Re: [STDS-802-3-400G] IEEE P802.3bs 400 Gb/s Ethernet SMF Ad Hoc conference call draft agenda
Dear Kikuchi-san
As far as I know, a small amount of oversampling still doesn't change the fact that the resulting phase detection process in the PAM4 CDR is noisy (low gain) and has a lot of data-dependent noise, compared to 2 samples per baud where one sample is always on the steep slope of the data transition (but PAM4 still has more noise in the estimate than NRZ).
This means a lot of averaging is needed to extract an accurate phase error estimate, which means low loop bandwidth, which means high sensitivity to REFCLK and VCO noise. The problem is not the algorithm but the performance in real life when considering phase noise requirements on REFCLK and VCO. It wasn't a problem for 10G but at 100G the phase noise and jitter requirements are 10x (20dB) more stringent, this can then become a major issue which I fear is not being properly considered.
If you have some results showing that this is not the case then I suggest you present them for comparison with other approaches such as NRZ, lower-rate PAM4, and DMT.
(actually, results should be presented for all systems to show that this is not a show-stopper for low-cost low-power realizations)
Best regards
Ian
-----Original Message-----
From: Nobuhiko Kikuchi [mailto:nobuhiko.kikuchi.ca@xxxxxxxxxxx]
Sent: 02 October 2014 05:22
To: Dedic, Ian; IEEE_listserver
Subject: Re: Re: [STDS-802-3-400G] IEEE P802.3bs 400 Gb/s Ethernet SMF Ad Hoc conference call draft agenda
Dear Ian, Winston, and Cole
Please excuse me to express my view.
As to the timing issues of Nyquist PAM signal, Ian pointed out, there will be some countermeasures as you know, one of it is the increase of the ADC sampling ratio a bit, for example to 1.1-sample-per-symbol (Ex: 64GSa/sec for 56GBaud Nyquist PAM4 with the roll-off factor of 0.1). Such a small increase is sufficient to cover all the Nyquist spectrum and will not cause significant cost nor bandwidth issues.
I think that not only us, but other PAM4 vendors also have solutions to their implementation issues.
Best regards,
Nobu
Nobuhiko Kikuchi,
Central Research Lab, Hitachi Ltd.
Yoshida-machi 292,
Totsuka, Kanagawa, 244-0817
TEL: +81-50-3135-3104/3486
E-mail: nobuhiko.kikuchi.ca@xxxxxxxxxxx
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