I. Introduction
The massive multiple-input multiple-output (MIMO) transmission technology has certainly emerged as one of the most important innovation for the next generation of wireless communication systems [1] . By well composing a large number of antennas on a site unit, the huge amount of space resources can be created so that multiple users can simultaneously share the whole time-frequency resources. Generally, the time division duplex (TDD) scheme is suggested for the massive MIMO systems since the property of channel reciprocity can facilitate the channel estimations [2]. However, the limited time-slot resources (i.e., the coherent interval) restricts the number of available pilot sequences for conducting the channel estimation. Accordingly, the necessary pilots reuse among cells unavoidably results in the serious pilot contamination problem [3].