Sub-Baudrate Sampling at DAC and ADC: Towards 200G per lane IM/DD Systems
01 January 2018
As optical transceivers evolve towards 200G per lane, the comparatively low bandwidth and sampling speed of the commercially available high-speed DACs and ADCs increasingly become the dominating obstacle towards success. On the other hand, DACs and ADCs are essential to make use of digital signal processing for impairment compensation and error correction; altogether driving systems engineers to a hard design dilemma. With the purpose of alleviating this compromise, we propose a transceiver structure - encompassing both hardware and software - enabling the generation, transmission, and detection of strongly over-filtered data signals whose baudrate exceeds the sampling rates of (potentially) both the DAC and ADC. We call this approach sub-baudrate sampling, a technique which does not increase the processing complexity at the transmitter side and does not even require the information about the channel state for pre-distortion or pre-coding purposes. We experimentally assess the performance of sub-baudrate sampled signals in two blocks of experiments: first the proof of concept in back-to-back configuration, including up to 112Gbaud on-off keying at 0.785 samples per symbol in 25-GHz aggregate bandwidth. And then the C-band transmission demonstration, showing up to 100Gbaud quaternary pulse-amplitude modulation (0.92 samples per symbol) and 125Gbaud on-off keying (0.736 samples per symbol) over 1 and 80-km of singlemode fiber respectively with more than 3-dB receiver sensitivity margin at the 7%-overhead hard-decision bit error-rate limit.
