Today’s systems simply can’t communicate any faster. Learn how some companies are getting creative and doubling their data rates using PAM4 – and the extra challenge this technology means for engineers.
Mike Hoffman and Daniel Bogdanoff sit down with PAM4 transmitter expert Alex Bailes and PAM4 receiver expert Steve Reinhold to discuss the trends, challenges, and rewards of this technology.
PAM isn’t just cooking spray.
What is PAM4? PAM stands for Pulse Amplitude Modulation, and is a serial data communication technique in which more than one bit of data can be communicated per clock cycle. Instead of just a high (1) or low (0) value, a in PAM4, a voltage level can represent 00, 01, 10, or 11. NRZ is essentially just PAM2.
We are reaching the limit of NRZ communication capabilities over the current communication channels.
2:10 PAM has been around for a while, it was used in 1000BASE-T. 10GBASE-T uses PAM16, which means it has 16 different possible voltage levels per clock cycle. It acts a bit like an analog to digital converter.
2:55 Many existing PAM4 specifications have voltage swings of 600-800 mV
3:15 What does a PAM4 receiver look like? A basic NRZ receiver just needs a comparator, but what about multiple levels?
3:40 Engineers add multiple slicers and do post-processing to clean up the data or put an ADC at the receiver and do the data analysis all at once.
PAM4 communicates 2-bits per clock cycle, 00, 01, 10, or 11.
4:25 Radio engineers have been searching for better modulation techniques for some time, but now digital people are starting to get interested.
4:40 With communications going so fast, the channel bandwidth limits the ability to transmit data.
PAM4 allows you to effectively double your data rate by doubling the amount of data per clock cycle.
5:05 What’s the downside of PAM4? The Signal to Noise Ratio (SNR) for PAM4 worse than traditional NRZ. In a perfect world, the ideal SNR would be 9.6 dB (for four levels instead of two). In reality, it’s worse, though.
5:30 Each eye may not be the same height, so that also has an effect on the total SNR.
6:05 What’s the bit error ratio (BER) of a PAM4 vs. NRZ signal if the transmission channel doesn’t change?
6:45 The channels were already challenged, even for many NRZ signals. So, it doesn’t look good for PAM4 signals. Something has to change.
7:00 PAM4 is designed to operate at a high BER. NRZ typically specified a 1E-12 or 1E-15 BER, but many PAM4 specs are targeting 1E-4 or 1E-5. It uses forward error correction (or other schemes) to get accurate data transmission.
7:50 Companies are designing more complex receivers and more robust computing power to make PAM4 work. This investment is worth it because they don’t have to significantly change their existing hardware.
8:45 PAM is being driven largely by Ethernet. The goal is to get to a 1 Tb/s data rate.
9:15 Currently 400 GbE is the next step towards the 1 Tbps Ethernet rate (terabit per second).
10:25 In Steve’s HP days, the salesmen would e-mail large pictures (1 MB) to him to try to fill up his drive.
11:10 Is there a diminishing rate of return for going to higher PAM levels?
PAM3 is used in automotive Ethernet, and 1000BASE-T uses PAM5.
Broadcom pushed the development of PAM3. The goal was to have just one pair of cables going through a vehicle instead of the 4 pairs in typical Ethernet cables.
Cars are an electrically noisy environment, so Ethernet is very popular for entertainment systems and less critical systems.
Essentially, Ethernet is replacing FlexRay. There was a technology battle for different automotive communication techniques. You wouldn’t want your ABS running on Ethernet because it’s not very robust.
14:45 In optical communication systems there is more modulation, but those systems don’t have the same noise constraints.
For digital communications, PAM8 is not possible over today’s channels because of the noise.
15:20 PAM4 is the main new scheme for digital communications
15:50 Baseband digital data transmission covers a wide frequency range. It goes from DC (all zeroes or all ones) to a frequency of the baud rate over 2 (e.g. 101010). This causes intersymbol interference (ISI) jitter that has to be corrected for – which is why we use transmitter equalization and receiver equalization.
16:55 PAM4 also requires clock recovery, and it is much harder to recover a clock when you have multiple possible signal levels.
17:35 ISI is easier to think about on an NRZ signal. If a signal has ten 0s in a row, then transitions up to ten 1s in a row, the channel attenuation will be minimal. But, if you put a transition every bit, the attenuation will be much worse.
19:15 To reduce ISI, we use de-emphasis or pre-emphasis on the transmit side, and equalization on the receiver side. Engineers essentially boost the high frequencies at the expense of the low frequencies. It’s very similar to Dolby audio.
20:40 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 potentially error-inducing transition bits, this circuitry drives a higher amplitude than a normal bit.
22:35 Clock recovery is a big challenge, especially for collapsed eyes. In oscilloscopes, there are special techniques to recover the eye and allow system analysis.
With different tools, you can profile an impulse response and detect whether you need to de-emphasize or modify the signal before transmission. Essentially, you can get the transfer function of your link.
23:45 For Ethernet systems, there are usually three equalization taps. Chip designers can modify the tap coefficients to tweak their systems and get the chip to operate properly. They have to design in enough compensation flexibility to make the communication system operate properly.
25:00 PAM vs. QAM? Is QAM just an RF and optical technique, or can it be used in a digital system?
25:40 Steve suspects QAM will start to be used for digital communications instead of just being used in coherent communication systems.
26:30 PAM4 is mostly applicable to the 200 GbE and 400 GbE, and something has to have to happen for us to get faster data transfer.
26:48 Many other technologies are starting to look into PAM4 – InfiniBand, Thunderbolt, and PCIe for example.