  In this article we compare the performance of two typical hyrbid beamforming
structures for multiuser massive MIMO systems, i.e., the full- and
partial-connection structures. Under the assumption of small angular spread for
mmWave channels, given the analog precoder formed towards users and the
zero-forcing digital precoder, we develop an explicit upper bound for the
analog-and-digital-precoded channel gain of users, based on which the
relationship between the two structures is investigated. The analysis results
show that the full-connection structure is not always better than the
partial-connection structure, and the regimes suitable for each structure are
revealed. Simulations are conducted to validate the analysis results.
