Downlink and Uplink Decoupling in Heterogeneous Networks for 5G and Beyond

To meet the explosively increased data traffic demand from 5G and beyond, one of the most promising technologies is densified heterogeneous networks (HetNets). In HetNets, base stations (BSs) are brought closer and closer to users by densifying small BSs, which results in considerable high spectral efficiency and energy efficiency for cellular systems. However, HetNet topology challenges the traditional cellular systems. One of the most important challenges is user association. Aiming to avoid the drawback brought by coupled access (CA) in user association of HetNets, a decoupled access (DA) approach has emerged recently. By allowing access points in uplink (UL) and downlink (DL) association to be different, DA significantly improves UL performance of HetNets. This article focuses on recent works in DA. First, typical scenarios of HetNets for DA are described, which is followed by presenting two major decoupled association policies under typical scenarios. Then, DA shows its superiority over CA under different scenarios and decoupled association policies via various performance metrics. Finally, future work on DA in HetNets is discussed.