Abstract
We report the successful transmission of ten 494.85 Gbit/s DWDM signals on the standard 50
GHz ITU-T grid over 32 × 100 km of ultra-large-area (ULA) fiber. A net spectral efficiency
(SE) of 8.25 b/s/Hz was achieved, after excluding the 20% soft-decision forward-error-correction
(FEC) overhead. Such a result was accomplished by the use of a recently proposed
polarization-division-μltiplexed (PDM) time-domain hybrid 32–64
quadrature-amplitude-modulation (QAM) format, along with improved carrier frequency and phase
recovery algorithms. It is shown that time-domain hybrid QAM provides a new degree of design
freedom to optimize the transmission performance by fine tuning the SE of the modulation format
for a specific channel bandwidth and FEC redundancy requirement. In terms of carrier recovery, we
demonstrate that 1) hardware efficient estimation and tracking of the frequency offset between the
signal and local-oscillator (LO) can be achieved by using a new feedback-based method, and 2) a
training-assisted two-stage phase estimation algorithm effectively mitigates cyclic phase slipping
problems. This new phase recovery algorithm not only improves the receiver sensitivity by
eliminating the need for differential coding and decoding, but also enables an additional
equalization stage following the phase recovery. We have shown that the introduction of this
additional equalization stage (with larger number of taps) helps reduce the implementation
penalty. This paper also presents the first experimental study of the impact of inphase (I) and
quadrature (Q) correlation for a high-order QAM. It is shown that an adaptive equalizer could
exploit the correlation between I and Q signal components to artificially boost the performance by
up to 0.7 dB for a PDM time-domain hybrid 32–64 QAM signal when the equalizer length is
significantly longer than I/Q de-correlation delay.
© 2013 IEEE
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