SSB direct detection scheme in duobinary-carrier-suppressed RZ transmission

Comment is made on the single sideband (SSB) direct detection scheme in duobinary-carrier-suppressed RZ (DCS-RZ) transmission (see Hirano et al., ibid., vol.38, no.12, p.585-7, 2002). Apparent differences between these results and those of other reports are clarified. It is shown that for DCS-RZ (termed modified duobinary (Cheng and Conradi, 2002)) alternate marks are inverted (AMI), whereas for duobinary marks are inverted only when there are an odd number of spaces in between (Shtaif and Gnauck, 1999). The benefit of having an alternate sign on a sequence of pulses is the suppression of a spurious pulse, caused by dispersion, that occurs midway between two adjacent pulses. Hence, AMI might be expected to outperform duobinary. Comparison is made of AMI and duobinary when the elementary pulse shape for both signals is RZ (before filtering) to clarify why Hirano et al. concluded that duobinary outperforms AMI, whereas Cheng and Conradi concluded the opposite.