Joint timing recovery and adaptive equalization based on training sequences for PM-16QAM faster-than-Nyquist WDM systems

Since the timing error detectors sensitivity (TEDS) of the timing recovery algorithm is close to zero under the singularity condition of azimuth theta about +/-pi/4 and differential group delay (DGD) about n x 1/2T (n is an odd number, T is the symbol period), it makes the squared Gardner phase detector (SGPD) timing recovery algorithm fail to achieve timing synchronization. What's worse, in the faster-than-Nyquist wavelength division multiplexing (FTN-WDM) systems, the tight filtering introduces inter-symbol-interference (ISI) so severe that the convergence cost of the SGPD timing recovery algorithm is extremely large even under the non-singularity condition. This paper proposes a joint timing recovery and adaptive equalization scheme for FTN coherent systems based on training sequences that could calculate channel matrix and indicate polarization characteristics, thereby avoiding the influence of azimuth on adaptive equalization and polarization demultiplexing (AEPD) embedded in the timing recovery feedback loop. Since embedded AEPD could compensate for most of DGD, the TEDS could be restored and timing synchronization could be achieved under the above adverse conditions. Thanks to the innovative scheme, which equalizes ISI and DGD during the feedback process of the loop, the convergence cost of timing recovery could be reduced with similar computational complexity compared with the conventional one. The simulation results of 128 GBaud polarization multiplexing (PM) 16-quadrature amplitude modulation (QAM) FTN-WDM transmission systems demonstrate that the proposed scheme could stably achieve timing synchronization under the singularity condition. And compared with the conventional scheme, the convergence cost is reduced by at least 42% @ 0.9 acceleration factor. In addition, 40 GBaud PM-16QAM FTN experiment results show that the proposed scheme could not only achieve timing synchronization stably but also exhibit an optical signal-to-noise ratio tolerance gain of 0.8 dB compared with the conventional scheme under 800 km transmission.