  Traffic load balancing and radio resource management is key to harness the
dense and increasingly heterogeneous deployment of next generation "$5$G"
wireless infrastructure. Strategies for aggregating user traffic from across
multiple radio access technologies (RATs) and/or access points (APs) would be
crucial in such heterogeneous networks (HetNets), but are not well
investigated. In this paper, we develop a low complexity solution for
maximizing an $\alpha$-optimal network utility leveraging the multi-link
aggregation (simultaneous connectivity to multiple RATs/APs) capability of
users in the network. The network utility maximization formulation has
maximization of sum rate ($\alpha=0$), maximization of minimum rate ($\alpha
\to \infty$), and proportional fair ($\alpha=1$) as its special cases. A closed
form is also developed for the special case where a user aggregates traffic
from at most two APs/RATs, and hence can be applied to practical scenarios like
LTE-WLAN aggregation (LWA) and LTE dual-connectivity solutions. It is shown
that the required objective may also be realized through a decentralized
implementation requiring a series of message exchanges between the users and
network. Using comprehensive system level simulations, it is shown that optimal
leveraging of multi-link aggregation leads to substantial throughput gains over
single RAT/AP selection techniques.
