Compensation of frequency offset for 16-QAM optical coherent systems using QPSK partitioning

The performance of a feed-forward carrier frequency recovery algorithm for 16-quadrature amplitude modulation (QAM) optical coherent systems is investigated using the quaternary phase-shift keying (QPSK) partition scheme. The frequency estimation technique, which employs only a subset of the 16-QAM symbols with the QPSK partition, is demonstrated to be effective when applied prior to the carrier phase recovery. Frequency offset of 10-1 times the symbol rate is shown to be tolerable at a bit-error rate (BER) of 10 -3 with combined linewidths symbol duration products of 10 -4. © 2011 IEEE