### Browsing by Department "Distributed and Networked Control Systems"

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Item Adaptive Coded Modulation for Stabilization of Wireless Networked Control Systems over Binary Erasure Channels(2019-09-01) Royyan, Muhammad; Vehkapera, Mikko; Charalambous, Themistoklis; Wichman, Risto; Dept Signal Process and Acoust; Distributed and Networked Control Systems; Risto Wichman Group; Department of Electrical Engineering and AutomationThis paper proposes adaptive coded modulation for stabilization of wireless networked control systems (WNCSs). We combine a well-known data rate theorem with adaptive coded modulation to guarantee stability and optimize the spectral efficiency in WNCSs. We believe that this is the first work in adaptive coded modulation for stabilization. In addition, we propose three schemes to optimize various objectives with given constraints. Our proposed schemes are as follows: maximizing throughput with energy constraint (MaxTEC), minimizing energy with throughput constraint (MinETC), and minimizing delay with energy constraint (MinDEC). The numerical results show that each scheme is able to select the optimal modulation to optimize objectives at given channel conditions and constraints.Item Analysis of contractions in system graphs(2021-10-06) Doostmohammadian, Mohammadreza; Charalambous, Themistoklis; Shafie-khah, Miadreza; Rabiee, Hamid R.; Khan, Usman A.; Bionic and Rehabilitation Engineering; Distributed and Networked Control Systems; University of Vaasa; Sharif University of Technology; Tufts University; Department of Electrical Engineering and AutomationObservability and estimation are closely tied to the system structure, which can be visualized as a system graph-a graph that captures the inter-dependencies within the state variables. For example, in social system graphs such inter-dependencies represent the social interactions of different individuals. It was recently shown that contractions, a key concept from graph theory, in the system graph are critical to system observability, as (at least) one state measurement in every contraction is necessary for observability. Thus, the size and number of contractions are critical in recovering for loss of observability. In this paper, the correlation between the average-size/number of contractions and the global clustering coefficient (GCC) of the system graph is studied. Our empirical results show that estimating systems with high GCC requires fewer measurements, and in case of measurement failure, there are fewer possible options to find substitute measurement that recovers the system's observability. This is significant as by tuning the GCC, we can improve the observability properties of large-scale engineered networks, such as social networks and smart grid.Item Centralized and Distributed Multi-Region Traffic Flow Control(2020-05) Boufous, Omar; Roncoli, Claudio; Charalambous, Themistoklis; Distributed and Networked Control Systems; Department of Built Environment; Department of Electrical Engineering and AutomationIn this paper, centralized and distributed multi-region perimeter flow control approaches are proposed for congestion avoidance in urban networks. First, multi-region network dynamics are modeled with Macroscopic Fundamental Diagrams (MFDs) and necessary stability conditions are derived using Lyapunov stability theory for a centralized perimeter controller. Later, an optimization problem is formulated, solved and the desired optimal states are reached by means of an algorithm based on Model Predictive Control (MPC). Finally, the paper combines the centralized controller for perimeter control as a first layer controller and a distributed controller managing the inter-transfers between regions, thus optimizing the overall state of the network. Simulations show that the distributed control scheme leads to good results maximizing the output of the traffic network, similar to the MPC controller.Item Consensus of General Linear Multi-Agent Systems with Heterogeneous Input and Communication Delays(IEEE, 2021-07) Jiang, Wei; Chen, Yiyang; Charalambous, Themistoklis; Distributed and Networked Control Systems; University of Southampton; Department of Electrical Engineering and AutomationThis letter studies the consensus tracking control for multi-agent systems (MASs) of general linear dynamics considering heterogeneous constant known input and communication delays under a directed communication graph containing a spanning tree. First, for open-loop stable MASs, a distributed predictive observer is proposed to estimate the consensus tracking error and to construct the control input that does not involve any integral term (which is time-efficient in calculation). Then, using the generalized Nyquist criterion, we derive the conditions for asymptotic convergence of the closed-loop system and show that is delay-independent. Subsequently, another observer is designed that allows the MASs to be open-loop unstable. Next, we use the generalized Nyquist criterion to compute the observer's gain matrix. Towards this end, we choose a specific structure with which the problem boils down to computing a single parameter, herein called the predictive observer parameter. Two algorithms are proposed for choosing this parameter: one for general linear systems and one for monotone systems. To the best of the authors' knowledge, this is the first work for which asymptotic convergence of consensus is proven for general linear MASs with arbitrary heterogeneous delays. Finally, the validity of our results is demonstrated