Constellation Expansion for Differentially Encoded 100G Transmission

Recent technological progress has enabled the implementation of powerful soft-decision forward error correction codes performing very close to the theoretical Shannon limit. However, recently we showed that, even when the most powerful binary codes are used, a significant potential gain remains unexploited in traditional, coherently detected, non differentially encoded, polarization division multiplexed 100G optical communication systems, when the modulation format is restricted to quadrature phase-shift keying. In this letter, we extend our previous work to the case of differentially encoded systems. Assuming again additive and phase white Gaussian noise, we show that an additional gain of 1-4.5 dB (depending on the amount of phase noise present) is available even when differential transmission is employed, if higher order constellations are used in place of quadrature phase-shift keying. Special care and suitable optimization criteria are required in the choice of the alternative signal constellations since, in contrast with the NDE case, the Euclidean distance is not a significant figure of merit anymore.