Showing papers by "Richard H. Middleton published in 2018"

Consensus of Multiagents in Switching Networks Using Input-to-State Stability of Switched Systems

Abstract: Solvability of a consensus problem for multiagent systems (MASs) in switching networks heavily relies on stablization techniques for switched systems. The consensus problem becomes challenging when the agent dynamics are complicated that requires stability of a switched system composed of unstable subsystems. Moreover, complicated dynamics bring other coupling structures into a switched system. To solve the consensus problem in such a scenario, we study the input-to-state stability property of a switched system with external influence and hence design a stablization controller for a switched system of interconnected components using the small gain theorem. The controller successfully leads to the solution to the consensus problem for a class of nonlinear, heterogeneous, and uncertain MASs in switching networks with jointly connected topology.

Energy Efficiency Maximization for Downlink Cloud Radio Access Networks With Data Sharing and Data Compression

Abstract: This paper aims to maximize the energy efficiency of a downlink cloud radio access network (C-RAN). Here, data is transferred from a baseband unit in the core network to several remote radio heads via a set of edge routers over capacity-limited fronthaul links. The remote radio heads then send the received signals to their users via radio access links. Both data sharing and compression-based strategies are considered for fronthaul data transfer. New mixed-integer nonlinear problems are formulated, in which the ratio of network throughput and total power consumption is maximized. These challenging problem formulations include practical constraints on routing, predefined minimum data rates, fronthaul capacity, and maximum remote radio head transmit power. By employing the successive convex quadratic programming, iterative algorithms are proposed with guaranteed convergence to the Fritz John solutions of the formulated problems. Significantly, each iteration of the proposed algorithms solves only one simple convex program. Numerical examples with practical parameters confirm that the proposed joint optimization designs markedly improve the C-RAN’s energy efficiency compared to benchmark schemes. They also show that the fronthaul data-sharing strategy outperforms its compression-based counterpart in terms of energy efficiency, in both single-hop and multi-hop network scenarios.

Spectral and Energy Efficiency Maximization for Content-Centric C-RANs With Edge Caching

Abstract: This paper aims to maximize the spectral and energy efficiencies of a content-centric cloud radio access network (C-RAN), where users requesting the same contents are grouped together. Data are transferred from a central baseband unit to multiple remote radio heads (RRHs) equipped with local caches. The RRHs then send the received data to each group’s user. Both multicast and unicast schemes are considered for data transmission. We formulate mixed-integer nonlinear problems in which user association, RRH activation, data rate allocation, and signal precoding are jointly designed. These challenging problems are subject to minimum data rate requirements, limited fronthaul capacity, and maximum RRH transmit power. Employing successive convex quadratic programming, we propose iterative algorithms with guaranteed convergence to Fritz John solutions. Numerical results confirm that the proposed joint designs markedly improve the spectral and energy efficiencies of the considered content-centric C-RAN compared to benchmark schemes. Importantly, they show that unicasting outperforms multicasting in terms of spectral efficiency in both cache and cache-less scenarios. In terms of energy efficiency, multicasting is the best choice for the system without cache whereas unicasting is best for the system with cache. Finally, edge caching is shown to improve both spectral and energy efficiencies.

LMI-Based Fixed Order Output Feedback Synthesis for Two-Dimensional Mixed Continuous-Discrete-Time Systems

Abstract: This paper addresses the problem of designing stabilizing fixed order output feedback controllers for two-dimensional mixed continuous-discrete-time systems. The paper starts by addressing this problem in a basic formulation, where the plant is supposed strictly proper, and the output feedback controller is supposed static. It is shown that a necessary and sufficient condition for establishing the existence of a stabilizing feedback controller can be obtained by solving two convex optimization problems with linear matrix inequality (LMI) constraints. The condition is mainly obtained by exploiting a reformulation of the problem based on the stability of a matrix over the complex unit disc, and by introducing suitable tables for establishing stability of polynomials with complex coefficients. The condition is sufficient for any size of the LMIs, and is also necessary for a size sufficiently large. Then, the paper proceeds by addressing a more general formulation, showing that the proposed approach can be used also when the plant is allowed to be nonstrictly proper, and when the output feedback controller is allowed to be dynamic.

Delay Skew Packet Flow Control in Wireless Systems With Dual Connectivity

Abstract: The paper presents a new data flow controller for use in 4G and 5G wireless applications where the incoming internet data flow is split in support of multipoint downlink wireless transmission. The different paths of the data from the node of the split to the wireless transmission nodes and on to the mobile may then result in different travel times. The mobile can cope with small values of this delay skew by means of buffers and reordering protocols. The control objective is therefore to keep the delay skew within a prespecified interval of time, when the packets arrive in the mobile. The proposed algorithm solves the problem by cascade control. The packet dwell time of each wireless transmission node data queue is controlled by an inner loop, with the reference dwell times being determined by an outer delay skew control loop. The delay skew control loop exploits a deadzone to achieve interval control. In a further contribution the $\mathcal{L}_2$ -stability of the controller is analyzed for the dual connectivity functionality of the coming 5G cellular network deployment. A combined use of the input–output Nyquist- and Popov-criteria is required for the analysis. The performance is also evaluated by a laboratory testbed implementation.

