Contents I. Introduction
The rising popularity of bandwidth-demanding services such as high-definition (HD) television streaming, cloud computing, interactive HD online gaming, and file sharing between large data centers is driving increasing demand for high capacity, low latency optical transport networks (OTN). To accommodate ever increasing data rates over the finite transparent spectrum of optical fiber, optical systems have had to increase spectral efficiency (SE) by using multilevel multidimensional modulation formats. Coherent detection [1] and digital signal processing (DSP) have so far enabled the capacity of experimental systems to keep pace with the 40% annual growth in global internet traffic.
