MHz. Normal two-tone measurements are also presented. 

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    RFMTC09
  • 34.
    Landin, Per N.
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Chalmers University of Technology, Göteborg, Sweden; KTH Royal Institute of Technology, Stockholm, Sweden; Vrije Universiteit Brussel, Belgium.
    Barbé, Kurt
    Vrije Universiteit Brussel, Belgium.
    Van Moer, Wendy
    Vrije Universiteit Brussel, Belgium.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    KTH Royal Institute of Technology, Stockholm, Sweden.
    Two novel memory polynomial models for modeling of RF power amplifiers2015In: International journal of microwave and wireless technologies, ISSN 1759-0795, E-ISSN 1759-0787, Vol. 7, no 1, p. 19-29Article in journal (Refereed)
    Abstract [en]

    Two novel memory polynomial models are derived based on physical knowledge of a general power amplifier (PA). The derivations are given in detail to facilitate derivations of other model structures. The model error in terms of normalized mean square error (NMSE) and adjacent channel error power ratio (ACEPR) of the novel model structures are compared to that of established models based on the number of parameters using data measured on two different amplifiers, one high-power base-station PA and one low-power general purpose amplifier. The novel models show both lower NMSE and ACEPR for any chosen number of parameters compared to the established models. The low model errors make the novel models suitable candidates for both modeling and digital predistortion.

  • 35.
    Landin, Per
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Nader, Charles
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Van Moer, Wendy
    Vrije Universiteit Brussel.
    Björsell, Niclas
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    KTH Royal Institute of Technology.
    Overview of Synergetic OFDM Crest Factor Reduction and Digital Pre-Distortion for RF PAs2011In: RFMTC11, 2011Conference paper (Other academic)
    Abstract [en]

    Digital baseband pre-distortion (DPD) is used to linearize nonlinear RF PAs, crest factor reduction (CFR) to increase the maximum output power level of RF PAs. Together they result in increased output power with maintained, or even, better linearity. This paper discuss some of the techniques for DPD and CFR methods currently under investigation. Measured results for a class-AB PA using OFDM signals indicate a possibility to increase the power added efficiency (PAE) by at least 15%-points, from 30% to 45%, and a power output increase of 6 dB while maintaining specified ACLR requirements.

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  • 36.
    Landin, Per Niklas
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Barbé, Kurt
    Vrije Universiteit Brussel.
    Van Moer, Wendy
    Vrije Universiteit Brussel.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    KTH Royal Institute of Technology.
    Memory Polynomial Baseband Modeling of RF Power Amplifiers2012Manuscript (preprint) (Other academic)
  • 37.
    Landin, Per Niklas
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Bengtsson, Olof
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Power Amplifier Behavioural Model Mismatch Sensitivity and the Impact on Digital Predistortion Performance2009In: Proc. 39th European Microwave Conference (EuMC), IEEE conference proceedings, 2009, p. 338-341Conference paper (Refereed)
    Abstract [en]

    This paper presents an investigation of poweramplifier behavioural model performance sensitivity to in-bandreflections. The measurement system and model extractionprocess is presented together with the results and an analysis ofthe effect of parameter variations in a digitally predistortedsystem. A load-pull system is used together with a digitalbaseband model extraction system to identify the impact onmodelling performance. The results show that the modelperformance varies greatly with in-band reflection magnitudeand phase on the amplifier output. It is also shown that a digitalpredistortion based on a model extracted at matched conditions,where it gives an excellent improvement of 20 dB in adjacentchannel leakage ratio (ACLR), gives a poor improvement of 7 dBfor miss-matched conditions with as low reflections as Γ=0.2(return loss 14 dB). This indicates that in-band reflections needto be considered and adaptive predistortion used also for lowVSWR system like base-stations for telecommunication.

