Training-aided frequency-domain equalization is proposed for coherent optical transmission systems. In particular, novel training sequences based on a special class of CAZAC codes, efficient 2x2 MIMO channel estimation algorithms, equalizer-coefficients updating solutions and a frame synchronization algorithm are derived from the theoretical linear fiber channel model. The robustness of the proposed methods is demonstrated in 100Gb/s, 200Gb/s, and 400Gb/s transmission systems with respect to the main degrading optical propagation effects.weiterlesen