Constrained coding for quasi-linear optical data transmission systems

This paper considers the application of constrained coding to 40-Gb/s dispersion-managed single-channel optical communication systems limited by intrachannel four-wave mixing (IFWM). It is shown that when transmitted sequences obey the so-called (2, infinity) constraint, runlength-limited codes can be designed to not only suppress ghost pulse formation but also improve the data rate by as much as 50% without increasing the transmission bandwidth. Since IFWM is a highly pattern-dependent effect, coding schemes with certain characteristics are preferable. Two different codes with simple encoding and decoding algorithms are constructed, and their performance is analyzed and compared to that of a realistic benchmark system. One of the codes turns out to be consistently better than the other due to its superior statistical properties. The qualitative conclusions are confirmed by numerical simulations. An extension of the method to multichannel links is also considered, and similar gain in data rate is demonstrated.