  In this paper, we propose a linear polarization coding scheme (LPC) combined
with the phase conjugated twin signals (PCTS) technique, referred to as
LPC-PCTS, for fiber nonlinearity mitigation in coherent optical orthogonal
frequency division multiplexing (CO-OFDM) systems. The LPC linearly combines
the data symbols on the adjacent subcarriers of the OFDM symbol, one at full
amplitude and the other at half amplitude. The linearly coded data is then
transmitted as phase conjugate pairs on the same subcarriers of the two OFDM
symbols on the two orthogonal polarizations. The nonlinear distortions added to
these subcarriers are essentially anti-correlated, since they carry phase
conjugate pairs of data. At the receiver, the coherent superposition of the
information symbols received on these pairs of subcarriers eventually leads to
the cancellation of the nonlinear distortions. We conducted numerical
simulation of a single channel 200 Gb/s CO-OFDM system employing the LPCPCTS
technique. The results show that a Q-factor improvement of 2.3 dB and 1.7 dB
with and without the dispersion symmetry, respectively, when compared to the
recently proposed phase conjugated subcarrier coding (PCSC) technique, at an
average launch power of 3 dBm. In addition, our proposed LPCPCTS technique
shows a significant performance improvement when compared to the 16-quadrature
amplitude modulation (QAM) with phase conjugated twin waves (PCTW) scheme, at
the same spectral efficiency, for an uncompensated transmission distance of
2800 km.
