Zero-forcing beamforming in massive MIMO systems with time-shifted pilots

By deploying a substantial number of antennas on the base station (BS) side, massive MIMO system has been demonstrated to achieve unprecedented spectral efficiency. However, its performance is still limited by pilot contamination due to unavoidable reuse of pilot sequences from user terminals in other cells. In this paper, we focus on the performance analysis of zero-forcing beamforming in a finite-antenna massive MIMO system using a time-shifted pilot scheme, which was shown to combat pilot contamination effectively with infinite BS antennas and conjugate beamforming. We derive rigorous expressions for sum-rate lower bounds and associated signal to interference plus noise ratios of both forward link and reverse link. Based on these expressions, we provide some engineering rules of thumb to design a time-shifted pilot system achieving higher sum-rate performance. Besides, our system model is proved to cover a series of previous works as special cases as well.