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  • 1.
    Alistarh, Dan
    et al.
    IST Austria, Klosterneuburg, Austria..
    Hoefler, Torsten
    Swiss Fed Inst Technol, Zurich, Switzerland..
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Khirirat, Sarit
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Konstantinov, Nikola
    IST Austria, Klosterneuburg, Austria..
    Renggli, Cedric
    Swiss Fed Inst Technol, Zurich, Switzerland..
    The Convergence of Sparsified Gradient Methods2018In: Advances in Neural Information Processing Systems 31 (NIPS 2018) / [ed] Bengio, S Wallach, H Larochelle, H Grauman, K CesaBianchi, N Garnett, R, Neural Information Processing Systems (NIPS) , 2018, Vol. 31Conference paper (Refereed)
    Abstract [en]

    Stochastic Gradient Descent (SGD) has become the standard tool for distributed training of massive machine learning models, in particular deep neural networks. Several families of communication-reduction methods, such as quantization, large-batch methods, and gradient sparsification, have been proposed to reduce the overheads of distribution. To date, gradient sparsification methods-where each node sorts gradients by magnitude, and only communicates a subset of the components, accumulating the rest locally-are known to yield some of the largest practical gains. Such methods can reduce the amount of communication per step by up to three orders of magnitude, while preserving model accuracy. Yet, this family of methods currently has no theoretical justification. This is the question we address in this paper. We prove that, under analytic assumptions, sparsifying gradients by magnitude with local error correction provides convergence guarantees, for both convex and non-convex smooth objectives, for data-parallel SGD. The main insight is that sparsification methods implicitly maintain bounds on the maximum impact of stale updates, thanks to selection by magnitude. Our analysis also reveals that these methods do require analytical conditions to converge well, justifying and complementing existing heuristics.

  • 2. Almstrom, Peter
    et al.
    Rabi, Maben
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Networked state estimation over a Gilbert-Elliot type channel2009In: Proceedings of the IEEE Conference on Decision and Control, 2009, p. 2711-2716Conference paper (Refereed)
    Abstract [en]

    We characterize the stability and achievable performance of networked estimation under correlated packet losses described by the Gilbert-Elliot model. For scalar continuous-time linear systems, we derive closed-form expressions for the mean-square distortion of the optimal estimator. The conditions for stable mean square estimation error are equivalent to those obtained previously for stability of 'peak distortions' [3]. We study how the estimator performance depends on loss probability and loss burst length, and show that the mean-square distortion remains bounded if the average burst length does not exceeda calculated bound. The main new finding is that, once we fix the mean length of loss bursts, the average packet loss rate influences the estimator's performance but not its stability. 

  • 3.
    Aminian, Behdad
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Araújo, José
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl H.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    GISOO: A virtual testbed for wireless cyber-physical systems2013In: Industrial Electronics Society, IECON 2013 - 39th Annual Conference of the IEEE, IEEE , 2013, p. 5588-5593Conference paper (Refereed)
    Abstract [en]

    The increasing demand for wireless cyber-physical systems requires correct design, implementation and validation of computation, communication and control methods. Traditional simulation tools, which focus on either computation, communication or control, are insufficient when the three aspects interact. Efforts to extend the traditional tools to cover multiple domains, e.g., from simulating only control aspects to simulating both control and communication, often rely on simplistic models of a small subset of possible communication solutions. We introduce GISOO, a virtual testbed for simulation of wireless cyber-physical systems that integrates two state-of-the art simulators, Simulink and COOJA. GISOO enables users to evaluate actual embedded code for the wireless nodes in realistic cyber-physical experiments, observing the effects of both the control and communication components. In this way, a wide range of communication solutions can be evaluated without developing abstract models of their control-relevant aspects, and changes made to the networking code in simulations is guaranteed to be translated into production code without errors. A double-tank laboratory experimental setup controlled over a multi-hop relay wireless network is used to validate GISOO and demonstrate its features.

  • 4.
    António, Gonga
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Landsiedel, O.
    Soldati, Pablo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Multi-channel communication vs. adaptive routing for reliable communication in WSNs2012In: IPSN'12 - Proceedings of the 11th International Conference on Information Processing in Sensor Networks, Association for Computing Machinery (ACM), 2012, p. 125-126Conference paper (Refereed)
    Abstract [en]

    Interference and link dynamics constitute great concerns for stability and performance of protocols in WSNs. In this paper we evaluate the impact of channel hopping and adaptive routing on delay and reliability focusing on delay critical applications.

  • 5.
    Aytekin, Arda
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Feyzmahdavian, Hamid Reza
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Asynchronous incremental block-coordinate descent2014In: 52nd Annual Allerton Conference on Communication, Control, and Computing (Allerton), IEEE conference proceedings, 2014, p. 19-24Conference paper (Refereed)
    Abstract [en]

    This paper studies a flexible algorithm for minimizing a sum of component functions, each of which depends on a large number of decision variables. Such formulations appear naturally in “big data” applications, where each function describes the loss estimated using the data available at a specific machine, and the number of features under consideration is huge. In our algorithm, a coordinator updates a global iterate based on delayed partial gradients of the individual objective functions with respect to blocks of coordinates. Delayed incremental gradient and delayed coordinate descent algorithms are obtained as special cases. Under the assumption of strong convexity and block coordinate-wise Lipschitz continuous partial gradients, we show that the algorithm converges linearly to a ball around the optimal value. Contrary to related proposals in the literature, our algorithm is delay-insensitive: it converges for any bounded information delay, and its step-size parameter can be chosen independently of the maximum delay bound.

  • 6. Belleschi, Marco
    et al.
    Balucanti, Lapo
    Soldati, Pablo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Abrardo, Andrea
    Fast power control for cross-layer optimal resource allocation in DS-CDMA wireless networks2009In: 2009 IEEE International Conference on Communications, 2009, p. 4341-4346Conference paper (Refereed)
    Abstract [en]

    This paper presents a novel cross-layer design for joint power and end-to-end rate control optimization in DS-CDMA wireless networks, along with a detailed implementation and evaluation in the network simulator ns-2. Starting with a network utility maximization formulation of the problem, we derive distributed power control, transport rate and queue management schemes that jointly achieve the optimal network operation. Our solution has several attractive features compared to alternatives: it adheres to the natural time-scale separation between rapid power control updates and slower end-to-end rate adjustments, and uses simplified power control mechanisms with reduced signalling requirements. We argue that these features are critical for a successful real-world implementation. To validate these claims, we present a detailed implementation of a crosslayer adapted networking stack for DS-CSMA ad-hoc networks in ns-2. We describe several critical issues that arise in the implementation, but are typically neglected in the theoretical protocol design, and evaluate the alternatives in extensive simulations.

