Alignment-Free Optical Combining with Scalar-Message Expectation Propagation Detection for Distributed Multi-Aperture Coherent Reception

Abstract

We propose an alignment-free distributed multi-aperture optical receiver with a single coherent front end, in which the differential delays between apertures are large. The branch signals are optically combined without inter-branch delay or phase alignment. Large differential delays produce separated tap clusters, forming a sparse ISI channel with long memory. Scalar-message expectation propagation exploits this sparsity to recover the transmitted symbols. Simulations show lower BER than equal-area single-aperture reception and no clear BER degradation over the tested delay spans. As the maximum differential delay increases from 100.5 to 200.5 symbol periods, the channel span nearly doubles, while the factor-graph edge count increases by only about 1.5\%. The trade-off is higher BER than ideal coherent combining schemes in exchange for fewer coherent front ends than digital combining and no inter-branch delay or phase alignment before optical combining.