The fifth generation (5G) mobile networks are envisioned to support the
deluge of data traffic with reduced energy consumption and improved quality of
service (QoS) provision. To this end, the key enabling technologies, such as
heterogeneous networks (HetNets), massive multiple-input multiple-output (MIMO)
and millimeter wave (mmWave) techniques, are identified to bring 5G to
fruition. Regardless of the technology adopted, a user association mechanism is
needed to determine whether a user is associated with a particular base station
(BS) before the data transmission commences. User association plays a pivotal
role in enhancing the load balancing, the spectrum efficiency and the energy
efficiency of networks. The emerging 5G networks introduce numerous challenges
and opportunities for the design of sophisticated user association mechanisms.
Hence, substantial research efforts are dedicated to the issues of user
association in HetNets, massive MIMO networks, mmWave networks and energy
harvesting networks. We introduce a taxonomy as a framework for systematically
studying the existing user association algorithms. Based on the proposed
taxonomy, we then proceed to present an extensive overview of the
state-of-the-art in user association conceived for HetNets, massive MIMO,
mmWave and energy harvesting networks. Finally, we summarize the challenges as
well as opportunities of user association in 5G and provide design guidelines
and potential solutions for sophisticated user association mechanisms.Comment: 26 pages; accepted to appear in IEEE Communications Surveys and
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