# Energy Efficient IoT Data Collection in Smart Cities Exploiting D2D Communications
## 2 Background and Related Work
### D2D Communications  
The number of recent papers addressing D2D communications and its applications is wide [29] and ranges from those addressing mobile data traffic offloading [30, 31], to those dealing with network coverage extension [32], or with content sharing in neighborhood areas [33, 34]. This new enabling technology was introduced in the 3 GPP Release-11 and further details have been defined as a technical work in 3 GPP Release-12 under the name of Proximity Services (ProSe) [35].  
A good taxonomy of D2D communications is given in [29], where a first distinction is made based on the spectrum adopted for D2D communications. This can be either cellular licensed spectrum, like for cellular communications (_i.e._, inband communication), or unlicensed bands such as Wi-Fi (_i.e._, outband communication). The inband solution, can be further classified in (i) underlay inband D2D mode and (ii) overlay inband D2D mode. In the former, D2D and cellular communications share the same licensed cellular spectrum, whereas in the latter, a portion of the cellular resources is dedicated to D2D communications for avoiding interference problems. The outband solution aims to eliminate the interference between D2D and cellular link, but needs extra interfaces such as Wi-Fi Direct or Bluetooth. Nevertheless, the research community is focusing on the inband LTE-Direct standardization (expected in the next Release-13 of the 3 GPP), whereas WiFi-Direct is at the moment the \"standard-de-facto\" due to the many prototypes already available [36, 37].  
Browsing the research contributions on D2D communications, most of the work focuses on single-hop downlink services [15], whereas two-hop communications (like the one proposed in this paper), have been investigated in a very few recent papers. Specifically, in [38] a network-assisted multihop D2D communication is addressed with an analysis on D2D power control and mode selection. Similarly, a multihop D2D communication scheme is proposed in [39] for end-to-end connectivity in M2M communication. Cooperative D2D communications and green cellular communications are investigated in [40], where mobile terminals are grouped together into cooperative clusters and proximity-based transmissions are used to share content of common interest in an LTE network. In [41]instead, D2D communications are addressed in the view of adopting a cooperative approach for energy efficiency management in LTE-A public safety networks. Finally, the role that D2D communications can play in the IoT is presented in [42]. In [43] instead, the ability for devices to communicate and collaborate with each other autonomously, without any centralized control, is underlined by focusing on how state-of-the-art routing algorithms can achieve multihop D2D communication in the IoT. As discussed in [42], the use of D2D and its potential benefits may support data aggregation, where small data from several objects are aggregated locally before sending it to the final destination. However, such a promising solutions has received little attention in the literature so far.