I. Introduction
The fast and continuous increase of traffic load in optical networks prompts the support of multiple line rates to enable adaptation to traffic variations [1]– [4] as well as of transmission techniques that improve the spectral efficiency of optical communications [5]– [9]. Examples of spectral efficient solutions are the use of high-order modulation formats (e.g., polarization multiplexing 16 quadrature amplitude modulation — PM-16QAM) or faster-than-Nyquist transmission techniques [5]. In both cases, more bits can be squeezed in a Hz of bandwidth, allowing for a flexible upgrade planning of the network. Thus, optical networks can be optimized by selecting the best rate and/or modulation format for each requested optical channel or lightpath, leading to a multi rate and multi modulation format design.