Downlink performance of superimposed pilots in massive MIMO systems in the presence of pilot contamination

In this paper, we analyze the downlink (DL) performance of superimposed pilots in time division duplexing massive multiple-input multiple-output (MIMO) systems, and show that superimposed pilots offer an increased resilience to pilot contamination with respect to time-multiplexed pilots and data. Based on a closed form expression for the DL signal-to-interference-plus-noise ratio (SINR) at the user terminal, we show that the DL SINR increases without bound with an increasing number of antennas at the base station (BS). In addition, we derive the Cramér-Rao lower bound (CRLB) for the channel estimator that uses superimposed pilots. The CRLB is compared with the mean-squared error and we show that the estimator achieves the CRLB asymptotically in the number of antennas at the BS. Simulation results validate our closed-form expressions and the performance of the proposed method.