The sub-Nyquist time-frequency packing (TFP) technique was demonstrated in a superchannel field trial transmission over long-haul distances. The technique allows high spectral efficiency even with low-order modulation formats. The transmission was successfully performed on an installed link between Milan and Finkenstein (Austria) in a loop back configuration, which included 2 Â 660 km of ITU-T G.655 fiber. The superchannel is composed by eight subchannels with low-level modulation format, i.e.,

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... polarization multiplexed (PM)-quadrature phase shift keying (QPSK). The use of low-order modulation format guarantees better robustness in terms of optical-signal-to-noise ratio (OSNR) and reduced complexity with respect to higher order formats. At the receiver side, coherent detection was used, together with iterative maximum a posteriori probability (MAP) detection and decoding. A 992-Gb/s PM-QPSK superchannel was successfully transmitted between Milan-Finkenstein-Milan with spectral efficiency (SE) of 6.2 bit/s/Hz. Long-term measurements confirm the system reliability.