# Improving RF Fingerprinting Methods by Means of D2D Communication Protocol
## 3 Material and Methods
### D2D Communication Aspects  
D2D communication has been specified in 3 GPP LTE Release 12, that enables direct communication between nearby devices to handle particular applications such as proximity services (ProSe), content sharing, multi-party gaming, etc [39]. It is an exciting and innovative feature of next-generation cellular networks. By exploiting D2D communication, it is possible to enhance throughput, spectrum utilization and energy efficiency of the cellular network. To address future network challenges of dynamic environment adaptability, and productive use of available resources in 5G, D2D communication is considered as a potential tool. Concerning spectrum usage, D2D communication is primarily classified into two types, e.g., inband and outband. In inband D2D communication, cellular communication and D2D communication use the same spectrum licensed to the network operator, whereas outband D2D communication uses unlicensed spectrum (e.g., the free 2.4 GHz ISM band) where cellular communication does not occur [39]. Use of outband eliminates the  
Figure 1: Diagram of matching-based RF fingerprint positioning system with examples of the radio map, operational fingerprint and estimated position. interference between D2D communication and network users. In terms of control, subband technology is divided into two categories, controlled and autonomous. In control mode, the radio interface is controlled by the cellular network while in autonomous mode, control of D2D communication belongs to the end users. The basic architecture of the 3 GPP D2D communication architecture is shown in Figure 2.  
A device that wants to establish D2D communication needs to contact the ProSe function through PC3 logical interface to obtain permission. After the authorization completed, the device can start the discovery process. From a user's perspective, there are two types of peer discovery techniques, Evolved Packet Core (EPC)-level discovery and direct discovery. At direct discovery, a device would search other devices with ProSe capability in their proximity using the PC5 interface. When two (or more) ProSe-enabled devices have discovered each other, they can start direct communication over the direct link between them. From a network perspective, peer discovery can be controlled hardly or softly by the base station [40]. The interface between the EPC and the ProSe function is called PC4. Generally, a D2D link connects a device to another following in a single-hop communication. A multi-hop network composed of D2D connections is also possible. In a multihop D2D network, the central devices act as relays between two devices. In 5G, cell size will be smaller, and the density of the devices will be higher, 150 devices per cell are assumed in 3 GPP for evaluating D2D discovery [41]. As a result, less inter-device distance is expected. Small inter-device distance increases the probability of Line of Sight (LOS) communication among devices that results in a better TOA-based ranging accuracy [42]. Without any relay, the effective D2D communication range is expected to be around 30 m [43]. Due to the distributed structure of control and mobile devices being computationally poor to apply complex encryption mechanisms and other security technologies, D2D communication can be vulnerable to malicious attacks. [PERSON] [44] mentioned three major security and privacy problems, these are ubiquitous data collection, unexpected uses of data, and heightened security risks. Many researchers [45; 46] have studied RF fingerprint based wireless security system. [PERSON] et al. [47] proposed a D2D communication security authentication process based on RF fingerprint identification. The advantage of using fingerprint for security purpose is that fingerprint computes the inherent hardware differences of the transmitter for identification, so this method cannot be completely copied. Their simulation results show that when Signal to Noise Ratio (SNR) is higher than 8 dB, the device recognition rate is 100%.