Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Reconfigurable Intelligent Surfaces for 6G Systems: Principles,
  Applications, and Research Directions   Reconfigurable intelligent surfaces (RISs) or intelligent reflecting surfaces
(IRSs), are regarded as one of the most promising and revolutionizing
techniques for enhancing the spectrum and/or energy efficiency of wireless
systems. These devices are capable of reconfiguring the wireless propagation
environment by carefully tuning the phase shifts of a large number of low-cost
passive reflecting elements. In this article, we aim for answering four
fundmental questions: 1) Why do we need RISs? 2) What is an RIS? 3) What are
RIS's applications? 4) What are the relevant challenges and future research
directions? In response, eight promising research directions are pointed out.

37
The categories of this paper are: supr-con
17
The categories of this paper are: q-fin
2
The categories of this paper are: math
29
The categories of this paper are: funct-an
0
The categories of this paper are: hep-ph
27
The categories of this paper are: comp-gas
3
The categories of this paper are: physics
12
The categories of this paper are: nucl-th
16
The categories of this paper are: stat
14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
5
The categories of this paper are: gr-qc
22
The categories of this paper are: chao-dyn
28
The categories of this paper are: alg-geom