via a vehicle platooning example.Item A Distributed Mode Selection Scheme for Full-Duplex Device-to-Device Communication(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2019-10-01) Penda, Demia Della; Wichman, Risto; Charalambous, Themistoklis; Fodor, Gabor; Johansson, Mikael; KTH Royal Institute of Technology; Risto Wichman Group; Distributed and Networked Control Systems; Department of Electrical Engineering and AutomationNetworks with device-to-device (D2D) technology allow for two possible communication modes: traditional communication via the base station, and direct communication between the users. Recent studies show that in-band full-duplex (IBFD) operations can be advantageously combined with D2D communication to improve the spectral efficiency. However, no algorithms for selecting the communication mode of mobile users in IBFD networks have yet appeared in the literature. In this paper, we design a distributed mode selection scheme for users in D2D-enabled IBFD networks. The proposed scheme maximizes the users' prob-ability of successful communication by leveraging only existing signaling mechanisms.Item Feasibility of Bluetooth Low Energy for motion capturing with Inertial Measurement Units(Academic Press, 2023-04) Veijalainen, Pyry; Charalambous, Themistoklis; Wichman, Risto; Distributed and Networked Control Systems; Department of Electrical Engineering and Automation; Risto Wichman Group; Department of Information and Communications EngineeringWireless Inertial Measurement Units provide motion capture data with a low hardware cost while offering a lot of mobility for the user. The current solutions rely on Wi-Fi or custom radio protocols, which are usually access point-centre having all the traffic routed through a single access point, limiting direct interaction capabilities with the surrounding devices. Bluetooth Low Energy (BLE) forms point-to-point networks directly between two devices without additional networking, thus, a BLE based motion capture suit could enable seamless direct cooperation between two robots or a human and a robot. In this paper, the feasibility of BLE 5 for motion capturing is investigated by designing and testing an implementation of such a motion capture system using existing commercial hardware. More specifically, a mobile phone was utilized as the receiver device for real-time visualization, whereas BLE sensors were used in order to test the feasibility of the technology for actual use and to identify the bottlenecks in the system for future optimizations. The designed prototype system achieved similar or better performance for the static accuracy and delays in the system when compared to the currently existing commercial suits, while providing a feasible throughput for real-time motion capturing.Item Generalizations of Nonanticipative Rate Distortion Function to Multivariate Nonstationary Gaussian Autoregressive Processes(2019-12-01) Charalambous, Charalambos D.; Kourtellaris, Christos; Charalambous, Themistoklis; Van Schuppen, Jan H.; University of Cyprus; Distributed and Networked Control Systems; Van Schuppen Control Research; Department of Electrical Engineering and AutomationThe characterizations of nonanticipative rate distortion function (NRDF) on a finite horizon are generalized to nonstationary multivariate Gaussian order L autoregressive, AR(L), source processes, with respect to mean square error (MSE) distortion functions. It is shown that the optimal reproduction distributions are induced by a reproduction process, which is a linear function of the state of the source, its best mean-square error estimate, and a Gaussian random process.Item Infinite horizon average cost dynamic programming subject to total variation distance ambiguity(Society for Industrial and Applied Mathematics Publications, 2019-01-01) Tzortzis, Ioannis; Charalambous, Charalambos D.; Charalambous, Themistoklis; University of Cyprus; Distributed and Networked Control Systems; Department of Electrical Engineering and AutomationWe analyze the per unit-time infinite horizon average cost Markov control model, subject to a total variation distance ambiguity on the controlled process conditional distribution. This stochastic optimal control problem is formulated as a minimax optimization problem in which the minimization is over the admissible set of control strategies, while the maximization is over the set of conditional distributions which are in a ball, with respect to the total variation distance, centered at a nominal distribution. We derive two new equivalent dynamic programming equations, and a new policy iteration algorithm. The main feature of the new dynamic programming equations is that the optimal control strategies are insensitive to inaccuracies or ambiguities in the controlled process conditional distribution. The main feature of the new policy iteration algorithm is that the policy evaluation and policy improvement steps are performed using the maximizing conditional distribution, which is obtained via a water filling solution of aggregating states together to form new states. Throughout the paper, we illustrate the new dynamic programming equations and the corresponding policy iteration algorithm to various examples.Item LoLA4SOR : A Low-Latency Algorithm for Successive Opportunistic Relaying(2019-04) Nomikos, Nikolaos; Charalambous, Themistoklis; Pappas, Nikolaos; Vouyioukas, Demosthenes; Wichman, Risto; University of the Aegean; Distributed and Networked Control Systems; Linköping University; Dept Signal Process and Acoust; Department of Electrical Engineering and