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We investigated through simulations the performance of Nyquist-WDM Terabit superchannels implemented using polarization-multiplexed phase shift-keying based on 2 (PM-BPSK) and 4 (PM-QPSK) signal points or polarization-multiplexed quadrature amplitude modulation based on 8 (PM-8QAM) and 16 (PM-16QAM) signal points. Terabit superchannels are obtained through the aggregation of multiple subcarriers using the Nyquist-WDM technique, based on a tight spectral shaping of each subcarrier which allows very narrow spacing. We first studied the optimum transmitter/receiver filtering in a back-to-back configuration. Then we investigated the maximum reach for different spectral efficiencies, after nonlinear propagation over uncompensated links with lumped amplification. Performance for systems based on both standard single-mode fiber (SSMF) and large effective area non-zero dispersion-shifted fiber (NZDSF) has been analyzed. Assuming SSMF with 25-dB span loss, we found that PM-BPSK can reach 6480 km at a net capacity of 4 Tb/s across the C band. Conversely, PM-16QAM can deliver 27 Tb/s, but over 270 km only. Note that a lower span length, the use of Raman amplification and/or pure silica-core fibers (PSCFs) can significantly increase the maximum reach, but without changing the hierarchy among the performance of modulation formats. We also show that the maximum reachable distance is approximately 2/3 of the one achievable in linear propagation at the optimum launch power, regardless of the modulation format, spacing and fiber type. As additional results, we also verified that the optimum launch power per subcarrier linearly depends on the span loss, varies with the fiber type, but it is independent of the modulation format, and that the relationship between the maximum reachable distance and the span loss is almost linear.