Experimental Study of a novel adaptive decision-directed channel equalizer in 28 GBaud RGI-DP-CO-OFDM transport systems

Abstract

We report and experimentally investigate the performance of an adaptive decision-directed channel equalizer (ADDCE) in reduced-guardinterval dual-polarization coherent-optical orthogonal-frequency-divisionmultiplexing (RGI-DP-CO-OFDM) transport systems. ADDCE retrieves an estimation of the phase noise value after an initial decision making stage by extracting and averaging the phase drift of all OFDM sub-channels. Moreover, it updates the channel transfer matrix on a symbol-by-symbol basis. We experimentally compare the performance of the ADDCE and the conventional equalizer (CE) combined with maximum-likelihood (ML) phase noise compensation and inter-subcarrier-frequency-averaging (ISFA) algorithms. The study is conducted at 28 GBaud for RGI-DP-CO-OFDM systems with quadrature-phase-shift-keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) formats. Using ADDCE, zero-overhead laser phase noise compensation is accomplished and the overhead due to training symbol (TSs) insertion is significantly reduced. In addition, ADDCE offers a superior performance over the CE in the presence of synchronization timing errors and residual chromatic dispersion (CD). We also achieve a longer transmission distance than when using the CE. At a forward-errorcorrection (FEC) threshold of 3.8 × 10 −3 , using a cumulative overhead of less than 2.6%, transmission distances of 5500 km and 400 km were achieved for the cases of QPSK and 16-QAM RGI-DP-CO-OFDM, respectively.

Time-Domain Adaptive Decision-Directed Channel Equalizer for RGI-DP-CO-OFDM

“…It is worth noting that the calculation of Q xy (t) or Q yx (t) is significantly simplified because they are both Toeplitz matrices and conjugated symmetric (see Properties 2 and 3 in Appendix for details). We also apply some basic characteristics of Fourier transform to calculate (3). The analysis of computational complexity of channel estimation and update for FD-ADDCE can be found in [3].…”

“…The OFDM symbols are then equalized using the CSI and subsequently the phase noise of the symbols is compensated by a common phase error (CPE) compensator based on PSCs. In [3], an adaptive decision-directed channel equalizer (ADDCE) in frequency domain (FD-ADDCE) is demonstrated to attain better performance than TSACE-ISFA. In [3], the phase noise is compensated by a decision-directed scheme.…”

“…In [3], an adaptive decision-directed channel equalizer (ADDCE) in frequency domain (FD-ADDCE) is demonstrated to attain better performance than TSACE-ISFA. In [3], the phase noise is compensated by a decision-directed scheme. Therefore, no PSCs are required for phase noise compensation.…”

Efficient Joint Carrier Frequency Offset and Phase Noise Compensation Scheme for High-Speed Coherent Optical OFDM Systems

“…In this case, the extracted RF-pilot will include all the phase impairments, which are induced by not only the laser phase noise but the IFO as well as residual FFO (RFFO). The combined phase impairments can be calculated as (7) where represents the operation of "return the absolute value of the input element", is the filtered signal (namely the extracted RF-pilot), is the estimated FO, and stand for the estimated phase noises of laser linewidth and amplified spontaneous emission (ASE) respectively.…”

“…1) Multi-pilot subcarriers phase estimation algorithm (MSPE), which compensates for only the common phase impairments after OFDM demodulation (e.g., that proposed in [5]- [7]). However, the laser linewidth must be small enough and the OFDM symbol should be short as well in order to mitigate the impact of the inter-carrier interference (ICI) and obtain a satisfying performance of phase noise compensation.…”