# A Survey of the Literature on Order-Picking Systems by Combining Planning Problems
## 3 Order-Picking System Research Background  
During the past decades, awareness about OPS problems has been increasing worldwide thanks to industry and research. However, despite many efforts over the past few decades, OPS has not yet effectively matured. Therefore, it is essential to pay attention to warehousing activities, such as parts receiving, warehousing, orders, and internal routing [2].  
It is also crucial to consider the OPS classification developed by researchers to understand previously conducted research on OPS and the boundaries of OPS research. For instance, [PERSON] and [PERSON] [15] proposed the problems related to school bus routing. In this matter, some limitations, such as the maximum capacity of the bus, the maximum time to get on and off a bus, and the time window, were discussed. This class of surveyed problems has several subsets, including device transportation selection, routing, transport schedules, and completion time of the operation. Meanwhile, [PERSON] et al. [2] developed a classification for warehouse managers' benefit. They stated that since warehouses offer several services to customers, the poor performance of warehouses may result in high costs and customer dissatisfaction with demand. [PERSON] et al. [2] also declared that new market developments force warehouses to handle many orders within tight time windows. Accordingly, they studied the related review and classification of the scientific literature with combinations of tactical and operational order-picking planning problems.  
Investigating a bunch of articles has also shown that the new policies on warehousing, ordering, routing, batching using fuzzy parameters, and innovative algorithms have been developed for planning resources and optimizing performance by considering the number of workers [16; 17; 18; 19]. On the other hand, most of the studies on OPS design with a combined program have been conducted to re-evaluate warehouse activities, routing problems, equipment, order selection in a warehouse, and warehouse design [2; 15]. Furthermore, some studies have been performed on the components of the order-picking process [20; 21; 22]. Some studies in recent years have sought to reduce travel time and makespan by integrating decisions on classification and scheduling of bodies [23]. [PERSON] et al. [24; 25] also investigated routing, batching, and sequencing to reduce travel time in the warehouse. [PERSON] et al. [2] proposed, on a large scale, tactical and strategic decision-making issues. The advantage of the present study over the one conducted by [PERSON] et al. [2] is addressing the issues of order tardiness and warehouses. Likewise, the research method used in their paper was mathematical programming, analytical models, and simulations that are different from the classification of our paper. In the current work, we tried to consider the necessity of internal aspects of procurement, the limitations, and available resources to reduce costs and increase adaptability to customer requirements by increasing the productivity, and the use of OPS systems. Although, in the review studies, the mentioned factors in the OPS system have been classified in terms of tactic, strategy, and design, they have failed to present a proper classification, including performance measurement indicators, problem-solving methods, planning, and combination. Thus, this paper reviewed a combination of cost, equipment, warehousing, assignment, routing, batching, sequencing, and tardiness in the scientific literature on a combination of planning problems. To achieve the aim of this research, which is the classification of the order-picking process, first, planning problems and their combination should be surveyed. After that, the performance measurement indicators and the chosen solution methods should be specified and investigated. The relationships between planning problems should be finally analyzed based on such indicators and solutions.