Adaptive QAM-OFDM Radio-over-FSO Fronthaul: Semi-Analytical EVM Modeling and Outage Analysis

Analog radio-over-free-space optical (RoFSO) fronthaul can transport radio waveforms over rapidly deployable optical beams, but atmospheric fading, receiver noise, orthogonal frequency-division multiplexing (OFDM) peaks, analog bandwidth, and channel-state uncertainty degrade modulation quality. This paper develops a 1 km intensity-modulation/direct-detection quadrature amplitude modulation (QAM)-OFDM RoFSO link together with a simulation-calibrated semi-analytical error vector magnitude (EVM)/outage approximation. The fixed-calibration approximation is assessed on previously untested numerical cases without refitting, with a maximum threshold-power difference of 0.29 dB. To address practical waveform and receiver effects, a separate root-mean-square (RMS)-referenced track preserves natural OFDM peak-to-average power ratio (PAPR), uses explicit pilot OFDM symbols and pilot-only least-squares gain estimation, accounts for pilot overhead in effective spectral efficiency, applies a partial RoFSO EVM budget, and derives a protection margin from empirical gain-conditioned channel-state information (CSI) errors. Under a common 5-dBm maximum active-frame mean-power ceiling, the margin-aware joint QAM/power controller achieves 1.608 b/s/Hz with 5.98% outage, compared with 1.130 b/s/Hz and 10.76% for static 16-QAM. A marginal-preserving turbulence–pointing dependence stress test produces only a modest additional outage penalty. These results are simulation-based sensitivity and consistency assessments rather than experimental validation.