# Secure Transmission for Buffer-Aided Relay Networks in the Internet of Things
## 1 Introduction  
The Internet of things (IoT) serves a crucial architecture in future wireless communication systems, which can connect all things (e.g., mobile devices, sensors, and vehicles) to the Internet and enable these physical devices with sensorial and computing capabilities to cooperate with each other and achieve common goals [1; 2; 3]. Numerous fields such as industry, medical and transportation are expected to deploy IoT applications widely [4]. Moreover, due to the resource constraints of IoT devices (e.g., energy and computing capability), relay transmission is seen as a promising solution to solve the problem above in IoT networks, which has attracted great research interest [5; 6; 7]. Specifically, in [6], the unmanned aerial vehicle (UAV) was considered as the relay node firstly, and then the outage probability and throughput was investigated in the UAV relay assisted IoT networks enhanced with energy harvesting. In [7], [PERSON] et al. considered both half-duplex (HD) and full-duplex (FD) decode-and-forward (DF) relaying schemes in multi-hop IoT networks, whose operating mode was similar to the one in [8], and studied the outage probability of the system with randomly located interferers. However, since the best link may be not available, the relay has to follow the fixed transmission strategy to transmit the data packet [9]. That is to say, the selected relay receives the data packets from the source node in the first hop and then forwards it to the destination node immediately in the second hop. Recently, equipping data buffer at the relays has drawn considerable attentions due to its ability of offering high performance gains and extra degrees of freedom, which is called \"buffer-aided relay\" [10; 11; 12; 13]. In [12], [PERSON] et al. proposed the max-max relay selection (MMRS) scheme forDF relay networks. In [13], the max-link relay selection scheme was proposed, which could achieve better performance than MMRS by selecting the best link among all the available links. Nowadays, several works have considered applying the buffer-aided relay to IoT for increasing the reliability of communication networks [14; 15; 16]. The buffer-aided successive relay selection scheme for energy harvesting IoT networks based on DF and amplify-and-forward (AF) relay is investigated in [15]. In [16], a novel prioritization-based buffer-aided relay selection scheme was proposed, which can seamlessly combine both non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) transmission in the IoT. On the other hand, the broadcast characteristics of the wireless channels makes the wireless networks vulnerable to malicious attacks by illegitimate nodes, which presents a new challenge for the security of data transmission [17; 18]. The encryption technique employed at the upper layer is a traditional method against eavesdropping [19]. However, this traditional technique not only imposes extra computational complexity resulted from the secret key management but can also be easily decrypted with the rapid improvement of the calculation level and thus being inappropriate to provide security services for IoT networks especially. Alternatively, physical layer security has been proposed as an effective approach to prevent the eavesdroppers from intercepting the information transmission by exploiting the randomness nature of the wireless channels [20; 21; 22]. Inspired by this, lots of research efforts have focused on the security of IoT networks from a physical layer security perspective [23; 24; 25; 26]. In [23], the secrecy outage performance was studied for the wireless communication in IoT under eavesdropper collusion. In [24], three different scheduling schemes were designed to perform secure communication in an untrusted-relay-aided IoT uplink transmission. An on-off based multiuser secure transmission scheme was proposed for the heterogeneous IoT downlink networks in [25]. Then, the authors optimized several parameters to maximize the network secrecy throughput. Additionally, [PERSON] et al.