  We propose a unitary precoding scheme, namely polar-precoding, to improve the
performance of polar-coded MIMO systems. In contrast to the traditional design
of MIMO precoding criteria, the proposed polar-precoding scheme relies on the
\emph{polarization criterion}. In particular, the precoding matrix design
comprises two steps. After selecting a basic matrix for maximizing the capacity
in the first step, we design a unitary matrix for maximizing the polarization
effect among the data streams without degrading the capacity. Our simulation
results show that the proposed polar-precoding scheme outperforms the
state-of-the-art DFT precoding scheme.
