# Wave Planning for Cart Picking in a Randomized Storage Warehouse
## 1 Introduction
### The Importance of Order Picking for a Warehouse and the Type of Order Picking  
Warehouse order picking is a simple concept, but in practice, the picking process can be complicated. Warehouse order picking refers to the necessary labor and parts involved in pulling items out of inventory to fulfill customer orders [1]. This process may sound like the simplest aspect of a business, but when one deals with numbers and realizes that in any given distribution center, order picking counts as the most time-consuming operation and an average of 55% of operating costs. A well-thought-out warehouse will soon find out these ineffective systems may prevent them from further innovation. For a business to develop a warehouse order picking strategy to meet the demand shifts of the future, the emphasizes on speediness, accuracy, and organization should always be bear in mind. There are many types of order picking. Some of the order pickings are classified by the customer order handling policies, some are classified by equipment used by pickers, some are classified by warehousing areas, and the others are classified by materials handling equipment. For example, discrete order picking and batch picking are those classified by the customer order handling policies; cart picking, radio frequency (RF) picking, voice picking, tote picking, and put to store are those classified by equipment used by pickers; pick and pass or zone order picking are classified by warehousing areas; and full pallet picking and materials handling equipment assisted picking are those classified by materials handling equipment used. Discrete picking is when a picker goes through one order at a time, grabbing an item line by line before moving onto the next order. It is the simplest of all order picking methods and can also be one of the most accurate. Discrete order picking with a RF scanner or voice assistant device helps to streamline the process and increase accuracy. Cases where discrete order picking can be ideal include (1) when a warehouse has a small staff, (2) when orders are for multiple items, and (3) when the items ordered are very large. Batch picking is the process of collecting inventory for identical orders at once, rather than picking items one order at a time or picking orders with different SKUs and quantities consecutively. Batch picking focused on less walking and faster order processing. It is particularly good for warehouses that stock small items. In pick and pack operations, instead of batching the items into an order tote, you can pick orders directly into a shipping tote. Put-to-store is often used with picking carts to facilitate batch picking of multiple orders in one pass. It is ideal for retail store replenishment. It also optimizes cross-docking operations where a percentage of full case quantities are broken down to store level cartons. Zone order picking is a picking technique in which warehouses have been sectioned into several zones, and pickers have been assigned to each zone. Pickers pick all of the items from a customer order that are located in their zone and then passing the order on to the next zone, so that zone order picking is also called pick and pass order picking. Zone picking is particularly useful in large warehouses that deal with a high number of SKUs. Because each picker is assigned to a designated area, it allows pickers to gain deep familiarity with their assigned zones and the SKUs stored there, which can increase picking speed and reduce errors. In this paper, we suggest looking at an order picking problem from forms and workflow of a warehouse management system (WMS) perspective. This would help one to tackle (i.e., finding out the considering factors/attributes) an order picking planning problem. From information technology perspective (i.e., data and workflow), the inputs of a WMS are customer orders and pre-packaged stock keeping units [2].