Hybrid precoding architectures are widely studied for millimeter wave (mmWave) massive MIMO systems. A major challenge in designing hybrid precoders is the practical constraints on the number of RF chains, which can have a direct impact on the spectral and energy efficiencies of the communication systems. In this paper, we investigate tradeoff between the two performance metrics in both static and mobile communication scenarios via closed-form expressions, when the number of active RF chains can be selected. Based on these expressions, the computational complexity to configure the hybrid precoder is reduced, which can be used to adaptively activate required RF chains for the given MIMO system and channel condition. Numerical results indicate that a certain number of RF chains should be activated in order to maximize energy efficiency at high SNRs, which is generally different from the optimal configuration to maximize spectral efficiency. Further-more, for low SNRs, we have shown that a simple analog beamforming, which uses only a single RF chain, is optimal for both spectral and energy efficiencies. In addition, the proposed mobility-aware hybrid precoding is shown to be capable of effectively achieving beamforming gain between high-speed mobile devices.
