  This paper studies the exploitation of triple polarization (TP) for
multi-user (MU) holographic multiple-input multiple-output surface (HMIMOS)
wireless communication systems, aiming at capacity boosting without enlarging
the antenna array size. We specifically consider that both the transmitter and
receiver are equipped with an HMIMOS comprising compact sub-wavelength TP patch
antennas. To characterize TP MUHMIMOS systems, a TP near-field channel model is
proposed using the dyadic Green's function, whose characteristics are leveraged
to design a user-cluster-based precoding scheme for mitigating the
cross-polarization and inter-user interference contributions. A theoretical
correlation analysis for HMIMOS with infinitely small patch antennas is also
presented. According to the proposed scheme, the users are assigned to one of
the three polarizations, which is easy to implement, at the cost, however, of
reducing the system's diversity. Our numerical results showcase that the
cross-polarization channel components have a nonnegligible impact on the system
performance, which is efficiently eliminated with the proposed MU precoding
scheme.
