400G optical modules widely use PAM4 (4-level Pulse Amplitude Modulation) technology, which transmits signals through four different signal levels, with each symbol cycle representing 2 bits of logical information (0, 1, 2, 3). This technique allows signals to be transmitted at only half the symbol rate of NRZ signals, which greatly reduces the losses caused by the transmission channel. However, the signal-to-noise ratio can deteriorate much more than with NRZ signals and there are significant differences in measurement methods.
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