Coding gain of FEC encoded 21.42 Gb/s RZ-DQPSK using an electrical differential quaternary precoder

We report a measured gross coding gain of 6.8 dB for 21.42 Gb/s RZ-DQPSK using standard FEC in combination with differential precoding. Furthermore we show that RZ-DQPSK has approximately a 2 dB higher FEC coding gain than RZ-DPSK.