  • 7.
    Belleschi, Marco
    et al.
    Ericsson AB, Sweden.
    Fodor, Gabor
    Ericsson AB, Sweden.
    Della Penda, Demia
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Pradini, Aidilla
    Ericsson AB, Sweden.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Abrardo, Andrea
    University of Siena, Italy.
    Benchmarking Practical RRM Algorithms for D2D Communications in LTE Advanced2014In: Wireless personal communications, ISSN 0929-6212, E-ISSN 1572-834X, Vol. 82, no 2, p. 883-910Article in journal (Refereed)
    Abstract [en]

    Device-to-device (D2D) communication integrated into cellular networks is an advanced tool to take advantage of the proximity of devices and allow for reusing cellular resources and thereby to increase the user bitrates and the system capacity. However, the introduction of D2D in legacy long term evolution (LTE) cellular spectrum requires to revisit and modify the existing radio resource management and power control (PC) techniques in order to fully realize the potential of the proximity and reuse gains and to limit the interference to the cellular layer. In this paper, we examine the performance of the legacy LTE PC tool box and benchmark it against an utility optimal iterative scheme. We find that the open loop PC scheme of LTE performs well for cellular users both in terms of the used transmit power levels and the achieved signal-to-interference-and-noise-ratio distribution. However, the performance of the D2D users as well as the overall system throughput can be boosted by the utility optimal scheme, by taking better advantage of both the proximity and the reuse gains. Therefore, in this paper we propose a hybrid PC scheme, in which cellular users employ the legacy LTE open loop PC, while D2D users exploits the utility optimizing distributed PC scheme. We also recognize that the hybrid scheme is not only nearly optimal, and can balance between spectral and energy efficiency, but it also allows for a distributed implementation at the D2D users, while preserving the LTE PC scheme for the cellular users.

  • 8. Bemporad, A.
    et al.
    Heemels, M.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Networked control systems2010Book (Other academic)
  • 9. Besselink, B.
    et al.
    Feyzmahdavian, Hamid Reza
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Håkan
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    D-stability and delay-independent stability of monotone nonlinear systems with max-separable Lyapunov functions2016In: 2016 IEEE 55th Conference on Decision and Control, CDC 2016, Institute of Electrical and Electronics Engineers (IEEE), 2016, p. 3172-3177, article id 2-s2.0-85010722892Conference paper (Refereed)
    Abstract [en]

    Stability properties of monotone nonlinear systems with max-separable Lyapunov functions are considered in this paper, motivated by the following observations. First, recent results have shown that such Lyapunov functions are guaranteed to exist for asymptotically stable monotone systems on compact sets. Second, it is well-known that, for monotone linear systems, asymptotic stability implies the stronger properties of D-stability and robustness with respect to time-delays. This paper shows that similar properties hold for monotone nonlinear systems that admit max-separable Lyapunov functions. In particular, a notion of D-stability for monotone nonlinear systems and delay-independent stability will be discussed. The theoretical results are illustrated by means of examples.

  • 10.
    Biel, Martin
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Aytekin, Arda
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    POLO.J1: Policy-based optimization algorithms in Julia2019In: Advances in Engineering Software, ISSN 0965-9978, E-ISSN 1873-5339, Vol. 136, article id UNSP 102695Article in journal (Refereed)
    Abstract [en]

    We present POLO. j1- a Julia package that helps algorithm developers and machine-learning practitioners design and use state-of-the-art parallel optimization algorithms in a flexible and efficient way. POLO. j1 extends our C+ + library POLO, which has been designed and implemented with the same intentions. POLO. j1 not only wraps selected algorithms in POLO and provides an easy mechanism to use data manipulation facilities and loss function definitions in Julia together with the underlying compiled C+ + library, but it also uses the policy-based design technique in a Julian way to help users prototype optimization algorithms from their own building blocks. In our experiments, we observe that there is little overhead when using the compiled C+ + code directly within Julia. We also notice that the performance of algorithms implemented in pure Julia is comparable with that of their C+ + counterparts. Both libraries are hosted on GitHub(1)under the free MIT license, and can be used easily by pulling the pre-built 64-bit architecture Docker images.(2)

  • 11.
    Biel, Martin
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Distributed L-shaped Algorithms in Julia2018In: PROCEEDINGS OF PAW-ATM18: 2018 IEEE/ACM PARALLEL APPLICATIONS WORKSHOP, ALTERNATIVES TO MPI (PAW-ATM) / [ed] NDERS JF, 2005, NUMER MATH, V2, P3 okhmal P., 2005, APPLICATIONS OF STOCHASTIC PROGRAMMING, V5, P609 nderoth J, 2003, COMPUTATIONAL OPTIMIZATION AND APPLICATIONS, V24, P207 well Warren B., 2005, APPLICATIONS OF STOCHASTIC PROGRAMMING, V5, P185, IEEE , 2018, p. 57-69Conference paper (Refereed)
    Abstract [en]

    We present LShapedSolvers.jl, a suite of scalable stochastic programming solvers implemented in the Julia programming language. The solvers, which are based on the L-shaped algorithm, run efficiently in parallel, exploit problem structure, and operate on distributed data. The implementation introduces several flexible high-level abstractions that result in a modular design and simplify the development of algorithm variants. In addition, we demonstrate how the abstractions available in the Julia module for distributed computing are exploited to simplify the implementation of the parallel algorithms. The performance of the solvers is evaluated on large-scale problems for finding optimal orders on the Nordic day-ahead electricity market. With 16 worker cores, the fastest algorithm solves a distributed problem with 2.5 million variables and 1.5 million linear constraints about 19 times faster than Gurobi is able to solve the extended form directly.

  • 12. Björkbom, M.
    et al.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Networked PID control: Tuning and outage compensation2010In: IECON 2010 - 36th Annual Conference on IEEE Industrial Electronics Society, IEEE , 2010, p. 168-173Conference paper (Refereed)
    Abstract [en]

    This paper deals with architecture and algorithm design of networked control systems (NCSs) to cope with network outages. A Networked PID controller and an internal model control based tuning scheme is proposed, using the jitter margin to ensure stability despite dropped packets. For longer periods of disconnection, an outage heuristic is proposed. The control design and outage heuristic need only the step response of the process and is as such simple to apply in practice. The effect of these outages on the stability and performance of the closed-loop control system are evaluated, and compared with alternative schemes via extensive simulations. The simple Networked PID with the outage heuristic is shown to perform well, compared to other techniques proposed in the NCS literature.

