Dancing with Chains: Spaceborne Distributed Multi-User Detection under Inter-Satellite Link Constraints

Mega constellation, as an extremely large-scale radio access network, faces severe multi-user interference when accommodating ubiquitous access. Distributed multi-user detection (MUD) can utilize the multi-satellite spatial diversities and processing capabilities to alleviate inter-user interference. However, the spaceborne nature makes it seriously chained by inter-satellite link (ISL) constraints including the limited number of ISL ports per satellite and the constrained bandwidth per ISL port. To this end, this paper proposes an efficient message passing (MP) based distributed MUD framework under stringent ISL constraints. First, the overheads introduced by the optimal fully-connected distributed MUD, including required ISL ports and bandwidth, are quantitatively characterized by employing distributed factor graph (FG) model and MP mechanism. By analyzing the FG representation, we propose two ISL-compatible design principles for distributed MUD, i.e., orchestrating message flow (MF) hierarchically among satellites to save ISL ports, and propagating messages selectively to save ISL bandwidth. Specifically, a novel multi-branch tree-like MF orchestration is proposed to forward and aggregate the locally generated detection messages in a partially-connected manner under limited ISL port number. The relationship between MF structure and overall performance is revealed via EXIT chart and a fairness-aware orchestration algorithm is developed. Further, we introduce a novel squeeze node into the distributed FG, compressing messages and facilitating selective MP under bandwidth constraint. Three criteria are correspondingly proposed to identify the most effective messages for distributed MUD. The proposed framework is then evaluated under practical simulation settings with satellite orbits and propagation environments generated via STK. Simulation results demonstrate that our framework reduces ISL costs by 50% with less than 1 dB loss in terms of BER performance compared to the fully-connected MUD, and achieves up to 5 dB gain in BER over the state-of-the-art distributed reception methods under fair comparisons.