Rate-Splitting Multiple Access With STAR RIS Over Spatially-Correlated Channels

Abstract

The paper explores simultaneous transmitting and reflecting (STAR) reconfigurable intelligent surface (RIS)-aided rate-splitting multiple access (RSMA) communication. A base-station (BS) transmits information to the cell-edge users which are assisted by the STAR RIS in such a way that two users, one placed in front of and other behind the RIS, are served by reflection and transmission, respectively. The energy splitting (ES) and mode switching (MS) configurations of the STAR RIS are considered for analysis. Owing to the close-placement of RIS elements and rectangular geometry of RIS, the Rician distributed RIS channels are assumed to be spatially-correlated. The new expression for the joint moment of spatially-correlated Rician channels is derived. Both, infinite and finite block-length transmissions are analyzed in this work. For infinite block-length transmission, the expressions for outage probability and channel capacity are derived, whereas for finite block-length transmission, the expressions for block error rate (BLER), system goodput and channel capacity are derived. To get further insights, the expression for asymptotic outage probability is also derived. Furthermore, the effects of various parameters, including RSMA power splitting factor, reflection coefficients, inter-element spacing, distances between STAR RIS and users, finite block-length parameters, imperfect channel state information (CSI), etc., are examined.