  This paper is concerned with asymptotic diversity analysis for
millimeter-wave (mmWave) massive MIMO systems. First, for a single-user mmWave
system employing distributed antenna subarray architecture in which the
transmitter and receiver consist of Kt and Kr subarrays, respectively, a
diversity gain theorem is established when the numbers of antennas at subarrays
go to infinity. Specifically, assuming that all subchannels have the same
number of propagation paths L, the theorem states that by employing such a
distributed antenna-subarray architecture, a diversity gain of KrKtL-Ns+1 can
be achieved, where Ns is the number of data streams. This result means that
compared to the co-located antenna architecture, using the distributed
antenna-subarray architecture can scale up the diversity gain or multiplexing
gain proportionally to KrKt. The diversity gain analysis is then extended to
the multiuser scenario as well as the scenario with conventional
partially-connected RF structure in the literature. Simulation results obtained
with the hybrid analog/digital processing corroborate the analysis results and
show that the distributed subarray architecture indeed yields significantly
better diversity performance than the co-located antenna architectures.