Agent-based Modeling of Inter-provincial Migration in the Mekong Delta, Vietnam: A Data Analytics Approach

Abstract: The Mekong Delta (MKD) region in Vietnam has experienced large-scale inter-provincial migration flows. This study employs agent-based modeling to explore migration dynamics across the provinces and cities in the MKD. An agent-based model, which incorporates the Theory of Planned Behaviour into the decision-making process to effectively break down migration intention into related components, is developed to simulate how an individual makes migration decisions considering different economic, social, and environmental circumstances. Outputs of the agent-based model are calibrated via real province-level data of in-, out-, and net-migration flows in the MKD region with the use of a data analytics approach, in an attempt to validate the results produced.

Stability properties of a MIMO data flow controller

Abstract: The increasing demand for data in mobile applications has created a need for better communications systems. To satisfy this growing data demand next generation wireless systems are planned to use millimetre wave carrier frequencies. At these frequencies, radio shadowing is severe. To compensate for shadowing, incoming data flows must therefore be split and sent over several radio interfaces, that may have different delay properties. This paper analyses the stability properties of a MIMO data splitter that controls the delay skew between the data paths. The stability analysis is performed using IQC stability theory.

Stability of Grid-Connected Permanent Magnet Synchronous Generator-Based Wind Turbines

Abstract: This work studies the stability of a wind turbine with permanent magnet synchronous generator (PMSG) for wind energy conversion systems (WECSs) integrated to power grids using a proposed control strategy. The cascaded control structure uses PI controllers and feedback linearisation which takes the nonlinear relationship between the generator speed and DC-link voltage into account. The stability analysis given here concentrates on the small signal model and utilises eigenvalue analysis to achieve a result. The relation between different modes and state variables is determined by calculating participation factors, based on which effective controller parameters can be selected to improve control performance. Achievable performance is illustrated on numerical simulations performed on a 2MW PMSG WECS model.

A port-Hamiltonian approach to the control of nonholonomic systems.

Abstract: In this paper a method of controlling nonholonomic systems within the port-Hamiltonian (pH) framework is presented. It is well known that nonholonomic systems can be represented as pH systems without Lagrange multipliers by considering a reduced momentum space. Here, we revisit the modelling of these systems for the purpose of identifying the role that physical damping plays. Using this representation, a geometric structure generalising the well known chained form is identified as \textit{chained structure}. A discontinuous control law is then proposed for pH systems with chained structure such that the configuration of the system asymptotically approaches the origin. The proposed control law is robust against the damping and inertial of the open-loop system. The results are then demonstrated numerically on a car-like vehicle.

Robust integral action of port-Hamiltonian systems

Abstract: Interconnection and damping assignment, passivity-based control (IDA-PBC) has proven to be a successful control technique for the stabilisation of many nonlinear systems. In this paper, we propose a method to robustify a system which has been stabilised using IDA-PBC with respect to constant, matched disturbances via the addition of integral action. The proposed controller extends previous work on the topic by being robust against the damping of the system, a quantity which may not be known in many applications.

Delay Alignment Control for 5G Multi Connectivity

Abstract: In fifth generation wireless systems, multi-point transmission will be used to counter the increasing radio shadowing at high carrier frequencies. Due to fading and varying delays over the IP-connections to the transmission nodes, the travel time of data from the source to the application may vary significantly between the different transmission paths. In case of e.g. video streaming this could result in packet reordering problems, leading to protocol resets. The paper therefore develops and evaluates a new delay alignment feedback controller that simultaneously aims to control the delay and the delay skew between the transmission paths. Simulations illustrate the performance of the proposed data flow delay controller. The delay skew controller is also analyzed and proved to yield a globally $\mathcal{L}_{2}$ stable feedback system.

Energy-Efficient Design for Downlink Cloud Radio Access Networks

Abstract: This work aims to maximize the energy efficiency of a downlink cloud radio access network (C-RAN), where data is transferred from a baseband unit in the core network to several remote radio heads via a set of edge routers over capacity-limited fronthaul links. The remote radio heads then send the received signals to their users via radio access links. We formulate a new mixed-integer nonlinear problem in which the ratio of network throughput and total power consumption is maximized. This challenging problem formulation includes practical constraints on routing, predefined minimum data rates, fronthaul capacity and maximum RRH transmit power. By employing the successive convex quadratic programming framework, an iterative algorithm is proposed with guaranteed convergence to a Fritz John solution of the formulated problem. Significantly, each iteration of the proposed algorithm solves only one simple convex program. Numerical examples with practical parameters confirm that the proposed joint optimization design markedly improves the C-RAN's energy efficiency compared to benchmark schemes.

Discontinuous energy shaping control of the Chaplygin sleigh

Abstract: In this paper we present an energy shaping control law for set-point regulation of the Chaplygin sleigh. It is well known that nonholonomic mechanical systems cannot be asymptotically stabilised using smooth control laws as they do no satisfy Brockett's necessary condition for smooth stabilisation. Here, we propose a discontinuous control law that can be seen as a potential energy shaping and damping injection controller. The proposed controller is shown to be robust against the parameters of both the inertia matrix and the damping structure of the open-loop system.