Combinatorial constructions of Schubert subspace codes

We study Schubert subspace codes, which are constant-dimension subspace codes with prescribed intersection conditions with a fixed subspace.

Our goal is to construct codes of maximum possible size in the extremal distance cases where a natural counting upper bound applies.

We give two families of constructions.

The first one uses a direct-sum decomposition of the ambient space, together with partial spreads and colorings of powers of $q$-Johnson graphs.

For this construction, we also prove necessary conditions, which show how chromatic and clique obstructions arise.

The second family is obtained by field reduction from evasive and scattered subspaces over extension fields.

This gives codes whose size can be computed exactly in the scattered case and recovers the only previously known construction as a special case.