AutomationBuffer-aided (BA) relaying has been proposed as an efficient way to improve the diversity of multi-hop networks. Nonetheless, it results in increased end-to-end delay, as packets tend to reside in the relays' buffers for many time-slots. In order to enable the adoption of BA relays in delay-intolerant applications, the combination with successive relaying can increase the number of packets that are transmitted. In this work, hybrid relay selection algorithms are studied, where delay- and diversity aware (DDA) half-duplex algorithms are complemented with successive BA relaying. Moreover, a hybrid DDA algorithm is presented, namely LoLA4SOR that adapts to the transmit conditions, switching between half-duplex (HD) and successive relaying (SuR). The performance of LoLA4SOR is compared to several HD, successive and hybrid algorithms, in terms of outage probability, throughput and delay. The results indicate that LoLA4SOR provides superior outage and throughput performance, while its delay performance is not affected by the buffer size and, as the number of relays increase, its delay reduces due to the high probability for successive transmissions.Item LoLa4SOR : Leveraging Successive Transmissions for Low-Latency Buffer-Aided Opportunistic Relay Networks(IEEE, 2021-05-03) Nomikos, Nikolaos; Charalambous, Themistoklis; Pappas, Nikolaos; Vouyioukas, Demosthenes; Wichman, Risto; University of Cyprus; Distributed and Networked Control Systems; Linköping University; University of the Aegean; Dept Signal Process and Acoust; Department of Electrical Engineering and AutomationBuffer-aided (BA) relaying improves the diversity of cooperative networks often at the cost of increasing end-to-end packet delays. This characteristic renders BA relaying unsuitable for delay-sensitive applications. However, the increased diversity makes BA relaying appealing for ultra-reliable communications. Towards enabling ultra-reliable low-latency communication (URLLC), we aim at enhancing BA relaying for supporting delay-sensitive applications. In this paper, reliable full-duplex (FD) network operation is targeted and for this purpose, hybrid relay selection algorithms are formulated, combining BA successive relaying (SuR) and delay-and diversity-aware (DDA) half-duplex (HD) algorithms. In this context, a hybrid FD DDA algorithm is presented, namely LoLa4SOR, switching between SuR and HD operation. Additionally, a low-complexity distributed version is given, namely d-LoLa4SOR, providing a trade-off among channel state information requirements and performance. The theoretical analysis shows that the diversity of LoLa4SOR equals to two times the number of available relays K, i.e. 2K, when the buffer size L is greater than or equal to 3. Comparisons with other HD, SuR and hybrid algorithms reveal that LoLa4SOR offers superior outage and throughput performance while, the average delay is reduced due to SuR-based FD operation and the consideration of buffer state information for relay-pair selection. d-LoLa4SOR, as one of the few distributed algorithms in the literature, has a reasonable performance that makes it a more practical approach.Item Observer-based Control for Vehicle Platooning with a Leader of Varying Velocity(2022-01) Jiang, Wei; Abolfazli, Elham; Charalambous, Themistoklis; Department of Electrical Engineering and Automation; Bionic and Rehabilitation Engineering; Distributed and Networked Control SystemsThis paper studies the internal stability and string stability of a vehicle platooning with a varying-velocity leader using a multiple-predecessor following strategy via vehicle-to-vehicle communication. Unlike the common case in which the leader’s velocity is constant and different kinds of PID controllers are implemented, in this case, the fact that the leader has a velocity that varies over time, necessitates the design of an observer. First, in order to estimate the position, velocity and acceleration error between the follower and the leader, an observer is designed whose parameters are the entries of a matrix. The matrix format of the observer is later reduced to a specific observer parameter matrix form. This new observer turns out to have a third order integrator dynamics, which is used to generate the parameter conditions to guarantee the string stability. Additionally, instead of a typical PID controller, an observer-based controller is developed which only needs local information. The effectivenessof the proposed observer and controller is demonstrated via an illustrative example.Item On the stability of the Foschini-Miljanic algorithm with uncertainty over channel gains(2019-06-01) Moutsinas, Giannis; Charalambous, Themistoklis; Guo, Weisi; University of Warwick; Distributed and Networked Control Systems; Department of Electrical Engineering and AutomationDistributed power control in wireless networks faces challenges related to its stability. When perfect information of channel states and transmitting agents are available, previous work has shown that the stability conditions can be known. When there is uncertainty over the parameter space, stability is not well understood. In this work, we study the impact of parameter uncertainty and network structure on the stability and scalability of a well known distributed power control, namely the Foschini-Miljanic algorithm. More specifically, we derive probabilistic conditions with respect to the parameters of the channel distributions for which the system is stable. Furthermore, we study the effects of these parameters for different node distribution on the plane. Numerical