The sixth-generation communication networks aim to integrate space, air, and ground communications into a robust network. However, maritime communications are confronted with the challenge of the atmospheric duct, which could capture wireless signals and propagate beyond the line of sight. In this paper, a parabolic equation-based modeling approach is proposed to characterize the maritime evaporation duct channel. The constant coefficients of each variable in the modeling are calculated by the least square method, and the specific distribution of shadow fading in channel modeling is determined by the Kullback-Leibler divergence method. Furthermore, the partitioning around medoids method is utilized to reduce the average deviation and improve the accuracy of the model. Numerical results illustrate that the statistical channel model possesses satisfactory calculation ability at the distance of signal interference caused by the evaporation duct. In addition, the paper analyzed the influence of the antenna height and downtilt angle on signal propagation.

The Reconfigurable Intelligent Surface (RIS) is seen as one of the most prospective technologies for next-generation networks. RIS can form virtual line-of-sight (LoS) link during non-line-of-sight (NLoS) transmission to improve system performance with low power consumption, especially for urban scenarios. In this paper, for an RIS-aided system, we extend the two-dimensional path-loss model to a more practical three-dimensional path-loss model. We further compare the system performance differences among RIS, relay and single-input single-output (SISO) systems. We also reveal proper deployment positions of RIS and derive the number of reflecting elements required under different constraints. Numerical results verify the complementarity between RIS and decode-and-forward (DF) relay. The performance of RIS-aided communication system can be significantly improved by optimizing RIS deployment locations and the number of reflecting elements.

This paper proposes solutions to channel prediction with atmospheric duct based on feedforward neural network (FNN) modeling. Specifically, FNN-based model is to produce accurate prediction by directly learning from large database rather than depending on any assumption. The prediction accuracy of the model applied to Sub-6 GHz and 28 GHz bands attain 88.72% and 94.77%, respectively. Besides, the paper also validates the difference of prediction performance of networks by comparisons of artificial neural network (ANN) and FNN-based networks. The results show that when the bands get higher, FNN-based framework would enhance the prediction accuracy while ANN-based framework would bring it down. It is demonstrated that the proposed FNN-based network obtain accuracy gain over 30% than ANN-based framework.

Remote interference, that the downlink signals from a remote base station (BS) propagate beyond guard period (GP) in the atmospheric duct layer and interfere with the local uplink signals, seriously degrades TDD-based system performance. And remote interference management (RIM) will be standardized in 3rd Generation Partnership Project (3GPP) Release 16. Mitigating the impact of remote BS will be one of the most challenging issues. In this paper, we establish a remote interference simulation system by combining refractivity estimation and parabolic equation (PE) methods. This novel simulation system includes dynamic meteorology measurement design, refractivity estimation, communication setting, and path loss computing. It also provides a traceable approach to remote interference. Moreover we also proved the minimum BS separation distance reference (in 3GPP Release 12) is far from satisfied when atmospheric duct happens. Long-term meteorological statistics and antenna parameters are discussed for mitigating remote interference. The traceable approach can be helpful for RIM and the next generation network deployment planning.

In this letter, we investigate the ergodic capacity of the Relay-Reconfigurable Intelligent Surface (Relay-RIS) system exploiting the three-dimensional channel model, which is different from existing works ignoring antenna height. Specifically, we propose a novel three-dimensional statistical channel for the Relay-RIS system. The upper bound expressions of the ergodic capacity are deduced in diverse propagation environments. Meanwhile, the impact of channel condition and deployment location on the capacity improvement is quantified, and the minimum number of reflecting elements to achieve a given rate threshold is obtained. Simulation results verify the effectiveness of the Relay-RIS system and the three-dimensional deployment strategy.

To realize the 6G maritime broadband communications, it is essential to address the challenge of limited coverage caused by the unique characteristics of maritime environment. Fortunately, as a prevalent maritime atmospheric layer with super refraction, evaporation ducts present an opportunity for extending long-distance coverage and enhancing the quality of maritime communications. In this letter, we propose a maritime three-ray model for path loss prediction in evaporation ducts. To accurately mimic the maritime wireless environment, this model incorporates the round earth geometric model, the equivalent third-ray model to account for radio wave propagation in evaporation ducts, the effective reflection coefficient to account for the effects of rough sea surface, the fading coefficient due to wave fluctuations, and the divergence factor due to the earth curvature. We utilize a shore-to-ship wireless measurement data with synchronous meteorological data to verify the accuracy of the proposed model. The range-dependent results at 12 frequency points from 5.25 GHz to 12.74 GHz, show excellent agreement with the measured path loss data both within and beyond line-of-sight distances, implying that the proposed model has better prediction performance than the latest two-ray maritime model.