Penalty-free transmission of multilevel 240 Gbit/s RZ-DQPSK-ASK using only 40 Gbit/s equipment

Combining four-level phase modulation with binary amplitude modulation and polarisation multiplexing at 40 Gbaud, we present the first experimental implementation of 240 Gbit/s RZ-DQPSK-ASK. We demonstrate transmission over a 50 km fibre span without sensitivity degradation.