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This letter focuses on the spectrum efficiency (SE) and energy efficiency (EE) of K-tier heterogeneous networks (HetNets), in which massive multiple-input multiple-output (MIMO) is employed in the macro cells. We consider the impact of massive MIMO on flexible cell association. We develop an analytical approach to examine SE and EE of HetNets. We confirm that using flexible cell association can improve the EE of HetNets by offloading data traffic to small cell. This is due to an increase of EE in the macro cell. Moreover, we show that serving moderate number of users in the macro cells with massive MIMO can boost both SE and EE.

Spectrum and Energy Efficiency in Massive MIMO Enabled HetNets: A Stochastic Geometry Approach