# IoT-Enabled Smart Cities: Evolution and Outlook
## 1 Introduction  
Historically, cities and their citizens have spearheaded the enormous technological and social changes that have been taking place continuously, especially since the transition from an agricultural economy to an industrial one [1]. This phenomenon is especially significant from the mid-eighteenth century and it will become more intense if the predictions that forecasts the progressive urbanization of the world population (e.g., around the year 2050, approximately 70% will concentrate in some type of city [2]) finally come true [3]. With these boundary conditions, it is evident that the achievement of more efficient and sustainable cities is an essential objective for which politicians, managers and technicians must work in order to guarantee the quality of life of their citizens [4]. Although this paradigm of sustainability and efficiency has always been important for the managers of cities, it has not been until very recently that technology has made available to the responsible parties a plethora of possibilities that, when properly employed, translate into significant savings [5]. At the same time, the day-to-day improvement of the citizens is consolidating a new urban concept in which the different processes and systems that occur in it are continuously monitored in both time and space [6]. The Internet of Things (IoT) is recognized as a game-changer technology that expands its applicability to a huge variety of scenarios and domains. According to a recent report from McKensey, the potential economic impact of IoT can reach as much as $11.1 trillion per year by 2025, being the biggest source of value of all disruptive technologies, ahead of mobile Internet, knowledge-work automation, cloud computing, and advanced robotics [7]. IoT provides the necessary information to support the context-aware services that are behind the development of smart spaces. With the evolution of IoT technologies, the visionof a Smart City is starting to become a reality. The goal of a Smart City is to establish a wealthy ecosystem where the operation of the city is optimized, increasing its efficiency, but also where new opportunities arise and improve the life of its citizens [8; 9]. In contrast to other IoT domains, the Smart City is an agglomeration of many different application areas, including traffic, public transportation, parking, water, energy, waste management, street lighting, public safety, and many more. To support all these application areas, infrastructure has to be deployed in order to enable sensing important parameters of the city, but also for controlling and actuating aspects. The true value of IoT in the Smart City will be achieved if infrastructure is not limited to silos but is made available across different application areas in a homogeneous and resilient manner. The biggest challenges lie in the integration of the heterogeneous underlying IoT technologies and achieving the required scalability and security so that information, even personal and/or critical, can be exchanged among authorized actors and protected from illicit access. In this sense, the key asset is the data that the myriad of sensors embedded in real-world spaces are constantly generating [10]. Moreover, higher level knowledge shall be extracted, giving insights that help in optimizing the operation of the city and quality of life of its citizens. Finally, mechanisms to track the provenance, quality, and value of the information exchanged have to be in place in order to allow trading operations where the raw material is precisely the data that fuel Smart City services. The contributions of the paper are the following:  
1. It gives an overview of the current landscape of IoT-enabled smart cities based on a selection of relevant smart cities initiatives around the world.