Deadline-Aware Graph-Guided Beam Hopping and Precoding for Dynamic Low Earth Orbit Downlinks

Dynamic multibeam low Earth orbit (LEO) downlinks must serve nonuniform traffic demand under power and spatial-multiplexing constraints. Beam hopping redistributes capacity across the satellite footprint, yet each cell-activation decision also reshapes co-channel interference. Aggregate backlog also conceals the remaining service opportunities of delay-sensitive traffic. We propose deadline-aware graph-guided fractional precoding (DAGFP), a causal cross-layer framework for joint cell activation and precoding. DAGFP maps age-resolved service slack into interference-compatible active-set construction, thereby linking queue urgency to the subchannel subsequently optimized by weighted fractional programming (FP). Precoding allocates power and suppresses residual interference under the same traffic weights. Under the normalized aperture model, DAGFP reduces the deadline-drop ratio by 5.51 percentage points relative to backlog-weighted Queue-Graph-FP at an offered-load factor of 2, starting from empty queues over a 1.024 s arrival window, illustrating the benefit of retaining traffic-age information under these conditions. Comparisons with independently tuned regularized zero-forcing (RZF) baselines also favor DAGFP at the evaluated congested loads. Controlled component comparisons further show that the contribution of age-resolved weighting depends on its placement in active-set construction and precoding. The benefit of age-resolved weighting depends on load, service overhead, and queue initialization: Queue-Graph-FP achieves lower drop at loads 8 and 36 in the main study and, in separate load-4 studies, under the tested overhead or warmed-up conditions. These findings indicate that coupling deadline urgency with channel-compatible active-set construction and subsequent precoding can improve timely service under suitable traffic and operating conditions in the evaluated LEO downlink model.