Improved Quadrature Duobinary System Performance Using Multi-Modulus Equalization

Quadrature duobinary (QDB) signals can be generated from strongly filtered quadrature phase shift keying whose constellation becomes a 9-ary quadrature amplitude modulation. In this letter, the performance of multi-modulus equalization (MMEQ) to suppress noise and crosstalk for this QDB signal is experimentally investigated and compared with that of the constant-modulus equalization (CMEQ) with post filtering, under different filter bandwidth and carrier spacing. The results show that the MMEQ scheme has better performance for noise and crosstalk suppression, resulting in improved filtering tolerance. This improved filtering tolerance is demonstrated in an 8-channel 112-Gb/s wavelength-division-multiplexing experiment with a 25-GHz grid over 2640-km single-mode fiber. Compared with the regular CMEQ with the post filtering scheme, 1-dB optical signal-to-noise ratio improvement at the bit-error-ratio (BER) of $1\times 10^{-3}$ and 32% increased transmission distance at the BER level of $3.8\times 10^{-3}$ for forward-error-correction are obtained for the MMEQ scheme.