ASE-noise limit of direct-detection receivers: duobinary vs. IMDD

The duobinary technique, at first proposed for radio communications has recently re-emerged in the field of optical communications as a possible candidate for next generation of ultra-DWDM systems. The purpose of this work is to derive, for the first time to our knowledge, the back-to-back sensitivity (quantum limit) of duobinary technique in ASE-noise-limited direct-detected optical systems: we found that it is at least 0.91 dB better than the IMDD limit and we show that a practical duobinary system setup could closely approach the quantum limit.