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Simplified digital coherent-based beyond-100G optical access systems for B5G/6G [Invited]

In this paper, we review the latest progress in the simplified digital coherent technologies suitable for future beyond-100G optical access systems, which will be a key optical transport platform for meeting the very wide range of extreme requirements of beyond-5G (B5G) and 6G users. As the centerpiece of beyond-100G optical access systems, the technical challenges and recent demonstrations of ultimate-capacity passive optical network systems carrying more than 100 Gb/s per wavelength by using simplified digital coherent technologies incorporating low-complexity digital signal processing (DSP) are presented. In addition, to facilitate the implementation of these relatively large-scale digital coherent technologies in the optical access area, which has strict cost, power consumption, and size restraints, the rapid progress in compact optical transceiver form factors and optical devices heading toward 400 Gb/s-based Ethernet transceivers for the 80 km range (400G-ZR) and over 800 Gb/s short-reach applications based on the digital coherent technologies are also discussed. Furthermore, the recent expansion of simplified digital coherent-based optical access systems such as optical space communications and their latest experimental demonstrations are introduced as an alternative evolution of beyond-100G optical access systems, going hand in hand with the reduction of complexity of the real-life digital coherent platforms such as the DSP large-scale integration and the large-scale field programmable gate arrays.