Abstract
Millimeter-wave (mmWave) communications, along with massive multiple-input multiple-output (MIMO) systems, are recently being considered extensively as enabling technologies for emerging 5G cellular networks. The existing literature shows that the capacity in massive MIMO systems is limited by the pilot contamination effect, caused by the reuse of pilot sequences during the uplink channel estimation stage. It has also been proved that the pilot contamination effect increases with the angular spread (AS) of the multipath clusters. In this work, we study the relation between the AS and the capacity of a cellular network, where we show that there exists an optimal AS that maximizes the capacity. Furthermore, we observe that the optimal AS depends on the specific configuration of the network, such as the number of antennas installed at the base station, and the number of cells and users per cell. Since the AS depends on the operating frequency, we argue that the system capacity can be maximized by proper selection of the carrier frequency.