RIS-Assisted Multicasting through Novel Group Formation for Highly-Directional Communications

Mobile network traffic has seen a significant surge, doubling in the past two years, due to increased adoption of fifth-generation ($\mathbf{5 G)}$ subscriptions. This emphasizes the need for efficient solutions like multicasting to manage escalating data demands. Multicast communications, however, encounter challenges in highly directional transmission scenarios, typical for 5G and sixth-generation (6G) systems. In such scenarios, characterized by the presence of obstacles or signal attenuation affecting one user, the performance degradation propagates across the entire multicast group. While Reconfigurable Intelligent Surfaces (RISs) have shown promise in mitigating blockage issues and optimizing wireless propagation environments, limited research has been done on their application to multicast communication. To address this gap, this study introduces a novel multicast group formation framework designed for highly directional RIS-enhanced multicast communications. The framework utilizes Self-Organizing Map (SOM) to organize close-range users into multicast groups based on the similarity of their azimuth angles relative to the RIS, while either microwave multicasting or mmWave unicasting are considered for users at greater distances. Furthermore, the study provides closed-form expressions for the transition threshold from SOM to either microwave multicasting or mmWave unicasting. Our results (in terms of data rate) demonstrate that the proposed approach with microwave technology outperforms all benchmark schemes and remains stable as the number of users increases, while the mmWave unicast approach exhibits performance similar to pure unicasting.