A Realistic Uplink–Downlink Coupled and Decoupled User Association Technique for K-tier 5G HetNets

Cell association policies in present-day heterogeneous networks (HetNets) cannot promise fairness and high capacity to all network users. To optimize this cell association, concept of downlink (DL)–uplink (UL) decoupling (DUDe) has been introduced where uplink and downlink channels can be associated with two different base stations (BSs) instead of existing policy of being associated with same BS. For this decoupled access, a number of theoretical uplink or downlink analysis frameworks have been proposed. However, these frameworks do not fully take into account the practical realization aspects. In these preceding works, all network users are assumed to employ decoupled access despite the fact that it is not preferred by all users. These models also ignore noise to get simplified solutions which can result into practical inaccuracies. In this paper, we propose a realistic user association technique in which both coupled access and decoupled access are permissible to users, depending upon their location and benefits. Therefore, the goal of the present paper is to present K-tier uplink analysis framework for network devices that prefer decoupled access. Simple closed-form solutions without ignoring noise are presented, which relate user performance to number of HetNet tiers, base-station densities and power levels. Moreover, derived distributions are also employed to compare performance of DUDe case with existing process of coupled access. Finally, decoupling impacts on K-tier network design have been presented. Based on conducted analysis, it can be concluded that decoupling is beneficial and can be easily realized in 5G K-tier HetNets.