examples validate our theoretical results.Item Power Allocation for ARQ Two-Hop Cooperative Networks for Ultra-Reliable Communication(IEEE, 2020-07) Dosti, Endrit; Charalambous, Themistoklis; Wichman, Risto; Sergiy Vorobyov Group; Distributed and Networked Control Systems; Dept Signal Process and Acoust; Department of Electrical Engineering and AutomationWe analyze the performance of amplify-and-forward (AF) automatic repeat request (ARQ) for a two-hop cooperative system with reliability constrains. For this setup, we first derive the closed-form outage probability expression. Next, we present a power allocation scheme that allows us to achieve a target outage probability, while minimizing the outage-weighted average power expenditure for asymmetric power allocation between the source and relay. This is cast as an optimization problem, and the optimal power allocation (OPA) is obtained in closed form by invoking the Karush-Kuhn-Tucker (KKT) conditions. We evaluate numerically the OPA strategy between different AFARQ transmission rounds and we show that the proposed scheme provides large power gains with respect to the optimized pointto-point ARQ scheme, as well as with respect to the equal power allocation (EPA) strategy.Item Power Allocation of Sensor Transmission for Remote Estimation Over an Unknown Gilbert-Elliott Channel(2020-05) Farjam, Tahmoores; Fardno, Fatemeh; Charalambous, Themistoklis; Distributed and Networked Control Systems; Sharif University of Technology; Department of Electrical Engineering and AutomationIn this paper, we consider the problem of scheduling the power of a sensor when transmitting over an unknown Gilbert-Elliott (GE) channel for remote state estimation. The sensor supports two power modes, namely low power and high power, which are to be selected for transmission over the channel in order to minimize a cost on the error covariance, while satisfying the energy constraints. The remote estimator provides error-free acknowledgement/negative-acknowledgement (ACK/NACK) messages to the sensor only when low power is utilized. We first consider the Partially Observable Markov Decision Process (POMDP) problem for the case of known GE channels and derive conditions for optimality of a stationary schedule. Next, a Bayesian inference approach is used through which the channels statistics are approximately learned when they are initially unknown. An algorithm is proposed in which the sensor adjusts its scheduling policy based on the energy constraint.Item Privacy-Preserving Event-Triggered Quantized Average Consensus(IEEE, 2020-12-14) Rikos, Apostolos I.; Charalambous, Themistoklis; Johansson, Karl H.; Hadjicostis, Christoforos N.; KTH Royal Institute of Technology; Distributed and Networked Control Systems; University of Cyprus; Department of Electrical Engineering and AutomationIn this paper, we propose a privacy-preserving event-triggered quantized average consensus algorithm that allows agents to calculate the average of their initial values without revealing to other agents their specific value. We assume that agents (nodes) interact with other agents via directed communication links (edges), forming a directed communication topology (digraph). The proposed distributed algorithm can be followed by any agent wishing to maintain its privacy (i.e., not reveal the initial value it contributes to the average) to other, possibly multiple, curious but not malicious agents. Curious agents try to identify the initial values of other agents, but do not interfere in the computation in any other way. We develop a distributed strategy that allows agents while processing and transmitting quantized information, to preserve the privacy of their initial quantized values and at the same time to obtain, after a finite number of steps, the exact average of the initial values of the nodes. Illustrative examples demonstrate the validity and performance of our proposed algorithm.Item Reducing time headway in platoons under the MPF topology in the presence of communication delays(Elsevier Science Publishers BV, 2020) Abolfazli, Elham; Besselink, Bart; Charalambous, Themistoklis; Bionic and Rehabilitation Engineering; University of Groningen; Distributed and Networked Control Systems; Department of Electrical Engineering and AutomationFor platoons under the multiple-predecessor following (MPF) topology, communication delays can compromise both the internal stability and string stability. The most straightforward solution to guarantee stability is by increasing the time headway. On the other hand, time headway plays a significant role in road capacity and increasing its value is in contrast with the idea of platooning. In this study, internal stability and string stability of platoons suffering from communication delays are investigated and a lower bound for the time headway is proposed. Using this bound, platoons do not need to massively increase the time headway in order to compensate for the effects of communications delays. Finally, we evaluate the proposed lower bound on the time headway and the simulation results demonstrate its effectiveness. Copyright (C) 2020 The Authors.Item Reducing time headway in platoons under the MPF topology when using sensors and wireless communications(IEEE, 2021-01-11) Abolfazli, Elham; Besselink, Bart; Charalambous, Themistoklis; Distributed and Networked Control Systems; University of Groningen; Department of Electrical Engineering and AutomationIn platoons under the multiple-predecessor following (MPF) topology, the exchange of information is usually assumed to be via