Abstract
Insufficient system bandwidth and chromatic dispersion (CD) of fiber are major limitations for high-speed four-level pulse amplitude modulation (PAM4) direct detection systems. In this article, we investigate C-band 112-Gb/s PAM4 optical transport systems over 10-km and 20-km standard single-mode fiber (SSMF) transmission, which suffers from both bandwidth limitation and CD-induced power fading. Advanced digital signal processing techniques are proposed to handle those channel distortions. At the transmitter, a low-complexity
$1+\alpha Z^{-1}$
finite impulse response (FIR) filter coarsely pre-compensates system bandwidth without a preliminary channel estimation. At the receiver, a modified feed-forward equalizer and decision feedback equalizer (FFE-DFE) is employed to compensate residual insufficient bandwidth and CD-induced power fading. Moreover, a novel multi-symbols joint decision is proposed for receiver-side FFE-DFE to mitigate symbol error propagation and correct possible previous error symbols. Experimental results show that the proposed pre-equalization technique exhibits a better transmission performance, which reduces bit error ratio (BER) from
$3.3\times 10^{-3}$
to
$5.5\times 10^{-4}$
for optical back to back transmission. As for 10-km transmission case, FFE-DFE with 5-symbols joint decision achieves a BER of
$7.3\times 10^{-5}$
, which is better than the BER of
$1.6\times 10^{-4}$
with ideal error-propagation-free FFE-DFE (EPF-FFE-DFE). Furthermore, for 20-km transmission scenario, FFE-DFE with 8-symbols joint decision achieves a BER of
$6.0\times 10^{-4}$
, which is also a superior result than the BER of
$9.1\times 10^{-4}$
with EPF-FFE-DFE.
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