Massive MIMO-OFDM with Adjustable Phase Shift Pilots'
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Adjustable phase shift pilots, massive MIMO-OFDM, channel estimation, channel prediction, channel acquisition, pilot phase shift scheduling.

Introduction
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5G cellular wireless systems are expected to support 1000 times faster data rates than the currently deployed 4G long-term evolution (LTE) system. To achieve the high data rates required by 5G, many technologies have been proposed [@Andrews14What; @Boccardi14Five; @Wang14Cellular]. Among them, massive multiple-input multiple-output (MIMO) systems, which deploy unprecedented numbers of antennas at the base stations (BSs) to simultaneously serve a relatively large number of user terminals (UTs), are believed to be one of the key candidate technologies for 5G [@Marzetta10Noncooperative; @Larsson14Massive; @Lu14overview].

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation technology suited for high data rate wideband wireless transmission [@Cimini85Analysis; @Stuber04Broadband]. Due to its robustness to channel frequency selectivity and relatively efficient implementation, OFDM combined with massive MIMO is a promising technique for wideband massive MIMO transmission [@Marzetta10Noncooperative]. As in conventional MIMO-OFDM, the performance of massive MIMO-OFDM is highly dependant on the quality of the channel acquisition. Pilot design and channel acquisition for massive MIMO-OFDM is of great practical importance.

Optimal pilot design and channel acquisition for conventional MIMO-OFDM has been extensively 