An enhanced digital coherent receiver including a novel blind equalization approach and maximum-likelihood sequence detection (MLSD) is proposed for dual-polarization quadrature duobinary (DP-QDB) systems. In virtue of the compact spectrum of DP-QDB signals approaching Nyquist limit, the equalization is implemented at symbol rate, which relaxes the demand on the sampling speed of analog-to-digital converters (ADCs). The proposed symbol-rate receiver is verified by 43Gb/s experiments in the presence of chromatic dispersion and polarization-mode dispersion. This paper also investigates the tolerances to narrowband optical filtering, ADC sampling phase, duobinary-generating filter bandwidth, laser linewidth, and carrier frequency offset. Detailed modulation format comparisons are made among DP-QDB, DP-quadrature-phase-shift keying (QPSK), DP-8 quadrature amplitude modulation (QAM), and DP-16QAM. It is shown that DP-QDB with MLSD well trades off major requirements on advanced modulation formats.
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