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  • 1. 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. 

  • 2.
    Johansson, Björn
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Rabi, Maben
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    A RANDOMIZED INCREMENTAL SUBGRADIENT METHOD FOR DISTRIBUTED OPTIMIZATION IN NETWORKED SYSTEMS2009In: SIAM Journal on Optimization, ISSN 1052-6234, E-ISSN 1095-7189, Vol. 20, no 3, p. 1157-1170Article in journal (Refereed)
    Abstract [en]

    We present an algorithm that generalizes the randomized incremental subgradient method with fixed stepsize due to Nedic and Bertsekas [SIAM J. Optim., 12 (2001), pp. 109-138]. Our novel algorithm is particularly suitable for distributed implementation and execution, and possible applications include distributed optimization, e.g., parameter estimation in networks of tiny wireless sensors. The stochastic component in the algorithm is described by a Markov chain, which can be constructed in a distributed fashion using only local information. We provide a detailed convergence analysis of the proposed algorithm and compare it with existing, both deterministic and randomized, incremental subgradient methods.

  • 3.
    Johansson, Björn
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Rabi, Maben
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    A simple peer-to-peer algorithm for distributed optimization in sensor networks2007In: PROCEEDINGS OF THE 46TH IEEE CONFERENCE ON DECISION AND CONTROL, 2007, p. 5402-5407Conference paper (Refereed)
    Abstract [en]

    We propose a distributed algorithm that solves a special class of optimization problems using only peer-to-peer communication. One application is parameter estimation problems in sensor networks. Current decentralized algorithms for solving this class of optimization problems typically rely on passing around a parameter estimate in a ring consisting of all network nodes. In our algorithm, which extends the randomized incremental subgradient method with fixed stepsize due to Nedic and Bertsekas, nodes maintain individual estimates and need to exchange information only with their neighbors. We establish approach of the solution to an interval around the optimum value. We illustrate the algorithm's performance, in terms of convergence rate and communication cost relative to alternative schemes, through several numerical examples.

  • 4.
    Rabi, Maben
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Baras, John S.
    Level-triggered control of a scalar linear system2007In: 2007 MEDITERRANEAN CONFERENCE ON CONTROL & AUTOMATION, VOLS 1-4, NEW YORK: IEEE , 2007, p. 1896-1901Conference paper (Refereed)
    Abstract [en]

    In networked control systems, because of limits on communication rates, control waveforms are frequently piece-wise constant with limits on rates at which they can switch levels. It is, therefore, natural to expect event-triggered control switchings to perform more efficiently than time-triggered ones. This article verifies the validity of this supposition. A method for designing good level-triggered control schemes is obtained by reducing the continuous time problem to one in discrete time. Then, by numerical procedures, the performance of the level-triggered scheme is computed for comparison with that of the periodically switched control scheme.

  • 5.
    Rabi, Maben
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Event-triggered strategies for industrial control over wireless networks2008Conference paper (Refereed)
    Abstract [en]

    New event-based sampling strategies can support the efficient use of radio resources in wireless control systems. Motivated by the recent introduction of wireless network nodes in process control industry, we consider the particular demands these closed-loop systems set on the wireless communication and the influence the communication has on the control performance. In the paper, it is pointed out that by letting sensor nodes transmit only when needed, it is possible to minimize the communication bandwidth utilization in these systems. We show how classical control strategies commonly based on periodic sampling, such as proportional-integralderivative control and minimum variance control, can be cast in an event-based setting in which decentralized communication decisions are taken suitable for commonly used contention-based medium access control protocols. Eventtriggered sampling for estimation is also reviewed. Simulated examples illustrate the results.

