Geometry-Aware Robust Scheduling for HAPS-to-Vehicle Links: Decision Validity Under Mobility and Feasible Load in Corridor Deployments

A high-altitude platform station (HAPS) can serve an extended motorway corridor from a single aperture, but stratospheric attitude disturbance introduces a pointing error that degrades the isolation between closely spaced beams. Robust user selection addresses this by requiring an explicit angular separation between co-scheduled beams, yet existing formulations are evaluated at a single scheduling instant over compact footprints. This paper asks how long a scheduling decision remains valid once the served vehicles move, and how footprint geometry bounds the number of vehicles that can be co-scheduled. A corridor mobility model is developed; the pairwise separation requirement is enforced by max–min angular dispersion, for which the chance-constrained separation margin acts as a feasibility certificate; and the margin is extended to the age of the decision. Monte Carlo evaluation at 28 GHz with zero-forcing precoding shows that the scheduling geometry evolves about $5\times {10}^4$ times more slowly than the small-scale fading, so that outage is statistically unchanged at a decision age of 100 ms and rises by 30% at 1 s. On the corridor, max–min selection yields lower outage than capped-sum angle-aware selection, channel-aware selection and random selection at every load from $K=4$ to $K=12$, at the cost of a 5% lower cell-edge spectral efficiency at intermediate load. Widening the margin to absorb angular drift gives no significant benefit at a realistic candidate-pool size and increases outage 2.7-fold at $K=10$, so decision age is better bounded through the scheduling interval than compensated through margin inflation.