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 and the constrained bandwidth of ISL ports. Therefore, this paper proposes an efficient message passing (MP) based distributed MUD framework under stringent ISL constraints. First, the overheads on ISL ports and bandwidth introduced by fully-connected distributed MUD are quantitatively characterized using distributed factor graph (FG) model. On this basis, we propose two ISL-compatible design principles for distributed MUD, i.e., orchestrating message flow (MF) hierarchically among satellites to save ports, and propagating messages selectively to save 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. 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. Our proposed MUD framework is evaluated under practical settings, which demonstrates a reduction of 50% ISL bandwidth costs with less than 1 dB loss in terms of BER compared to the fully-connected MUD, and achieves up to 5 dB gain in BER over the state-of-the-art distributed reception methods.