Construction Of SLF Magnetic Induction Communication System And Research On Its Propagation Characteristics

Super low frequency(SLF)communication is widely used in military submarine communication and through the earth communication because of its good penetration,strong anti-interference ability and stabilization.At present,most of the SLF communication systems are equipped with electrically adjustable antennas of the order of 100 meters or even kilometers as transmitting antennas,which are large and unable to move flexibly.Therefore,the Defense Advanced Research Projects Agency(DARPA)proposed to develop a portable SLF communication system with the frequency below 30 k Hz in 2017.In order to reduce the size of the antenna,this paper replaces the traditional electrically adjustable antenna with the compact coil.The receiving and transmitting coils use the induced magnetic field to transmit information.There is little theoretical study on the loss law of the SLF signal in this magnetic induction communication mode,so it is of great significance to study the propagation characteristics of SLF magnetic induction communication.In this paper,a complete small SLF magnetic induction communication system is built based on MSP430F149 MCU.The propagation characteristics of SLF signal is studied through the combination of theory and experiment.The antenna selected is multi-layer cylindrical coil,and the modulation method is Minimum Shift Keying(MSK).The main work of this paper mainly includes:(1)The basic principle of magnetic induction communication is analyzed in detail,and the equivalent circuit model of magnetic induction communication is decomposed into power model and coupling model.Then the relationship between the transmitted signal and the received signal in the system is analyzed,and the calculation formula of signal transmission loss is obtained.(2)The SLF magnetic induction communication system is designed.For the transmitter,the scheme of generating modulated waveform data in the master computer is proposed.The MCU receives the data sent by the master computer and controls the output of the D/A module without complex modulation function operation,which solves the problems of small memory and slow operation speed.For the receiver,the principle of synchronization head positioning and the selection method of synchronization head duration are analyzed,which can automatically determine the threshold and realize real-time demodulation.Finally,a simulation model of the communication system is established,and the bit error rate under different signal-to-noise ratio is obtained,which shows the feasibility of the system scheme.(3)By modeling the antenna composed of multilayer cylindrical coil,the empirical calculation formulas of coil parameters are given,and the influence of coil cross-sectional area and length on the parameters such as resistance,self-inductance and mutual inductance is analyzed,which lay a foundation for the calculation of loss model.(4)This paper studies the propagation characteristics of SLF magnetic induction communication system by combining theory and experimental tests,mainly analyzes the influence of coil size and distance on transmission loss.Through experiments,it is found that the loss is proportional to the cubic of distance at the short distance,and is proportional to the second power of distance at the long distance.Then the loss model is summarized according to the experimental results.(5)The system performance evaluation model is established and verified,which can estimate the bit error rate at a given distance according to the system parameters.At the same time,this model can also be used to design the system parameters according to the bit error rate requirements at the expected communication distance,which provides guidance for the design of SLF magnetic induction communication system.