Enabling Technologies for Next-Generation Smart Cities: A Comprehensive Review and Research Directions  
[PERSON]  
1 Department of Electronics and Communications Engineering, Zagazig University, Zagazig 44519, Egypt; [EMAIL_ADDRESS] (S.A.A.); [EMAIL_ADDRESS] (S.A.E.)  
[PERSON]  
1 Department of Electronics and Communications Engineering, Zagazig University, Zagazig 44519, Egypt; [EMAIL_ADDRESS] (S.A.A.); [EMAIL_ADDRESS] (S.A.E.)  
[PERSON]  
1 Department of Electronics and Communications Engineering, Zagazig University, Zagazig 44519, Egypt; [EMAIL_ADDRESS] (S.A.E.)  
[PERSON]  
4E1 Department of Electronics and Communications Engineering, Zagazig University, Zagazig 44519, Egypt; [EMAIL_ADDRESS] (S.A.E.)  
[PERSON]  
4E1 Department of Electronics and Communications Engineering, Zagazig University, Zagazig 44519, Egypt; [EMAIL_ADDRESS] (S.A.E.)  
Footnote 1: email: {45,5,5,5}  
Footnote 2: email: {45  
enabling informed decision making in infrastructure management, transportation, energy consumption, public safety, and environmental protection [4].  
Enhanced sustainability is a key advantage of smart cities, which is achieved through technologies that reduce the city's energy consumption, waste, and carbon footprint. For instance, smart illumination systems use sensors to detect human presence and automatically adjust the lighting to conserve energy [5]. Smart cities focus on providing personalized and responsive services to citizens, often through mobile apps, enabling access to information, payment for services, and feedback submission, leading to improvements in service quality and overall livability [6].  
As an emerging urban planning and development paradigm, smart cities offer solutions to pressing urban challenges, particularly in the transportation sector [7]. By employing sophisticated technologies, smart cities can manage traffic flow efficiently, reduce congestion, and improve intermodal connectivity, thereby minimizing carbon emissions and air pollution [8].  
Smart cities have garnered considerable attention from academic and industrial communities in recent years. Many proposals were presented to develop robust and reliable smart city networks or specific smart city applications [9].  
There is no standardization of smart cities yet; nevertheless, the main features of smart cities can be viewed as six different axes. These axes can be defined as follows [10; 11]:  
1. Smart economy: This axis comprises economic competitiveness, such as tourism services and outdoor digital marketing. This includes innovation, entrepreneurial business, and digital currency.
2. Smart governance: this axis consists of the services that serve the citizens in a smart city, allowing for the participation of citizens in political opinions and authority performance.
3. Smart environment: this axis characterizes natural conditions and resources, e.g., weather, green areas, energy, and water, to reduce bad effects, better utilize resources, and meet nations' sustainability policies.
4. Smart living: this axis considers the quality of life, including many aspects such as culture, safety, security, health, and education.
5. Smart mobility: this axis considers transportation aspects, including safe transportation sustainability and infrastructure systems.
6. Smart people: this axis is related to human and social technology solutions. It is introduced to get people to be more creative, qualified, and engaged in life-long learning.  
This work reviews the key technologies of smart cities, including the Internet of things (IoT), artificial intelligence (AI), distributed computing, network function virtualization (NFV), blockchain, and software-defined networking (SDN). Moreover, it shows the main features of each technology and the objectives of deploying it to enable smart cities.