  In this paper, we focus on one of the representative 5G network scenarios,
namely multi-tier heterogeneous cellular networks. User association is
investigated in order to reduce the down-link co-channel interference. Firstly,
in order to analyze the multi-tier heterogeneous cellular networks where the
base stations in different tiers usually adopt different transmission powers,
we propose a Transmission Power Normalization Model (TPNM), which is able to
convert a multi-tier cellular network into a single-tier network, such that all
base stations have the same normalized transmission power. Then using TPNM, the
signal and interference received at any point in the complex multi-tier
environment can be analyzed by considering the same point in the equivalent
single-tier cellular network model, thus significantly simplifying the
analysis. On this basis, we propose a new user association scheme in
heterogeneous cellular networks, where the base station that leads to the
smallest interference to other co-channel mobile stations is chosen from a set
of candidate base stations that satisfy the quality-of-service (QoS) constraint
for an intended mobile station. Numerical results show that the proposed user
association scheme is able to significantly reduce the down-link interference
compared with existing schemes while maintaining a reasonably good QoS.
