User association techniques in future heterogeneous networks

The exponential increase in the demand of data rate and scarcity of the available spectrum have evolved to the more advanced network topology which efficiently utilizes the radio spectrum such as Heterogeneous Networks (HetNets). HetNets consist of many tiers of base stations, having different transmit powers. The macro base stations transmit at high power level whereas pico and femto base stations have much less power as compared to macro. An efficient user association becomes necessary to exploit the actual benefits of the HetNets. However, the user association is challenging task and recently a number of techniques have been proposed. In this study, we summarize the recent developments in user association methods under different constraints.