Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Small-Scale Spatial-Temporal Correlation and Degrees of Freedom for
  Reconfigurable Intelligent Surfaces   The reconfigurable intelligent surface (RIS) is an emerging promising
candidate technology for future wireless networks, where the element spacing is
usually of sub-wavelength. Only limited knowledge, however, has been gained
about the spatial-temporal correlation behavior among the elements in an RIS.
In this paper, we investigate the spatial-temporal correlation for an
RIS-enabled wireless communication system. Specifically, a joint small-scale
spatial-temporal correlation model is derived under isotropic scattering, which
can be represented by a four-dimensional sinc function. Furthermore, based upon
the spatial-only correlation at a certain time instant, an essential RIS
property -- the spatial degrees of freedom (DoF) -- is revisited, and an
analytical expression is propounded to characterize the spatial DoF for RISs
with realistic hence non-infinitesimal element spacing and finite aperture
sizes. The results are vital to the accurate evaluation of various system
performance metrics.

5
The categories of this paper are: gr-qc
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
24
The categories of this paper are: dg-ga
17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
7
The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
21
The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
22
The categories of this paper are: chao-dyn
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
2
The categories of this paper are: math