  • 13. Carlsson, Mats
    et al.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Larson, Jeffrey
    Scheduling double round-robin tournaments with divisional play using constraint programming2017In: European Journal of Operational Research, ISSN 0377-2217, E-ISSN 1872-6860, Vol. 259, no 3, p. 1180-1190Article in journal (Refereed)
    Abstract [en]

    We study a tournament format that extends a traditional double round-robin format with divisional single round-robin tournaments. Elitserien, the top Swedish handball league, uses such a format for its league schedule. We present a constraint programming model that characterizes the general double round-robin plus divisional single round-robin format. This integrated model allows scheduling to be performed in a single step, as opposed to common multistep approaches that decompose scheduling into smaller problems and possibly miss optimal solutions. In addition to general constraints, we introduce Elitserien-specific requirements for its tournament. These general and league-specific constraints allow us to identify implicit and symmetry-breaking properties that reduce the time to solution from hours to seconds. A scalability study of the number of teams shows that our approach is reasonably fast for even larger league sizes. The experimental evaluation of the integrated approach takes considerably less computational effort to schedule Elitserien than does the previous decomposed approach.

  • 14.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hadjicostis, Christoforos N.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Distributed Minimum-Time Weight Balancing over Digraphs2014Conference paper (Refereed)
    Abstract [en]

    We address the weight-balancing problem for a distributed system whose components (nodes) can exchange information via interconnection links (edges) that form an arbitrary, possibly directed, communication topology (digraph). A weighted digraph is balanced if, for each node, the sum of the weights of the edges outgoing from that node is equal to the sum of the weights of the edges incoming to that node. Weight-balanced digraphs play a key role in a variety of applications, such as coordination of groups of robots, distributed decision making, and distributed averaging which is important for a wide range of applications in signal processing. We propose a distributed algorithm for solving the weight balancing problem in a minimum number of iterations, when the weights are nonnegative real numbers. We also provide examples to corroborate the proposed algorithm.

  • 15. Charalambous, Themistoklis
    et al.
    Hadjicostis, Christoforos N.
    Rabbat, Michael G.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Totally asynchronous distributed estimation of eigenvector centrality in digraphs with application to the PageRank problem2016In: 2016 IEEE 55th Conference on Decision and Control, CDC 2016, Institute of Electrical and Electronics Engineers (IEEE), 2016, p. 25-30, article id 7798241Conference paper (Refereed)
    Abstract [en]

    We propose a distributed coordination mechanism which enables nodes in a directed graph to accurately estimate their eigenvector centrality (eigencentrality) even if they update their values at times determined by their own clocks. The clocks need neither be synchronized nor have the same speed. The main idea is to let nodes adjust the weights on outgoing links to compensate for their update speed: the higher the update frequency, the smaller the link weights. Our mechanism is used to develop a distributed algorithm for computing the PageRank vector, commonly used to assign importance to web pages and rank search results. Although several distributed approaches in the literature can deal with asynchronism, they cannot handle the different update speeds that occur when servers have heterogeneous computational capabilities. When existing algorithms are executed using heterogeneous update speeds, they compute incorrect PageRank values. The advantages of our algorithm over existing approaches are verified through illustrative examples.

  • 16.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Kalyviannaki, Evangelia
    City University London.
    Hadjicostis, Christoforos N.
    Electrical and Computer Engineering Department, University of Cyprus.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Distributed Offline Load Balancing in MapReduce Networks2013In: 2013 IEEE 52ND ANNUAL CONFERENCE ON DECISION AND CONTROL (CDC), IEEE conference proceedings, 2013, p. 835-840Conference paper (Refereed)
    Abstract [en]

    In this paper we address the problem of balancing the processing load of MapReduce tasks running on heterogeneous clusters, i.e., clusters with different capacities and update cycles. We present a fully decentralized algorithm, based on ratio consensus, where each mapper decides the amount of workload data to handle for a single user job using only job specific local information, i.e., information that can be collected from directly connected neighboring mappers, regarding their current workload and capacity. In contrast to other algorithms in the literature, the proposed algorithm can be deployed in heterogeneous networks and can operate asynchronously in both directed and undirected communication topologies. The performance of the proposed algorithm is demonstrated via simulation experiments on large-scale strongly connected topologies. 

  • 17.
    Charalambous, Themistoklis
    et al.
    Aalto Univ, Dept Elect Engn & Automat, Espoo, Finland..
    Kim, Su Min
    Korea Polytech Univ, Dept Elect Engn, Shihung, South Korea..
    Nomikos, Nikolaos
    Univ Aegean, Dept Informat & Commun Syst Engn, Samos, Greece..
    Bengtsson, Mats
    KTH, School of Electrical Engineering and Computer Science (EECS), Information Science and Engineering.
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Relay-pair selection in buffer-aided successive opportunistic relaying using a multi-antenna source2019In: Ad hoc networks, ISSN 1570-8705, E-ISSN 1570-8713, Vol. 84, p. 29-41Article in journal (Refereed)
    Abstract [en]

    We study a cooperative network with a buffer-aided multi-antenna source, multiple half-duplex (HD) buffer-aided relays and a single destination. Such a setup could represent a cellular downlink scenario, in which the source can be a more powerful wireless device with a buffer and multiple antennas, while a set of intermediate less powerful devices are used as relays to reach the destination. The main target is to recover the multiplexing loss of the network by having the source and a relay to simultaneously transmit their information to another relay and the destination, respectively. Successive transmissions in such a cooperative network, however, cause inter-relay interference (IRI). First, by assuming global channel state information (CSI), we show that the detrimental effect of IRI can be alleviated by precoding at the source, mitigating or even fully cancelling the interference. A cooperative relaying policy is proposed that employs a joint precoding design and relay-pair selection. Note that both fixed rate and adaptive rate transmissions can be considered. For the case when channel state information is only available at the receiver side (CSIR), we propose a relay selection policy that employs a phase alignment technique to reduce the IRI. The performance of the two proposed relay pair selection policies are evaluated and compared with other state-of-the-art relaying schemes in terms of outage and throughput. The results show that the use of a powerful source can provide considerable performance improvements.

