Pilot Contamination Reduction in Very Large MIMO Multi-Cell TDD Systems

n very large MIMO time-division duplex system with L cells，the sum-rate in downlink is bottlenecked by the problem of pilot contamination as the base station’s antennas number M grows. Assuming uplink training with orthogonal training sequence in one cell while aligned pilots are used in all cells, the close form analytical expression of the mean square error (MSE) of the channel estimators M^MSE( for all channels) and M_ι^MSE(only for desired channels) were proposed, respectively. And M^MSE has a lower bound M（L-1） while M_ι^MSE has an upper bound M（L-1）/L. We found that when the length of training sequence t is relatively large with the antennas number M’s increasing, t has little effect on the performance of  M_ι^MSE and  M_ι^MSE . Thus, a pilot power control scheme in uplink was proposed, which dividing the training phase into two parts and making the pilot transmissions in the cells with relatively big cross gain staggered. The performance of M^MSE and M_ι^MSE were improved by it so that the base station could design much better precoding, which greatly enhancing the sumrate in downlink precell. The theoretical analysis and numerical results both verified the results.