## 1 Introduction  
When it comes to the secure transmission of information, cryptographic algorithms are traditionally employed as the security technology to achieve information confidentiality [1]. Information encryption usually involves the use of a key to encrypt and decrypt the messages. Unlike conventional wired communication that uses wired media (optical cable, copper, etc.) for data transmission, the broadcast nature of wireless channels allows an illegal receiver to wiretap legal messages easily during the transmission from the source to the destination. Moreover, with the development of cryptography, new algorithms and methods to decipher traditionally encrypted messages have been developed, which means old encryption methods are faced with higher risk of being cracked. Cryptographic methods face numerous risks due to the rapid development of the computing capabilities of eavesdroppers [2]. Besides, complex cryptographic algorithms require devices capable of dealing with heavy computing loads, leading to high power and resource costs. The physical layer security adopts the physical characteristics of wireless channels to guarantee its security through coding and signal processing, in which confidential message can be decoded only by their legitimate receiver and avoid complex computing. In this way, physical layer security has been proposed to safeguard wireless transmission, for instance, 5G wireless communication networks and heterogeneous cellular networks [3, 4]. Physical layer security was first introduced by [PERSON] [5], who adapted [PERSON]'s concepts of entropy and equivocation [6] to investigate the physical layer security. The achieved maximum transmission rate of confidential message communication is defined by [PERSON] as the secrecy capacity. However, the feasibility of physical layer security is hampered by the limitation when the channel between the source and the destination (the main channel) is worse than that between the source and the eavesdropper (the wiretap channel). An effective way to improve the main channel condition is cooperative communication with a relay's help [7]. Cooperative relays can enhance the secrecy capacity of wireless communications by exploiting decode and forward (DF), amplify and forward (AF) [8]. In the DF and AF cooperative strategies, the relay utilizes its overheard information to reinforce the transmitting signal of the source [9, 10, 11]. In [12, 13, 14], the physical layer security is improved by the advantages of large scale MIMO arrays of relays with AF and DF strategies. In [15, 16], the optimal transmission with maximized secrecy rate under energy harvesting requirements is investigated, where the secrecy capacity is sensitive to the position among the nodes. It was found that the distance from the source to the relay has an important influence on secrecy capacity. Therefore, to find the optimal relay location is an important requirement to achieve maximum secrecy capacity [17]. In this paper, we investigated a cooperative communication system with one source, one relay, one destination and one eavesdropper with experiments for further discussion. In practical applications, relays may not always select a cooperative strategy because of its own benefits. Some research has investigated the secrecy capacity with untrusted relays. In [18, 19] the authors discussed the case of untrusted relays with both a cooperator and an eavesdropper. The secrecy capacity in a selfish behavior relay scheme in which the relay wants to save its own resources [20, 21] was also discussed. Theoretical analysis usually guides the real implementation with simplified or ideal physical factors, while applying them in practice may lead to different results. The authors in [22] firstly analyzed the physical layer security with the probability of a non-zero secrecy capacity (\(P(C_{S}>0)\)) of a wiretap channel by experimental measurements. It was found that the shadowing deviation can influence the security, which was seldom considered by theoretical analysis.