This paper describes a series of experiments of underwater radio frequency communications both in fresh water and sea water. We observed than due to skin effect, radio waves attenuated rapidly at first before skin depth appears and then attenuated gently just as theoretical prediction. The relationship between skin depth and frequency in water is discussed and graphical relationship is also given. It's turned out that radio waves propagate much harder in sea water than in fresh water.
Quadrature phase shift keying (QPSK) modulation has been widely applied in communication systems. With the increasing development of QPSK modulation recognition, it is meaningful for ensuring validity and accuracy of recognition method. Therefore, the method of signal recognition has been presented based on the features of QPSK modulated signal, in which the feature parameters of QPSK signal are extracted. Besides, signal classification is fulfilled through thresholds, and modulation recognition is completed with quartic spectrum. The simulation results show that, the method can recognize QPSK modulation effectively in less sample space condition and can be more accurate.
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