  This paper is concerned with spatial multiplexing analysis for
millimeter-wave (mmWave) massive MIMO systems. For a single-user mmWave system
employing distributed antenna subarray architecture in which the transmitter
and receiver consist of Kt and Kr subarrays, respectively, an asymptotic
multiplexing gain formula is firstly derived when the numbers of antennas at
subarrays go to infinity. Specifically, assuming that all subchannels have the
same average number of propagation paths L, the formula implies that by
employing such a distributed antenna-subarray architecture, an exact average
maximum multiplexing gain of KrKtL can be achieved. This result means that
compared to the co-located antenna architecture, using the distributed
antenna-subarray architecture can scale up the maximum multiplexing gain
proportionally to KrKt. In order to further reveal the relation between
diversity gain and multiplexing gain, a simple characterization of the
diversity-multiplexing tradeoff is also given. The multiplexing gain analysis
is then extended to the multiuser scenario. Moreover, simulation results
obtained with the hybrid analog/digital processing corroborate the analysis
results.
