Underwater acoustic signal denoising is crucial for enhancing the reliability of underwater communication and monitoring systems by mitigating the effects of noise and improving signal clarity. The complex and dynamic nature of underwater environments presents unique challenges that make effective denoising essential for accurate data interpretation and system performance. This paper comprehensively reviews recent advances in underwater acoustic signal denoising, focusing on its critical role in improving these systems. The review begins by addressing the fundamental challenges in underwater acoustic signal processing, such as signal attenuation, noise variability, and environmental impacts. It then categorizes and analyzes various denoising algorithms, including conventional, decomposition-based, and learning-based approaches, discussing their applications, strengths, and limitations. Additionally, the paper reviews evaluation metrics and experimental datasets used in the field. The conclusion highlights key open questions and suggests future research directions, emphasizing the development of more adaptive and robust denoising techniques for dynamic underwater environments.

This paper investigates secure hybrid beamforming with subarray architectures in a millimeter-wave (mmWave) integrated sensing and communication (ISAC) system. To achieve secure communication against a potential eavesdropper, we design the sensing signal in a dual-functional form, serving both to detect the target and to generate artificial noise (AN) that disrupts the eavesdropper. With fixed subarray architectures, the security and sensing performance are analyzed, including the impact of imperfect channel state information (CSI). Moreover, the design of analog and digital beamformers is formulated as the problem of minimizing transmit power while satisfying the constraints of sensing beampattern and secrecy rate. To solve the non-convex problem, semi-definite relaxation (SDR) and successive convex approximation (SCA) techniques are employed to transform the original problem and obtain the solution. For the scenario with imperfect CSI, the robust hybrid beamforming is designed based on S-procedure. Additionally, to optimally utilize spatial domain resources provided by multiple antennas, a dynamic subarray architecture is introduced, along with two dynamic subarray partitioning algorithms. This flexible architecture can be dynamically adjusted according to the CSI, ensuring a bidirectional alignment with the actual channels and the proposed scheme. Specifically, the greedy algorithm and the secrecy rate increment maximization (SRIM) algorithm are designed to partition the dynamic subarray, and the alternating optimization is utilized to further improve the quality of the solution. Numerical results demonstrate the effectiveness of the proposed schemes.

In this paper, we propose a physical layer secure transmission technology based on multi-components for multiple-input single-output (MISO) systems. The scheme mainly realizes physical layer security (PLS) in General Multi-Fractional Fourier Transform (GMFRFT) signal domain, which is different from traditional schemes that rely on the spatial domain. Different GMFRFT components are transmitted by multiple antennas at the transmitter, then the legitimate receiver and the eavesdropper receive the superposition of multiple non-orthogonal components. The resulting mutual interference will reduce the signal-to-interference and noise (SINR) at the eavesdropper, but without affecting the legitimate receiver. Because the legitimate receiver receives the non-contaminated GMFRFT signal due to designing multi-components at the transmitter, while the eavesdropper receives the signal whose constraint relations in the GMFRFT signal domain are destroyed, which will cause the energy loss of the information bearing signal. Furthermore, in order to achieve the maximum secrecy capacity, the selection method of GMFRFT transform order is given to adjust the power allocation among GMFRFT components. Compared with the scheme based on artificial noise (AN), the advantages of our scheme are: 1) our scheme can further reduce the capacity of wiretap channel while not requiring the transmitter to use partial power to transmit meaningless artificial noise signals; 2) our scheme outperforms AN-based schemes when the available spatial degrees of freedom are limited. Simulation results are provided to demonstrate the secrecy performance of the proposed scheme.

In this paper, we investigate secure communication for the downlink rate-splitting multiple access (RSMA) system by integrating both the spatial domain and weighted fractional Fourier transform (WFRFT) signal domain to enhance its physical layer security (PLS) performance. To counter the external passive eavesdropper, we propose an RSMA-CM-WFRFT scheme to promote the dual use of the common message (CM), thereby improving the secrecy rate of both common and private messages. Specifically, the common message is encoded based on WFRFT, enabling all received power of legitimate users for signal reconstruction, while introducing inter-component interference at the eavesdropper. Furthermore, the inevitable serial interference caused by the WFRFT-encoded CM sequence overlays onto the private message (PM), thus enhancing its security. To counter internal eavesdroppers, we propose an RSMA-PM-WFRFT scheme to achieve parallel encryption by encoding the private message of different users into different WFRFT domains. Additionally, we formulate and solve a non-convex optimization problem aimed at minimizing the required power under both security and reliability constraints. Numerical results demonstrate the superior performance of the two proposed schemes.

In this paper, we investigate hybrid precoder design to enhance the physical layer security of the millimeter-wave (mmWave) system with multiple eavesdroppers. Differing from traditional spatial domain-based schemes, we introduce an extra signal domain dimension and propose a multi-component-based security mechanism for the hybrid precoding system model. The model integrates both signal and spatial domains for designing baseband and analog precoders, ensuring that the received signal at the legitimate user complies specific signal domain characteristics. Consequently, all received energy becomes available for signal reconstruction, whereas the distorted signal at eavesdroppers inevitably forms inter-component interference. By introducing general multi-fractional Fourier transform (GMFRFT) signal domain into the model, we propose a specific GMFRFT-based scheme for subarray architectures. The secrecy performance is analyzed, and power allocation methods between multiple components are designed. Additionally, to optimize the utilization of spatial domain resources, we propose a GMFRFT-based scheme with dynamic subarrays, along with a low-complexity subarray partitioning algorithm. Furthermore, a GMFRFT-based scheme for fully connected architectures is investigated. It is verified to be equivalent to a lower-complexity weighted fractional Fourier transform (WFRFT)-based analog beamforming scheme, the efficacy of which hinges on the adherent secrecy provided by WFRFT signals. Both theoretical analysis and numerical results demonstrate the effectiveness of the proposed schemes.