ave channels, e.g., [@3GPPTR389011411; @3GPPTR389001420; @3GPPR1-164975]. Different from low-frequency communications, mmWave carrier frequencies are relatively very high. The high frequency results in some propagation challenges, such as large pathloss and severe shadowing [@rappaport2014millimeter]. Meanwhile, the small wavelength enables a large number of antennas to be closely packed to form mmWave large multi-antenna systems, which can be utilized to overcome the propagation challenges and provide reasonable signal to noise ratios (SNRs) [@rappaport2013millimeter]. The multi-input multi-output (MIMO) technology has already been standardized and widely adopted in commercial WLAN and cellular systems at sub-6 GHz frequencies (IEEE 802.11n/ac, IEEE 802.16e/m, 3GPP cellular LTE, and LTE Advanced) [@Kim7060495; @Li5458368]. However, the performance of mmWave MIMO systems is still considerably limited due to the sparsity of the channels and the stringent constraint of using radio frequency (RF) chains in mmWave transceivers. The directional propagations and clustered scattering make the mmWave paths to be highly sparse [@Pi11Aninmmvmbs]. More importantly, the high cost and power consumption of RF components and data converters preclude the adoption of fully digital processing for mmWave MIMO to achieve large beamforming gains [@Pi11Aninmmvmbs; @Doan04dcf60gcmosra], and low-complexity transceivers relying heavily on analog or hybrid (analog-digital) processing are often adopted [@Venkateswaran10ancpsnoce; @Ayach_14_SpatiallySparsePrecodingmmMIMO; @Liu06STTrecbocpf].

The limited beamforming capability and performance of mmWave MIMO motivate us to investigate the potential benefits of employing reconfigurable antennas for mmWave MIMO in this work. Different from conventional antennas with fixed radiation characteristics, reconfigurable antennas can dynamically change their radiation patterns, polarizations, and/or frequencies [@Cetiner_04_MEMS_Magazine; @Grau_08_AreMIMOcym], and offer an additional degree of