The Internet of Underwater Things (IoUT) technology has been gaining importance in smart ocean investigation in recent years. The integrated system for underwater detection and communication (ISUDC) is considered a crucial technique for IoUT due to its potential to decrease equipment size and power consumption. The linear frequency modulation (LFM) signal has the characteristics of large time-bandwidth product and has great advantages in detection. However, it has high complexity and low robustness of communication in ISUDC. To address this issue, this article proposes a sector-modulated LFM (SM-LFM) signal for ISUDC. In terms of communication, it divides Fresnel domain into equal length sectors. Then, data is loaded by activating different sector. At the communication receiver, by establishing a relationship between the Dirac function in the Fresnel domain and the channel impulse response (CIR) in the time domain, data is demodulated by finding CIR’s main path without channel estimation or equalization. In terms of detection, SM-LFM has an improvement in the maximum unambiguous velocity by calculating relative radial velocity in the Fresnel domain instead of the frequency domain. Numerical simulations and lake experiments have confirmed that the SM-LFM signal can deliver superior robustness and easy-to-use advantages for underwater active detection and communication.

In underwater Internet of Things (IoT) applications, efficient and reliable transmission is demanding. Recently, the non-orthogonal multicarrier (MC) modulation is utilized as a promising spectrum-efficient solution in the UWA channel, where generally, the constellation mapping (CM) methods (such as 8-PSK and 16-QAM) are utilized to further increase the transmission rate. In this paper, we focus on the transmission of sonar images among IoT nodes and a constellation diagram learning-based adaptive sparse non-orthogonal wavelet division multiplexing (CDLA-SNOWDM) system is proposed to combat complex channel states between IoT nodes. We found that different CM methods contribute to unique time-frequency characteristics in constellation diagrams. We first utilize the time-frequency characteristics in designing the non-orthogonal subcarriers. The complex-valued CM signals are regarded as a new type of image. We introduce dictionary learning (DL) to non-orthogonal subcarrier designing, with the idea that designing CM signal adaptive non-orthogonal subcarriers can be regarded as designing an adaptive subcarrier dictionary. The CDLA-SNOWDM modulation is performed as a projection of the CM image to the subcarriers with an adaptive dictionary. The learned adaptive subcarriers can represent constellation diagrams more efficiently and improve transmission reliability. Both simulations and experiments show that the proposed scheme improved the reliability in transmitting sonar images over other orthogonal and non-orthogonal modulation schemes in UWA scenarios.

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.