Time Switching-Variable Mode Splitting Protocol for STAR-RIS-aided SWIPT in NOMA Networks

This paper presents a novel transmission protocol for simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) that jointly exploits traditional time switching and mode splitting protocol. The proposed approach, namely time-switching-variable mode splitting (TVM), allows to dynamically allocate STAR-RIS resources (i.e., time slot duration, and number of reflecting and transmitting elements) based on variable environment conditions. Energy harvesting (EH) and information transfer (IT) phases alternate to each other in downlink according to a variable time frame structure, while the energy harvested in downlink is exploited for IT in uplink. The elements of the STAR-RIS are then dynamically switched from transmission to reflection in different time slots based on the demand of the network, allowing the transmission users (TUs) and the reflection users (RUs) to be connected to the base station (BS) simultaneously. The proposed approach is investigated under the hypothesis of a non-orthogonal multiple access (NOMA) network, and the Nakagami-m distributed channel model with attenuation loss. The analytical performance metrics are obtained considering dynamic time slots and link duration for both IT and EH downlink phases, as well as a variable number of transmission and reflection elements for each time slot. The assessment of the proposed TVM protocol is carried out by means of numerical and Monte Carlo (MC) simulation results of the joint outage probability, the sum throughput, and the age of information in case of multiple reflecting and transmitting users.