In this work, we focus on enhancing the network reach in terabit superchannel transmission by using a noise-suppressed Nyquist wavelength division multiplexing （NS-N-WDM） technique for polarization multiplexing quadrature phase-shift keying （PM-QPSK） subchannels at different symbol-rate-to-subchannel-spacing ratios up to 1.28. For the first time, we experimentally compare the transmission reach of this emerging technique with that of no-guard-interval coherent optical orthogonal frequency-division multiplexing （NGI-CO-OFDM） on the same testbed. At BER of 2 x 10 3 and 100 Gbit/s per channel, an NGI-CO-OFDM terabit superchannel can transmit over a maximum of 3200 km SMF-28 with EDFA-only amplification, and an NS-N-WDM terabit superchannel can transmit over a maximum of 2800 km SMF-28 with EDFA-only amplification. Assuming different coding gain, 11 x 112 Gbit/s per channel with hard-decision （HD） forward-error correction （FEC） and 11 × 128 Gbit/s per channel NS-N-WDM transmission with soft-decision （SD） FEC can be achieved over a maximum of 2100 km and 2170 km, respectively. These are almost equal and were achieved using digital noise filtering and one-bit maximum likelihood sequence estimation （MLSE） at the receiver DSP. Characteristics including the back-to-back （BTB） curves, the ADC bandwidth requirement, and the tolerance to unequal subchannel power of an NS-N-WDM superchannel were also evaluated.
We propose a joint scheme for symbol, sampling clock, and carrier frequency synchronization in a polarization division multiplexing coherent optical orthogonal frequency division multiplexing （PDM-CO-OFDM） system. Unlike other existing algorithms designed for specific impairment, the scheme can estimate and compensate for the interactional synchronization errors effectively without extra training overhead by building a comprehensive error model. The simulation shows that symbol synchronization error and sample timing error can be corrected by channel equalizer, and the estimation ranges of sampling frequency offset and normalized carrier frequency offset are about （-2000, 2000） ppm, and （-0.04 （-3.5）, 0.04 （3.5）） Ghz, respectively.
