  Multi-layer ultra-dense satellite networks (MLUDSNs) have soared this
meteoric to provide vast throughputd for globally diverse services. Differing
from traditional monolayer constellations, MLUDSNs emphasize the spatial
integration among layers, and its throughput may not be simply the sum of
throughput of each layer. The hop-count of cross-layer communication paths can
be reduced by deploying inter-layer connections (ILCs), augmenting MLUDSN's
throughput. Therefore, it remains an open issue how to deploy ILCs to optimize
the dynamic MLUDSN topology to dramatically raise throughput gains under
multi-layer collaboration. This paper designs an ILC deployment strategy to
enhance throughput by revealing the impacts of ILC distribution on reducing
hop-count. Since deploying ILCs burdens the satellite with extra communication
resource consumption, we model the ILC deployment problem as minimizing the
average hop with limited ILCs, to maximize throughput. The proposed problem is
a typical integer linear programming (ILP) problem, of which computational
complexity is exponential as the satellite scale expands and the time evolves.
Based on the symmetrical topology of each layer, we propose a two-phase
deployment scheme to halve the problem scale and prioritize stable ILCs to
reduce handover-count, which decreases the exponential complexity to a
polynomial one, with 1% estimation error: Simulation results based on realistic
megaconstellation information confirm that the optimal number of ILCs is less
than P.S/2, where P and S are orbits and satellites per orbit. Besides, these
ILCs deploy uniformly in each layer, which raises over 1.55x throughput than
isolated layers.
