Boosting free-space optical links by repurposing a weak channel for error correction

Orbital angular momentum (OAM) multiplexing increases the capacity of free-space optical (FSO) links, but higher-order modes exhibit substantially lower coupling efficiencies and higher noise compared to lower-order modes, making them inefficient for direct data transmission. Conventional strategies either discard these degraded modes or treat all modes uniformly, leaving potential coding gain untapped. We propose a hybrid maximum-ratio combining (MRC) and classical surface code (CSC) architecture that repurposes the weakest OAM mode as a carrier of structured topological redundancy. The syndrome of a distance-3 surface code, transmitted over the outer mode \(l=2\) , is fused as a graded Hamming-weight penalty in the Viterbi branch metric of the two data modes ( \(l=0\) and \(l=1\) ) combined via MRC. Unlike conventional repetition codes, our scheme exploits the full syndrome information to bias the Viterbi decoder toward sequences consistent with the surface code parity checks. The proposed scheme achieves \(\textrm{BER}=10^{-3}\) at 10.8 dB, gaining 1.7 dB over pure MRC and 0.9 dB over a repetition-code baseline. At \(10^{-6}\) , the advantage grows to 2.0 dB. Both coded schemes exhibit an error floor near \(2\times 10^{-8}\) , consistent with the undetected error probability of the \(d=3\) surface code. The framework converts a low-capacity OAM mode into a source of coding gain without additional spectral resources, retransmissions, or feedback. All reported gains are obtained from Monte Carlo simulation of the channel model; a benchtop free-space link is used solely to validate the Gaussian channel model and verify the decoder implementation, not to demonstrate OAM multiplexing.