Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A New Look at Physical Layer Security, Caching, and Wireless Energy
  Harvesting for Heterogeneous Ultra-dense Networks   Heterogeneous ultra-dense networks enable ultra-high data rates and ultra-low
latency through the use of dense sub-6 GHz and millimeter wave (mmWave) small
cells with different antenna configurations. Existing work has widely studied
spectral and energy efficiency in such networks and shown that high spectral
and energy efficiency can be achieved. This article investigates the benefits
of heterogeneous ultra-dense network architecture from the perspectives of
three promising technologies, i.e., physical layer security, caching, and
wireless energy harvesting, and provides enthusiastic outlook towards
application of these technologies in heterogeneous ultra-dense networks. Based
on the rationale of each technology, opportunities and challenges are
identified to advance the research in this emerging network.

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The categories of this paper are: alg-geom
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The categories of this paper are: dg-ga
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The categories of this paper are: eess
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The categories of this paper are: plasm-ph
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The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
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The categories of this paper are: econ
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The categories of this paper are: q-fin
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The categories of this paper are: solv-int
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The categories of this paper are: cmp-lg
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The categories of this paper are: math-ph
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The categories of this paper are: q-alg
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph
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The categories of this paper are: nucl-th