# Experimental Validation of Peer-to-Peer Distributed Voltage Control System
## 5 Device-to-Device Communication  
### Background  
Device-to-device communications typically refer to cellular communications technologies enabling direct transmission between proximate devices, without relaying information through the cellular base station [31]. However, D2D communications is not the exclusive domain of cellular networks and generally relates to the ability of peer devices to directly communicate with one another without having to relay the actual data through a central coordinator device, as e.g., used in [32; 33]. The paper [31] presents a survey of the current state of the art for cellular D2D communications and points out that cellular D2D communications are much more efficient than communications on unlicensed spectrum as the communication interference is controllable at the licensed spectrum. The paper categorizes cellular D2D communications into four categories based on the level of control the base station has on them. The first category is device relaying with base station assisted controlled link. Here the base station allocates the channel resources for user equipment communications so that user equipment in poor coverage can maintain connectivity with the network. Direct communication between devices with base station assisted controlled link is the second category, where user equipment exchange data directly and some of these features have already been standardized by 3 rd Generation Partnership Project (3 GPP) release 13 proximity services and in release 14 for cellular vehicle-to-everything (C-V2X) communications (mode 3). The C-V2X uses outband communications as the actual data communications occur at the intelligent transportation system licensed radio frequency band and not on the cellular bands. The third category is relaying device with device assisted controlled link, where the user equipment communicate with one another using relays and without base station control. The fourth category is direct D2D with device assisted controlled link, where the user equipment communicate directly with one another without base station provision of control links. The paper [34] proposes a solution combining categories three and four for smart grid demand response scenarios for increased resiliency of smart grid operations. The D2D communications required for P2P voltage control need not be based on cellular technologies and currently, no commercial-of-the-shelf cellular D2D chipsets are available. The key criteria for the selection of appropriate communication technology to adopt arise from the distances and placement of the observer and actuator agents. Common unlicensed band communication technologies like the IEEE 802.11 family (WiFi) or the IEEE 802.15.4 family (low rate personal area network) can be utilized if the distances between agents are within a few hundred meters and they have been installed outdoors or inside buildings near the exterior walls. Even then mesh type network where devices communicate in ad hoc fashion are required to ensure reliable connectivity. In other cases low power wide area (LPWA) communication technologies need to be utilized. [PERSON] et al., [35] provide a survey on LPWA networks and claim that they represent a novel communication paradigm, which will complement traditional cellular and short range wireless technologies in addressing diverse requirements of Internet of Things (IoT) applications. This applies to smart grids in particular as LPWA technologies offer unique sets of features including wide-area connectivity for low power and low data rate devices, not provided by legacy wireless technologies. As an example, [33] proposes a gateway assisted D2D communications solution utilizing Long Range Wide-area network (LoRaWAN)technology and the work is a basis for the D2D communications scheme used in this paper.