  Although wireless communications in complex environments, such as
underground, underwater, and indoor, can enable a large number of novel
applications, their performances are constrained by lossy media and complicated
structures. Magnetic Induction (MI) has been proved to be an efficient solution
to achieve reliable communication in such environments. However, due to the
small coil antenna's physical limitation, MI's communication range is still
very limited if devices are required to be portable. To this end,
Metamaterial-enhanced Magnetic Induction (M$^2$I) communication has been
proposed and the theoretical results predict that it can significantly increase
the communication performance, namely, data rate and communication range.
Nevertheless, currently, the real implementation of M$^2$I is still a challenge
and there is no guideline on design and fabrication of spherical metamaterials.
In this paper, a practical design is proposed by leveraging a spherical coil
array to realize M$^2$I. We prove that the effectively negative permeability
can be achieved and there exists a resonance condition where the radiated
magnetic field can be significantly amplified. The radiation and communication
performances are evaluated and full-wave simulation is conducted to validate
the design objectives. By using the spherical coil array-based M$^2$I, the
communication range can be significantly extended, exactly as we predicted in
the ideal M$^2$I model. Finally, the proposed M$^2$I antenna is implemented and
tested in various environments.
