Performance assessment of XPM-limited direct-detection short-reach DSB OFDM optical systems

A closed-form expression for the normalized variance of the fluctuations induced by cross-phase modulation (XPM) in direct-detection (DD) double-sideband (DSB) orthogonal frequency-division multiplexing (OFDM) systems is proposed and validated. This expression is obtained by adapting the analytical model for XPM-induced intensity-modulated fluctuations derived in the past for conventional on-off keying systems to DSB-OFDM systems employing DD receivers. It is shown that the normalized variance can be used as an error vector magnitude (EVM) estimator in DD DSB-OFDM systems dominantly impaired by XPM. This conclusion is drawn after validating the proposed closed-form expression in two application scenarios: (1) distribution of DSB-OFDM ultra-wideband signals using a radio-over-fiber infrastructure, and (2) transmission of 10 Gb/s DSB-OFDM signals in access networks. Excellent accuracy between the EVM estimates provided by the normalized variance expression and the actual EVM of the OFDM system is demonstrated through numerical simulation for a large set of system parameters. When the carrier-to-signal power ratio (CSPR) is high, an EVM discrepancy not exceeding 0.3 dB is obtained in most of the analyzed cases. When the CSPR decreases to 0 dB, the discrepancy between the actual EVM and the normalized variance does not exceed 0.5 dB. A maximum discrepancy between the actual EVM and the normalized variance around 1 dB is also shown when the spectral occupancy of the DSB-OFDM signal leads to a dispersion-induced power fading in the most affected subcarrier as high as 20 dB. These results suggest that the proposed closed-form expression is a powerful tool to provide fast and accurate estimates of the XPM-limited capacity of wavelength division multiplexing DSB-OFDM systems employing direct detection.