Constellation expansion: Theoretical limits and practical examples

In optical communications the advent of coherent transmission techniques has been shortly followed by the breakthrough of soft decision (SD) forward error correction (FEC). The optical community put much effort into the design and implementation of powerful FEC codes. However, we show that, for the exemplary case of 100G polarization division multiplexed (PDM) systems, both with differential and non-differential transmission, a potential gain of more than 1 dB is still available if coding and modulation are jointly optimized. The benefits of this approach are characterized in terms of the capacity limits over a simplified channel model and are illustrated by means of practical examples and experimental results.