  • 18.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Krikidis, I.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Precoding decision for full-duplex X-relay channel with Decode-And-Forward2014In: IWCMC 2014 - 10th International Wireless Communications and Mobile Computing Conference, IEEE , 2014, p. 791-796Conference paper (Refereed)
    Abstract [en]

    In this paper, we study a simple X-relay configuration where the shared relay operates in full-duplex (FD) mode. The relay node may have limited spatial degrees of freedom, and as a result, it may not be able to handle both the loop interference and the multiuser interference. Hence, a decision on the precoding scheme is necessitated. It is often the case that the relay does not have the option of real-Time switching between different precoding schemes, either due to hardware limitations of the relay or increased complexity of the problem. Hence, we investigate a 'static' precoding decision where the relay node decides on its precoding scheme based only on statistical knowledge of the channel conditions. To perform this decision, the behavior of the system is formulated as a Markov chain and the outage probability of the system is derived in a closed-form with the precoding decision as a parameter. The outage probability is minimized by optimally choosing the precoding scheme, using easily verifiable conditions on the statistical knowledge of the channel conditions. Simulations validate the investigated scheme.

  • 19.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Nomikos, Nikolaos
    Krikidis, Ioannis
    Vouyioukas, Demosthenes
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Modeling Buffer-Aided Relay Selection in Networks With Direct Transmission Capability2015In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 19, no 4, p. 649-652Article in journal (Refereed)
    Abstract [en]

    We consider a wireless relay network that consists of a source, half-duplex decode-and-forward buffer-aided relays and a destination. While the majority of previous works on relay selection assume no direct transmission between source and destination in such a setting, we lift this assumption and propose a link selection policy that exploits both the buffering ability and the opportunity for successful reception of a packet directly from the source. The proposed relay selection scheme incorporates the instantaneous strength of the wireless links and adapts the relay selection decision based on the strongest available link. The evolution of the network as a whole is modeled by means of a Markov chain and thus, the outage probability is associated with the steady state of the Markov chain. It is deduced that even if the link between the source and the destination is in principle a very unreliable link, it is always beneficial for the source to multicast a packet to both the relay with the strongest available link and the destination.

  • 20.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rabbat, M. G.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hadjicostis, C. N.
    Distributed Finite-Time Computation of Digraph Parameters: Left-Eigenvector, Out-Degree and Spectrum2016In: IEEE Transactions on Control of Network Systems, ISSN 2325-5870, Vol. 3, no 2, p. 137-148, article id 7100912Article in journal (Refereed)
    Abstract [en]

    Many of the algorithms that have been proposed in the field of distributed computation rely on assumptions that require nodes to be aware of some global parameters. In this paper, we propose algorithms to compute some network parameters in a distributed fashion and in a finite number of steps. More specifically, given an arbitrary strongly connected network of interconnected nodes, by adapting a distributed finite-time approach, we develop distributed strategies that enable nodes to compute the following network parameters: the left-eigenvector, the out-degree, and the spectrum of weighted adjacency matrices.

  • 21.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Yuan, Ye
    University of Cambridge.
    Yang, Tao
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Pan, Wei
    Imperial College London.
    Hadjicostis, Christoforos N.
    University of Cyprus.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Decentralised Minimum-Time Average Consensus in Digraphs2013In: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 2617-2622Conference paper (Refereed)
    Abstract [en]

    Distributed algorithms for average consensus in directed graphs are typically asymptotic in the literature. In this work, we propose a protocol to distributively reach average consensus in a finite number of steps on interconnection topologies that form strongly connected directed graphs (digraphs). The average consensus value can be computed, based exclusively on local observations at each component, by running a protocol that requires each component to observe and store its own value over a finite and minimal number of steps, and to have knowledge of the number of its out-going links (i.e., the number of components to which it sends information). The proposed algorithm is demonstrated via illustrative examples.

  • 22.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Yuan, Ye
    Yang, Tao
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Pan, Wei
    Hadjicostis, Christoforos N.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Distributed Finite-Time Average Consensus in Digraphs in the Presence of Time Delays2015In: IEEE TRANSACTIONS ON CONTROL OF NETWORK SYSTEMS, ISSN 2325-5870, Vol. 2, no 4, p. 370-381Article in journal (Refereed)
    Abstract [en]

    Most algorithms for distributed averaging only guarantee asymptotic convergence. This paper introduces a distributed protocol that allows nodes to find the exact average of the initial values in a finite and minimum number of steps on interconnection topologies described by strongly connected directed graphs (digraphs). More specifically, under the assumption that each component has knowledge of the number of its outgoing links (i.e., the number of components to which it sends information), we show that the average value can be computed based on local observations over a finite time interval. The average can be obtained in a finite number of steps even when the information exchange is subject to delays. The proposed algorithm is the first in the literature that allows for distributed computation of the exact average in digraphs in finite time, with and without delays.

  • 23. Chen, S.
    et al.
    Dunkels, A.
    Österlind, F.
    Voigt, T.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Time synchronization for predictable and secure data collection in wireless sensor networks2007In: Proceedings of MedHocNet, 2007, p. 165-172Conference paper (Refereed)
    Abstract [en]

    Wireless sensor networks are trying to findtheir way from relatively undemanding applications suchas environmental monitoring to applications such as industrialcontrol, which have stronger requirements in termsof security and predictability. Predictability cannot beachieved without coordination and the coordination of distributedentities and events requires time synchronization.Towards this end, we present a secure time synchronizationservice, that as our experimental results show does notdegrade time synchronization accuracy. Based on the timesynchronization service we implement time slotted datacollection and present results that show that this way wecan provide a predictable data collection service.

