PAPR-aware OFDM signalling for hybrid fibre–free-space-optical links: a comparative simulation study of companding and selected mapping

Direct-detection optical OFDM (DCO-OFDM) suits hybrid fibre–free-space-optical (FSO) links because simple per-subcarrier equalisation tolerates chromatic dispersion and turbulence, but its peak-to-average power ratio (PAPR) drives the Mach–Zehnder modulator into nonlinearity and degrades bit-error rate (BER). This Monte Carlo study compares clipping and filtering, μ-law companding, selected mapping (SLM) and a hybrid SLM + μ-law scheme for 16-QAM DCO-OFDM over single-mode fibre cascaded with a Gamma–Gamma turbulence channel. SLM + μ-law gives the largest PAPR reduction, from 13.50 dB to 7.83 dB at 10 −3 , yet PAPR alone does not predict link performance. SLM gives the most consistent BER gain (0.3–0.8 dB SNR at the 3.8 × 10 −3 FEC threshold), whereas companding’s receiver-side expansion amplifies noise and loses its advantage as turbulence strengthens. Clipping and filtering adds little. Under strong turbulence (Rytov variance σ R 2 σ R² = 4) no scheme reaches the threshold, so diversity is required. An ablation shows that SLM selection is the only stage with a benefit in every reported condition, at four IFFTs per block and 0.39 % side-information overhead. SLM is the robust choice for fibre-fed FSO fronthaul and backhaul links.