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

Channel Estimation for RIS Assisted Wireless Communications: Part II -- An Improved Solution Based on Double-Structured Sparsity