# A Simulation-Based Optimization Method for Warehouse Worker Assignment  
[PERSON]  
1 Department of Industrial Systems Engineering and Management, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore; [EMAIL_ADDRESS] (K.K.); [EMAIL_ADDRESS] (H.L.)1  
[PERSON]  
1 Department of Industrial Systems Engineering and Management, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore; [EMAIL_ADDRESS] (K.K.); [EMAIL_ADDRESS] (H.L.)1  
[PERSON] and [PERSON]  
1 Department of Industrial Systems Engineering and Management, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore; isekk  
In the traditional assignment optimization problem, the benefit can be directly calculated given a fixed worker-task pairing. Even with this assumption, the assignment problem is NP-hard [7]. The problem in the warehouse is more complicated due to the precedence constraints, whereby the succeeding activity cannot start until the preceding activity is completed. Another related problem is the assembly line balancing, which considers the precedence constraints. However, oftentimes such problems do not consider worker capability and capacity factors, which are critical to human resource allocation problems [8].  
Discrete-event simulation (DES) is one of the popular modeling techniques in which a model changes only at a discrete random set of time points [9]. Nowadays, many commercial simulation software packages such as Arena integrate optimization techniques with DES [10, 11]. Although such commercial software packages are designed to provide users with functionality to create their desired simulation models based on processes, oftentimes the users are not allowed to customize the event logic, which is an integral part of each DES model [10]. In this study, we adopt the object-oriented discrete-event simulation (O2 DES in \(C\#\)) framework developed by [PERSON] et al. [12]. This novel framework features a flexible simulation modeling environment, which allows the user to customize the event logic, configure simulation parameters, and incorporate add-on algorithms, including optimization models.  
Our research aim was to develop a simulation-based optimization method to improve the warehouse service level, i.e., daily productivity, by optimally allocating warehouse workers into inbound and outbound activities, while at the same time considering all operational constraints of the warehouse's operation. In addressing this aim, a warehouse manpower planning tool was developed based on the simulation-based optimization method, which combines the O2 DES simulation framework and a random neighborhood search method. Our study is the first in the operations research domain to introduce a decision support tool for warehouse worker allocation using a simulation-based optimization method.  
The manpower allocation tool was tested in a warehouse located at Singapore. It is now used to support the warehouse managers' decisions on a weekly and/or daily basis.  
The rest of the paper is organized as follows. In Section 2, a literature review on the related topics is presented. Next, the worker assignment problem in the warehouse is described in Section 3. Section 4 outlines the simulation-based optimization method. The O2 DES framework and the warehouse simulation model are introduced in Section 5, followed by the simulation-based optimization algorithm specifications in Section 6. Section 7 presents the numerical results.