# Energy Efficient IoT Data Collection in Smart Cities Exploiting D2D Communications
## 2 Background and Related Work  
### Energy Efficiency  
Energy efficiency is of high interest in the context of Smart Cities and can strongly benefit from the adoption of the IoT paradigm [17] offering the opportunity to effectively manage the energy consumption of a city. For instance, city authorities and citizens might be provided with a clear overview of the amount of energy required and consumed by the different city services (e.g., public lighting, lights, surveillance camera, heating/cooling of public buildings, and so on). Moreover, the IoT can provide a useful help to identify all the entities that badly influence the city energy consumption and help the authorities to elaborate strategies to optimize their behaviors [18].  
Extending the analysis to wireless networks in general, energy efficiency is of high interest in the research community to cope with the continuous growth of energy demanding applications in limited energy resource scenarios (e.g., a Smart City environment) [19]. In particular, green radio solutions are being studied for future wireless systems [20] and in [21] a discontinuous transmission  
Figure 1: Reference scenario IoT data collection in an LTE-A cellular environment.  
mode (DTX) is introduced on the base station side to reduce the consumption of LTE communications. The discontinuous reception/transmission (DRX/DTX) mechanism in the LTE-A standard is also defined to allow devices to go in sleep mode when no data need to be received or transmitted from/to the base station. Undoubtedly, this feature is of special interest for MTCs over cellular infrastructures. In particular, in [22], the DRX/DTX optimization is investigated to maximize the sleep periods of devices while guaranteeing their Quality of Service (QoS).  
Other solutions for energy efficient communication over the IoT have also been proposed in the literature e.g., based on the time reversal technique [23], whereby energy of all multi-path signals at the receiver can be harvested. Further inspiration for energy efficient solutions for data collection in the IoT can be obtained from other networking scenarios, like wireless sensor networks [24]. When considering the cellular environment, in [25] the impact of MTC on energy and delay performance is investigated from the point of view of the overload of the random access channel (RACH) whereas in [3, 26] a contention-based LTE transmission mechanism is proposed showing an improvement in network resource consumption, device energy efficiency, and mean data access delay. In [27] a method to reduce the device battery consumption to efficiently support M2M communication is presented based on the proper configuration of discontinuous reception cycle length. The authors in [8] instead, propose a group based MTC approach for supporting a large number of MTC devices with small data transmissions and with different QoS requirements. In particular, MTC devices are grouped into a cluster according to their QoS characteristics and requirements. So doing, the LTE-Advanced station can manage radio resources on a cluster basis instead of a single MTC device, with a consequence complexity reduction. A further group-based scheme is proposed in [28] to minimize the overall energy consumption for the MTC devices. The proposal considers a group-based solution with the group head guaranteeing low energy consumption by limiting the access among the MTC devices and the eNB.