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Through graphics-processing-unit-based simulations with different numbers of copropagating channels (1--81), the dependence of the nonlinear threshold on channel count, as well as on the fiber polarization mode dispersion (PMD) coefficient, is investigated for both dispersion-managed and DCM-free 40 and 100 Gb/s coherent-detected polarization-multiplexed quadrature phase-shift keying (CP-QPSK) transmission systems. Different fiber types including standard single-mode fiber (SSMF), large effective area fiber (LEAF), and truewave classic fiber (TWC) are considered and compared. Our investigations show that the required number of simulated copropagating channels to correctly simulate the nonlinear penalty caused by interchannel nonlinearities on CP-QSPK modulation is strongly dependent on the fiber type. The generally used assumption of around ten channels for simulating interchannel nonlinearities is only valid for the SSMF with relative low channel input power. For transmission links consisting of fiber types with low dispersion or high nonlinear coefficients, such as the LEAF or TWC, ten copropagating channels are clearly not sufficient. In dispersion-managed systems with DCMs, the required number of simulated copropagating channels is not only dependent on fiber types and data rates but also strongly on PMD present in the links. Our investigations have indicated that for transmission over fibers with very low PMD (this is the case of most new fibers), ten copropagating channels are not sufficient to correctly characterize the interchannel nonlinearities even for high-dispersion fiber types, such as the SSMF, and hence causes a clear underestimation of the nonlinearity penalty. Finally, synchronized and interleaved CP-QPSK is compared. We show that despite the depolarization effect of PMD, there are still some benefits of using interleaved RZ-CP-QPSK systems.