Digital Compensation of Bandwidth Limitations for High-Speed DACs and ADCs

In this paper, we present a novel digital preemphasis algorithm to compensate for the electrical bandwidth limitations at the transceiver also by taking into account the quantization noise introduced by the signal digitalization. The proposed method is based on the minimization of the mean square error between the desired input signals and the output signals of the digital-to-analog converter/analog-to-digital converter (DAC/ADC), when assuming the knowledge of the DAC/ADC frequency responses. Though this paper focuses on the DAC/ADC compensation, the introduced method could be applied to electrical bandwidth limitations caused by any other component within the transponder. The performance of the algorithm is assessed in optical back-to-back configuration by comparing it against the case without digital preemphasis and with a previously published method to compensate for DAC bandwidth limitations. Our analysis shows that when utilizing realistic descriptions of the DAC/ADC, the proposed digital preemphasis (at the transmitter) or digital compensation (at the transmitter and receiver) can considerably increase the maximum transmittable symbol rate for the case of advanced modulation formats. For example, the maximum symbol rate can be ideally increased up to $\sim$ 60% for the case of 16QAM when employing the high-speed DAC with a $-$ 3 dB electrical bandwidth of $\sim$ 16 GHz and with six effective number of bits. Furthermore, we evaluate the impact of additional noise sources, based on experimental measurements, envisioning potential for further improvements of the digital preemphasis module. Finally, we experimentally verified our algorithm, for the specific case of polarization-multiplexed 16QAM, showing a considerable match between simulation and lab results.