All-Digital Timing Recovery and Adaptive Equalization for 112 Gbit∕s POLMUX-NRZ-DQPSK Optical Coherent Receivers

An all-digital timing recovery loop is employed in optical coherent receivers, which depends on the number-controlled oscillator and the interpolator to complete timing adjustment in an all-digital way instead of the traditional hybrid way using an analog device (a voltage-controlled oscillator) to adjust the sampling frequency. Therefore it avoids introducing the nonideal characteristics of an analog device and is beneficial to all-digital implementation of optical coherent receivers. Furthermore, in order to meet the requirements of low complexity and sampling rate, the Gardner timing error detector is adopted in the all-digital timing recovery loop. However, it has low tolerance of dispersion, which requires another device to compensate the large residual dispersion prior to it. Although the adaptive equalizer can compensate signal for dispersion impairment, its effective operation must rely upon synchronized signals. For solving the incompatible problem without any extra cost, a joint scheme that embeds an adaptive equalizer into the all-digital timing recovery loop is proposed to accomplish synchronization and compensation of linear transmission impairments. Finally, the feasibility and effectiveness of the proposed scheme are demonstrated in a 112 Gbit/s polarization-multiplexing non-return-to-zero differential-quadrature phase-shift-keying (POLMUX-NRZ-DQPSK) optical coherent receiver by simulation.