Defining of the optimal user association is the prime
area of concern which depends on the signal to interfer-
ence and noise ratio from every single user to every single
base station, the selections of other users in the network,
the load at every single base station, and the prerequisite
to apply the same base station and standard in both uplink
and downlink for simplifying the operation of control
channels for resource allocation and feedback [69], [70].
So, certain procedures must be implemented to overcome
these issues.