Polarization multiplexing of QPSK (PolMux QPSK) is a good
candidate for the implementation of next generation high-speed transmission systems.
It helps to reduce the requirements on electrical and opto-electrical components
because it requires a symbol rate of only one-fourth of the bit rate. Digital
signal processing (DSP) combined with coherent detection has the potential to
mitigate the impact of transmission impairments. This example illustrates the
usage of DSP to mitigate the impact of phase noise*, chromatic dispersion (CD)
and polarization-induced distortions (PMD and polarization crosstalk**).
* Phase and frequency mismatch between laser transmitter and local oscillator.
** Crosstalk between orthogonally polarized channels resulting from the misalignment between the states of polarization of the local oscillator and the detected signal.
The simulation setup is displayed in
It consists of three 112-Gb/s PolMux QPSK transmitters at 100 GHz channel spacing, a transmission line
and polarization diversity receivers that include two 90° hybrids and a DSP unit (see
The transmission line is composed of a SSMF, a polarization tracker (to control the alignment between
the signal SOP and the polarization beam splitter at the receiver), a PMD emulator and an ASE source
(to set the OSNR). After detection the signal is sampled at 56 Gsamples/s (2 samples per symbol) and is
passed to the DSP unit. Residual CD is compensated using a FIR filter with Nt taps. Polarization effects
(PMD and polarization crosstalk) are mitigated using a MIMO structure consisting of 5-tap FIR filters. The
coefficients of the MIMO structure are optimized using the constant modulus algorithm (CMA). The phase
noise is corrected using a multisymbol phase estimation (MSPE) technique based on the Viterbi & Viterbi
algorithm. The estimated phase error is averaged over Ns symbols. The impact of CD for different number
of taps Nt is displayed in
The influence of polarization effects and phase noise (for different values of Ns) on the BER and
constellation diagram is illustrated in
Keywords: Coherent, Polarization Multiplexing (PolMux), Quadrature Phase-Shift Keying (QPSK), Digital Signal Processing (DSP), Finite Impluse Response (FIR), Constant Modulus Algorithm (CMA), Multisymbol Phase Estimation (MSPE)
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.