Enhanced time-shifted pilot based channel estimation in massive MIMO systems with finite number of antennas

Time-shifted pilot scheme can efficiency combat pilot contamination. However, when the number of base station (BS) antennas is finite, the pilot interference due to the downlink transmission symbols cannot be mitigated in the time-shifted scheme. This paper proposed a novel two-stage channel estimation algorithm based on time-shifted pilot to reduce the effect of pilot interference. In the first stage, beamforming vectors are determined to reduce the interference from other cells by introducing null symbol periods. Then, receive beamforming is carried out. In the second stage, channel estimation is performed. In the proposed scheme, the pilots of different users in the same cell are orthogonal and the pilots are reused in other cells. The proposed scheme can reduce the effect of pilot interference and improve spectrum efficiency, when the number of BS antennas is finite. The proposed channel estimation algorithm is presented in detail, and the channel estimation performance and achievable spectrum efficiency are analyzed. Simulation results validate the proposed scheme.