## 2 Synchronization Issues with Various Part-to-Picker Systems and a Literature Review
### Synchronization Problems in Warehouses
Warehouses and manufacturing facilities frequently use online vehicle dispatching rules to control the movements of vehicles. The vehicle reassignment dispatching rules are effective in reducing the typical load waiting time, but frequent reassignment is not a good strategy for guided vehicles. [PERSON], [PERSON], and [PERSON]. [PERSON] provided a thorough analysis of the state of the art in warehouse operation planning research and identified future researched possibilities [7]. The research results are not evenly distributed among warehouse operational issues; the SLAP (storage location assignment problem) and routing have been given more attention than zoning. The focus of research should be on the operational management of warehouse systems, where multiple processes are jointly considered and multiple objectives are taken into account at once. In 2007, [PERSON] and [PERSON] reviewed research on conveyor system models used in semiconductor manufacturing facilities [20]. They provide a comprehensive introduction to simulation-based models, general analytical models of closed-loop conveyors, and pinpoints. Specific research issues and demands are addressed in the development and management of closed-loop conveyors. Furthermore, their work identifies and describes the research needs related to the implementation of closed-loop conveyor-based automated material handling systems (AMHSs). And this research concludes that the majority of the existing literature focuses on difficult-to-generalize simulation-based studies that are application-specific in nature. [PERSON] et al. developed a Hungarian algorithm-based reassignment technique for overhead hoist transport (OHT). This one outperforms current reassignment-based rules and conventional dispatching rules in terms of vehicle requirements, mean lead time, and variance of lead time. It is easy to incorporate into any AGV/OHT system and can reduce overall vehicle costs by USD 1.3 million and USD 1.0 million [21]. In 2010, [PERSON] and [PERSON] focused on the issue of coordinating workflow for the completion of small orders through order release control. A quantitative model was developed to produce prescriptive control guidelines, and a rigorous performance comparison was conducted between wave-based and waveless operations [22]. [PERSON], [PERSON], and [PERSON] proposed a robust solution approach to improve internal transports at one of the main parcel sorting centers of Deutsche Post World Net [23]. The mixed-integer linear programming (MILP) model minimizes manual transportation effort by modifying the layout and taking into account the specific characteristics and requirements of the parcel sorting center. The robustness concept is suggested to take into account uncertainty in internal transports, and the calculated robust solution is close to every optimal-scenario objective value. The focus of this research is on the problems of release time scheduling and hub location. By the next day, the total cargo amount is guaranteed and delivered to every potential destination. This situation must meet a certain threshold and the total routing cost must be below that threshold. [PERSON], [PERSON], and [PERSON] suggested that the assumption of fixed truck release times should be relaxed and truck departures should be synchronized [24]. The assumptions of the hub network, single allocation, and flows between all pairs in the current study have drawbacks. But the elimination of these presumptions will make future research directions more challenging and achievable. The circumstance is uncertain and dynamic. The most significant, labor-intensive, and expensive activities for warehouses are order picking and sorting operations. [PERSON], [PERSON], and [PERSON] presented an innovative way to integrate these operations [25]. By calculating the ideal wave size, the trade-off between picking and sorting was resolved. The purpose of this study is to aid in the decision making process regarding the ideal wave size for order picking and order sortation operations.