  • 38.
    Landin, Per Niklas
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Fager, Christian
    Chalmers.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Andersson, Kristoffer
    Chalmers.
    Power Amplifier Behavioral Modeling Performance Comparison of the LSNA and the Modulation-Domain System2008In: 72nd ARFTG Microwave Measurement Symposium, 2008, Portland, OR., 2008, p. 73-78Conference paper (Refereed)
  • 39.
    Landin, Per Niklas
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Fritzin, Jonas
    Linköping University.
    Jung, Ylva
    Linköping University.
    Enqvist, Martin
    Linköping University.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Alvandpour, Atila
    Linköping University.
    Digital baseband pre-distortion of class-D outphasing amplifiers2012In: Proceedings Gigahertz Symposium 2012, 2012Conference paper (Other academic)
    Abstract [en]

    This paper presents a novel structure and method for modeling and digitally pre-distorting outphasing amplifiers. The models and pre-distortion are evaluated on a CMOS low-power class-D outphasing amplifier using uplink WCDMA signals. Using the proposed pre-distorter improves the ACLR by no less than 12 dB making the amplifier fulfill the 3GPP requirements on ACLR.

  • 40.
    Landin, Per Niklas
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Fritzin, Jonas
    Linköping University.
    Van Moer, Wendy
    Vrije Universiteit Brussel.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Alvandpour, Atila
    Linköping University.
    Modeling and digital predistortion of class-D outphasing RF power amplifiers2012In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 60, no 6, p. 1907-1915Article in journal (Refereed)
    Abstract [en]

    This paper presents a direct model structure for describing class-D outphasing power amplifiers (PAs) and a method for digitally predistorting these amplifiers. The direct model structure is based on modeling differences in gain and delay, nonlinear interactions between the two paths, and differences in the amplifier behavior. The digital predistortion method is designed to operate only on the input signals' phases, to correct for both amplitude and phase mismatches. This eliminates the need for additional voltage supplies to compensate for gain mismatch. Model and predistortion performance are evaluated on a 32-dBm peak-output-power class-D outphasing PA in CMOS with on-chip transformers. The excitation signal is a 5-MHz downlink WCDMA signal with peak-to-average power ratio of 9.5 dB. Using the proposed digital predistorter, the 5-MHz adjacent channel leakage power ratio (ACLR) was improved by 13.5 dB, from -32.1 to -45.6 dBc. The 10-MHz ACLR was improved by 6.4 dB, from -44.3 to -50.7 dBc, making the amplifier pass the 3GPP ACLR requirements.

  • 41.
    Landin, Per Niklas
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    A review of validation criteria for behavioral power amplifier models2008Conference paper (Other academic)
  • 42.
    Landin, Per Niklas
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Isaksson, Magnus
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Händel, Peter
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Comparison of Evaluation Criteria for Power Amplifier Behavioral Modeling2008In: IEEE MTT-S International Microwave Symposium Digest; 2008, Atlanta, GA, USA, 2008, p. 1441-1444Conference paper (Refereed)
  • 43.
    Landin, Per Niklas
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Parameter Extraction and Performance Evaluation Method for Increased Performance in RF Power Amplifier Behavioral Modeling2010In: International Journal of RF and Microwave Computer-Aided Engineering, ISSN 1096-4290, E-ISSN 1099-047X, Vol. 20, no 2, p. 200-208Article in journal (Refereed)
  • 44.
    Landin, Per Niklas
    et al.
    University of Gävle, Center for RF Measurement Technologies.
    Nader, Charles
    University of Gävle, Center for RF Measurement Technologies.
    Björsell, Niclas
    University of Gävle, Center for RF Measurement Technologies.
    Isaksson, Magnus
    University of Gävle, Center for RF Measurement Technologies.
    Wisell, David
    Ericsson AB.
    Händel, Peter
    University of Gävle, Center for RF Measurement Technologies.
    Andersen, Olav
    Ericsson AB.
    Keskitalo, Niclas
    University of Gävle, Center for RF Measurement Technologies.
    Wideband Characterization of Power Amplifiers Using Undersampling2009In: 2009 IEEE MTT-S International Microwave Symposium, Boston: IEEE MTT , 2009, p. 1365-1368Conference paper (Refereed)
    Abstract [en]

    In this paper a radio frequency power amplifier is measured and characterized by the use of undersampling based on the generalized Zhu-Frank sampling theorem. A test system has been designed allowing the bandwidth of the stimuli signal to be 100 MHz in the characterization process. That would not be possible with any vector signal analyzer on the market. One of the more challenging problem within the proposed concept is the model validation process. Here, two different techniques for model validation are proposed, the multitone and the spectrum scan validation methods.

