The evaporation duct (ED) is generated by the evaporation of seawater and can be an influential factor of electromagnetic (EM)-wave propagation. Rainfall also affects atmospheric factors and EM-wave propagation. However, the distribution of the ED and path loss (PL) during rainfall has rarely been reported. This paper analyzes the distribution of the atmospheric factors and ED in the South China Sea (SCS). The results show that the evaporation duct height (EDH) in the area of rainfall is generally lower. The effect of the ED on the over-the-horizon (OTH) propagation reaches 0.69 dB km⁻¹ on average, which is 4.3 times stronger than the maximum rain attenuation (0.16 dB km⁻¹) when the rainfall is less than 5 mm h⁻¹. In the SCS, a 53 km long OTH link was established between Donghai Island and Jizhao Bay to observe the PL. The measurement results show that the nearly saturated relative humidity (RH) leads to a high PL. The results also show that the change in the direction of the sea–land breeze causes a 42.4 dB decrease of PL by transferring the moist patches. Rainfall has an attenuation effect on OTH propagation in ED, mainly owing to the high RH.

In this paper, by allocating the limited subchannel resources based on interference alignment (IA) with clustering to mitigate the severe co-tier interference in dense small cell networks (SCNs), we aim at maximizing the number of data streams achieved by all small cell user equipments (SUEs) while guaranteeing the data stream requirement for each SUE. As the corresponding optimization problem is NP-hard, we propose a two-phases algorithm based on graph theory to obtain a suboptimal but efficient solution, which requires much lower complexity and notably reduced feedback overhead. In the first phase, all SBS-SUE pairs are grouped into disjoint clusters by the recursive partition of the constructed graph, where the edge weight is only determined by the path losses from small cell base stations (SBSs) to SUEs, so the estimation of perfect global channel state information (CSI) is avoided. Furthermore, when guaranteeing that IA is feasible within each cluster, the number of formed clusters and the size of each cluster are determined by the distributions of dense SCNs instead of being specified. In the second phase, by further treating each cluster as a new vertex, graph coloring algorithm is proposed for subchannel allocation, which only requires to estimate the perfect CSI within each cluster. The analysis of computational complexity demonstrates that the proposed two-phases algorithm has a much lower complexity compared with the optimal solution obtained by the exhaustive search. Numerical results show that the proposed solution outperforms other related schemes and exhibits a performance close to the optimal solution.

Evaporation duct is formed over the sea surface owing to the evaporation of seawater, and the increase in the evaporation duct height (EDH) will lead to the sharp decrease in the profiles of atmospheric modified refractivity near sea surface. Evaporation duct reduces the path loss (PL) of electromagnetic (EM) wave propagation, which has a significant effect on the performance of radar and communication systems. The trapping ability of evaporation duct fluctuates greatly within a single day; however, the effect of the diurnal variation of evaporation duct with measured PLs has seldom been investigated. In this paper, the diurnal variations of measured PLs of three over-the-horizon links are recorded by the developed EM propagation systems for two days in the northern part of the South China Sea. Combined with the data of meteorological observation and reanalysis results, the diurnal variation of evaporation duct is investigated through numerical simulations. The results demonstrate that the measured PLs exhibit strong diurnal variation with a fluctuation about 50 dB. Besides, the EDH shows a good correlation with the variation of PLs: in one day, during 06:00-08:00, the measured PLs are high, and the EDH is low; during 17:00-19:00, the measured PLs are low, and the EDH is high.

With the rapid development of vibration and noise reduction technologies, underwater target detection is facing great challenges. Particularly, the task of high-resolution direction of arrival (DOA) estimation with sonar array is becoming more and more tough. In recent years, unmanned underwater vehicles (UUVs) have been developed considerably, with the improvements of target localization performance in terms of adaptability, detection range, operation efficiency, and anti-interference ability. Nevertheless, in general, the size of UUV is small such that current passive sonar systems usually have relatively limited localization accuracy, detection distance, and environmental robustness in complex ocean noise. This motivates us to present a new approach to construct a large-aperture virtual array with multiple small-aperture arrays of unmanned underwater vehicle swarm (UUVS) which consists of multiple UUVs in this paper. However, for the UUVS array, the received data could suffer from unobserved and corrupted samples. This makes it challenging to analyze and process large-aperture array data. Towards this end, the matrix completion technique is employed to recover the unobserved and corrupted data for virtual array construction based on the low rank property of array data matrix. The recovered matrix is then exploited for underwater target bearing estimation using the traditional DOA estimation approach. Numerical results verify that the proposed method is capable of detecting underwater targets with high precision and resolution.

An evaporation duct is a persistent phenomenon that is formed by evaporation over the sea surface, and influences marine communication and radar detection through its trapping effects. A cold wave is cold air that rapidly intensifies and intrudes a region with strong winds, a dry air mass and precipitation, which affects the air-sea interactions and eventually affects the propagation of electromagnetic waves over the sea. However, there are few studies of over-the-horizon propagation in evaporation duct during a cold wave. In this paper, the path loss and atmospheric factors of two over-the-horizon propagation links over the Bohai Strait and Yellow Sea were observed during a cold wave in November 2021. The cold wave caused path loss to decrease over 35.9 dB and 44.2 dB respectively at links over the Bohai Strait and Yellow Sea, as the evaporation duct height simulated from the measured data increased 5.9 m and 7.6 m. According to the sensitivity analysis, high wind speed was the main factor promoting seawater evaporation during the cold wave, while a decrease in air temperature had the opposite effect. These effects of cold wave should be taken into consideration in marine communication and radar detection systems to enable the optimal use.

This paper investigates subchannel allocation based on clustered interference alignment in dense small cell networks when all small cell user equipments(SUEs) have differentiated requirements for data streams. By imposing the condition that each cluster has a size not exceeding the maximum value achieved when each SUE needs only one data stream, we maximize the number of SUEs with guaranteed requirements for data streams, which is NP-hard. Hence, we propose a two-phase efficient solution with much lower complexity and reduced feedback overhead. First, similarity clustering is performed by graph partition, and then, subchannel allocation is done through a coloring algorithm. Numerical results show that the proposed solution offers a performance better than the related schemes and close to the approximate optimal solution.