Principles of digital coherent receivers for optical communications

▪ Coherent detection enables the highest spectral efficiency by allowing information be encoded in amptitude and phase. ▪ Homodyne or heterodyne downcoverter recovers the field in each polarization, and sampling above Nyquist rate enables compensation in DSP with no loss of information. ▪ CD and PMD are linear distortions and can be compensated by a linear filter in both single-carrier and OFDM transmission. ▪ Single-carrier and QFDM have similar computational requirements. ▪ Phase-noise tolerance of feedforward carrier recovery is superior to PLL. ▪ In the presence of dispersion, phase noise tolerance scales inversely with impulse response duration. ▪ Digital BP is a universal method of compensating linear and nonlinear impairments for both single-carrier and OFDM. Enables greater transmission distance, at the expense of complexity.