Routing and Downlink Resource Allocation for Multicast Traffic in LEO Satellite Constellations

In this work, we optimize unicast and multicast traffic delivery over non‐terrestrial networks (NTNs) to users on the ground through direct satellite downlink (DL). We consider a gateway‐free mesh architecture with low Earth orbit (LEO) satellites interconnected via inter‐satellite links (ISLs), where unicast and multicast services compete for both ISL and DL resources. The (E2E) problem is decomposed into: (i) a space‐segment multicast distribution problem over capacity‐constrained ISLs, modeled via Steiner‐tree approximations; and (ii) a DL resource allocation problem aligned with a 3GPP cell‐based NTN RAN model, where we formulate a demand‐aware allocation scheme for mixed multicast/unicast traffic. Results show that Steiner‐based multicast reduces ISL utilization and risk of bottlenecks, achieving up to lower load at the cost of a moderate delay increase below compared to shortest‐path unicast routing. These gains are further amplified in scenarios with spatially correlated satellites. In the DL segment, demand‐aware allocation maximizes service availability and improves throughput by up to 700% compared to a proportional fair benchmark through flexible resource allocation.