Antenna Tilt Adaptation for Multi-Cell Massive MIMO Systems

This letter proposes an antenna tilt adaptation approach for multi-cell massive multiple-input multiple-output systems. The asymptotic spectral efficiency of the system with pilot contamination in the limit of the antennas is first derived. The analysis demonstrates that the spectral efficiency is a concave function of the tilts with some specific requirements being satisfied. As a result, a gradient descent-based method is proposed to approach the optimal tilt. Numerical results verify that the proposed approach performs better than a traditional low-complexity approach.

I. Introduction

Massive multiple-input multiple-output (MIMO) systems can provide high spectral efficiency and energy efficiency, and may be implemented in the future wireless systems [1], [2]. However, the limited channel coherence interval makes the orthogonal pilots too long to be employed, which causes interference in channel estimation. Furthermore, this interference constrains the performance improvement of massive MIMO systems, and this phenomenon is named pilot contamination. In order to mitigate the interference in massive MIMO systems, several methods have been proposed [3]. However, these methods only consider the transmission in the horizontal plane, while the degree of freedom (DoF) in the vertical dimension has not been used.