  Reconfigurable intelligent surfaces (RISs) have attracted the attention of
academia and industry circles because of their ability to control the
electromagnetic characteristics of channel environments. However, it has been
found that the introduction of an RIS may bring new and more serious network
coexistence problems. It may even further deteriorate the network performance
if these new network coexistence problems cannot be effectively solved. In this
paper, an RIS network coexistence model is proposed and discussed in detail,
and these problems are deeply analysed. Two novel RIS design mechanisms,
including a novel multilayer RIS structure with an out-of-band filter and an
RIS blocking mechanism, are further explored. Finally, numerical results and a
discussion are given.
