Coexistence of Multicast and Unicast Services in mmWave/sub-THz Self-Backhauled Systems: User Associations and Performance Gains

The use of millimeter wave (mmWave) and sub-terahertz (sub-THz) bands in 5G/6G cellular networks offers high data rates but demands infrastructure densification. To reduce deployment costs, the 3GPP has introduced Integrated Access and Backhaul (IAB) enabling multi-hop connectivity via low-cost IAB relay nodes. Multicast communications appear particularly advantageous in IAB systems as they permit saving spectrum resources both in access and backhaul. This work aims at evaluating the impact of the multicast transmission mode and multicast-aware user association in 5G/6G mmWave/sub-THz self-backhauled systems serving unicast and multicast traffic. We develop an optimization framework for user association and multicast grouping in IAB taking into account the specifics of the propagation in mmWave/sub-THz spectrum, antenna design, and resource sharing between access and backhaul. The resulting optimization problem is solved using an approximate dynamic programming approach capable of handling the 3GPP-recommended numbers of cell users. Our results show that in an IAB network unicast and multicast users differ in their optimal associations, namely, multicast users tend to be associated more frequently with the relay nodes compared to unicast users, and this difference is more notable in low blockage scenarios. Quantitatively, the use of multicasting in an IAB system permits saving up to 40% of resources.