## 3 Challenges with Smart City Applications  
Smart city applications have grown rapidly with sensory manufacturing advances and wireless networking innovations [17]. Figure 4 presents the main smart city applications. Figure 4: Main categories of smart city applications. **Smart grid:** This comprises three main systems, namely, generation, transportation, and consumption [18]. The consumer system deploys sensing devices that control and monitor the machines on premises. One or more nodes act as a gateway and provide connectivity to the Internet. Cloud or edge computing platforms provide processing, analysis, and storage to the smart grid system. Also, a control unit can achieve real-time system management [19]. Buildings, homes, businesses, medical facilities, educational institutions, and other establishments depend on energy. Therefore, a sustainable smart city must have effective power management systems with environment-friendly energy sources, such as photovoltaic cells (PVCs), solar panels, and windmills [20]. **Smart living and infrastructures:** This domain provides public services in tourism, education, and cultural activities, as well as smart city infrastructure, including smart buildings and homes. Inside the home, everyday tasks are automated using smart technologies, including AI and the IoT [21]. Sensors, IoT gadgets, and network connections are the foundation of smart homes [22; 23]. Modern smart homes incorporate solar energy. Additionally, residents must feel secure while they are at home. This necessitates the installation of a continuous monitoring and emergency system. The generated data must be processed and stored in a safe, trusted environment. A smart home is essential to ensuring residents' quality of life. This infrastructure must, therefore, be upgraded over time in response to user demand [17]. Smart buildings and homes are the major applications of smart cities. Smart homes depend on the capabilities of the control systems to make an environment comfortable at home [24]. Smart buildings are integrated into two levels: physical and virtual levels. The physical level consists of the network capabilities through wired and wireless buildings that combine the power network and transportation system [25]. The virtual level consists of the information shared and operated between residents and used in buildings. **Smart transportation:** The most recent advancement in intelligent and autonomous transportation is the development of vehicular ad hoc networks, which allows for inter- and intra-vehicular communications [26; 27]. Vehicle connectivity makes real-time traffic analysis easier than the traditional transportation system. Traffic analysis assists in managing traffic congestion, responding to emergencies, and spotting illegal activity on the road [28]. Potential traffic monitoring for airlines makes smooth, secure, and adaptable air travel possible. Several sensors gather weather information, analyze it, and communicate their decision to the air traffic controllers. Onboard computers make intelligent decisions using the same data. **Smart healthcare:** Humans have a basic need for healthcare. Measuring a patient's condition with sensor-enabled IoT devices is helpful [29]. Patients' heart rates, oxygen saturation levels, blood pressure, and other parameters can be measured by sensors. Historical patient information can aid in a precise diagnosis. The locations of doctors and nurses can be determined using sensors and paging devices, which can aid in the dispatch of emergency medical teams. Data generated by sophisticated medical and healthcare technologies will be substantial. Medical data are extremely sensitive and require improved privacy and security [30]. Table 2 summarizes the main networking features required for different smart city applications. The main design challenges associated with the evolution of the previously introduced applications are presented in Figure 5.  
To address these challenges, we introduce different key technologies in the following sections.