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Hi Brian, The basic parameters of the simulation have been given in my previous email replying to George. We can compare the absolute SNRs of PAM3 and PAM4, with
respect to each required SNR at BER of 10-10, giving SNR margin. The points at different symbol rates below the SNR threshold line requires FEC to provide coding gain. Just make sure that we compare the results on the same page. Best wishes, Tingting 发件人: Murray, Brian <Brian.Murray@xxxxxxxxxx>
Tingting Yes, you are correct in general the difference in absolute SNR between PAM-3 and PAM-4 should be about the same as the difference is the signal power. And as PAM-4 should operate at a lower symbol rate it has some advantage
here which should also translate into improved SNR. At lower noise levels in my sims I see between 0.5 and 1.5dB between PAM-3 at 75 MSym/s and PAM-4 at 62.5 MSym/s.
However, the most important difference between PAM-3 and PAM-4 is that the PAM-4 levels are closer together and you require a higher SNR, by 3.5 dB to get the same bit error rate. The 3.5 dB, is compensated by the 0.7
dB difference in lower PAM-4 signal power so this translates to about 2.8 dB. So we can’t compare absolute SNR levels when comparing PAM-3 and PAM-4. The next level of difference is probably down to implementation details. In my presentations I have outlined the basic parameters of the data path of the time domain simulation. I would need to see some more details your
data path parameters to comment. And you have only given a single cable length and noise level, which makes comparison difficult. For PAM-4 at 62.5 MSym/s for 500m at noise levels of -113 dBm/Hz over 100 MHz ( 7 mV rms) I am seeing error propagation in my
time domain simulations, which is degrading the SNR. But this is a real issue at high noise levels, once the SNR get a few dB below the 10-10 BER rate we will start to see this effect, which is a few dB below 23 dB for PAM-4.
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