PCI Express 6.0 Reaches Version 0.5 Ahead Of Finalization Next Year

[bug77]

There is always a tradeoff to be made, nothing comes for free.
Usually higher-order modulation works better than doubling the clock, which comes with its own set of downsides.
To achieve acceptable robustness, PCIe 6.0 will very likely also add Forward-Error-Correction, which will decrease the net-throughput.

Yup, FEC is mentioned in there, too.

Most importantly, PAM-4 is a very very simple modulation, which makes it reasonably cheap. Doubling the clock would require quite a bit more expensive hardware.

I'm not sure what "reasonably cheap" means, looking at what PCIe4 did to motherboards. But who knows, maybe enterprise will take care of costs this time around and it will be affordable once we need it in a PC.

[ms178]

Speaking of being cheap, I hope they found a way to decrease costs, I am not a fan of inflating price levels due to higher costs from build materials and using retimers. The engineers should have come up with something more clever than that.

[tg--]

Yup, FEC is mentioned in there, too.

I'm not sure what "reasonably cheap" means, looking at what PCIe4 did to motherboards. But who knows, maybe enterprise will take care of costs this time around and it will be affordable once we need it in a PC.

Oh, PCIe 6.0 will be MUCH more expensive than 5.0 hardware, PAM-4 coding costs realestate in the chipsets, and FEC even more.
However, there the usual gamble mostly works: silicon will get cheaper, and sooner or later this isn't a problem.

In contrast to doubling the clock, which needs board-realestate instead of (or in addition to) silicon, which won't come down in price, since the limit here is already physics.

[brouhaha]

For wireless communications, PAM-2 is called BPSK (Binary Phase Shift Keying), two points on a unit circle spaced 180 degrees apart, which happen two lie on a straight line through the circle center.

I may be nitpicking a bit here, but that's not quite correct, because PAM is amplitude modulation, not phase modulation (or phase keying). The radio equivalent of PAM-2 is OOK (on-off keying), which is used by many remote controls but not for high-data-rate communication. There is no commonly used radio modulation equivalent to PAM-3 or higher; high-data-rate wireless uses QAM, which modulates both amplitude and phase, or OFDM, which is multiple QAM carriers, as used by e.g. WiFi and LTE.

[willmore]

I may be nitpicking a bit here, but that's not quite correct, because PAM is amplitude modulation, not phase modulation (or phase keying). The radio equivalent of PAM-2 is OOK (on-off keying), which is used by many remote controls but not for high-data-rate communication. There is no commonly used radio modulation equivalent to PAM-3 or higher; high-data-rate wireless uses QAM, which modulates both amplitude and phase, or OFDM, which is multiple QAM carriers, as used by e.g. WiFi and LTE.

In radio modulation PAM generally means Phase Angle Modulation. It's the class of modulations of which BPSK is but one. PAM-2 would be BPSK (0 and 180 degrees). PAM-4 would be QPSK (0, 90, 180, 270 degrees). I've seen PAM-8 and 16 used before. Generally beyond that, more complex modulations are used like QAM--which varies the phase and amplitude of the modulated signal. PAM has the property that the signal it generates is phase coherent which lets it be amplified by non-linear amplifiers--which are cheaper to design and can be more efficient.

[StefanBruens]

I may be nitpicking a bit here, but that's not quite correct, because PAM is amplitude modulation, not phase modulation (or phase keying). The radio equivalent of PAM-2 is OOK (on-off keying), which is used by many remote controls but not for high-data-rate communication. There is no commonly used radio modulation equivalent to PAM-3 or higher; high-data-rate wireless uses QAM, which modulates both amplitude and phase, or OFDM, which is multiple QAM carriers, as used by e.g. WiFi and LTE.

This depends on if your two levels are [0,1] or [1,-1]. The latter obviously is the same as phase inversion, i.e. BPSK. So OOK is another PAM-2 modulation, while ASK (Amplitude Shift Keying) would be a third one. For BPSK you obviously need to decode/recover the phase of the signal, while for OOK/ASK a trivial envelope/power detector is sufficient. If you want to go dirt cheap, use OOK/ASK.

There is actually no difference between Amplitude/Phase and Quadrature modulation if you just look at the radio signal, these are just different coordinate systems - polar vs carthesian. You can not tell QPSK and QAM-4 apart. SDR receivers are typically build as quadrature receivers, and if the signal of interest uses phase shift keying the phase is computed from the quadrature signals.

In wired communication, both [0,1] and [1,-1] are used. RS232 signals use e.g. +5/-5 volts, TTL signals are 0V/+5V.

[brouhaha]

In radio modulation PAM generally means Phase Angle Modulation.

Interesting. While I've heard of "phase angle modulation", I've never seen it abbreivated "PAM". All of the phase-only modulation techniques I've seen for radio have been called PSK or PM, or variants thereof. I've only ever seen "PAM" used for Pulse Amplitude Modulation, which is the kind of PAM used in PCIe 6.

[brouhaha]

This depends on if your two levels are [0,1] or [1,-1]. The latter obviously is the same as phase inversion, i.e. BPSK.

Thanks, you're obviously correct. I hadn't considered the [+1, -1] case.

[torsionbar28]

Phoronix: PCI Express 6.0 Reaches Version 0.5 Ahead Of Finalization Next Year

Meanwhile, Intel will still be stuck at PCIe 3.0 and 14 nm CPU's, lol.

[zxy_thf]

There are motherboard options with passive cooling.

In addition, Epyc is SOC and no chipset on motherboard. I'm hoping next TR follows this.