Millimeter-wave small cells: base station discovery, beam alignment, and system design challenges

MmWave communications applied to small cells has been recognized recently as an important means to break the spectrum gridlock and to dramatically scale up the system capacity for the evolving 5G mobile networks. However, significant technical challenges must be resolved to fully exploit the potential of mmWave small cells. Among them, base station discovery is particularly challenging due to the severe propagation loss at mmWave bands. Beam alignment arises as another critical problem that must be solved adaptively for 5G mobile access. This article discusses protocols and techniques for these two aspects, and presents important system design insights. Some open research problems are also highlighted in base station discovery, multi-user beam alignment, and network management.