Research On Brillouin Pulse-based Short Distance Underwater Communication

In recent years, the analysis and experiments of electromagnetic precursors fieldpropagation in Lorentz and Debye dispersive medium have proved Brillouinprecursors’ peak amplitude algebraic attenuation property. This research has arousedgreat interest of researchers in the fields of radar remote sensing, microwave imaging,ultra-wideband communications, and has became a new research focus. However,underwater communications has not been studied deeply, so Brillouin precursor fieldpropagation characteristics in freshwater and seawater were analyzed from the pointof view of the underwater communication, and the feasibility of underwater shortdistance pulse communication was discussed.The electromagnetic oscillations before the arrival of the steady-state portion ofthe electromagnetic field are called electromagnetic precursor field, which includesthe Sommerfeld precursor with high spectral energy and the Brillouin precursor withlow spectral energy. Brillouin precursor has been more concerned for itslow-attenuation in the dispersive medium. Freshwater and seawater are dispersivemedia. The electromagnetic wave propagating in a dispersive medium is interacted byphase dispersion and frequency dispersion, after spreading several absorption depthdistances, which will gradually evolves into a local mature dispersion regime with arelatively fixed frequency and decay rate at a constant speed. Brillouin precursor fieldpropagation is affected by the dispersion characteristics of the dielectric and the initialsignal energy spectrum distribution. Therefore, the depth study of the propagationeffects of the dielectric dispersion characteristics and the spectrum of the differentsignals on Brillouin precursor field has great significance for the selection or designof the underwater communication pulse signal, and then achieve the underwatercommunication in short distance with high bandwidth. The main factor that affects the Brillouin precursor propagation is the dispersioncharacteristics of the dielectric, which is mainly influenced by permittivity. Thedielectric constant models of freshwater and seawater have both the simple theoreticalmodel and the practical model using measurement data fitting. On the base of thedifference analysis between the dielectric constant models, approximaterepresentation and dynamic description of the electromagnetic field propagation havebeen completed in the form of a simple theoretical model; The approximate formulasof saddle point dynamics, complex phase function, as well as electromagnetic fieldpropagation are given with different dielectric constantmodels for freshwater andseawater; Saddle point dynamics, complex phase function, and Brillouin precursorpropagation dynamics are verified in freshwater and seawater with different dielectricmodel; Influence of different the dielectric constant model on Brillouin precursorpropagation are analyzed.The spectral energy distribution of the initial signal is another main factor toaffect the Brillouin precursor propagation, so the suitable signal form for underwatercommunication must be selected or designed. The amplitude attenuationcharacteristics and temporal width spreading characteristics of signal are standards tomeasure whether a signal is suitable for underwater communication. In the analysisprocess, three types of eight kinds of signals of single pulse ultra-wideband signals,carrier modulation signals and double Brillouin pulse signals are selected to analyzepropagation dynamics with relative propagation distance in freshwater and seawater,and to get the amplitude attenuation factor and the temporal width spreading factor ofthe propagation process, and the double trapezoidal pulse is selected as the candidateunderwater pulse communication signal.In underwater communication natural environment, it is necessary to considerthe influences of water salinity and temperature on the underwater communications,and the determination of the operating frequency and pulse intervals which determinethe communication range and bandwidth of the channel. On the basis of the analyzingof the influence of water salinity, temperature and pulse frequency of the pulse propagation, the study is focused on the double trapezoidal pulse propagation within50meters in freshwater and within1meter in seawater; the maximum possiblecommunication bandwidth within different communication range is predicted bycomprehensive consideration of the pulse amplitude with the actual propagationdistance and the pulse propagation characteristics with relative propagation distance,and then the important conclusion that the communication bandwidth can reach100MHz within5meters in freshwater and within0.1meter range in seawater isderived.As the basic research in underwater pulse communication, the details of thedielectric characteristics of freshwater and seawater, the Brillouin precursor fieldpropagation dynamics and the propagation characteristics of signals are analyzed, thecandidate pulse signal underwater communication is selected, and the communicationrange and channel bandwidth are predicted, which provide a reference for nextexperiments and underwater communication channel modeling.