An Iterative Frequency Offset Estimation Method for Coherent Optical OFDM System

In this letter, we propose and experimentally investigate a training-aided iterative frequency offset estimation (IFOE) method, which can track dynamic frequency offset (FO) drift in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. In contrast to existing FO estimation algorithms that compute specific FO values, in IFOE, only the sign value of FO is judged and recorded without any phase calculation. The estimated FO is updated once for every data block based on FO signs obtained from multiple previous blocks, which helps combat noise and allows the shortening of the employed training symbol. We experimentally investigate the performance of IFOE in a 160-km CO-OFDM system. The results show that IFOE achieves superior bit error rate performance to a one-time FO estimator (without FO tracking) and approaches the performance of multiple separate FO estimators that use longer training symbols.