## 2 Synchronization Issues with Various Part-to-Picker Systems and a Literature Review
### Synchronization Problems in Airports
This paper proposed an alternative approach for reducing the complexity of computations by approximately solving the nonlinear optimization problem using a mixed-integer linear programming problem. A static mixed-integer model was introduced by [PERSON] and his colleagues to solve the transfer baggage problem. The proposed model combined robustness and workload allocation with efficiency goals in order to decrease missing bags and transit time in 2013 [45]. The model's operating environment includes a significant amount of problems related to uncertainties. Data analysis reveals that these uncertainties contain a significant amount of information that the model can utilize to enhance its performance. A straightforward stochastic model can be used to enhance the model, although this might be time-consuming and require changing business procedures. Simulation can be used to evaluate the value of a modification. The airport gate assignment problem (AGAP) is a daily challenge for operations managers. [PERSON] et al. surveyed the state of the art of these problems and the various methods to obtain a solution [46]. Their work addressed the following research questions: Is AGAP NP-hard? What formulation can be defined for such a problem? The algorithms that yield an optimal solution are the exact ones. The heuristic and metaheuristic approaches use systematic rules to avoid local optimals and temporarily accept moves that cause the worsening of the objective function value. This survey examines the contributions and trends in the research using exact or approximate methods to AGAP or other related problems. [PERSON] et al. analyzed how conveyor-based baggage handling systems' merging bottlenecks are designed and controlled. This demonstration shows that input variance had little bearing on bag throughput performance in their research in 2015 [47]. The effectiveness of two merging control techniques can be evaluated by measuring bag throughput. This study examined two algorithms to regulate a merge using a first-in first-out (FIFO) dispatch mechanism and found that the variable length approach outperformed the other algorithm. In the same year, SysML was utilized to model and analyze the baggage handling system, resulting in a formal representation and validation of the system. [PERSON] et al. proposed a modeling framework that uses system modeling language (SysML) to estimate the system's performance. The complex system consisted of lengthy conveyors and human decisions [48]. The integration of SysML and simulation was used to model a baggage handling system (BHS). And it also redefined critical performance for evaluation, such as maximum baggage traveling time and on-time delivery rate. [PERSON] and his colleague presented an empirical study of interval release time using simulation with full factorial design of the BHS. The best combination of interval release time and its interactions was determined in this research. In addition, this also minimized congestion time in the airport baggage handling system at the Taiwan Taoyuan Airport during the 2015 simulation [49]. The simulation time for each alternative was 5.32 min, with a total time of 35.9 h required. The outbound airport baggage handling system (BHS) consists of a set of unloading zones (chutes) assigned to outgoing flights. To enhance this system, solutions such as the vector assignment problem (VAP) and stochastic vector assignment problem (SVAP) are necessary. [PERSON] et al. provided a two-stage stochastic programming model to optimally solve the airport chute assignment problem, outperforming the current solution [50]. A hybrid heuristic was proposed by [PERSON] et al. to optimize the inbound baggage handling process at Munich's Franz Josef Strauss Airport [51]. It was based on an extensive simulation incorporating real-world data. And it provided a mathematical model to assign incoming flights to infeed stations, baggage carousels, and set the infeeding order of baggage tugs arriving at the same time.