arXiv subjects

Shukai Li

Publications and source records attributed to Shukai Li.

3 recordsLinked to original sources

arXiv2026

Koopman-based NMPC for Virtually Coupled Train Control System

This paper investigates an analytical Koopman-based nonlinear model predictive control (K-NMPC) approach for tracking control of virtually coupled train systems. A nonlinear train movement model incorporating train dynamics, speed and control input limits, passenger comfort constraints, and collision avoidance is systematically lifted into a finite-dimensional Koopman space through closed-form observable functions. After freezing the affine parameter-varying lifted predictor along the shifted predicted trajectory, the online optimal control problem is solved as a quadratic program that can be solved efficiently. The proposed KNMPC is benchmarked against a time-discrete NMPC scheme, demonstrating comparable control performance with significantly reduced online computation time and strong potential for real-time implementation in practical virtually coupled train control systems.

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arXiv2023

Optimizing Equitable Resource Allocation in Parallel Any-Scale Queues with Service Abandonment and its Application to Liver Transplant

We study the problem of equitably and efficiently allocating an arriving resource to multiple queues with customer abandonment. The problem is motivated by the cadaveric liver allocation system of the United States, which includes a large number of small-scale (in terms of yearly arrival intensities) patient waitlists with the possibility of patients abandoning (due to death) until the required service is completed (matched donor liver arrives). We model each waitlist as a GI/MI/1+GI queue, in which a virtual server receives a donor liver for the patient at the top of the waitlist, and patients may abandon while waiting or during service. To evaluate the performance of each queue, we develop a finite approximation technique as an alternative to fluid or diffusion approximations, which are inaccurate unless the queue's arrival intensity is large. This finite approximation for hundreds of queues is used within an optimization model to optimally allocate donor livers to each waitlist. A piecewise linear approximation of the optimization model is shown to provide the desired accuracy. Computational results show that solutions obtained in this way provide greater flexibility, and improve system performance when compared to solutions from the fluid models. Importantly, we find that appropriately increasing the proportion of livers allocated to waitlists with small scales or high mortality risks improves the allocation equity. This suggests a proportionately greater allocation of organs to smaller transplant centers and/or those with more vulnerable populations in an allocation policy. While our motivation is from liver allocation, the solution approach developed in this paper is applicable in other operational contexts with similar modeling frameworks.

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arXiv2022

A New Method for Computing Stationary Distribution and Steady-State Performance Measures of a Continuous-State Markov Chain with a Queuing Application

Applications of stochastic models often involve the evaluation of steady-state performance, which requires solving a set of balance equations. In most cases of interest, the number of equations is infinite or even uncountable. As a result, numerical or analytical solutions are unavailable. This is true even when the system state is one-dimensional. This paper develops a general method for computing stationary distributions and steady-state performance measures of stochastic systems that can be described as continuous-state Markov chains supported on R. The balance equations are numerically solved by properly constructing a proxy Markov chain with finite states. We show the consistency of the approximate solution and provide deterministic non-asymptotic error bounds under the supremum norm. Our finite approximation method is near-optimal among all approximation methods using discrete distributions, including the empirical distributions generated by a simulation approach. We apply the developed method to compute the stationary distribution of virtual waiting time in a G/G/1+G queue and associated performance measures under certain mild but general differentiability and boundedness assumptions on the inter-arrival, service, and patience time distributions. Numerical experiments validate the accuracy and efficiency of our method, and show it outperforms a standard Markov chain Monte Carlo method by several orders of magnitude. The developed method is also significantly more accurate than the available fluid approximations for this queue.

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