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
.