IAB vs. RIS: Performance-Cost Tradeoffs in 5G/6G Systems with Multicast and Unicast Traffic in Roadside Deployments

The introduction of millimeter wave (mmWave) and sub-terahertz (sub-THz) frequency bands in 5G and future 6G networks promises an unprecedented capacity enhancement at the air interface driven by highly directional transmissions. While this facilitates interference suppression and increased deployment density, it also presents challenges in multicast service delivery, particularly due to the use of directional antennas and environmental factors that can cause signal blockage. Technologies such as Integrated Access and Backhaul (IAB) and Reconfigurable Intelligent Surfaces (RISs) have emerged to address these challenges and reduce capital expenditure. This study comprehensively compares IAB- and RIS-based designs for cost-efficient densification in mmWave and sub-THz 5G/6G systems, focusing on both unicast and multicast traffic in roadside-type deployments. Two deployment scenarios with tunable parameters are analyzed, and optimization frameworks are formulated for each approach, accounting for propagation characteristics, radio properties, and antenna directionality. The evaluation metrics not only assess the performance of each technology (IAB and RIS) but also consider the deployment costs required to achieve equivalent performance levels. Numerical results show that both RIS- and IAB-based deployments can effectively support multicast and unicast traffic, with IAB systems demonstrating superior performance in terms of overall resource utilization up to 50% in sparse deployment scenarios. Instead, in highly dense deployment scenarios, RISs exhibit superior scalability and resource efficiency compared to IABs, achieving up to a 3 times improvement factor. Furthermore, unlike IAB deployments, the performance of RIS-based systems continuously improves as the number of RIS nodes increases. From a capital expenditure perspective, RIS deployments prove to be more cost-efficient than IAB systems, provided that the unit cost of RIS is lower.