# A Survey of the Literature on Order-Picking Systems by Combining Planning Problems
## 5 Discussion
### Investigating the Problems of Order-Picking Planning and Their Combination
#### 5.1.1 Study the Warehouse Layout  
A warehouse comprises various areas and domains, including transportation operations, parts receiving area, parts warehousing area, and order-picking area [28]. Aisle is also referred to as the path between the rows of a cell. Moreover, in the recent literature related to customized picking, three types of assignments have been introduced to determine the optimum anchors.  
1. Assignment of storage and batching
2. Allocating storage and routing location
3. Wrapping and routing  
Generally, in the design of a warehouse, all operational processes and the type of system that uses the design must be considered. Product characteristics and demand can also affect the selected system. On the other hand, in a design, compatibility between areas and requirements should be considered and the design should be so as to improve the performance of the system [29]. Figure 5 graphically depicts a typical order-picking area.  
Figure 4: Number of Qualitative articles in journals.  
According to Table 2, most of the studies in the warehouse were on parallel aisles and other methods were less favored by the researchers.  
[PERSON] et al. [2] and [PERSON] et al. [20] stated that the process of picking and selecting pieces for picking is complete when the picker returns to the original location after selecting the last batch. In these papers, the authors used the mathematical model to solve small samples. In contrast, for larger samples, they used a variable neighborhood descent (VND) algorithm followed by numerical experiments, showing that the algorithm offers excellent solutions for path optimization. Figure 6 introduce items, such as storage location assignment, zone assignment of the warehouse, picking area, ordering, routing, labor assignment, labor force level, and labor assignment in an interconnected structure.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Auditors** & **Layout Method** & **Number** \\ \hline [PERSON] et al. [12], [PERSON] et al. [13], [PERSON] et al. [22], [PERSON] et al. [30], [PERSON] et al. [31], [PERSON] et al. [32], [PERSON] \& [PERSON] [33], [PERSON] [14], [PERSON] \& [PERSON]. [34], [PERSON]. [35], [PERSON] et al. [36], [PERSON] [37], [PERSON] et al. [38], [PERSON] et al. [39] & Parallel aisle & 14 \\ [PERSON] \& [PERSON] [40], [PERSON] \& [PERSON] [41], [PERSON] et al. [42] & cross aisle & 3 \\ [PERSON] et al. [43], [PERSON] et al. [20] & Parallel aisle with a middle aisle & 2 \\ [PERSON] \& [PERSON] [44], [PERSON] et al. [19] & parallel aisle with cross aisle & 2 \\ [PERSON] et al. [45] & Parallel aisle with 3 cross aisles & 1 \\ [PERSON] et al. [46] & parallel aisles and two front and & 1 \\ [PERSON] et al. [8], [PERSON] \& [PERSON] [47] & parallel aisles and two or more & 2 \\ [PERSON] \& [PERSON] [48] & [PERSON] & 1 \\ [PERSON] et al. [49], [PERSON] et al. [50] & picker-to-parts & 2 \\ [PERSON] \& [PERSON] [51] & Mixed-width aisle & 1 \\ [PERSON] et al. [52] & pick-and-pass & 1 \\ [PERSON] [53], [PERSON] et al. [29] & multiple cross aisles & 2 \\ [PERSON] et al. [24] & single-block and multiple-block & 1 \\ [PERSON] et al. [54] & U-shaped central aisle & 1 \\ Total & 34 & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Areas of study of warehouse layout types in picking.  
Figure 5: Order-picking Area in Stock [29].