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Physical layer security is an attractive security mechanism, which exploits the randomness characteristics of wireless transmission channel to achieve security. However, it is hampered by the limitation of the channel condition that the main channel must be better than the eavesdropper channel. To alleviate the limitation, cooperative communication is introduced. Few studies have investigated the physical layer security of the relay transmission model. In this paper, we performed some experiments to evaluate the physical layer security of a cooperative communication system, with a relay operating in decode-and-forward (DF) cooperative mode, selfish and malicious behavior in real non-ideal transmission environment. Security performance is evaluated in terms of the probability of non-zero secrecy capacity. Experiments showed some different results compared to theoretical simulation: (1) to achieve the maximum secrecy capacity, the optimal relay power according to the experiments result is larger than that of ideal theoretical results under both cooperative and selfish behavior relay; (2) the relay in malicious behavior who forwards noise to deteriorate the main channel may deteriorate the eavesdropper channel more seriously than the main channel; (3) the optimal relay positions under cooperative and selfish behavior relay cases are both located near the destination because of non-ideal transmission.

Underwater acoustic (UWA) communication has been developing rapidly over the past decades for its crucial position in resource exploration, environmental monitoring, and scientific research. However, the transmission data rate of UWA communication is limited by the narrow bandwidth of the underwater acoustic channel. Here, the generation of quadrature acoustic frequency combs (AFCs) is first reported. Massive parallel channels achieved with multiplexing of AFCs for UWA communication are demonstrated. The generated AFCs have unique characteristics which have a stable and precise division in frequency and time domain simultaneously with carrier spacing of 1 Hz with the stability of 10 −8 . The different frequency spacing in combs leads to different teeth spacings in time, which is orthogonal and separable in the time domain. The orthogonality among AFCs provides a new dimension for multiplexing and can drastically increase channel efficiency. The underwater acoustic communication experiments demonstrate that it is drastically increasing the transmission rate to 45.33 kb s −1 and high spectral efficiency of 1.51 (channel s −1 ) Hz −1 without utilizing other modulation techniques by a single transducer. Since AFCs multiplexing is a completely independent degree of freedom that can be readily integrated with other high order modulation techniques, such as quadrature amplitude modulation and phase-shift keying, in a single channel, AFCs multiplexing opens a new dimension for acoustic communication.

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