Digital Signal Processing in Spatially Multiplexed Coherent Communications

Based on recent experimental results for spatially multiplexed transmission, we analyze the impulse response matrix of few-mode fiber links that support the propagation of LP01 and LP11 modes over up to 1,200-km. We characterize the channel's modal delay spread and mode-dependent loss. Introduction Spatially multiplexed transmission over the full set of coupled modes of a few-mode fiber (FMF) was first demonstrated in Ref. 1, and recently extended to a reach of up to 1,200 km in Ref. 2. These results were enabled by multiple-inputmultiple-output (MIMO) digital signal processing (DSP)1-5 in combination with differential group delay (DGD) compensated fiber spans. In Ref. 6, 120 equalizer taps were required to achieve a transmission distance of about 100 km. In order to render the implementation of such systems feasible for long-haul distances, it is key to reduce the required equalizer complexity. Two approaches need to be investigated in this context. First, optical means to minimize the modal delay spread (MDS), i.e., the width of the impulse response, must be explored. This includes the design of low-DGD FMFs as well as the optimized design of DGDcompensated fiber spans2,7. In a second step, the performance-complexity trade-offs of efficient equalizer structures such as the frequency-domain equalizer (FDE) needs to be studied, taking into account static as well timevarying channel impairments5,8,9. In this paper, we study the channel impulse response of an up to 1,200-km long transmission link based on channel estimations from measured waveforms.