  • 24. Daafouz, J.
    et al.
    Benedetto, M. D. Di
    Blondel, V. D.
    Ferrari-Trecate, G.
    Hetel, L.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Juloski, A. L.
    Paoletti, S.
    Pola, G.
    Santis, E. De
    Vidal, R.
    Switched and  piecewise affine systems2009In: Handbook of Hybrid Systems Control: Theory, Tools, Applications / [ed] Lunze, J.; Lamnabhi-Lagarrigue, F., Cambridge Press , 2009, p. 87-138Chapter in book (Other academic)
  • 25.
    Della Penda, Demia
    et al.
    Ericsson AB, S-16480 Kista, Sweden..
    Abrardo, Andrea
    Univ Siena, Dept Informat Engn, I-53100 Siena, Italy.;Consorzio Nazl Interuniv Telecomunicaz, I-43124 Parma, Italy..
    Moretti, Marco
    Consorzio Nazl Interuniv Telecomunicaz, I-43124 Parma, Italy.;Univ Pisa, Dept Informat Engn, I-56122 Pisa, Italy..
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Distributed Channel Allocation for D2D-Enabled 5G Networks Using Potential Games2019In: IEEE Access, E-ISSN 2169-3536, Vol. 7, p. 11195-11208Article in journal (Refereed)
    Abstract [en]

    Frequency channel allocation is a key technique for improving the performance of cellular networks. In this paper, we address the channel allocation problem for a 5G multi-cell system. We consider a heterogeneous network in which cellular users, micro-cell users, and device-to-device (D2D) communications coexist within the radio footprint of the macro cell. We maximize the aggregate transmission rate, exploiting channel diversity and managing both the inter-cell interference, typical of cellular networks and the intra-cell interference generated by the nonorthogonal transmissions of the small-cell and D2D users. By modeling the allocation problem as a potential game, whose Nash equilibria correspond to the local optima of the objective function, we propose a new decentralized solution. The convergence of our scheme is enforced by using a better response dynamic based on a message passing approach. The simulation results assess the validity of the proposed scheme in terms of convergence time and achievable rate under different settings.

  • 26.
    Della Penda, Demia
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Abrardo, Andrea
    University of Siena.
    Moretti, Marco
    University of Pisa.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Potential games for subcarrier allocation in multi-cell networks with D2D communications2016In: 2016 IEEE International Conference Communications (ICC), Institute of Electrical and Electronics Engineers (IEEE), 2016, article id 7511458Conference paper (Refereed)
    Abstract [en]

    This paper investigates the subcarrier allocation problem for uplink transmissions in a multi-cell network, where device-to-device communications are enabled. We focus on maximizing the aggregate transmission rate in the system accounting for both inter- and intra-cell interference. This problem is computationally hard due to its nonconvex and combinatorial nature. However, we show that it can be described by a potential game, and thus a Nash equilibrium can be found using iterative algorithms based on best/better response dynamics. In particular, we propose a simple iterative algorithm with limited signaling that is guaranteed to converge to an equilibrium point, corresponding to a local maximum of the potential function. Using extensive simulations, we show that the algorithm converges quickly also for dense networks, and that the distance to the true optimum is often small, at least for the small-sized networks for which we were able to compute the true optimum.

  • 27.
    Della Penda, Demia
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Fu, Liqun
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Energy efficient D2D communications in dynamic TDD systems2017In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 65, no 3, p. 1260-1273Article in journal (Refereed)
    Abstract [en]

    Device-to-Device (D2D) communication is a promising technology for improving the performance of proximity-based services. This paper demonstrates how the integration of D2D communication in cellular systems operating under dynamic Time Division Duplex (TDD) can improve energy efficiency. We perform joint optimization of mode selection, uplink/downlink transmission period, and power allocation to minimize the transmission energy consumption while satisfying a traffic requirement. Solutions are developed for two scenarios: with and without interference among D2D communications. Both formulations are expressed as mixed-integer nonlinear programming problems, which are NP hardin general. We exploit problem structure to develop efficient solutions for both scenarios. For the interference-free case, we develop algorithms that find the optimal solution in polynomial time. When considering interference, we propose a customized solver based on branch-and-bound that reduces the search complexity by taking advantage of the problem-specific proprieties. We complement this solver by a more practical heuristic algorithm. Simulation results demonstrate that D2D communications in dynamic TDD systems can yield significant energy savings and improved spectral efficiency compared to traditional cellular communication. Furthermore, we give analytical characterizations of the receiver locations relative to a given transmitter where D2D communication is optimal. These regions can be surprisingly large and not necessarily circular.

  • 28.
    Della Penda, Demia
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Fu, Liqun
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Mode Selection for Energy Efficient D2DCommunications in Dynamic TDD Systems2015In: Communications (ICC), 2015 IEEE International Conference on, 978-1-4673-6432-4: IEEE conference proceedings, 2015, p. 5404-5409Conference paper (Refereed)
    Abstract [en]

    Network-assisted Device-to-Device (D2D) communicationis a promising technology for improving the performanceof proximity-based services. This paper demonstrates how D2Dcommunication can be used to improve the energy-efficiencyof cellular networks, leading to a greener system operationand a prolonged battery life of the mobile devices. Assuminga flexible TDD system, we develop optimal mode selectionpolicies for minimizing the energy cost (either from the systemor from the device perspective) while guaranteeing a certainrate requirement. The jointly optimal transmit power and timeallocation, as well as the optimal mode selection, is found bysolving a small convex optimization problem. Special attentionis given to the geometrical interpretation of the obtained results.We show that when network energy is the primary concern, D2Dmode is preferable in a large portion of the cell. When the deviceenergy consumption is most important, on the other hand, thearea where D2D mode is preferable shrinks and becomes close tocircular. Finally, we investigate how network parameters affectthe range where direct communication is preferred.

  • 29.
    Della Penda, Demia
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Nomikos, Nikolaos
    Charalambous, Themistoklis
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Minimum Power Scheduling under Rician Fading in Full-Duplex Relay-Assisted D2D Communication2017In: 2017 IEEE Globecom Workshops, GC Wkshps 2017 - Proceedings, IEEE , 2017Conference paper (Refereed)
    Abstract [en]

    In cellular systems, the combination of Device-to-Device (D2D) communication and relaying is an efficient means for improving network coverage and transmissions quality without additional infrastructure deployment. It enables communication between user pairs in situations when both their direct D2D transmission and the traditional communication via the base station experience poor channel quality. In this paper, we propose a joint relaying-operation selection and power-allocation scheme, herein called HyD2D, for relay-assisted D2D communication in Rician fading environment. The target is to choose the set of communication links that minimizes the power consumption, while ensuring a minimum success probability. To overcome the nonconvexity of the outage probability constraints under Rician fading, we use the concept of coherent-measure-of-risk from the field of finance. We therefore obtain a linear programming formulation that we can efficiently solve. Simulations show that HyD2D selects the most energy-efficient relaying operation that satisfies the success probability requirement, while leveraging only statistical channel state information.

