We describe the first on-line 256 QAM digital coherent optical transmission system with an ECLD light source and DFB LD-based injection-locking, and an FPGA-based transceiver. In this system, precise optical phase control was achieved with an optical injection-locking circuit and a phase-locked loop circuit at the receiver, which helped to reduce the complexity of digital signal processing in the FPGA-based receiver. The precise polarization-demultiplexing of a 256 QAM signal was realized by using a training symbol in the FPGA. With these techniques, we successfully demonstrated the on-line detection of a QAM signal with a multiplicity as high as 256. We then present a 10 channel WDM 80 Gbit/s/ch, 256 QAM bi-directional coherent transmission as a high capacity next-generation mobile fronthaul by using the FPGA-based transmission system. We achieved error-free operations with a 14% overhead FEC for 10 km down-link (on-line) and up-link (off-line) transmissions for all channels.
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