# IoT-Enabled Smart Cities: A Review of Concepts, Frameworks and Key Technologies
## 3 Review of IoT-Enabled Smart City Components and Solutions  
There is a wide range of research literature regarding IoT application in smart city contexts. A search of the Web of Science (WoS) database for papers containing the keywords \"smart city\" OR \"smart cities\" AND \"IoT\" OR \"Internet of things\" in their topic (i.e., the union of the \"title\", \"abstract\" and \"keywords\" search fields) resulted in a total of 5285 articles, published from 2010 to 2021, which is a very large number of resources to be extensively and systematically reviewed, or even to be filtered in a supervised way to select and consider the most relevant papers. In order to provide an overview of the growing interest around these topics, the temporal evolution of the above-mentioned papers (grouped by year of publication) is reported in Figure 2. It is to be noted that the apparent decrease in the number of published papers over the last year (depicted with a dotted line in Figure 2) may be due to the fact that the count for 2021 was incomplete (since the search was performed in October 2021).  
On the other hand, many different approaches have been proposed in the literature for classifying smart city frameworks and solutions in a variety of application domains. For this reason, we focused on reviews and surveys as a starting point for our study. Therefore, the literature research was conducted adopting the following criteria:  
1. The WoS database was used for searching for reviews and survey articles containing the keywords \"smart city\" OR \"smart cities\" AND \"IoT\" OR \"Internet of things\" in at least one of the following fields: title; abstract; and paper keywords. Subsequently, a supervised overview and filter was performed in order to assure that each paper topic actually fit the subject of this review;
2. Recent literature was the main object of the present review, i.e., papers published from 2018 to the present (2021) were selected from the initial search;
3. Papers from Q1 and Q2 journals (as ranked in the SCImago index) were given priority over those from Q3 and Q4.  
Following these criteria, a total of 52 surveys and reviews on IoT-enabled smart cities were considered as the baseline for the survey presented in this paper. It is to be noted that most of the 52 surveys reviewed often aimed to review one or some specific IoT-smart city domains and use cases without providing a more general or comprehensive overview, which is one of the aims of this paper. In addition, in our opinion, the few general surveys that were retrieved lacked in addressing the relationships between each smart city domain (with related sub-domains and scenarios) with the related IoT technologies that have been employed in each specific context.  
In order to describe the wide landscape that we found by reviewing the selected literature in the most comprehensive way, the following eight domains were identified (as depicted in Figure 3), which are typically used to classify smart city components and application areas: governance; living and infrastructures; mobility and transportation; economy; industry and production; energy; environment; and healthcare. This approach extends the six-domain classification presented in [1; 37]. The classification proposed in this paper is not meant to be exhaustive and, in some cases, these domains may not necessarily be orthogonal as they may overlap in several contexts and applications.  
Figure 2: The temporal evolution of research papers related to IoT-enabled smart cities, as retrieved from the WoS database (collected in October 2021).  
In the next subsections, each of the above identified smart city domains is discussed in terms of general scenarios, features and services provided, the IoT technologies employed, the frameworks or solutions adopted and real-world case studies of smart cities implementing solutions for that specific domain.