I. Introduction
Massive multiple-input multiple-output (MIMO) is believed to be one of the key technologies to drastically enhance the transmission rate as well as system capacity brought by its significant array gain and spatial multiplexing gain. Since its fundamental concept has been conceived [1], research field now covers a wide variety of topics, e.g. hybrid beamforming [2], complexity reduction [3], mobility robustness [4] and overhead reduction [5]. In any case, a large number of base stations (BSs) having massive antenna arrays will be extensively deployed and this scenario leaves the unavoidable problem still being discussed so called pilot contamination [6]. It is caused by inter-cell interference (ICI) among pilot signals which must be reused due to the limitation of orthogonal training sequence set, see Fig. 1a. Contaminated pilots deteriorate the channel state information (CSI) accuracy. Therefore in uplink data transmission phase (Fig. 1b), CSI-aided postcoding weight cannot well suppress inter-user interference (IUI) for accommodating user terminals (UTs), in addition to ICI.