# Energy Efficient IoT Data Collection in Smart Cities Exploiting D2D Communications
## 1 Introduction  
The Internet of Things (IoT) holds the promise to improve our lives by providing innovative services conceived for a wide range of application domains, ranging from personal to industrial environments, and facing several societal challenges in various everyday-life human contexts [1]. The large number of heterogeneous and pervasive IoT devices continuously generating sensing data and connecting different \"technological islands\", offers opportunities for applications and services only limited by the designer fantasy. In particular, this avalanche of new \"smart\" devices, opportunities and services is expected to provide a new radical evolution of what the current and future cities will be. The idea behind the new vision of \"Smart Cities\" [2] is to enable an improved wellbeingto the citizens from economic, social, and environmental perspective. This vision pairs well with the opportunities offered by the IoT paradigm where a large number of \"smart\" devices is able to monitor and process information from the surrounding environment. These devices typically operate through their virtual representations within a digital overlay information system that is built over the physical world. Therefore, the vision of our everyday furniture, food containers, and paper documents accessing the Internet is not a mirage anymore [3]. However, strong challenges arise when considering the current estimates of about 10 billions of IoT connected devices and between 24 billions and 50 billions total connections expected within the next five years [4; 5]. In particular, a key concern for \"Smart Cities\" is the need of effective solutions to process the huge amount of information coming from their infrastructure (e.g., roads, bridges, rail/subways, airports, buildings) to then efficiently provide services to the city users. With respect to this, one of the biggest challenges is represented by the coexistence of different communication technologies (e.g., LTE, D2D, WiFi) that need to cooperatively interact to efficiently collect (in real time) the required data, to process them, and deliver the information of interest to the citizens [6]. To cope with this and according to the current Fifth Generation (5G) vision, the IoT infrastructure has been identified as a possible key technology able to provide data processing and management, actuation and analytics for real time information to be converted to usable knowledge. Therefore, we strongly believe that research is needed to enable energy efficient solutions for effective IoT communications and data forwarding to support the decision-making of city management and citizens and by this turn the city _smart_[7]. The capability of IoT devices to communicate and interact with each other without the human intervention is also known as Machine to Machine (M2M) communications [8] or Machine Type Communication (MTC) according to the 3 rd Generation Partnership Project (3 GPP) definition. In the context of this paper both IoT and MTC acronyms will be used depending on the specific context. In particular, IoT will be used when the focus is on the general capability of a device to connect to the Internet, whereas MTC is used when the focus is on the communication for the IoT devices [9; 10]. A variety of short-range wireless technologies are proposed to guarantee connectivity among IoT devices, whereas the long-range connectivity is enabled by a wireless gateway which acts as a gate to the Internet. Many researchers and experts in the field share the opinion that the next-to-come fifth generation (5G) cellular systems will be a strong boost for the IoT deployment, where Long Term Evolution (LTE) is deemed to play a fundamental role. In particular, LTE-Advanced (LTE-A) [11], is the version of the 3 GPP standards that completely fulfills the requirements set by ITU for IMT Advanced for 4G systems and guarantees high data rate, low latency, low cost per bit, high spectrum efficiency, and high system capacity.