## 2 Synchronization Issues with Various Part-to-Picker Systems and a Literature Review
### Synchronization Problems in Warehouses
Despite the lack of academic research on integrated order picking and sortation systems, three different operation strategies for sorting systems have been proposed. Automated storage/retrieval systems (ASRSs) are frequently utilized in warehouses and distribution centers, according to [PERSON] and [PERSON]. The effectiveness of these systems is due to high space utilization, low labor costs, quick retrieval times, and enhanced inventory management [26]. This work proposed a unique categorization method for describing different ASRS settings and summarizing existing and innovative outcomes, based on the well-known triple notation for machine scheduling. It was determined that further investigation was necessary. [PERSON], [PERSON], and [PERSON] analyzed the scheduling of inbound trucks at hub terminals [27] to examine the advantages of an interval scheduling strategy over randomly generated solutions [27]. The sortation performance could be improved by up to 10% without spending money on technical equipment. [PERSON] and [PERSON] investigated several closed-loop tilt tray sortation conveyor design options to increase throughput in parcel distribution facilities [28]. However, it should look at different design options, as the unique destination assignment problem (DAP) described in this work significantly increases package throughput in all layout settings. [PERSON] and [PERSON] presented an exact and heuristic solution to organize efficient and timely deliveries of parts and subassemblies to final assembly [29] in Figure 1. The optimization problem of scheduling in-plant transport vehicles to feed parts to mixed-model assembly lines was investigated in this paper. The result demonstrated that simple cyclic schedules are not optimal and that decreasing stopover time can decrease inventory. [PERSON], [PERSON], and [PERSON] explored order processing at the picking stations in a Kiva warehouse, where mobile robots move racks straight to stationary pickers [30]. Compared to a rule-based system, efficient order processing enables one or more robots to decrease the fleet size of robots needed to promptly supply a picking station. Warehouses that store small-sized items and handle orders from a limited number of order lines are the most suitable for this system. Figure 2 describes picker-to-parts systems that minimize investment costs and are more scalable. Parts-to-picker systems eliminate wasteful picker travel and high staff expenses while minimizing the idle time of expensive equipment. Figure 1: Tow trains deliver items to an assembly line [29]. Figure 3 illustrates how the order and rack sequences affect the number of rack trips needed to complete an order. Minimizing the number of rack visits is a good goal for the order picking process since it decreases the danger of bottleneck situations and the make span of the order picking schedule. Furthermore, minimizing the number of rack visits saves waiting time since all other time components are fixed once the picking list is specified and are not changed by the order sequence. The warehouse layout in Figure 4 has more space than is required to house m = 5 or 100 racks. But it is irrelevant for our simulation as Kiva robots can move underneath the racks. [PERSON] et al. investigated workplace design for order pickers in order to reduce ergonomic risks and improve the well-being of employees [31]. The workplace design of an order picking setting requires the integration of ergonomics aspects, which is often repeated and poses significant ergonomic risks. Computerized solution algorithms are necessary for minimizing the maximum ergonomic risk. It is important to examine ergonomic risks at workplaces, which are planned according to picking time, supplementing time measurement. The ergonomic risks of designing workplaces with acceptable physical loads should be compared to this risk. Batching and zoning tactics are frequently used by online merchants to speed up their picker-to-parts order fulfillment processes. However, these manual consolidation processes are not scalable.