  By flexibly manipulating the radio propagation environment, reconfigurable
intelligent surface (RIS) is a promising technique for future wireless
communications. However, the single-side coverage and double-fading attenuation
faced by conventional RISs largely restrict their applications. To address this
issue, we propose a novel concept of multi-functional RIS (MF-RIS), which
provides reflection, transmission, and amplification simultaneously for the
incident signal. With the aim of enhancing the performance of a non-orthogonal
multiple-access (NOMA) downlink multiuser network, we deploy an MF-RIS to
maximize the sum rate by jointly optimizing the active beamforming and MF-RIS
coefficients. Then, an alternating optimization algorithm is proposed to solve
the formulated non-convex problem by exploiting successive convex approximation
and penalty-based method. Numerical results show that the proposed MF-RIS
outperforms conventional RISs under different settings.
