Nowadays

in the era of big data and cloud computing

improving the data transmission rate has become a critical issue. High-order modulation technique

e.g. PAM4 modulation

provides a solution to achieve more data transmission rate while keeping the same bandwidth of optical fiber. We perform the first real-time demonstration of a 4-Tb/s metro-DCI system with commercial QSFP28 optical modules

equipped with 40 x100 G dual wavelength 56-Gb/s PAM4. We evaluate the system performance over 80 km transmission distance comprehensively and quantify the system tolerance to the chromatic dispersion and optical nonlinearity over a wide diversity of fiber types.