Channel estimation based on time-shifted pilots in multicell massive MIMO systems

Massive multiple-input multiple-output (MIMO) is a promising technique for future wireless communication systems. The principal benefits of using massive MIMO systems include the increase in the uplink achievable rate when the number of antennas at the base station (BS) increases. However, pilot contamination is a significant obstacle to obtaining the benefits of massive MIMO system. Specifically, the achievable rate affected by pilot contamination will saturate to a certain point even when the number of antennas increases. Recendy, the concept of time-shifted pilots was proposed. According to the idea, uplink training sequences from the users in a certain cell are transmitted to their BS during the downlink period of other cells. In this paper, we propose two uplink training strategies by combining the time-shifted pilots and the conventional time-aligned pilots to improve the performance of channel estimation in massive MIMO systems. Furthermore, we also derive a lower bound of the uplink achievable rate for the case where the number of antennas at the BS is finite. Numerical results are presented to verify the analysis.