Joint Non-Data-Aided Clock Recovery for Space-Division Multiplexed Optical Transmission Systems

In recent years, space-division multiplexing (SDM) has been proposed as a technique to cope with the increasing demand for higher per-fiber capacity in optical networks by modulating multiple independent signals onto multiple spatial paths. This can be accomplished by using specialized fibers that carry multiple signals in a number of fiber cores, fiber modes, or a combination of both types. In such fibers, strong spatial coupling of the signals requires for a joint digital signal processing (DSP) at the receiver. While research has mainly focused on system performance and multiple-input multiple-output (MIMO) equalizers, a reliable joint clock recovery tolerant to spatial-and-polarization-mode dispersion is an active field of research with recent progress. In this paper, we present a novel digital non-data-aided joint clock recovery that is tolerant to polarization-and-spatial-mode dispersion. The joint clock recovery is implemented in a feedforward architecture, which allows simple implementation. We provide a detailed analysis of the algorithm complexity for hardware implementation. In simulations, we show low clock phase jitter for fiber lengths up to 10,000$\,$km. Finally, we demonstrate clock recovery for a 90-GBd 16-QAM signal over a 150-km randomly-coupled 4-core fiber (RC-4CF) resulting in a total data rate of 2.88$\,$Tbit/s per wavelength and analyze equalizer convergence using a dedicated joint clock recovery.