  • 30.
    Della Penda, Demia
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Risuleo, Riccardo Sven
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Valenzuela, Patricio E.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Optimal Power Control for D2D Communications under Rician Fading: a Risk Theoretical Approach2017In: 2017 IEEE Global Communications Conference, GLOBECOM 2017 - Proceedings, Institute of Electrical and Electronics Engineers (IEEE), 2017Conference paper (Refereed)
    Abstract [en]

    Device-to-device communication is a technology that allows users in close proximity to establish a direct communication link instead of passing through the base station. Because direct communications are likely to have a strong line-of-sight component in the received signal, it is reasonable to model the direct channel with Rician fading. In this paper, we propose a power-control scheme for device-to-device communications on a shared channel. Our allocation minimizes the total power consumption while limiting the link outage probability due to Rician fast fading. By leveraging the concept of conditional-value-at-risk from the field of finance, we obtain a linear programming formulation which can be efficiently solved. Through simulation results we show the benefit of the proposed power allocation compared to a deterministic power control that does not account for the random channel variations. Moreover, we provide insights into how the network topology and the parameter settings affect the performance and feasibility of the power allocation.

  • 31.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Aytekin, Arda
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Quevedo, D. E.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    To wait or to drop: On the optimal number of retransmissions in wireless control2015In: 2015 European Control Conference, ECC 2015, IEEE , 2015, p. 962-968Conference paper (Refereed)
    Abstract [en]

    The dimensioning of wireless communication protocols for networked control involves a non-trivial trade-off between reliability and delay. Due to the lossy nature of wireless communications, there is a risk that sensor messages will be dropped. The end-to-end reliability can be improved by retransmitting dropped messages, but this comes at the expense of additional delays. In this work, we determine the number of retransmissions that strikes the optimal balance between communication reliability and delay, in the sense that it achieves the minimal expected linear-quadratic loss of the closed-loop system. An important feature of our setup is that it accounts for the random delays and possible losses that occur when unreliable communication is combatted with retransmissions. The resulting controller dynamically switches among a set of infinite-horizon linear-quadratic regulators, and is simple to implement. Numerical simulations are carried out to highlight the trade-off between reliability and delay. © 2015 EUCA.

  • 32.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Aytekin, Arda
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Quevedo, Daniel E.
    Univ Newcastle, Sch Elect Engn & Comp Sci, Callaghan, NSW 2308, Australia..
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    To wait or to drop: on the optimal number of retransmissions in wireless control2015In: 2015 EUROPEAN CONTROL CONFERENCE (ECC), IEEE , 2015, p. 962-968Conference paper (Refereed)
    Abstract [en]

    The dimensioning of wireless communication protocols for networked control involves a non-trivial trade-off between reliability and delay. Due to the lossy nature of wireless communications, there is a risk that sensor messages will be dropped. The end-to-end reliability can be improved by retransmitting dropped messages, but this comes at the expense of additional delays. In this work, we determine the number of retransmissions that strikes the optimal balance between communication reliability and delay, in the sense that it achieves the minimal expected linear-quadratic loss of the closed-loop system. An important feature of our setup is that it accounts for the random delays and possible losses that occur when unreliable communication is combatted with retransmissions. The resulting controller dynamically switches among a set of infinite-horizon linear-quadratic regulators, and is simple to implement. Numerical simulations are carried out to highlight the trade-off between reliability and delay.

  • 33.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Briat, C.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Deterministic and stochastic approaches to supervisory control design for networked systems with time-varying communication delays2013In: Nonlinear Analysis: Hybrid Systems, ISSN 1751-570X, E-ISSN 1878-7460, Vol. 10, no 1, p. 94-110Article in journal (Refereed)
    Abstract [en]

    This paper proposes a supervisory control structure for networked systems with time-varying delays. The control structure, in which a supervisor triggers the most appropriate controller from a multi-controller unit, aims at improving the closed-loop performance relative to what can be obtained using a single robust controller. Our analysis considers average dwell-time switching and is based on a novel multiple Lyapunov-Krasovskii functional. We develop stability conditions that can be verified by semi-definite programming, and show that the associated state feedback synthesis problem also can be solved using convex optimization tools. Extensions of the analysis and synthesis procedures to the case when the evolution of the delay mode is described by a Markov chain are also developed. Simulations on small and large-scale networked control systems are used to illustrate the effectiveness of our approach.

  • 34.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Briat, Corentin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Supervisory control design for networked systems with time-varying communication delays2012In: Analysis and Design of Hybrid Systems, IFAC , 2012, p. 133-140Conference paper (Refereed)
    Abstract [en]

    This paper proposes a supervisory control structure for networked systems with time-varying delays. The control structure, in which a supervisor triggers the most appropriate controller from a multi-controller unit, aims at improving the closed-loop performance relative to what can be obtained using a single robust controller. Our analysis considers average dwelltime switching and is based on a novel multiple Lyapunov-Krasovskii functional. We develop analysis conditions that can be verified by semi-definite programming, and show that associated state feedback synthesis problem also can be solved using convex optimization. Small and large scale networked control systems are used to illustrate the effectiveness of our approach.

  • 35.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Feyzmahdavian, Hamid Reza
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Ghadimi, Euhanna
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Stability Analysis of Discrete-Time Linear Systems with Unbounded Stochastic Delays2015In: 5th IFAC Workshop on Distributed Estimation and Control of Networked Systems (NECSYS), Elsevier, 2015, Vol. 48Conference paper (Refereed)
    Abstract [en]

    This paper investigates the stability of discrete-time linear systems with stochastic delays. We assume that delays are modeled as random variables, which take values in integers with a certain probability. For the scalar case, we provide an analytical bound on the probability to guarantee the stability of linear systems. In the vector case, we derive a linear matrix inequality condition to compute the probability for ensuring the stability of closed-loop systems. As a special case, we also determine the step size of gradient algorithms with stochastic delays in the unconstrained quadratic programming to guarantee convergence to the optimal solution. Numerical examples are provided to show the effectiveness of the proposed analysis techniques.