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    FULLTEXT01
  • 45.
    Landin, Per Niklas
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Van Moer, Wendy
    Vrije Universiteit Brussel.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Händel, Peter
    KTH Royal Institute of Technology.
    Peak-Power Controlled Digital Predistorters for RF Power Amplifiers2012In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 60, no 11, p. 3582-3590Article in journal (Refereed)
    Abstract [en]

    This paper investigates the issue of “predistorter blow-up,” i.e., uncontrolled peak expansion caused by the predistorter. To control the peak expansion, an extension of the multistep indirect learning architecture (MS-ILA) is proposed by adding a constraint that describes the allowed peak power of the predistortion signal. The resulting optimization problem is shown to be convex and an optimization method is formulated to solve it. Measurements on a class-AB power amplifier (PA) using orthogonal frequency-division multiplex signals show that the peak control works as intended and prevents the MS-ILA from generating high peaks when the PA is operated in compression. The restriction on the peak power also prevents the performance degradation occurring due to the “blow-up” problem. This makes the proposed controlled MS-ILA a safer option than the standard MS-ILA. In addition to controlling the peak input power to the PA, using the proposed method it was possible to increase the output power by 1.3 dB while fulfilling requirements of less than 40-dB adjacent channel leakage power ratio, compared to the standard five-step MS-ILA. Reduced peak power also reduces the requirements on linearity in signal generation, resolution in computations, and analog-to-digital and digital-to-analog conversion.

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  • 46.
    Masud, Nauman
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics. KTH.
    Mattsson, Per
    Uppsala University.
    Smith, Christian
    KTH.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    On stability and performance of disturbance observer-based-dynamic load torque compensator for assistive exoskeleton: A hybrid approach2020In: Mechatronics (Oxford), ISSN 0957-4158, E-ISSN 1873-4006, Vol. 69, article id 102373Article in journal (Refereed)
    Abstract [en]

    A disturbance observer-based-dynamic load-torque compensator for current-controlled DC-drives, as joint actuator of assistive exoskeletons, has been recently proposed. It has been shown that this compensator can effectively linearize and decouple the coupled nonlinear dynamics of the human-exoskeleton system, by more effectively compensating the associated nonlinear load-torques of the exoskeleton at the joint level. In this paper, a detailed analysis of the current controlled DC drive-servo system using the said compensator, with respect to performance and stability is presented, highlighting the key factors and considerations affecting both the stability and performance of the compensated servo system. It is shown both theoretically and through simulation results that the stability of the compensated servo system is compromised as performance is increased and vice-versa. Based on the saturation state of the servo system, a new hybrid switching control strategy is then proposed to select stability or performance-based compensator and controller optimally. The strategy is then experimentally verified both at the joint and task space level by using the developed four active-degree of freedom exoskeleton test rig.

  • 47.
    Masud, Nauman
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics. KTH.
    Rafique, Sajid
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Smith, Christian
    KTH.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Design control and actuator selection of a lower body assistive exoskeleton with 3-D passive compliant supports2023In: Journal of the Brazilian Society of Mechanical Sciences and Engineering, ISSN 1678-5878, E-ISSN 1806-3691, Vol. 45, no 12, article id 611Article in journal (Refereed)
    Abstract [en]

    Physical human–robotic interaction is a crucial area of concern for robotic exoskeletons. Lower weight requirement for the worn exoskeletons limits the number and size of joint actuators, resulting in a low active degree of freedom for the exoskeletons with joint actuators having limited power and bandwidth. This limitation invariably results in reduced physical human–robotic interaction performance for the exoskeleton. Recently several techniques have been proposed for the low active degree of freedom exoskeletons with improved physical human–robotic interaction performance using better load torque compensators and improved active compliance. However, effective practical implementation of these techniques requires special hardware and software design considerations. A detailed design of a new lower body exoskeleton is proposed in this paper that can apply these recently developed techniques to practically improve the physical human–robotic interaction performance of the worn exoskeletons. The design presented includes the exoskeleton's structural design, new joint assemblies and the design of novel 3-D passive, compliant supports. A methodology of selecting and verifying the joint actuators and estimating the desired assistive forces at the contact supports based on human user joint torque requirements and the degree of assistance is also thoroughly presented. A new CAN-based master–slave control architecture that supports the implementation of recent techniques for improved physical human–robotic interaction is also fully presented. A new control strategy capable of imparting simultaneous impedance-based force tracking control of the exoskeleton in task space using DOB-based-DLTC at joint space is also thoroughly presented. Simulation verification of the proposed strategy based on the actual gait data of elderly is presented lastly.

