The eye diagram for signal ML shows that the amplitude is still the same as it was for signals M and L, but the period is now T/2. If we flip that number upside down, we get the bandwidth, 2/T. We retain the SNR requirement related to A, but the required bandwidth for the signal has doubled. So it's good news and bad news on SNR and bandwidth, respectively.
We need a way to double the bit rate in the channel without doubling the required bandwidth, and that's where PAM4 enters the picture. PAM4 takes the L (Least Significant Bit) signal, divides it in half, and adds it to the M (Most Significant Bit) signal. The result is four signal levels instead of two, with each signal level corresponding to a two-bit symbol.
The PAM4 signal looks like trace M+L/2 in Figure 1. At the lowest level is 00, followed by 01, 10, and 11, respectively. PAM4 indicates pulse-amplitude modulation, with the "4" indicating four levels of pulse modulation.
An eye diagram for a PAM4 signal is unusual, with three eye openings and four levels stacked vertically as shown in the figure. The bit period (or symbol period) is T. However, the opening of each of these three eyes is A/3. For bandwidth requirement, we roll back to 1/T. Thus, this signal, which moves 56 Gb/s, does so using the same amount of bandwidth as did the ML signal that moved 28 Gb/s. But with the SNR related to A/3, we find that our M+L/2 signal is three times m.
We have, in effect, traded off SNR for bandwidth. Many serial links are bandwidth-constrained, as it's difficult to move much more than 28 Gb/s over any length of copper. But when you have some SNR headroom, it may well pay off to consider a PAM4 modulation scheme.