  In downlink multiuser multiple-input multiple-output (MU-MIMO) systems, users
are practically heterogeneous in nature. However, most of the existing user
scheduling algorithms are designed with an implicit assumption that the users
are homogeneous. In this paper, we revisit the problem by exploring the
characteristics of heterogeneous users from a subspace point of view. With an
objective of minimizing interference non-orthogonality among users, three new
angular-based user scheduling criteria that can be applied in various user
scheduling algorithms are proposed. While the first criterion is heuristically
determined by identifying the incapability of largest principal angle to
characterize the subspace correlation and hence the interference
non-orthogonality between users, the second and third ones are derived by
using, respectively, the sum rate capacity bounds with block diagonalization
and the change in capacity by adding a new user into an existing user subset.
Aiming at capturing fairness among heterogeneous users while maintaining
multiuser diversity gain, two new hybrid user scheduling algorithms are also
proposed whose computational complexities are only linearly proportional to the
number of users. We show by simulations that the effectiveness of our proposed
user scheduling criteria and algorithms with respect to those commonly used in
homogeneous environment.