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  • 48.
    Masud, Nauman
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Senkic, Dario
    Smith, C.
    Royal Institute of Technology, Stockholm.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electrical Engineering, Mathematics and Science, Electronics.
    Modeling and control of a 4-ADOF upper-body exoskeleton with mechanically decoupled 3-D compliant arm-supports for improved-pHRI2021In: Mechatronics (Oxford), ISSN 0957-4158, E-ISSN 1873-4006, Vol. 73, article id 102406Article in journal (Refereed)
    Abstract [en]

    Safe physical human-robotic interaction is a crucial concern for worn exoskeletons where lower weight requirement limits the number and size of actuators to be used. A novel control strategy is suggested in this paper for the low degree of freedom exoskeletons, by combining proposed mechanically decoupled passive-compliant arm-supports with active compliance, to achieve an improved and safer physical-human-robotic-interaction performance, while considering the practical limitations of low-power actuators. The approach is further improved with a novel vectoral-form of disturbance observer-based dynamic load-torque compensator, proposed to linearize and decouple the nonlinear human-machine dynamics effectively. The design of a four-degree of freedom exoskeleton test-rig that can assure the implementation of the proposed strategy is also shortly presented. It is shown through simulation and experimentation, that the use of proposed strategy results in an improved and safer physical human-robotic interaction, for the exoskeletons using limited-power actuators. It is also shown both through simulation and experimentation, that the proposed vectoral-form of disturbance based dynamic load-toque compensator, effectively outperforms the other traditional compensators in compensating the load-torques at the joints of the exoskeleton.

  • 49.
    Masud, Nauman
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics. Department of Robotics, Perception and Learning, Royal Institute of Technology, Stockholm, Sweden.
    Smith, Christian
    Department of Robotics, Perception and Learning, Royal Institute of Technology, Stockholm, Sweden.
    Isaksson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Electronics.
    Disturbance observer based dynamic load torque compensator for assistive exoskeletons2018In: Mechatronics (Oxford), ISSN 0957-4158, E-ISSN 1873-4006, Vol. 54, p. 78-93Article in journal (Refereed)
    Abstract [en]

    In assistive robotics applications, the human limb is attached intimately to the robotic exoskeleton. The coupled dynamics of the human-exoskeleton system are highly nonlinear and uncertain, and effectively appear as uncertain load-torques at the joint actuators of the exoskeleton. This uncertainty makes the application of standard computed torque techniques quite challenging. Furthermore, the need for safe human interaction severely limits the gear ratio of the actuators. With small gear ratios, the uncertain joint load-torques cannot be ignored and need to be effectively compensated. A novel disturbance observer based dynamic load-torque compensator is hereby proposed and analysed for the current controlled DC-drive actuators of the exoskeleton, to effectively compensate the said uncertain load-torques at the joint level. The feedforward dynamic load-torque compensator is proposed based on the higher order dynamic model of the current controlled DC-drive. The dynamic load-torque compensator based current controlled DC-drive is then combined with a tailored feedback disturbance observer to further improve the compensation performance in the presence of drive parametric uncertainty. The proposed compensator structure is shown both theoretically and practically to give significantly improved performance w.r.t disturbance observer compensator alone and classical static load-torque compensator, for rated load-torque frequencies up to 1.6 Hz, which is a typical joint frequency bound for normal daily activities for elderly. It is also shown theoretically that the proposed compensator achieves the improved performance with comparable reference current requirement for the current controlled DC-drive.

  • 50.
    Nader, Charles
    et al.
    University of Gävle, Center for RF Measurement Technologies. University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Landin, Per Niklas
    University of Gävle, Center for RF Measurement Technologies. University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Björsell, Niclas
    University of Gävle, Center for RF Measurement Technologies. University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Isaksson, Magnus
    University of Gävle, Center for RF Measurement Technologies. University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för elektronik.
    Wisell, David
    Ericsson AB.
    Händel, Peter
    KTH, EES, Signal processing lab.
    Andersen, Olav
    Ericsson AB.
    Keskitalo, Niclas
    University of Gävle, Center for RF Measurement Technologies.
    Wideband Power Amplifiers Characterization by Undersampling: Zhu-Frank Sampling Theorem2009Conference paper (Other academic)
12 1 - 50 of 69
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