Assessment of 40 Gb/s techniques for metro/regional applications

40 G is the next upgrade for optical transport with respect to the per-wavelength bit rate. Connecting high-end core routers is the first relevant driver for this from the applications viewpoint. Many deployments however will take place in the metro and regional space first, requiring distance capabilities of up to a few 100 km only. Various new modulation techniques like CSRZ, Duobinary, and DPSK have been developed for 40 G that have different capabilities with respect to maximum distances, and different hardware effort and resulting price as well. In the metro space, 40 G is also regarded as an upgrade to existing NRZ links, hence the respective technology must be able to co-exist with 10 G and even 2G5 systems. This paper identifies the 40 G modulation and transmission techniques that are most suitable for metro and regional applications.