  • 36.
    Demirel, Burak
    et al.
    Paderborn Univ, Chair Automat Control EIME, D-33098 Paderborn, Germany..
    Ghadimi, Euhanna
    Huawei Technol Sweden AB, SE-16494 Kista, Sweden..
    Quevedo, Daniel E.
    Paderborn Univ, Chair Automat Control EIME, D-33098 Paderborn, Germany..
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control. KTH, School of Electrical Engineering and Computer Science (EECS), Centres, ACCESS Linnaeus Centre.
    Optimal Control of Linear Systems With Limited Control Actions: Threshold-Based Event-Triggered Control2018In: IEEE Transactions on Big Data, ISSN 2325-5870, E-ISSN 2168-6750, Vol. 5, no 3, p. 1275-1286Article in journal (Refereed)
    Abstract [en]

    We consider a finite-horizon linear-quadratic optimal control problem where only a limited number of control messages are allowed for sending from the controller to the actuator. To restrict the number of control actions computed and transmitted by the controller, we employ a threshold-based event-triggering mechanism that decides whether or not a control message needs to be calculated and delivered. Due to the nature of threshold-based event-triggering algorithms, finding the optimal control sequence requires minimizing a quadratic cost function over a nonconvex domain. In this paper, we first provide an exact solution to this nonconvex problem by solving an exponential number of quadratic programs. To reduce computational complexity, we then propose two efficient heuristic algorithms based on greedy search and the alternating direction method of multipliers technique. Later, we consider a receding horizon control strategy for linear systems controlled by event-triggered controllers, and we further provide a complete stability analysis of receding horizon control that uses finite-horizon optimization in the proposed class. Numerical examples testify to the viability of the presented design technique.

  • 37.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Gupta, V.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    On the trade-off between control performance and communication cost for event-triggered control over lossy networks2013In: 2013 European Control Conference, ECC 2013, IEEE , 2013, p. 1168-1174Conference paper (Refereed)
    Abstract [en]

    This paper develops a theoretical framework for quantifying the trade-off between communication cost and control performance in event-triggered control over lossy networks. We consider a system where the communication between the controller and actuator is dictated by a threshold-based event-triggering algorithm, and develop a Markov-chain model that describes the attempted and successful transmissions of control messages over the lossy communication channel. A feature of our model is that it considers retransmissions of unsuccessful messages and that it accounts for the delay associated with such retransmissions. A systematic framework for analyzing the trade-off between the communication rate and control performance and for optimal tuning of the event threshold emanates by combining this model with an analytical model of the closed-loop performance. Numerical examples demonstrate the effectiveness of the proposed framework.

  • 38.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. University of Paderborn, Germany.
    Gupta, V.
    Quevedo, D. E.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    On the Trade-Off between Communication and Control Cost in Event-Triggered Dead-Beat Control2016In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. PP, no 99, article id 7562376Article in journal (Refereed)
    Abstract [en]

    We consider a stochastic system where the communication between the controller and the actuator is triggered by a thresholdbased rule. The communication is performed across an unreliable link that stochastically erases transmitted packets. To decrease the communication burden, and as a partial protection against dropped packets, the controller sends a sequence of control commands to the actuator in each packet. These commands are stored in a buffer and applied sequentially until the next control packet arrives. In this context, we study dead-beat control laws and compute the expected linear-quadratic loss of the closed-loop system for any given event-threshold. Furthermore, we provide analytical expressions that quantify the trade-off between the communication cost and the control performance of event-triggered control systems. Numerical examples demonstrate the effectiveness of the proposed technique.

  • 39.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. Univ Paderborn.
    Gupta, Vijay
    Quevedo, Daniel E.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Threshold Optimization of Event-Triggered Multi-Loop Control Systems2016In: 2016 13TH INTERNATIONAL WORKSHOP ON DISCRETE EVENT SYSTEMS (WODES), IEEE conference proceedings, 2016, p. 203-210Conference paper (Refereed)
    Abstract [en]

    This paper considers multiple linear stochastic control systems whose feedback loops are closed over a shared communication medium. A threshold-based event-triggering rule is used to transmit control commands from the controllers to the actuators, and network access is arbitrated using a static priority mechanism. Under these conditions, we study deadbeat control laws and compute the expected linear-quadratic loss of the closed-loop system as a function of the event-thresholds of the individual loops. Also, we present analytical expressions that quantify the trade-off between the communication cost and the control performance of such event-triggered control systems. Using a multi-dimensional exhaustive search method, we determine the set of event thresholds that attains the minimal expected linear-quadratic loss of the closed-loop systems. Simulation studies highlight the trade-off between the communication and control cost.

  • 40.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Zou, Zhenhua
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Soldati, Pablo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Modular Co-Design of Controllers and Transmission Schedules in WirelessHART2011In: IEEE Conference on Decision and Control, 2011, p. 5951-5958Conference paper (Refereed)
    Abstract [en]

    We consider the joint design of transmissionschedules and controllers for networked control loops that useWirelessHART communication for sensor and actuator data. Byparameterizing the design problem in terms of the samplingrate of the control loop, the co-design problem separates intotwo well-defined subproblems which admit optimal solutions:transmission scheduling should be done to maximize the delayconstrained reliability while the control design should optimizeclosed-loop performance under packet loss. We illustrate howthese problems can be solved and demonstrate our co-designframework for the case of linear-quadratic contro

  • 41.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Zou, Zhenhua
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Soldati, Pablo
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Modular Design of Jointly Optimal Controllers and Forwarding Policies for Wireless Control2014In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 59, no 12, p. 3252-3265Article in journal (Refereed)
    Abstract [en]

    We consider the joint design of packet forwarding policies and controllers for wireless control loops where sensor measurements are sent to the controller over an unreliable and energy-constrained multi-hop wireless network. For fixed sampling rate of the sensor, the co-design problem separates into two well-defined and independent subproblems: transmission scheduling for maximizing the deadline-constrained reliability and optimal control under packet loss. We develop optimal and implementable solutions for these subproblems and show that the optimally co-designed system can be efficiently found. Numerical examples highlight the many trade-offs involved and demonstrate the power of our approach.

  • 42.
    Di Benedetto, Maria Domenica
    et al.
    Univ Aquila, Coll Engn, Ctr Excellence DEWS, I-67100 Laquila, Italy..
    Johansson, Karl H.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Santucci, Fortunato
    Univ Aquila, Coll Engn, Ctr Excellence DEWS, I-67100 Laquila, Italy..
    Industrial control over wireless networks2010In: International Journal of Robust and Nonlinear Control, ISSN 1049-8923, E-ISSN 1099-1239, Vol. 20, no 2, p. 119-122Article in journal (Other academic)
  • 43. Duchi, J. C.
    et al.
    Agarwal, A.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Jordan, M. I.
    Ergodic mirror descent2011In: Annual Allerton Conference on Communication, Control, and Computing, 2011, p. 701-706Conference paper (Refereed)
    Abstract [en]

    We generalize stochastic subgradient methods to situations in which we do not receive independent samples from the distribution over which we optimize, but instead receive samples that are coupled over time. We show that as long as the source of randomness is suitably ergodic - it converges quickly enough to a stationary distribution - the method enjoys strong convergence guarantees, both in expectation and with high probability. This result has implications for high-dimensional stochastic optimization, peer-to-peer distributed optimization schemes, and stochastic optimization problems over combinatorial spaces.