  • 6.
    Rabi, Maben
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Optimal stopping for updating controls2009Conference paper (Refereed)
  • 7.
    Rabi, Maben
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Scheduling packets for event-triggered control2009In: Proceedings of the European Control Conference 2009. Budapest, Hungary, August 23-26, 2009, EUCA , 2009, p. 3779-3784Conference paper (Refereed)
  • 8.
    Rabi, Maben
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Karl Henrik
    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.
    Optimal stopping for event-triggered sensing and actuation2008In: Proceedings of the 47th IEEE Conference on Decision and Control, IEEE , 2008, p. 3607-3612Conference paper (Refereed)
    Abstract [en]

    Novel event-triggered sensing and actuation strategies are presented for networked control systems with limited communication resources. Two architectures are considered: one with the controller co-located with the sensor and one with the control co-located with the actuator. A stochastic control problem with an optimal stopping rule is shown to capture two interesting instances of these architectures. The solution of the problem leads to a parametrization of the control alphabet as piecewise constant commands. The execution of the control commands is triggered by stopping rules for the sensor. In simple situations, it is possible to analytically derive the optimal controller. Examples illustrate how the new event-based control and sensing strategies outperform conventional time-triggered schemes.

  • 9.
    Rabi, Maben
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Stabellini, Luca
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Almström, Peter
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Analysis of networked estimation under contention-based medium access2008In: 17th World Congress, International Federation of Automatic Control, IFAC, 2008Conference paper (Refereed)
    Abstract [en]

    We treat a problem in networked estimation where the focus is on sampling and transmitting measurements of the plant over a shared medium. The object is to choose the sampling rate at the sensor while taking the statistics of a contention based MAC protocol into account. In particular, we seek a trade-off between the reliable delivery of individual data packets and the input data load on the communication medium. Our analysis of the shared channel computes a probability of packet loss for an assumed IID loss process. This loss rate depends on the input packet stream intensities. We compute the estimation distortion with IID losses of periodically generated samples, as a function of the packet loss rate. This throws up a condition for stable estimator performance. We investigate the scalability limits of this stability as a function of the number of nodes. When stable estimation is possible, we provide a procedure that computes the input packet rate for the network that minimizes the average estimation distortion. We reproduce the analysis of estimation performance when the sensors sample asynchronously according to independent Poisson counters.

  • 10.
    Rabi, Maben
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Stabellini, Luca
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Proutiere, Alexandre
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Networked estimation under contention-based medium access2010In: International Journal of Robust and Nonlinear Control, ISSN 1049-8923, E-ISSN 1099-1239, Vol. 20, no 2, p. 140-155Article in journal (Refereed)
    Abstract [en]

    This paper studies networked estimation over a communication channel shared by a contention-based medium access protocol. A collection of N identical and physically decoupled scalar systems are sampled without sensor noise and transmitted over a common channel, using a contention-based medium access mechanism. We first carry out a calculation of the average distortion in estimation with irregular samples. Given the rate of packet generation at sensors, we characterize the traffic characteristics of the some contention-based MAC schemes. This lets us derive the statistics of inter-arrival times which in turn allows us to compute the packet loss rates and also the statistics of delay within a sample period. Using these results, we track the estimation performance as the sample generation rate and the number of contending nodes are varied. We provide a heuristic rule-of-thumb for choosing the sampling interval which minimizes the average distortion. By combining the network traffic characterization with that of the estimation performance, we show this rule performs pretty well. Carrying along the same lines, we are able to compute the scaling limits of estimation performance with respect to the number of contending nodes.

  • 11.
    Sandberg, Henrik
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Automatic Control.
    Rabi, Maben
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Automatic Control.
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Automatic Control.
    Estimation over heterogeneous sensor networks2008In: Proceedings of the IEEE Conference on Decision and Control, 2008, p. 4898-4903Conference paper (Refereed)
    Abstract [en]

    Design trade-offs between estimation performance, processing delay and communication cost for a sensor scheduling problem is discussed. We consider a heterogeneous sensor network with two types of sensors: the first type has low-quality measurements, small processing delay and a light communication cost, while the second type is of high quality, but imposes a large processing delay and a high communication cost. Such a heterogeneous sensor network is common in applications, where for instance in a localization system the poor sensor can be an ultrasound sensor while the more powerful sensor can be a camera. Using a time-periodic Kalman filter, we show how one can find an optimal schedule of the sensor communication. One can significantly improve estimation quality by only using the expensive sensor rarely. We also demonstrate how simple sensor switching rules based on the Riccati equation drives the filter into a stable time-periodic Kalman filter. ᅵ 2008 IEEE.

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