  In this paper, we propose a new model for indoor THz communication assisted
by RIS. We conduct a realistic modeling of indoor obstacles and analyze their
impact on performance. Order statistics are applied to calculate the cumulative
distribution functions (CDFs) of distances from the transmitter to the selected
RIS, i.e., the nearest RIS in the bounded indoor environment to the
transmitter, and from the selected RIS to the receiver. We calculate the
coverage probability (CP) as a function of RIS number, obstacle density, room
size, and the transmitter's location. By comparing the numerical results
obtained from the analytical expressions with Monte Carlo simulations, we
verify the accuracy of our analysis. Through numerical results, it is observed
that room size and obstacle density affect the CP in a significant way.
However, by optimizing the transmitter's location and increasing the RIS number
deployed in the room, the CP can be significantly improved (e.g., an increase
of around 15% by optimizing the transmitter's location, and an increase of
around 30% by increasing the RIS number deployed in the room).
