The Development of Novel Marine Communications Channel Models Using Theoretically-Based and Numerical Electromagnetic Simulation Methods.

摘要

Maritime communications is fast becoming a growing area of interest. Use of a "commercial-off-the-shelf" (COTS) integration approach to system design, increasing interest in maritime security and demanding bandwidth requirements of sensors make understanding the effects of the sea on the communications channel an important design consideration when developing reliable and high bandwidth communications links. Conventional VHF communications are being replaced with SATCOM and cellular technologies for a variety of vehicular, sensor, life craft, and survival suit systems. Considering this occurrence, the marine communications channel and the effects of the sea surface have remained an area of limited study, particularly in comparison to the efforts placed on research for terrestrially-based communications channels. Urban environments, mountainous terrain, seasonal issues, and foliage are well studied in regard to effects on communications channels. To support design of systems for marine applications, the contribution of this research effort is the development of communications channel models by novel theoretical and numerical methodologies. The results of these efforts are models suitable for use in quantifying sea surface shadowing effects on communication channel performance in fully developed deep sea locations.;Second, a marine communications channel modeling methodology is developed using transient electromagnetic simulation methods to simulate overwater propagation of VHF to 3 GHz signals above a realistic fully developed random deep sea surface. The field of computational electromagnetics is focused upon use of numerical methods to obtain solutions to Maxwell's equations for problems not addressed easily analytically, or for which no analytic solution is possible. This is very much the case with the marine propagation environment. The complexity of the sea surface makes analytical solutions extremely difficult, and the stochastic nature of the surface makes detailed knowledge of the sea over the entire physical channel at the precise time of measurement nearly impossible. The proposed methodology of using the Finite Difference - Time Domain (FDTD) method allows high accuracy propagation analysis over a well-known realistic random sea surface. In the post analysis segment of the FDTD simulation effort, conventional wireless communications channel measurement analysis methods are applied to characterize channel performance using the path loss equation. This proposed methodology solves the key historical problem of conducting marine propagation studies. Specifically, propagation analysis can hereafter be conducted whereby detailed knowledge of the sea surface over which propagation occurs is readily available. From the collective analysis of multiple communications channels consisting of various random sea surfaces, novel parameterized channel models are developed. The end result of the numerical segment of this research is compact generalized models that are functions of both frequency and wave height that quantify marine communications channel performance during sea surface shadowing conditions.;For both analytical and numerical methods, the Pierson-Moskowitz sea surface spectral functions are used to develop the required sea surface physical profile and subsequent random sea surfaces. Although this research effort has been conducted in support of the application of Unmanned Aerial Vehicles for maritime surveillance, the proposed methods may be used to estimate channel performance for any wireless technology operating in the marine environment, however, the Pieron-Moskowitz model is regionally specific to the north North Atlantic. The results are suitable within that geography for circumstances as outlined per the channel topology with considerations to the frequency limitations of the geometrical theory of diffraction and the conducted FDTD simulations. Both models are verified for validity by direct comparison to the well-known analytic knife edge diffraction model.;First, a theoretically-based marine geometrical theory of diffraction (Marine GTD) model is developed, whereby a diffraction methodology is devised specifically for single sea surface waves. For this segment, a sea surface wave is considered as an obstructing object between transmitter and receiver creating a shadowing condition. The physical sea surface attributes are studied using the modified Pierson-Moskowitz model for the north North Atlantic such that a novel Geometrical Theory of Diffraction wedge is synthesized based exclusively upon sea surface height. The wedge is thus physically representative of a fully-developed deep sea surface wave, and may be used to estimate diffraction loss. Complete formulations of the generalized model are given such that path loss effects due to diffraction are easily determined requiring only the height of the sea and the positions of the transmitter and receiver.