  Reconfigurable intelligent surfaces (RISs) have been deemed as one of
potential components of future wireless communication systems because they can
adaptively manipulate the wireless propagation environment with low-cost
passive devices. However, due to double fading effect, the passive RIS can
offer sufficient signal strength only when receivers are nearby and located at
the same side as the incident signals. Moreover, RIS cannot provide service
coverage for the users at the back side of it. In this paper we introduce a
novel reflection and relay dual-functional RIS architecture, which can
simultaneously realize passive reflection and active relay functionalities to
enhance the coverage. The problem of joint transmit beamforming and
dual-functional RIS design is investigated to maximize the achievable sum-rate
of a multiuser multiple-input single-output (MU-MISO) system. Based on
fractional programming (FP) theory and majorization-minimization (MM)
technique, we propose an efficient iterative transmit beamforming and RIS
design algorithm. Simulation results demonstrate the superiority of the
introduced dual-functional RIS architecture and the effectiveness of the
proposed algorithm.
