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PAM-4 Signaling

A high-speed signal encoding technique.
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Description

Pulse Amplitude Modulation with Four Levels, or PAM-4, is a signal encoding technique that uses four voltage levels to represent four combinations of two bits logic (00, 01, 10, and 11). It is used for some 56GHz channels and all 112GHz channels.

Non-Return-To-Zero (NRZ, sometimes also referred to as PAM-2), the previous widely used scheme, by contrast has two voltage levels to represent 0 and 1. It is commonly used in the 28GHz range and for some 56GHz channels.

PAM-4 has half the Nyquist frequency and twice the throughput for the same Baud rate compared to NRZ.


PAM-4 divides the normal voltage swing into three, for a total of four distinct values. For example, the number 3 would be transmitted using two consecutive “1” symbols in NRZ; that becomes a single symbol with PAM-4. The eye diagrams are significantly compressed for the smaller divisions. The binary values are extracted at the receiver by a “slicer.” Source: Bryon Moyer/Semiconductor Engineering

With the move from NRZ to PAM-4, the signal-to-noise ratio (SNR) is reduced by 9dB, meaning a more powerful receiver is required, increasing energy consumption. Forward error correction (FEC) is also used to reduce the effect of noise. The FEC block itself adds to power consumption, but it may allow for a lower-power receiver.


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