On the performance of companding techniques for DMT optical links

Abstract

In this paper the performance of Discrete Multi-Tone (DMT) optical links using nonlinear companding schemes for peak-to-average power ratio (PAPR) reduction is investigated. First, it is developed an analytical expression to evaluate the total distortion at the demodulation stage introduced by the expansion process, and also to estimate the BER vs. SNR performance curves for companded DMT systems. The analytical expression and procedure for BER estimation is tested with three compressing functions: μ-Law, Erf and Trapezoidal. The obtained results show a very good agreement between simulations and predictions, validating this simple, quick and accurate BER estimation tool for the considered system. Next, the effects produced by a nonlinear laser diode (modeled as a lower hard limiter) that is directly modulated, are investigated in terms of its input back-off (IBO). A trade-off between the distance that can be reached in an optical fiber link and the allowed BER degradation is numerically demonstrated. These results are compared against a nonlinear amplifier model (soft limiter), which results in a slight improvement in performance. Finally, it is shown that even though companding schemes degrade the BER for nearly linear operation, they can improve the BER performance of a DMT link if a moderate back-off value is allowed (particularly for an IBO=7 dB when the SNR>20 dB for a system with an Erf companding).