  This paper studies how to maximize the per-user-based throughput in an M-tier
heterogeneous wireless network (HetNet) by optimally managing traffic flows
between the access points (APs) in the HetNet. The APs in the first M-1 tiers
can use the licensed spectrum at the same time whereas they share the
unlicensed spectrum with the APs in the Mth tier by the proposed opportunistic
carrier sense multiple access with collision avoidance (CSMA/CA) protocol. The
APs that access the licensed and unlicensed spectra simultaneously are able to
integrate their spectrum resources by the carrier aggregation technique. We
first characterize the distribution of the cell load and the channel access
probability of each AP using a generalized AP association scheme. For an AP in
each tier, the tight lower bounds on its mean spectrum efficiencies in the
licensed and unlicensed spectra are derived for the general random models of
the channel gain and AP association weights. We define the per-user link
throughput and per-user network throughput based on the derived the mean
spectrum efficiencies and maximize them by proposing the decentralized and
centralized traffic management schemes for the APs in the first M-1 tiers under
the constraint that the per-user link throughput of the tier-M APs must be
above some minimum required value. Finally, a numerical example of coexisting
LTE and WiFi networks is provided to validate our derived results and findings.
