# Energy Efficient IoT Data Collection in Smart Cities Exploiting D2D Communications
## 1 Introduction
Moreover, it offers ubiquitous coverage, operates in licensed spectrum thus controlling interference, is supported by operator-based business models which offer contractual Service Level Agreements (SLAs) and ensures very high reliability. Nonetheless, such high performance comes at the cost of a high energy consumption at the device side, which can compromise its suitability for the IoT. To overcome the high energy consumption limitation for LTE-A in the IoT, recent studies on power saving schemes have been performed by the standardization entity (_i.e._, 3 GPP) in the Release 12 [12]. In particular, technical specifications have been provided in order to help the operators to migrate MTC traffic from 2G to LTE networks via a new UE category (_UE Category \"0\"_) for low data rates and delay tolerant transmissions, setting the performance requirements to reduce complexity and power consumption [12]. Further progress will also be provided in Release 13, leading to: (i) 75% complexity reduction compared to Cat-1 UE; (ii) 10+ years battery life; and (iii) 15-20 dB coverage enhancement. Besides the standardization activities, some studies presented in the literature try to reduce the energy consumption of LTE-A for supporting MTC communications by introducing new (discontinuous) transmissions and reception modes in LTE [13]. In this paper we follow an alternative research direction that is based on two main components: (i) the use of short-range D2D communications for energy efficient IoT data collection; and (ii) the wise use of Modulation and Coding Scheme (MCS) to convey the data in an efficient way over both the D2D and the uplink LTE-A towards the eNodeB. We focus on a typical reference IoT environment where different types of devices produce small data (in the order of few bytes) to be uploaded to the network over the LTE infrastructure, see Figure 1. Since LTE-A was originally designed for supporting data-intensive Human-Type Communications (HTC), the minimum amount of radio resources that can be allocated to a single device in LTE-A could actually be too big for the actual needs of IoT applications. Nevertheless, by smartly adapting the MCS on a communication link, it is possible to maximize the radio resource utilization and also reduce the transmission power according to the total amount of data to send [14]. This approach guarantees better energy efficiency w.r.t. classic cellular-mode uplink transmission. On the other hand, the IoT devices will be clustered together in smaller groups based on their proximity, and direct short-range D2D communications [15] are used within the cluster to improve the communication efficiency. One of the IoT smart objects in a cluster will be appointed as collector of all data from the cluster thus acting as an _aggregator_ towards the eNodeB. As we will show the coupled use of D2D links [16] and wise MCS selection on all the activated links will introduce important benefits in terms energy efficiency and resource utilization. The remainder of the paper is organized as follows. In Section 2, the background and motivations for our work are presented browsing the related work on energy efficiency in the IoT and D2D communications. In Section 3, the reference system model and service configuration are described. In Section 4, the proposed D2D cluster-based IoT data uploading scheme is presented in detail. The performance evaluation results are summarized in Section 5. The last section concludes the paper.