An Efficient and Frequency-Offset-Tolerant Channel Estimation and Synchronization Method for PDM CO-OFDM Transmission

We propose a computationally efficient and frequency-offset-tolerant channel estimation and synchronization method for polarization-division-multiplexed (PDM) coherent optical OFDM (COOFDM). Using this method, we demonstrate the transmission a 43-Gb/s PDM CO-OFDM signal with 16-QAM subcarrier modulation over 5,200-km of ultra-large-area fiber. Introduction Polarization-division-multiplexed coherent optical orthogonal frequency division multiplexing (PDM CO-OFDM) has actively been investigated for applications in future high1,2 speed optical transport systems . PDM COOFDM offers high spectral efficiency and high tolerance to chromatic dispersion and polarization mode dispersion. In OFDM, channel estimation and frame synchronization3 are important digital signal processing (DSP) processes. For PDM CO-OFDM, timeinterleaved single-polarization training symbols (TSs)4 and correlated dual-polarization (CDP) TSs5 have been proposed for efficient channel estimation. The use of CDP TSs makes the average power of TSs to be equal to that of payload symbols, thereby offering an additional benefit of reduced cross-phase modulation (XPM) penalty to other wavelength channels6. In this paper, we propose and demonstrate a novel channel estimation scheme based on subcarrier-interleaved dual-polarization (SI-DP) TSs, which offers doubled tolerance to the frequency offset between the transmit laser and the optical local oscillator (OLO) as compared to the previous methods reported in Refs. 4 and 5, while attaining the same high computational efficiency.