Estimating the Performance of DD OFDM Optical Systems Impaired by In-Band Crosstalk Using the Moment Generating Function

A theoretical method to assess the impact of in-band crosstalk on the performance of orthogonal frequency division multiplexing (OFDM) communication systems with direct-detection (DD) is presented. The proposed method relies on the moment generating function (MGF) to estimate the bit error rate (BER) of the DD OFDM communication system and takes into account in-band crosstalk, detected noise, and arbitrary electrical filtering at the receiver. By comparison with Monte Carlo simulation, we show that the MGF-based method predicts accurately the BER for different crosstalk levels, subcarriers numbers, 4-ary, 16-ary, and 64-ary quadrature amplitude modulation mappings, carrier-to-signal power ratios, ideal and nonideal electrical filtering, and different types of linearly mapped OFDM signals used in DD. The accuracy of the proposed method may be severely diminished in presence of significant signal distortion due to the nonlinearity of the optical modulator as, in this case, the assumptions of the proposed method do not hold. We also show that the DD OFDM optical systems tolerance to in-band crosstalk is lower than in single-carrier coherent detection optical systems, and it is significantly lessened when the modulation order in the OFDM subcarriers increases.