Polarization-Time Code and 4 x 4 Equalizer-Decoder for Coherent Optical Transmission

A linear polarization-time code in conjunction with an adaptive multi-tap 4 x 4 joint equalizer-decoder is proposed to average the effects of polarization-dependent impairments on two polarizations. For the first time in the context of polarization-time coding, there is no assumption of polarization-dependent loss (PDL) of knowledge at the receiver. This method compensates the performance loss penalty due to polarization-dependent impairments in polarization division multiplexed coherent optical single-carrier systems. Using the proposed method, a 1.25-dB optical signal-to-noise ratio gain is achieved in the presence of 6-dB PDL. The proposed 4 x 4 joint equalizer-decoder has twice the complexity of conventional time-domain equalizers. Further changes at the receiver including the demodulator are not required. Since the outputs of the equalizer are noisy quadrature phase shift keying signals, adoption of the method to work with both hard and soft forward error corrections is straightforward.