## 6 Challenges and Future Works  
The main challenges of the current study, as well as for the autonomous vehicular world, which we examined in depth and which will be targeted in the near future, are as follows:  
1. As several well-known vehicular companies are racing towards producing AVs [54], it is very interesting to have a smart-parking application installed in and supported with autonomous-vehicle built-in software; for example, the robot operating system (ROS), which is a flexible platform or tool to support and control AVs [55], and which can perform the functions of parking whenever is required by AVs. For example, whenever AVs are in an idle state, or not available for any service, the SP application can search for a closer available parking spot (e.g., either indoors or outdoors), and the interesting part is its electric charging services. The autonomous vehicle could obtain the parking spot and fulfill the demands of electric charging if required by the autonomous vehicle, as most AVs are supposed to be electric. However, if, for example, the autonomous vehicle has enough electric charging and thus parks in a parking spot that has no electric charging capabilities, then parking spots will be available for other AVs that have limitations to their electric charging services. In the future, there will be high demand for electric vehicles globally; as the report states, almost 130,000 electric vehicles were sold out in 2021, with about 6.6 million sold [56];
2. In metropolitan cities around the globe, multilevel parking is one of the best ways to offer and manage a large number of vehicles that are searching for parking, as per the indented closer locations. However, in the case of AVs, it is a challenging problem to offer indoor or multilevel parking, as the GPS is not functional inside the premises of multilevel buildings [57; 58]. To address this major issue, indoor driving technology, including indoor positioning systems (IPS) and smart sensor technology, can be employed as a resilient solution to conquer the challenge of indoor multilevel parking [59; 60; 61; 62; 63; 64]. Moreover, in large cities, where there are supposed to be numerous parking stations, these designs, or maps, can be varied as per the indoor or multilevel architectures, as it is a challenging issue for AVs to follow distinct parking designs to perform accurate parking in short periods of time. Among other things, in various essential parking designs, each autonomous vehicle should have installed, for example, an individual IPS for each parking station that functions on the road, or an autonomous vehicle design (i.e., firmware) that is fully functional or/and has capabilities to auto-install the IPS, or indoor driving map, of the parking station before entering; however, the best solution is that AVs should be installed with the IPS of each parking station, or with the IPSs of the parking stations that are essential for use in daily service routines; therefore, by having this feature, the autonomous vehicle can obtain information on closer parking stations, with the availability of parking slots;  
Figure 11: Performance Evaluation. 3. The exponential developments of 5G, the IoT, M2M, and other modern technologies can play important roles in enabling fully autonomous vehicles on the road with entirely safe features; however, having millions of AVs on the road will come with several challenges, which include, but are not limited to, the interoperability for both hardware and software. There are several vehicular companies that have already started or that are in the process of building fully AVs, with distinct vendors and vehicular companies. AVs on the road need QoS and intelligent information sharing for the safety of humans, and also for the safety of the AV itself.