Digital Pre-Emphasis in Optical Communication Systems: On the Nonlinear Performance

Digital signal pre-processing at the transmitter is a key enabler for next-generation optical transceivers. One of the major challenges faced by these transponders is the limited resolution and -3 dB electrical bandwidth of the digital-to-analog converters (DAC), severely limiting the transmission performance. On the other hand, these spectrally efficient flexible transport networks are extremely sensitive to nonlinear channel impairments, essentially limiting the maximum system reach. In this paper, we employ a simple digital pre-emphasis (DPE) algorithm to mitigate DAC-induced signal distortions, and further report on the impact of DPE algorithm on nonlinear transmission performance. We demonstrate, both numerically and experimentally, that DPE not only counteracts DAC low-pass response, allowing for higher baud rate transmission, but also enables better nonlinear channel tolerance. In particular, we first establish the maximum transmittable baud rates in back-to-back configuration for polarization multiplexed m-state quadrature amplitude modulation (PM-mQAM) formats, allowing up to 50, 46 and 44 Gbaud transmission for PM-4QAM, PM-8QAM, and PM-16QAM, respectively assuming typical DAC specifications of 16 GHz -3 dB bandwidth and 5.5 effective number of bits. Furthermore, we show that the performance improvements from DPE increase with increasing modulation order and decreasing span count, with maximum improvements of up to ~2.2 dB. In a conventional standard single mode fiber transmission link with lumped amplification, employing DPE, we report maximum reach of up to ~10 000, ~5000, and ~3000 km, for PM-4QAM, PM-8QAM, and PM-16QAM, respectively. Moreover, at pre-FEC bit-error-rate, relative reach improvements from DPE are found to be up to 21%, 25%, and 27%, respectively. Finally, we show that DPE gains are constant beyond a minimum required channel spacing, with minimum spacing requirements of 1.1 × baud rate, 1.15 × baud rate, and 1.2 × baud rate for PM-4QAM→PM-16QAM, respectively.