Determining the optimal user association is, for general
utility functions, a massive combinatorial optimization problem
that depends on the SINR from every user to every BS, the
instantaneous load at each BS, the choices of other users in the
network, and possibly other constraints such as the requirement to utilize the same BS and standard in both uplink and downlink
(to facilitate functioning control channels for resource allocation
and feedback) [35], [36]. Therefore, simplified procedures
must be adopted [37], an example of which appears in this
special issue [38]. The key such simplified procedures are
“biasing” and macrocell “blanking” or “muting”. Biasing refers
to associating with a small cell even if it provides a lower
SINR than the macrocell, and is useful for pushing users off of
the heavily loaded macrocell and onto the lightly loaded small
cell. Everyone wins: the remaining macrocell users get more
resources while the biased users have a lower SINR/spectral
efficiency but can utilize a large number of resource blocks on
the small cell, ultimately attaining a higher data rate. Blanking
refers to shutting off the macrocell transmissions for some
fraction of the time, preferably while the biased small cell
users are being served. This raises all the small-cell SINRs
considerably—enough to justify actually shutting down even
congested macrocell BSs—while also providing a mechanism
for the biased users to hear common control channels that
would otherwise be swamped by the macrocells.