PAM stands for Pulse Amplitude Modulation

PAM has been around for a long time, since 1000BASE-T

PAM4 specs have a voltage swing of 600 to 800 mV

What does a PAM4 reciver look like?

You add multiple slicers and do some post processing or put an ADC and do the receiving all at once

With communications going so fast, the channel bandwidth limits the ability to transmit data. PAM4 allows you to effectively double your data rate

What's the downside of PAM4? The Signal to Noise Ratio (SNR) is not as good as NRZ. The ideal SNR is 9.6 dB by factoring in four levels instead of two.

Eyes may not be the same height, so that effects SNR

The channels were already impaired already, even for NRZ

PAM4 is designed to operate at a high BER

Companies are having to design more complex receivers and more robust computing power to make PAM4 work. But, it's worth it because they don't have to boost their existing hardware

PAM is being driven by ethernet. The goal is to get to 1 Tb/s

In Steve's HP days, the disk salesmen would e-mail large files to him to try to fill up his disk.

Is there a diminishing return for going to higher PAM levels?

In optical communications, they modulate much more but don't have the same noise concerns. For digital communications PAM8 is not possible over current channels because of noise

PAM4 is the main new computing scheme for digital communications

This causes intersymbol interference (ISI) that has to be corrected for - which is why we use transmitter equalization and receiver equalization

PAM4 also requires clock recovery, and it is much harder to recover when you have multiple levels

ISI is easier to think about on an NRZ signal. If you have ten 0s in a row, then it transitions up to ten 1s in a row, loss will be minimal.

To reduce ISI you use de-emphasis or pre-emphasis on the transmit side, and equalization on the receiver side

How do you boost only the high frequencies? There are circuits you can design that react based on the history of the bit stream. At transition bits, this circuitry puts out a higher amplitude than a normal bit.

Clock recovery is a big challenge, especially for collapsed eyes. In oscilloscopes, there are special techniques to recover the eye for analysis. With different tools you can profile an impulse response, and detect whether you need to de-emphasize or modify the signal for transmission. Essentially engineers get the transfer function of their link.

For Ethernet, there are typically three equalization taps. Chip designers can modify the tap coefficients to make their chips work. You have to design in enough compensation flexibility

PAM vs. QAM? Is QAM just an RF technique, or can it be used in a digital setting? Steve suspects QAM will break into the digital communication space instead of just being used in coherent coms

A number of other technologies are starting to look into PAM4. InfiniBand, Thunderbolt, PCIe, etc.