  • 44. Duchi, John C.
    et al.
    Agarwal, Alekh
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Jordan, Michael I.
    Ergodic mirror descent2012In: SIAM Journal on Optimization, ISSN 1052-6234, E-ISSN 1095-7189, Vol. 22, no 4, p. 1549-1578Article in journal (Refereed)
    Abstract [en]

    We generalize stochastic subgradient descent methods to situations in which we do not receive independent samples from the distribution over which we optimize, instead receiving samples coupled over time. We show that as long as the source of randomness is suitably ergodic it converges quickly enough to a stationary distribution-the method enjoys strong convergence guarantees, both in expectation and with high probability. This result has implications for stochastic optimization in high-dimensional spaces, peer-to-peer distributed optimization schemes, decision problems with dependent data, and stochastic optimization problems over combinatorial spaces.

  • 45. Elmusrati, M
    et al.
    Eriksson, L
    Gribanova, K
    Pohjola, M
    Jäntti, R
    Koivo, H
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Zander, J.
    Wireless automation: opportunities and challenges2007In: Finnish Automation Days, 2007Conference paper (Refereed)
    Abstract [en]

    In this paper we discuss the opportunities of replacing wirelines by wireless connections in automation systems.However, there are several challenges inherently associated with these opportunities. One of the major challengesis how to select a proper wireless connection protocol that achieves at least the minimum requirements ofthe automation system. These system requirements are discussed in the paper. Some wireless communicationsystems which could be used for wireless automation are briefly revised. Finally, we discuss the applicability ofcontemporary wireless protocols for wireless automation and whether a new wireless protocol needs to be definedfor wireless automation systems. The augmentation of wireless technology to automation systems will improvethe performance of those systems and also many new applications may be defined.

  • 46. Eriksson, Lasse M.
    et al.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    PID Controller Tuning Rules for Varying Time-Delay Systems2007In: 2007 AMERICAN CONTROL CONFERENCE, VOLS 1-13, 2007, p. 619-625Conference paper (Refereed)
    Abstract [en]

    This paper considers the design of PID controllers for systems with varying time-delays. Using the concept of jitter margin combined with the AMIGO tuning rule methodology, novel tuning rules that are robust to varying time-delays are derived. In addition, we give an expression for the expected lower bound of the jitter margin as these tuning rules are applied. Extensive numerical evaluations demonstrate that, for wide range of processes, the new tuning rules achieve significant improvements in jitter margin at the expense of only slight decreases in other performance criteria.

  • 47. Eriksson, Lasse M.
    et al.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Simple PID tuning rules for varying time-delay systems2007In: PROCEEDINGS OF THE 46TH IEEE CONFERENCE ON DECISION AND CONTROL, VOLS 1-14, 2007, p. 1801-1807Conference paper (Refereed)
    Abstract [en]

    This paper discusses tuning of PID controllers for varying time-delay systems. We analyze the properties of the AMIGO tuning rules of Astrom and Hagglund applied to varying time-delay systems and propose improved tuning rules which increase the robustness to delay variations at the expense of a small degradation in nominal performance. We suggest a tuning scheme that uses the simple AMIGO tuning on an extended plant, and define the design concepts for extending the plant. This approach allows treating the maximum time-delay as a design parameter for the tuning rules. The proposed tuning rules are compared via simulations.

  • 48. Farokhi, Farhad
    et al.
    Shames, Iman
    Rabbat, Michael G.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Automatic Control.
    On reconstructability of quadratic utility functions from the iterations in gradient methods2016In: Automatica, ISSN 0005-1098, E-ISSN 1873-2836, Vol. 66, p. 254-261Article in journal (Refereed)
    Abstract [en]

    In this paper, we consider a scenario where an eavesdropper can read the content of messages transmitted over a network. The nodes in the network are running a gradient algorithm to optimize a quadratic utility function where such a utility optimization is a part of a decision making process by an administrator. We are interested in understanding the conditions under which the eavesdropper can reconstruct the utility function or a scaled version of it and, as a result, gain insight into the decision-making process. We establish that if the parameter of the gradient algorithm, i.e., the step size, is chosen appropriately, the task of reconstruction becomes practically impossible for a class of Bayesian filters with uniform priors. We establish what step-size rules should be employed to ensure this. 

  • 49.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Aytekin, Arda
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    A delayed proximal gradient method with linear convergence rate2014In: IEEE International Workshop on Machine Learning for Signal Processing (MLSP), IEEE conference proceedings, 2014, p. 1-6Conference paper (Refereed)
    Abstract [en]

    This paper presents a new incremental gradient algorithm for minimizing the average of a large number of smooth component functions based on delayed partial gradients. Even with a constant step size, which can be chosen independently of the maximum delay bound and the number of objective function components, the expected objective value is guaranteed to converge linearly to within some ball around the optimum. We derive an explicit expression that quantifies how the convergence rate depends on objective function properties and algorithm parameters such as step-size and the maximum delay. An associated upper bound on the asymptotic error reveals the trade-off between convergence speed and residual error. Numerical examples confirm the validity of our results.

  • 50.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Aytekin, Arda
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    An Asynchronous Mini-Batch Algorithm for Regularized Stochastic Optimization2015In: 2015 54th IEEE Conference on Decision and Control (CDC), IEEE , 2015, p. 1384-1389Conference paper (Refereed)
    Abstract [en]

    Mini-batch optimization has proven to be a powerful paradigm for large-scale learning. However, the state of the art mini-batch algorithms assume synchronous operation or cyclic update orders. When worker nodes are heterogeneous (due to different computational capabilities, or different communication delays), synchronous and cyclic operations are inefficient since they will leave workers idle waiting for the slower nodes to complete their work. We propose an asynchronous mini-batch algorithm for regularized stochastic optimization problems that eliminates idle waiting and allows workers to run at their maximal update rates. We show that the time necessary to compute an ϵ-optimal solution is asymptotically O(1/ϵ2), and the algorithm enjoys near-linear speedup if the number of workers is O(1/√ϵ). Theoretical results are confirmed in real implementations on a distributed computing infrastructure.

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