Concatenation of space-time coding and FEC for Few-mode fiber systems

Space-division multiplexing (SDM) is becoming essential to increase the capacity of future optical communication systems. Propagating modes through multimode fibers are affected by a non-unitary crosstalk known as mode dependent loss (MDL), that requires either optical or digital signal processing solutions to be reduced. Space-time (ST) coding designed and used for wireless communications was proved to be efficient to reduce polarization-dependent loss in PolMux systems and MDL in SDM systems. In this letter, we study a complete transmission scheme, including forward error correction (FEC) and ST coding. FEC is used to protect the transmission from errors, but in the presence of MDL, system's performance is reduced. We show by simulations and error probability derivation that coding gain summation is obtained, which makes both codes essential for the next generation of optical transmission systems.