Spatial orthogonality-based pilot reuse for multi-cell massive MIMO transmission

This paper addresses the problem of pilot reuse among the inter-cell user terminals (UTs) for massive multi-input multi-output (MIMO) transmission. Firstly, we derive the closed-form expression of the non-asymptotic downlink (DL) achievable rate by using the second-order channel statistical information. Based on this, we present the spatial orthogonality condition for inter-cell pilot reuse without rate loss, which offers a powerful way to discriminate UTs in the same time-frequency interval. Regarding that the spatial orthogonality condition can not always be met over realistic channels, we further propose a spatial orthogonality-based greedy pilot assignment algorithm which exploits the statistical channel covariance information. The simulation results indicate that, in the large-number-of-antennas, the pilot contamination effect can be vanished completely under the spatial orthogonality condition presented, and pilot reuse among inter-cell UTs will not incur any rate loss. The proposed spatial orthogonality-based greedy pilot assignment algorithm can provide a substantial gain in system rate performance.