Pilot Contamination Problem in Multi-Cell TDD Systems

This paper considers a multi-cell multiple antenna system with precoding at the base stations for downlink transmission. To enable precoding, channel state information (CSI) is obtained via uplink training. This paper mathematically characterizes the impact that uplink training has on the performance of multi-cell multiple antenna systems. When non-orthogonal training sequences are used for uplink training, it is shown that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells. This problem of {em pilot contamination} is analyzed in this paper. A multi-cell MMSE-based precoding is proposed that mitigate this problem.