Millimeter wave (mmWave) links will offer high capacity but are poor at penetrating into or diffracting around solid objects. Thus, we consider a hybrid cellular network with traditional sub 6 GHz macrocells coexisting with denser mmWave small cells, where a mobile user can connect to either opportunistically. We develop a general analytical model to characterize and derive the uplink and downlink cell association in view of the SINR and rate coverage probabilities in such a mixed deployment. We offer extensive validation of these analytical results (which rely on several simplifying assumptions) with simulation results. Using the analytical results, different decoupled uplink and downlink cell association strategies are investigated and their superiority is shown compared to the traditional coupled approach.Finally, small cell biasing in mmWave is studied, and we show that unprecedented biasing values are desirable due to the wide bandwidth. is with OFCOM, UK. M. Dohler (mischa.dohler@kcl.ac.uk) is with the Centre for Telecommunications Research (CTR), King's College London, UK. I. INTRODUCTIONTwo key capacity-increasing techniques for future cellular networks including 5G will be network densification and the use of higher frequency bands, such as millimeter wave (mmWave) [1], [2]. The main challenges to using mmWave frequencies are their high near-field pathloss (due to small effective antenna aperture) and very poor penetration into buildings. However, it is increasingly believed these challenges can be overcome, at least for outdoor-to-outdoor cellular networks, using high gain steerable antennas in a dense enough network with sufficient scattering[3]- [11]. Further, recent studies have shown that with such highly directional transmissions and sensitivity to blockage, a positive side effect is that interference is greatly reduced, and so in many or most cases, mmWave networks will be noise rather than interference-limited [10]- [13].Nevertheless, it is unrealistic to expect universal coverage with mmWave, especially indoors, and so a likely deployment scenario is that mmWave will co-exist with a traditional sub-6GHz cellular network. The mmWave small cells will be used opportunistically when a connection is possible, with the sub-6GHz base stations providing universal coverage, for both control signaling and for data when a mmWave connection is not available. The goal of this paper is to model and analyze such a hybrid network, considering in particular how user equipments (UEs) should associate with the two types of BSs in the uplink (UL) and downlink (DL). A. Related WorkDownlink and uplink associations are typically coupled, i.e. a UE connects to the same BS in the DL and UL. In the context of a heterogeneous network, downlink-uplink decoupling (DUDe) has been recently shown to significantly improve the network capacity (especially in the UL) by considering different association criteria for the UL and DL [14]. DUDe has been discussed in [2], [15], [16] as an interesting component for future cellular netw...

Abstract-Cell association in cellular networks has traditionally been based on the downlink received signal power only, despite the fact that up 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.Index Terms-5G, Heterogeneous Networks, downlink and uplink decoupling.

Ever since the inception of mobile telephony, the downlink and uplink of cellular networks have been coupled, i.e. mobile terminals have been constrained to associate with the same base station (BS) in both the downlink and uplink directions. New trends in network densification and mobile data usage increase the drawbacks of this constraint, and suggest that it should be revisited. In this paper we identify and explain five key arguments in favor of Downlink/Uplink Decoupling (DUDe) based on a blend of theoretical, experimental, and logical arguments. We then overview the changes needed in current (LTE-A) mobile systems to enable this decoupling, and then look ahead to fifth generation (5G) cellular standards. We believe the introduced paradigm will lead to significant gains in network throughput, outage and power consumption at a much lower cost compared to other solutions providing comparable or lower gains.