  In this paper, to the best of our knowledge, we propose the first multi-rate
Nyquist-subcarriers modulation (SCM) for C-band 100Gbit/s signal transmission
over 50km dispersion-uncompensated link. Chromatic dispersion (CD) introduces
severe spectral nulls on optical double-sideband signal, which greatly degrades
the performance of intensity-modulation and direct-detection systems. Based on
the prior knowledge of the dispersive channel, Nyquist-SCM with multi-rate
subcarriers is proposed to keep away from the CD-caused spectral nulls
flexibly. Signal on each subcarrier can be individually recovered by a digital
signal processing, including the feed-forward equalizer with no more than 31
taps, a two-tap post filter, and maximum likelihood sequence estimation with
one memory length. Combining with entropy loading based on probabilistic
constellation shaping to maximize the capacity-reach, the C-band 100Gbit/s
multi-rate Nyquist-SCM signal over 50km dispersion-uncompensated link can
achieve 7% hard-decision forward error correction limit and average normalized
generalized mutual information of 0.967 at received optical power of -4dBm and
optical signal-to-noise ratio of 47.67dB. In conclusion, the multi-rate
Nyquist-SCM shows great potentials in solving the CD-caused spectral
distortions.
