  Reconfigurable intelligent surface (RIS) can manipulate the wireless
communication environment by controlling the coefficients of RIS elements.
However, due to the large number of passive RIS elements without signal
processing capability, channel estimation in RIS assisted wireless
communication system requires high pilot overhead. In the second part of this
invited paper, we propose to exploit the double-structured sparsity of the
angular cascaded channels among users to reduce the pilot overhead.
Specifically, we first reveal the double-structured sparsity, i.e., different
angular cascaded channels for different users enjoy the completely common
non-zero rows and the partially common non-zero columns. By exploiting this
double-structured sparsity, we further propose the double-structured orthogonal
matching pursuit (DS-OMP) algorithm, where the completely common non-zero rows
and the partially common non-zero columns are jointly estimated for all users.
Simulation results show that the pilot overhead required by the proposed scheme
is lower than existing schemes.
