  In this letter, we investigate the millimeter wave (mmWave) downlink
multiuser multiple-input multiple-output (MU-MIMO) system, adopting the dynamic
subarray architecture at the base station and considering the multi-stream
communication for each user. Aiming at maximizing the system spectral
efficiency, we propose a novel hybrid beamforming design. First, assuming no
inter-user interference (IUI), we easily get the optimal fully-digital
beamformers and combiners using the singular value decomposition of each user
channel and the waterfilling algorithm. Then, based on the obtained
fullydigital beamformers, we propose a Kuhn-Munkres algorithmassisted dynamic
hybrid beamforming design, which guarantees that each radio-frequency chain is
connected to at least one antenna. Finally, we propose to further project each
obtained digital beamformer onto the null space of all the other equivalent
user channels to cancel the IUI. Numerical results verify the superiority of
our proposed hybrid beamforming design.
