Research On Hilbert-Huang Transform And Its Application In Direction Of Arrival Estimation And Underwater Acoustic Communication

The non-stationary signals with the characteristic of time variant widely exist in manyfields, and time-frequency analysis methods are essential to reveal the essential characteristicof the non-stationary signals. Hilbert-Huang transform is a new kind of time-frequencyanalysis method and it can adaptively decompose any complicated non-stationary signal intosome intrinsic mode functions without any prior knowledge of the signal. Any intrinsic modefunction only has one vibration mode at any time, and instantaneous frequency with physicalmeaning can be obtained. Hilbert-Huang transform has the advantages of decomposing signaladaptively and having higher time-frequency resolution. Compared with other time-frequencyanalysis methods, Hilbert-Huang transform has many advantages in non-stationary signalanalyzing and processing. Since its birth, Hilbert-Huang transform attracts many attentionsfrom researchers, and more and more researchers use this method to process thenon-stationary signals in their fields.This paper discussed some problems such as mean curve fitting, end effect and modemixing, and proposed corresponding solutions. A new method of directly fitting mean curveby piecewise high order spline was proposed in this paper, in which more accurate meanpoints were obtained by piecewise high order spline and mean curve was obtained by directlyfitting mean point sequence. The method made EMD method obtain more accurate meancurve when sifting intrinsic mode function and accomplish accurate mode extraction.Compared with traditional method, the new method had higher decomposing speed. Thispaper also discussed mode mixing problem, analyzed the basic reason leading to mode mixing,and proposed a method of eliminating mode mixing based on translation invariant wavelettransform and EMD scale filtering. The method could effectively remove high frequencydiscontinuous signal from useful signal, which is the main cause of mode mixing, andaccomplish the goal of eliminating mode mixing. Traditional Hilbert-Huang transformalgorithm had the disadvantage of too much computation, and to reduce the computationalburden of Hilbert-Huang transform and make it realizable on hardware realtime, this papermade some improvements on Hilbert-Huang transform, which included mean curve fitting,end effect treatment, terminating conditions of sifting intrinsic mode function, terminatingconditions of EMD decomposition and the calculation of Hilbert transform. Theseimprovements greatly reduced the computational burden of Hilbert-Huang transform, withperformance basically remaining unchanged.According to the characteristics of underwater acoustic signals, Hilbert-Huang transformwas introduced into underwater acoustic signal processing field in this paper. A target azimuth estimating method and a method of underwater acoustic communication based onHilbert-Huang transform were proposed in this paper. By combining Hilbert-Huang transformwith vector underwater acoustic signal processing technology, a new target azimuthestimation method based on Hilbert-Huang transform was proposed in this paper. The newmethod decomposed the three channel signals into intrinsic mode functions by EMD,calculated the complex instantaneous acoustic energy flux by the analytic signals of the sameorder intrinsic mode functions, and then got the instantaneous azimuth of a certain target. Themethod decomposed the synthesizing vectors from vector hydrophone into the sum of simplecomponent signals and realized the identification of targets at the different azimuths.The combining signal of two LFM signals, which had different initiation frequencies andchirp rates, had some time of local inversed phase, which resulted from the different speed ofphase variation. Thus the combining signal had many small scale signals in some localregions. After performing EMD on the combining signal, some intrinsic mode functions couldbe obtained. Perform Hilbert transform on the first order intrinsic mode function, and itsinstantaneous frequency curve could be obtained, in which there were some peaks. Modifyingthe combining signal by some methods could make the regions with peaks become flat oreven concave. Based on these characteristics, a new method of underwater acousticcommunication based on Hilbert-Huang transform was proposed in this paper, in whichinformation was encoded in the scale domain and decoded by the instantaneous frequency ofintrinsic mode function. The underwater acoustic communication method was validated bysimulation experiments, water pool experiments and lake experiments.