  Reconfigurable intelligent surfaces (RISs) have recently emerged as a
promising technology that can achieve high spectrum and energy efficiency for
future wireless networks by integrating a massive number of low-cost and
passive reflecting elements. An RIS can manipulate the properties of an
incident wave, such as the frequency, amplitude, and phase, and, then, reflect
this manipulated wave to a desired destination, without the need for complex
signal processing. In this paper, the asymptotic optimality of achievable rate
in a downlink RIS system is analyzed under a practical RIS environment with its
associated limitations. In particular, a passive beamformer that can achieve
the asymptotic optimal performance by controlling the incident wave properties
is designed, under a limited RIS control link and practical reflection
coefficients. In order to increase the achievable system sum-rate, a modulation
scheme that can be used in an RIS without interfering with existing users is
proposed and its average symbol error ratio is asymptotically derived.
Moreover, a new resource allocation algorithm that jointly considers user
scheduling and power control is designed, under consideration of the proposed
passive beamforming and modulation schemes. Simulation results show that the
proposed schemes are in close agreement with their upper bounds in presence of
a large number of RIS reflecting elements thereby verifying that the achievable
rate in practical RISs satisfies the asymptotic optimality.
