Low-Frequency Wireless Communication System Technology Across Seawater To Air Interface

Sea-to-air low-frequency wireless communication refers to the use of low-frequency(30 kHz-300 kHz)electromagnetic waves through the medium of seawater and the medium of air to transmit information in a wireless communication mode,which can be applied to scenarios such as integrated sea-to-air networks and submarine communications,helping to maintain national security and improve maritime control capabilities.However,due to the strong attenuation of electromagnetic wave propagation in seawater and the peculiarities of penetrating the seawater-air interface,research on sea-to-air electromagnetic wave communication has been slow.In order to explore the prospects of electromagnetic wave communication in the ocean,this thesis conducts research on technologies related to low-frequency wireless communication systems over seawater-air,including a layered model of low-frequency electromagnetic wave propagation over sea-air,a wireless communication system combining spread spectrum and multi-carrier modulation,and a low-overhead pilot design for channel estimation.The main research work in this thesis is as follows.(1)Based on the characteristics of the over-the-air channel path loss and subject to frequency-selective fading,the thesis designs a low-frequency over-the-air wireless communication system using polarisation code,spread spectrum and multi-carrier modulation as the main techniques.According to the propagation paths,the propagation characteristics of electromagnetic waves are studied in a hierarchical manner by dividing them into airborne propagation,seawater-air interface propagation and underwater propagation,and the formulae for calculating the path loss and signal-to-noise ratio of the trans-sea-air channel are derived.The analysis concludes that the attenuation of electromagnetic waves in seawater results in high path loss and low signal-to-noise ratio at the receiving end of the trans-sea-air path.The rapidly changing underwater frequency response with frequency results in the air-sea EM wave propagation being affected by frequency selective fading.The study of the air-sea propagation characteristics of electromagnetic waves can provide a theoretical basis for the design of communication systems.(2)A wireless communication system with polar code,spread spectrum and multicarrier modulation as the main techniques is designed according to the characteristics of the over-the-air channel with low signal-to-noise ratio and subject to frequency-selective fading.The coding and decoding methods of polar codes are analysed and the BER performance of different code rates and code lengths is tested to provide a basis for the selection of channel coding parameters.The characteristics of spread spectrum technology are analysed and direct sequence spread spectrum technology is selected to combat noise interference and multipath effects.Filter Bank Multicarrier(FBMC)modulation with higher spectral efficiency and bit rate is selected as the multicarrier modulation technique for the communication system.The performance of the FBMC system with polar code and spread spectrum in Gaussian channels and time-frequency dual selection channels is simulated and tested.The results show that the system can effectively combat both large scale fading and small scale fading and improve the reliability of communication.It is estimated that the bit rate of the communication system can reach 7 kbps when one antenna is located 1 m underwater and the other antenna is located on land,with a horizontal distance of 10 km between the transmitting and receiving antennas.(3)The thesis proposes a new pilot design to eliminate FBMC false-part interference for FBMC systems combined with spread spectrum.The theoretical analysis of the causes of false part interference in FBMC modulation and its effect on channel estimation,as well as the shortcomings of the conventional auxiliary and coding methods of pilot design are presented.The performance of the three pilot designs is analysed qualitatively and quantitatively.The results show that the new pilot design has advantages in terms of spectral efficiency,power efficiency,computational complexity,accurate channel estimation results,and low BER and channel mean square error.