Channel Estimation on Individual Subcarrier Using Image Processing in Optical OFDM System

In an optical orthogonal frequency division multiplexing (OFDM) system, channel impairments, such as residual chromatic dispersion, residual synchronization error, and clock frequency offset, induce phase shifts over subcarriers. We propose a method of using image processing for phase channel estimation with negligible overhead, which is applicable to various subcarrier modulation formats. We conduct the experiment of 32-, 40-, and 48-Gbit/s direct detection optical OFDM with 16QAM, 32QAM, and 64QAM subcarrier modulation and simulation of 112-Gb/s coherent optical OFDM to verify this method and compare the results with those of the conventional method. Then, we investigate the effects of the numbers of required test phases and OFDM symbols with experimental data. We also present an approach of removing the inner part of constellation points to reduce the complexity.