  The rapid advance of mega-constellation facilitates the booming of
direct-to-satellite massive access, where multi-user detection is critical to
alleviate the induced inter-user interference. While centralized implementation
of on-board detection induces unaffordable complexity for a single satellite,
this paper proposes to allocate the processing load among cooperative
satellites for finest exploitation of distributed processing power. Observing
the inherent disparities among users, we first excavate the closed-form
trade-offs between achievable sum-rate and the processing load corresponding to
the satellite-user matchings, which leads to a system sum-rate maximization
problem under stringent payload constraints. To address the non-trivial integer
matching, we develop a quadratic transformation to the original problem, and
prove it an equivalent conversion. The problem is further simplified into a
series of subproblems employing successive lower bound approximation which
obtains polynomial-time complexity and converges within a few iterations.
Numerical results show remarkably complexity reduction compared with
centralized processing, as well as around 20\% sum-rate gain compared with
other allocation methods.
