Riding on the coat-tails of the noise problem is power. PAM-4 is an easy win because 2-level signalling is "over-margined" (which is why you don't need error co

Riding on the coat-tails of the noise problem is power. PAM-4 is an easy win because 2-level signalling is "over-margined" (which is why you don't need error correction with 2 levels, but you start to rely on it more and more as the # of levels goes up). The "easiest" way to get around noise problems is to put more signal in, but power consumption increases with the square of voltage -- so it's extremely costly to overcome noise with more signal. However, noise is a fundamental limit; there's a set of tricks you can maybe do to improve it but for example, kT/q (thermal voltage of noise at roughly room temp) is about 25mV and that plays a role in a lot of device physics. Thus most signals engineers would steer away from higher-order PAM and go to QAM, where they use phase to encode one dimension of data, and voltage to encode the other. This gets you out of a one-dimensional line of voltages and into a two-dimensional matrix of voltage vs. time to represent information. This helps fight against that square-law term for power, but it comes at the expense of more precise timing sources. Time also has noise mechanisms, that are typically only overcome at the expense of -- you guessed it -- power. At the end of the day, probably the most fundamental limit all computation will run into is power -- both how much you can afford to put in, and more importantly, the limit of what you can pull out reliably and efficiently. I'd be curious to see the plot of PCI-express energy-per-bit over its generations...I suspect it improves over time, but not improving as fast as bandwidth is going up, which means in net each link should run hotter.