Massive MIMO and Small Cells: How to Densify Heterogeneous Networks

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The authors propose a Time Division Duplex (TDD) based network architecture where a macrocell tier with a "Massive" Multiple-Input Multiple-Output (MIMO) Base Station (BS) is overlaid with a dense tier of Small Cells (SCs). In this context, the TDD protocol and the resulting channel reciprocity have two compelling advantages. First, a large number of BS antennas can be deployed without incurring a prohibitive overhead for channel training. Second, the BS can estimate the interference covariance matrix from the SC tier which can be leveraged for downlink pre-coding.