The role of massive MIMO and small cells in ultra-dense networks

Massive multiple-input multiple-output (MIMO) employs hundreds of antenna elements at the base station (BS) to serve tens of users simultaneously at the same time-frequency resource block. This chapter proposes a downlink heterogeneous cellular network (HCN) where the user location as well as the deployment of multi-antenna macro and small BSs are modeled as independent Poisson point processes (PPPs). It introduces the HCN framework, the line-of-sight (LOS)/non-line-of-sight (NLOS) transmission model, and the user association policy. The chapter derives a tight approximation of the average downlink rate accounting for channel estimation with uplink training. It provides the optimal small cell density that maximizes the average downlink rate, which can be used as a guideline for practical small cell deployment. Moreover, the optimal bias factor is also provided for reference of the practical massive MIMO configuration. The chapter provides numerical results to validate the analytical results and further study the performance of the HCN.