  Joint Spatial Division and Multiplexing (JSDM) is a recently proposed scheme
to enable massive MIMO like gains and simplified system operations for
Frequency Division Duplexing (FDD) systems. The key idea lies in partitioning
the users into groups with approximately similar covariances, and use a two
stage downlink beamforming: a pre-beamformer that depends on the channel
covariances and minimizes interference across groups and a multiuser MIMO
precoder for the effective channel after pre-beamforming, to counteract
interference within a group. We first focus on the regime of a fixed number of
antennas and large number of users, and show that opportunistic beamforming
with user selection yields significant gain, and thus, channel correlation may
yield a capacity improvement over the uncorrelated "isotropic" channel result
of Sharif and Hassibi. We prove that in the presence of different correlations
among groups, a block diagonalization approach for the design of
pre-beamformers achieves the optimal sum-rate scaling. Next, we consider the
regime of large number of antennas and users, where user selection does not
provide significant gain. Here, we propose a simplified user grouping algorithm
to cluster users into groups when the number of antennas becomes very large, in
a realistic setting where users are randomly distributed and have different
angles of arrival and angular spreads depending on the propagation environment.
Our subsequent analysis leads to a probabilistic scheduling algorithm, where
users within each group are preselected at random based on probabilities
derived from the large system analysis, depending on the fairness criterion.
This is advantageous since only the selected users are required to feedback
their channel state information (CSIT).
