k from the receiver to the transmitter is available for the reconfiguration state selection and transmit beam selection, which will be detailed later in Section \[Sec:Architecture\]. We would like to point out that the assumption of full CSI at the receiver or even at both the transmitter and the receiver has often been adopted in the existing papers on mmWave systems and reconfigurable antennas, see, e.g., [@Sohrabi7913599; @Rusu7579557; @Sohrabi7389996; @Chen7055330; @Amadori_15_LowRDBStion; @Ayach_14_SpatiallySparsePrecodingmmMIMO; @Grau_08_AreMIMOcym; @Fazel09stsbcmmioyra; @Vakilian_15_SThmmra; @Vakilian_15_ThmmraAr]. On the other hand, it is worth mentioning that the full CSI assumption is not easy to achieve in practice. The full CSI requires user equipments to conduct channel estimation. However, channel estimation is relatively challenging for mmWave MIMO systems. Different from sub-6 GHz systems, the precoder for mmWave MIMO with a limited number of RF chains is usually not fully digital. Due to the constrained precoding structure, the channel has to be estimated via the use of a certain number of RF beams, and each beam only presents a projection of the channel matrix rather than the full channel matrix itself. To obtain the full CSI, the channel estimation needs to be conducted over enough number of RF beams so that the full channel matrix can be estimated. The same process has to be repeatedly conducted for all the reconfiguration states as well. Thus, the channel estimation process to obtain full CSI would incur a considerable latency, and the assumption of full CSI is not easy to achieve in practice.

Channel Model
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For low-frequency $N_r\times N_t$ MIMO systems in an ideally rich scattering environment, the channel for each reconfiguration state is usually modelled by a full rank $N_r\times N_t$ matrix with i.i.d. entries [@Grau_08_AreMIMOcym], e.g., i.i.d. complex Gaussian entries for Rayleigh fading channels. For mmWave communications in the clustered scattering environment, it i