What else might we do to double the bit rate? One approach is to serialize the two bit streams. Instead of two 28-Gb/s channels, we create one 56-Gb/s channel. As a result, in the same period in which we had one bit transmitted at 28 Gb/s, we now have two bits transmitted at 56 Gb/s. That would look like the bit stream ML in Figure 1.
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