## 1 Introduction  
Wireless sensor networks (WSNs) have been used in various fields including environment monitoring, home automation, healthcare, agriculture, military, smart grids, and smart cars [1, 2]. In these applications, sensors are equipped with wireless radio interfaces in order to form a wireless network and communicate with other sensors or a data aggregator. WSN will also play an important role to open the early Internet of Things (IoT) market [1, 3]. Some technical barriers, which should be overcome to use WSN as a network infrastructure of the early IoT market, still remain open issues. A representative open issue is the autonomous communications among sensors. It is inefficient for a central node to control all sensors in WSN. Thus device-to-device (D2D) communication is considered as a rising technology in WSNs to solve this problem [4; 5]. In D2D communications, sensor nodes or devices are able to communicate with each other through autonomous manner. D2D communication enables sensor nodes in close proximity to establish a direct link with each other as opposed to being routed by a controller or central node. Recently, D2D communication technologies have been actively studied in oneM2M standard for IoT [6; 7] and 3 rd Generation Partnership Project (3 GPP) standard for Long Term Evolution Advanced (LTE-A) systems [8; 9]. Another open issue of WSN to boost IoT is the network coverage extension and interworking with heterogeneous networks. The cellular networks can be an excellent candidate to overcome this problem. The cellular networks usually cover the majority of countries and interoperate with other networks. Thus, there is a growing trend to interwork WSNs with cellular networks. For these backgrounds, this paper considers D2D communication technologies for wireless sensor networks. In particular, D2D communication technologies underlying LTE-A systems are investigated to apply wireless sensor networks to IoT [8; 10]. D2D communication underlying LTE-A systems can be defined as a technology enabling direct communication between proximal sensor nodes or user equipment (UE). In D2D communication, sensor nodes do not need to pass through cellular infrastructural nodes such as an enhanced node-B (eNB) or mobility management entity (MME) to setup a D2D communication link. In the beginning of D2D study, integrating D2D in an LTE-A network was strongly supported by Qualcomm, which previously developed FlashLinQ. FlashLinQ is a proprietary technology which allows cellular devices to discover each other automatically and communicate with thousands of other FlashLinQ enabled devices within 1 km range [4; 11]. To avoid the loss of network throughput in D2D communication underlying LTE-A systems, the licensed spectrum of cellular networks should be shared with D2D communications. Unfortunately, the interference among cellular user equipment (CUE) and D2D user equipment (DUE) is inevitable. Because of this, a considerable amount of research on the interference management has been conducted with regard to D2D communications being laid into LTE-A cellular networks where CUEs and DUEs share the same resources. In [12], the authors proposed a radio resource allocation scheme for D2D communication underlying cellular networks using strict fractional frequency reuse (FFR) to reduce the inter-cell interference of a DUE on receiving mode (DUE-Rx) at the cell edge area. In [13], the authors proposed the modified FFR to improve the coverage of CUEs and DUEs in cell edge areas. In this scheme, different resources were allocated to DUEs based on their location. [PERSON] et al. proposed a resource allocation scheme in which dedicated frequency resource blocks (FRBs) are assigned to D2D links to avoid interference [14]. In [15], an intelligent power control mechanism has been proposed. It is based on the soft FFR that allocates radio resources to CUEs and DUEs with variable ratios. The drawback of prior systems, which are based on strict FFR, is the inefficient use of radio resources and the decrease of throughput.