Long-distance magnetic induction through-the-earth communication method based on OFDM

Magnetic induction through-the-earth (MI-TTE) communication provides a stable transmission channel but suffers from limited available bandwidth, making it challenging to support the high data rates required for real-time voice communication over long distances. This limitation poses difficulties for emergency rescue operations in underground mining environments. To enable long-distance, real-time voice transmission within restricted bandwidth, a long-distance MI-TTE communication method based on orthogonal frequency-division multiplexing (OFDM) is proposed. Given the weak coupling between transmitter and receiver coils at extended distances, a MI-TTE communication system comprising a single-axis transmitting coil and a tri-axial orthogonal receiving coil is proposed, and the equivalent circuit model of the corresponding transmit-receive loop is established. On this basis, the characteristics of path loss and the bandwidth of the MI-TTE communication channel when using the three-axis orthogonal receiving antenna are characterized. According to the characteristics that the channel bandwidth of the MI-TTE communication is narrow and the path loss will increase rapidly with the increase of the communication distance, a secondary modulation method using OFDM is proposed to increase bandwidth utilization. The OFDM design parameters that are more suitable for MI-TTE communication are presented, thereby compressing the communication bandwidth occupied by the MI-TTE communication system when using low-level symbols for long-distance voice calls within the available bandwidth of the MI-TTE communication channel. Through simulation analysis, the transmission effects of speech information in MI-TTE communication system using single carrier modulation based on high-level symbols and using OFDM based on low-level symbols were compared. The results show that OFDM can effectively increase the real-time voice calls distance of the MI-TTE communication system.