Uplink Asynchronous Fractional Pilots Scheduling in Massive MIMO System

Pilot contamination (PC) is one of the main obstacles that limit the performance of massive multiple-input multiple-output (MIMO) systems. In this paper, we propose the asynchronous scheduling which is based on the fractional pilot reuse so that the users can be free from the pilot contamination during the uplink transmission. According to the level of interference, the users are divided into two groups, which are referred as the center users, who suffer from the mild pilot contamination, and the edge users, who suffer from the severe pilot contamination. Based on this distinction, a cell-center pilot set is reused for all the center users in all cells, whereas a cell-edge pilot set is applied for the edge users in the adjacent cells. In this case, the pilots used by the cell-edge users are orthogonal to each other. So the edge users can transmit the pilots at any time. But the pilot set for the center users are reused for all the cells, the cell-center users send their pilots in the non-overlapped time periods in order to avoid the pilot contamination. With this scheduling, the cost of the orthogonal pilots for each cell is reduced obviously. And the base station (BS) can easily recover the estimation of the pilots as it knows there is no pilot contamination. Simulation results show that the proposed asynchronous fractional pilots scheduling (AFPS) outperforms the other conventional pilot assignment schemes.