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Polarization-multiplexed quaternary phase-shift keying (PM-QPSK) modulation associated with coherent detection and digital signal processing are the underlying technologies for 100-GE optical channels. Linear impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD) can now be easily overcome by adaptive digital filtering. Intra-channel nonlinear distortions such as the nonlinear phase noise (NLPN) and self-phase modulation (SPM) are the next limiting factors. Two of the highest performance intra-channel nonlinear equalizers proposed so far are the backward propagation split-step Fourier (BP-SSF) method and the frequency-domain Volterra series equalizer (FD-VSNE). In this paper we make a brief overview of digital signal processing for 100-GE optical carriers, and we test the performance limits of intra-channel nonlinear equalization algorithms, comparing the BP-SSF and FD-VSNE approaches in polarization-multiplexed transmission.