  In future networks, an operator may employ a wide range of access points
using diverse radio access technologies (RATs) over multiple licensed and
unlicensed frequency bands. This paper studies centralized user association and
spectrum allocation across many access points in such a heterogeneous network
(HetNet). Such centralized control is on a relatively slow timescale to allow
information exchange and joint optimization over multiple cells. This is in
contrast and complementary to fast timescale distributed scheduling. A queueing
model is introduced to capture the lower spectral efficiency, reliability, and
additional delays of data transmission over the unlicensed bands due to
contention and/or listen-before-talk requirements. Two optimization-based
spectrum allocation schemes are proposed along with efficient algorithms for
computing the allocations. The proposed solutions are fully aware of traffic
loads, network topology, as well as external interference levels in the
unlicensed bands. Packet-level simulation results show that the proposed
schemes significantly outperform orthogonal and full-frequency-reuse
allocations under all traffic conditions.
