Before revision:  However, existing wireless communication techniques do not work in such environments due to the harsh transmission medium with very high material absorption and the inaccessible nature of underground environment that requires extremely small device size. Although Magnetic Induction ( MI) communication has been shown to be a promising technique in underground environments, the existing MI system utilizes very large coil antennas, which are not suitable for deployment in underground.
After revision:  However, the transmission distance of MI techniques is limited since magnetic field attenuates very fast in the near field. To this end, this paper proposes Metamaterial-enhanced Magnetic Induction ( MI) communication has been shown to be a promising technique in underground environments, the existing MI system utilizes very large coil antennas, which are not suitable for deployment in underground.