Why is coherent transmission here today? Consider the advantages and disadvantages of coherent optical transmission compared to intensity modulated direct detection (IMDD). Also estimate the power budget limited length and dispersion limited length of an IMDD fibre optic link for bit rates of 2.5 Gbit/s, 10 Gbit/s, 40 Gbit/s, and 100 Gbit/s. Discuss advanced modulation formats and the fundamental limit to the capacity of a channel with given bandwidth and signal to noise ratio. Finally, analyze the relative merits of compensation of channel impairments in the optical domain and in the electrical domain in the context of advances in digital signal processing.