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An adaptive array design is proposed for hybrid beamforming in millimeter-wave (mmWave) communications in the context of cloud radio access networks (C-RAN). More explicitly, the adaptive design focuses on the physical layer aspect of C-RAN. The adaptation is performed at two levels, depending on whether the channel is of line-of-sight (LOS) or non-line-of-sight (NLOS) nature. First, the antenna array architecture can be adapted between a fully connected and a sub-array-connected architecture. Then, the employment of a digital precoder in the baseband is decided based on both the channel conditions and the architecture employed. We show that the proposed adaptive design performs better than the fully connected and sub-array-connected constituent designs, when the channel varies between LOS and NLOS scenarios. Then, we extend our proposed adaptive design to mmWave communications in the …