  Reconfigurable Intelligent Surface (RIS) has become a popular technology to
improve the capability of a THz multiuser Multi-input multi-output (MIMO)
communication system. THz wave characteristics, on the other hand, restrict THz
beam coverage on RIS when using a uniform planar array (UPA) antenna. In this
study, we propose a dynamic RIS subarray structure to improve the performance
of a THz MIMO communication system. In more details, an RIS is divided into
several RIS subarrays according to the number of users. Each RIS subarray is
paired with a user and only reflects beams to the corresponding user. Based on
the structure of RIS, we first propose a weighted minimum mean square error -
RIS local search (WMMSE-LS) scheme, which requires that each RIS element has
limited phase shifts. To improve the joint beamforming performance, we further
develop an adaptive Block Coordinate Descent(BCD)-aided algorithm, an iterative
optimization method. Numerical results demonstrate the effectiveness of the
dynamic RIS subarray structure and the adaptive BCD-aided joint beamforming
scheme and also show the merit of our proposed system.
