  Underwater magnetic induction communications (UWMICs) provide a low-power and
high-throughput solution for autonomous underwater vehicles (AUVs), which are
envisioned to explore and monitor the underwater environment. UWMIC with
tri-axis coils increases the reliability of the wireless channel by exploring
the coil orientation diversity. However, the UWMIC channel is different from
typical fading channels and the mutual inductance information (MII) is not
always available. It is not clear the performance of the tri-axis coil MIMO
without MII. Also, its performances with multiple users have not been
investigated. In this paper, we analyze the reliability and multiplexing gain
of UWMICs with tri-axis coils by using coil selection. We optimally select the
transmit and receive coils to reduce the computation complexity and power
consumption and explore the diversity for multiple users. We find that without
using all the coils and MII, we can still achieve reliability. Also, the
multiplexing gain of UWMIC without MII is 5dB smaller than typical terrestrial
fading channels. The results of this paper provide a more power-efficient way
to use UWMICs with tri-axis coils.
