# RIS Beams & MIMO (Primer)

## Concept
A Reconfigurable Intelligent Surface (RIS) applies per-element phases $\theta_{n}$ to reflect incident waves. The effective scalar channel (TX $\rightarrow$ RIS $\rightarrow$ UE) is:
$h_\text{eff} = \sum_{n=1}^{N} h_{\mathrm{RIS}\to \mathrm{UE},n}\, e^{j \theta_n}\, h_{\mathrm{TX}\to \mathrm{RIS},n}$
## RIS and MIMO
<p align="center">
  <img src="../_static/img/ris_diagram.svg" alt="ris" width="500px">
</p>

<p align="center">
  <img src="../_static/img/mimo_block.svg" alt="ris" width="500px">
</p>