Two-Dimensional Precoding for 3-D Massive MIMO

A 2-D precoding scheme is proposed for 3-D massive multiple-input multiple-output (MMIMO) systems for efficiently exploiting the 2-D antenna array of the base station. Specifically, by exploiting the Kronecker structure of the 3-D MIMO channel matrix, the transmit precoding operation is divided into elevation-domain precoding and azimuth-domain precoding. Explicitly, in contrast to the existing beamforming schemes, precoding is also performed in the vertical dimension. Consequently, the proposed scheme is capable of fully exploiting the extra degrees of freedom provided by the vertical dimension for avoiding the interuser interference to improve the attainable system performance. Compared to the conventional scheme relying on the equivalent 1-D precoding, the proposed 2-D precoding scheme offers an improved performance in severe intercell interference-contaminated environments, despite its lower complexity.