# A Simulation-Based Optimization Method for Warehouse Worker Assignment
## 3 Problem Description  
Warehouses typically handle a variety of stock-keeping units (SKUs), which require a range of activities to be executed. Different products may require different activities. Considering the space and workforce constraints, the warehouse aims to maximize its productivity with via appropriate worker assignment. As warehouse workers may have different skills, it is not only the number of the workers that count, but also every specific worker-activity pairing becomes critical. Under the conditions of high fluctuations in daily workload, it is important for the warehouse planner to obtain an optimal worker assignment in a rather short period of time. To address this problem, a simulation-based optimization method was developed. Specifically, the optimal worker assignment to warehouse activities/tasks should be achieved while satisfying all operational and worker skill set constraints. In our showcase, the following operational assumptions are considered:  
* Product types: We chose to consider all product types, i.e., franchises, that the warehouse handles. There are in total nine franchises being handled in the warehouse. Different franchises require different processing times for different activities. More details are provided in Section 5. * Inbound activities: Inbound activities start with the arrival of shipments at the warehouse. Immediately after the dock-in, the products in pallets are unloaded at the inbound staging area and later moved to the sorting workstation. After manual counting and checking for defects and damage, the products are moved to the goods receipt (GR) workstation. At the GR station, workers scan the barcode on each item and register the items in the warehouse management system (WMS), while putaway slips with storage bins are generated for each pallet load. After the GR, they are ready for putaway to the storage area(s). The putaway worker puts away the pallet in its designated storage area following the storage bin information printed on the putaway slip. Putaway activity is denoted as the terminal inbound activity. Figure 1 shows the entity flow diagram for inbound shipments. * Outbound activities: Outbound activities start when order information arrives into the WMS and a pick slip is generated. One order is assigned to one picker. After the order picking is completed, depending on the labeling requirements, the items need to be labeled before scanning starts. Otherwise, the picked items are sent to the scanning station. There are two types of scanning, each for a certain type of product, i.e., manual scanning and scanning via auto-scanning tunnel (AST). The scanning activity makes sure that all the items are picked against the order lists, and it generates slips that denote items which need to be packed together based on pallet or case dimension constraints. Then the items are packed into pallets and cases accordingly. A release worker moves the order to the outbound staging, from where the order is shipped out with an outbound truck. Figure 2 describes the outbound entity flow diagram. * Three types of storage areas: There are three storage areas in the warehouse, i.e., racking, long-span shelving (LSS), and vertical lift modules (VLMs). Racking is designed to store SKUs in full pallets, LSS contains SKUs in loose boxes, and VLMs are for loose boxes. Each storage type is dedicated to one or more product types. * Shift configuration and working days: There are two shifts operating during business days (excluding weekends and holidays): the morning shift operates from 8:00 a.m. to 6:00 p.m. with a one-hour break during 12:00-1:00 p.m., and the night shift operates from 9:00 p.m. to 7:00 a.m. (next day) with a one-hour break during 1:00-2:00 a.m.
* Worker numbers and shift assignment: The total number of workers is fixed (31 workers in total). Worker numbers assigned to each shift are pre-fixed; 21 workers are assigned to the morning shift and 5 workers are assigned to the night shift.