Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Downlink Channel Covariance Matrix Reconstruction for FDD Massive MIMO
  Systems with Limited Feedback
15
The categories of this paper are: nucl-ex
14
The categories of this paper are: math-ph
18
The categories of this paper are: econ
6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
3
The categories of this paper are: physics
25
The categories of this paper are: patt-sol
11
The categories of this paper are: hep-lat
7
The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
26
The categories of this paper are: bayes-an
12
The categories of this paper are: nucl-th
20
The categories of this paper are: acc-phys
37
The categories of this paper are: supr-con