  We propose an algorithm for carrying out joint frame and frequency
synchronization in reduced-guard-interval coherent optical orthogonal frequency
division multiplexing (RGI-CO-OFDM) systems. The synchronization is achieved by
using the same training symbols (TS) employed for training-aided channel
estimation (TA-CE), thereby avoiding additional training overhead. The proposed
algorithm is designed for polarization division multiplexing (PDM) RGI-CO-OFDM
systems that use the Alamouti-type polarization-time coding for TA-CE. Due to
their optimal TA-CE performance, Golay complementary sequences have been used
as the TS in the proposed algorithm. The frame synchronization is accomplished
by exploiting the cross-correlation between the received TS from the two
orthogonal polarizations. The arrangement of the TS is also used to estimate
the carrier frequency offset. Simulation results of a PDM RGI-CO-OFDM system
operating at 238.1 Gb/s data rate (197.6-Gb/s after coding), with a total
overhead of 9.2% (31.6% after coding), show that the proposed scheme has
accurate synchronization, and is robust to linear fiber impairments.
