Joint IQ imbalance compensation and channel estimation in coherent optical OFDM systems

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) is considered to be promising technique for meeting the ever increasing demand of data rate. It exhibits many advantages of coherent detection and OFDM, making the optical transmission over fiber extremely robust against distortions such as chromatic dispersion and polarisation mode dispersion. However, CO-OFDM is sensitive to IQ imbalance, in particular, when a homodyne (or direct down-conversion) receiver is used. In this paper, we study the impact of transmitter and receiver side IQ imbalance on coherent optical transmission over a single-mode fiber. Moreover, we investigate different methods, particularly in the frequency domain, to jointly compensate the IQ imbalance and to equalize the optical fiber channel, taking the effect of chromatic dispersion into account. The proposed minimum mean square error (MMSE) compensation algorithm shows superior performance, compared to conventional method. Furthermore, an optimum design of training sequence is derived and expressed in a closed-form, which provides significant performance gains even without applying IQ imbalance compensation.