NRZ is a modulation technique that has two voltage levels to represent logic 0 and logic 1. PAM4 uses four voltage levels to represent four combinations of two bits logic – 11, 10, 01, and 00.
PAM4 has the advantages of halving the Nyquist frequency and doubling the throughput for the same Baud rate. This alleviates the need for designers to have to invent infrastructure like silicon and cables that would go up to 50GHz bandwidth.
The SNR loss of a PAM4 signal compared to an NRZ signal is ~9.5 dB.
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