Vehicle-to-Vehicle (V2V) communication links. Communication delays, however, deteriorate internal and string stability and, as a consequence, to guarantee stability of the platoon the time headway between vehicles should be increased. Autonomous vehicles are nowadays equipped with a multitude of sensors, the three primary being radar, lidar and camera. The combination of such sensors allows a vehicle, among others, to detect the distance and speed of nearby objects. In this paper, we incorporate the available sensors of vehicles to anticipate the communication delays vehicles experience with their preceding vehicles, thus improving the stability conditions and subsequently reducing the time headway. More specifically, by incorporating sensors we alleviate the communication delays between neighboring vehicles. We demonstrate that (i) the system is internally stable irrespective of the size of the communication delays, and (ii) the time headway can be reduced, by deriving a sufficient condition which provides a lower bound on the time headway that guarantees string stability. With this new lower bound on the time headway, platoons do not need to massively increase the time headway in order to compensate for the effects of communications delays. Simulations demonstrate the effectiveness of the proposed lower bound.Item Robust trajectory planning of autonomous vehicles at intersections with communication impairments(2019-09-01) Chohan, Neha; Nazari, Mohammad A.; Wymeersch, Henk; Charalambous, Themistoklis; Distributed and Networked Control Systems; Chalmers University of Technology; Department of Electrical Engineering and AutomationIn this paper, we consider the trajectory planning of an autonomous vehicle to cross an intersection within a given time interval. The vehicle communicates its sensor data to a central coordinator which then computes the trajectory for the given time horizon and sends it back to the vehicle. We consider a realistic scenario in which the communication links are unreliable, the evolution of the state has noise (e.g., due to the model simplification and environmental disturbances), and the observation is noisy (e.g., due to noisy sensing and/or delayed information). The intersection crossing is modeled as a chance constraint problem and the stochastic noise evolution is restricted by a terminal constraint. The communication impairments are modeled as packet drop probabilities and Kalman estimation techniques are used for predicting the states in the presence of state and observation noises. A robust sub-optimal solution is obtained using convex optimization methods which ensures that the intersection is crossed by the vehicle in the given time interval with very low chance of failure.Item Simultaneous distributed estimation and attack detection/isolation in social networks(2021-10-06) Doostmohammadiany, Mohammadreza; Charalambous, Themistoklis; Shafie-khah, Miadreza; Meskin, Nader; Khan, Usman A.; Bionic and Rehabilitation Engineering; Distributed and Networked Control Systems; University of Vaasa; Qatar University; Tufts University; Department of Electrical Engineering and AutomationThis paper considers distributed estimation of linear systems when the state observations are corrupted with Gaussian noise of unbounded support and under possible random adversarial attacks. We consider sensors equipped with single time-scale estimators and local chi-square $(\chi^{2})$ detectors to simultaneously observe the states, share information, fuse the noise/attack-corrupted data locally, and detect possible anomalies in their own observations. While this scheme is applicable to a wide variety of systems associated with full-rank (invertible) matrices, we discuss it within the context of distributed inference in social networks. The proposed technique outperforms existing results in the sense that: (i) we consider Gaussian noise with no simplifying upper-bound assumption on the support; (ii) all existing $\chi^{2}$-based techniques are centralized while our proposed technique is distributed, where the sensors locally detect attacks, with no central coordinator, using specific probabilistic thresholds; and (iii) no local-observability assumption at a sensor is made, which makes our method feasible for large-scale social networks. Moreover, we consider a Linear Matrix Inequalities (LMI) approach to design block-diagonal gain (estimator) matrices under appropriate constraints for isolating the attacks.Item Structural Properties of Nonanticipatory Epsilon Entropy of Multivariate Gaussian Sources(IEEE, 2020) Charalambous, Charalambos D.; Charalambous, Themistoklis; Kourtellaris, Christos; Van Schuppen, Jan H.; University of Cyprus; Distributed and Networked Control Systems; Company Van Schuppen Control Research; Department of Electrical Engineering and AutomationThe complete characterization of the Gorbunov and Pinsker [1], [2] nonanticipatory epsilon entropy of multivariate Gauss-Markov sources with square-error fidelity is derived, which remained an open problem since 1974. Specifically, it is shown that the optimal matrices of the stochastic realization of the optimal test channel or reproduction distribution, admit spectral representations with respect to the same unitary matrices, and that the optimal reproduction process is generated, subject to pre-processing and post-processing by memoryless parallel additive Gaussian noise channels. The derivations and analyses are new and bring out several properties of such optimization problems over the space of conditional distributions and their realizations.