  This paper focuses on the design of transmission methods and reflection
optimization for a wireless system assisted by a single or multiple
reconfigurable intelligent surfaces (RISs). The existing techniques are either
too complex to implement in practical systems or too inefficient to achieve
high performance. To overcome the shortcomings of the existing schemes, we
propose a simple but efficient approach based on \textit{opportunistic
reflection} and \textit{non-orthogonal transmission}. The key idea is
opportunistically selecting the best user that can reap the maximal gain from
the optimally reflected signals via RIS. That is to say, only the channel state
information of the best user is used for RIS reflection optimization, which can
in turn lower complexity substantially. In addition, the second user is
selected to superpose its signal on that of the primary user, where the
benefits of non-orthogonal transmission, i.e., high system capacity and
improved user fairness, are obtained. Additionally, a simplified variant
exploiting random phase shifts is proposed to avoid the high overhead of RIS
channel estimation.
