Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: PDRS: A Fast Non-iterative Scheme for Massive Grant-free Access in
  Massive MIMO   Grant-free multiple-input multiple-output (MIMO) usually employs
non-orthogonal pilots for joint user detection and channel estimation. However,
existing methods are too complex for massive grant-free access in massive MIMO.
This letter proposes pilot detection reference signal (PDRS) to greatly reduce
the complexity. In PDRS scheme, no iteration is required. Direct weight
estimation is also proposed to calculate combining weights without channel
estimation. After combining, PDRS recovery errors are used to decide the pilot
activity. The simulation results show that the proposed grant-free scheme
performs good with a complexity reduced by orders of magnitude.

25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
6
The categories of this paper are: astro-ph
30
The categories of this paper are: q-alg
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The categories of this paper are: alg-geom
29
The categories of this paper are: funct-an
5
The categories of this paper are: gr-qc
24
The categories of this paper are: dg-ga
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The categories of this paper are: plasm-ph
3
The categories of this paper are: physics
8
The categories of this paper are: hep-ex
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The categories of this paper are: solv-int
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The categories of this paper are: bayes-an
9
The categories of this paper are: nlin