text: Our traditional notion of a cell is changing dramatically given the increasing degree of heterogeneity in 4G and emerging 5G systems. Rather than belonging to a specific cell, a device would choose the most suitable connection from the plethora of connections available. In such a setting, given the transmission powers differ significantly between downlink (DL) and uplink (UL), a wireless device that sees multiple Base Stations (BSs) may access the infrastructure in a way that it receives the downlink (DL) traffic from one BS and sends uplink (UL) traffic through another BS. This situation is referred to as Downlink and Uplink Decoupling (DUDe). In this paper, the capacity and throughput gains brought by decoupling are rigorously derived using stochastic geometry. Theoretical findings are then corroborated by means of simulation results. A further constituent of this paper is the verification of the theoretically derived results by means of a real-world system simulation platform. Despite theoretical assumptions differing from the very complete system simulator, the trends in the association probabilities and capacity gains are similar. Based on the promising results, we then outline architectural changes needed to facilitate the decoupling of DL and UL.
  @cite_15: Cell association in cellular networks has traditionally been based on the downlink received signal power only, despite the fact that uplink and downlink transmission powers and interference levels differed significantly. This approach was adequate in homogeneous networks with macro base stations all having similar transmission power levels. However, with the growth of heterogeneous networks where there is a big disparity in the transmit power of the different base station types, this approach is highly inefficient. In this paper, we study the notion of Downlink and Uplink Decoupling (DUDe) where the downlink cell association is based on the downlink received power while the uplink is based on the pathloss. We present the motivation and assess the gains of this 5G design approach with simulations that are based on Vodafone's LTE field trial network in a dense urban area, employing a high resolution ray-tracing pathloss prediction and realistic traffic maps based on live network measurements.
  @cite_21: Wireless cellular networks evolve towards a heterogeneous infrastructure, featuring multiple types of Base Stations (BSs), such as Femto BSs (FBSs) and Macro BSs (MBSs). A wireless device observes multiple points (BSs) through which it can access the infrastructure and it may choose to receive the downlink (DL) traffic from one BS and send uplink (UL) traffic through another BS. Such a situation is referred to as decoupled DL UL access . Using the framework of stochastic geometry, we derive the association probability for DL UL. To maximize the average received power, as the relative density of FBSs initially increases, a large fraction of devices chooses decoupled access, i.e., receive from a MBS in DL and transmit through a FBS in UL. We analyze the impact that this type of association has on the average throughput in the system.
summary: Notably, the most recent contributions @cite_15 @cite_21 have studied the impact of having a cell association which is different for downlink and uplink. The authors of @cite_15 have investigated the throughput and outage gains via the real-world simulation tool Atoll; the simulation results have yielded important performance gains which increase with an increasing density of small cells. The authors of @cite_21 then studied the association probabilities in more details, i.e. the probabilities that a device would associate i) both DL and UL to the Mcell; ii) DL to Scell and UL to Mcell; iii) DL to Mcell and UL to Scell; and iv) both DL and UL to the Scell.