A feed-forward pilot-symbols aided carrier phase recovery scheme is described. The approach relies on pilot symbols that are time-division multiplexed with the transmitted data. The main advantage of the proposed solution is that of avoiding the phase ambiguity problem after a cycle slip. For homogeneous PM-QPSK transmission the proposed scheme outperforms blind carrier recovery with differential decoding.
Coherent optical communication systems promise superior performance, but their realization in real time also poses big technical challenges. After an introduction the potential of coherent optical transmission systems is shown as manifested in offline experiments. Then we present key components that are necessary to realize these systems in real time. We review recent achievements in realtime coherent communication and finally present the results of a realtime QPSK transmission system with a 3x3 coupler in the receiver. The achieved BER at a data rate of 1.4 Gbit/s is well below the FEC threshold.
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