Chaotic Modulation Detection For Underwater Acoustic Communications

With the understanding of human development of the marine resources and strategic position,the related marine technology attracts more and more attentions from academia,industry,military and other areas.As the basic aspect of underwater technology,underwater acoustic communications have important research value and status.Underwater noncooperative communication is a concrete research direction for ensuring the safety of the oceans,which is designed to intercept and decrypt communication signals from unauthorized receivers in underwater communications and recover communication information.The modulation detection of signals for underwater acoustic communications is a key enabler to accomplish the tasks,including detecting modulation types and orders of signals while the information of the transmitters is unknown,to aid demodulation and interference identifications subsequently.The underwater acoustic channel is characterized as narrow-bandwidth,long time-delay,and strongly multipath.Meanwhile,the underwater environment is complex and dynamic.Moreover,modulation detection is “blind” because the receiver does not have prior knowledge.In addition,with the deep research of underwater secure communication in recent years,advanced and novel modulation methods come into view.Therefore,we need to study algorithms to face with the great challenges and solve the difficulties of modulation detection in underwater acoustic communications.In this paper,a modulation detection algorithm is studied in underwater non-cooperative communications,which intercepts two kinds of signals at the receiver.These two types of modulation are based on chaotic sequences and used for underwater confidential communications,one is called chaotic M-ary frequency shift keying(CMFSK),the other one is called chaotic M-ary phase shift keying(CMPSK).Study results are summarized as follows:We study the problem of chaotic modulation detection in underwater acoustic communications via instantaneous features.According to the signal models of CMFSK and CMPSK proposed by covert underwater acoustic communications,we design a detection algorithm based on instantaneous characteristics and use this algorithm to recognize the modulation type of signals in underwater non-cooperative communications.The main principle of the algorithm is:extracting instantaneous phases and instantaneous frequencies of the intercepted signals,then designing three related instantaneous features.The three instantaneous features are hierarchical,multistage detected,and reasonable thresholds are designed.We compare the instantaneous features with thresholds to distinguish among eight types of modulated signals: 2FSK,4FSK,C2 FSK,C4FSK,2PSK,4PSK,C2 PSK,C4PSK.The simulations through the AWGN channel and Bellhop channel confirm the effectiveness of the proposed method.The chaotic M-ary phase modulation detection based on cyclostationary features in underwater acoustic communications is studied.The detection and recognition algorithm according to the instantaneous features can only detect the modulation type of the chaotic phase modulation signals,but cannot recognize the modulation order,so we use the cyclostationary feature of signals to detect.We design a new cyclostationary feature using the spectral correlation density function's maximum values in the cyclic frequency domain and the frequency domain to discriminate between C2 PSK and C4 PSK.Then we use the probability density function of cyclostationary features of C2 PSK and C4 PSK signals to achieve proper threshold.Therefore,we integrate the modulation detection algorithm based on instantaneous characteristics and achieve a set of integrity,multi-level modulation detection method,which can detect a total of eight signal types including chaotic modulation,and can distinguish modulation orders.This method is simple to implement.When it is designed properly,the performance is near-optimal.We develop the platforms of underwater acoustic modems and the proposed modulation detection algorithm based on the features of instantaneous and cyclostationary is proved by experiments.We give a brief introduction to the developing software-defined underwater acoustic modem,and give detailed scheme,hardware architecture,software interface and applications of key technologies.Using AquaSeNT underwater acoustic modem,we carry out lake experiments to verify the modulation detection algorithm proposed based on characteristics of instantaneous and cyclostationary,and the experimental results indicate the algorithm has good detection effectiveness in modulation detection.