Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: User Association and Resource Allocation in Unified NOMA Enabled
  Heterogeneous Ultra Dense Networks   Heterogeneous ultra dense networks (HUDNs) and non-orthogonal multiple access
(NOMA) have been identified as two proposing techniques for the fifth
generation (5G) mobile communication systems due to their great capabilities to
enhance spectrum efficiency. This article investigates the application of NOMA
techniques in HUDNs to support massive connectivity in 5G systems.
Particularly, a unified NOMA framework is proposed, including power-domain NOMA
and code-domain NOMA, which can be configured flexibly to serve different
applications scenarios. As a further advance, the unified NOMA framework
enabled HUDNs is further investigated, with particular focuses on the user
association and resource allocation. Two case studies are provided for
demonstrating the effectiveness of the unified NOMA enabled HUDNs. Finally,
some main challenges and promising research directions in NOMA enabled HUDNs
are identified. Index Terms Heterogeneous ultra dense networks, non-orthogonal
multiple access, massive connectivity, user association, and resource
allocation

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The categories of this paper are: supr-con
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The categories of this paper are: plasm-ph
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The categories of this paper are: q-bio
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The categories of this paper are: alg-geom
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The categories of this paper are: atom-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: funct-an
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The categories of this paper are: q-fin
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The categories of this paper are: nucl-ex
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The categories of this paper are: econ
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The categories of this paper are: cond-mat
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The categories of this paper are: adap-org
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The categories of this paper are: hep-th
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The categories of this paper are: bayes-an
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The categories of this paper are: chao-dyn