  Magnetic induction (MI) is an effective technique in emergency
through-the-earth communications due to the higher penetration efficiency and
lower propagation loss as compared with electromagnetic wave communication. How
to cancel the interference between different users and enhance the
effectiveness of multi-user transmissions is imperative for the practical
application of MI communication. In this paper, we use multi-frequency resonant
circuit to establish multiple resonant frequencies for MI communication. The
transmissions corresponding to different users operate at different resonant
frequencies and multi-user interferences can be naturally mitigated. Numerical
results verify our theoretical analyses and show that the proposed system can
significantly enhance the performance.
