Pilot scheduling schemes for multi-cell massive multiple-input–multiple-output transmission

This study addresses the pilot scheduling problem in multiuser multi-cell massive multiple-input-multiple-output (MIMO) systems, aiming at mitigating the inter-cell interference (pilot contamination), which constitutes a bottleneck of the system performance. First, the authors investigate the pilot reuse and scheduling problem in cellular systems with unlimited number of base station antennas, only considering the large fading coefficients. Three low-complexity pilot scheduling schemes are then proposed by maximising the system achievable sum rate, including the greedy algorithm, the tabu search (TS) algorithm and the greedy TS algorithm. Second, they investigate the pilot reuse problem among the inter-cell user terminals (UTs) under spatially correlated channels for massive MIMO transmission, trying to distinguish UTs sharing the same time-frequency resources from the spatial domain. A closed-form expression of the non-asymptotic downlink achievable rate is derived by exploiting the second-order channel statistical information. On the basis of the degree of the UTs´ covariance matrices overlap with each other, they further propose a spatial orthogonality-based greedy pilot scheduling algorithm. The proposed approaches can provide much better performance in the presence of pilot contamination. Theoretical analysis and numerical results both verify the effectiveness of the proposed algorithms.