Re: PAM-5, what are your BERs ?
Hello Vivek,
A few months ago there was a discussion on the Reflector where
some members claimed that the BERs in some optical backbones
reached easily the 10^(-15). The repply to these claims was that
although this might be true, the philosophy of the 802.3, that made
its standards so successful, was to design based on worst case
conditions. Therefore, the Ethernet optical networks were specficied
to a BER of only 10^(-12).
The same holds for the Ethernet Copper links.
Although it might be true that some links of 100BASE-TX
reach a BER of 10^(-12), the official BER adopted in the
Standard was only 2.5*10^(-10). And the same for the
1000BASE-T, where the Standard adopted to support a
BER of 10^(-10).
The reason for this higher BERs in the Copper networks is
ISI (Inter Symbol Interference). Or, in layman's language:
is the eye open or not at the input of the receiver ?
Jaime
Jaime E> Kardontchik
Micro Linear
San Jose, CA 95131
Vivek Telang wrote:
> Hello Jaime and 10G'ers,
>
> I had to jump in to try and correct this misconception of the "eye opening requirement". While it is true that the "open eye" is always a good thing, it is definitely not a requirement for low BER reception. If it were a requirement, the tried and true 100BaseTX Ethernet would not be functioning today. Many existing implementations of 100BaseTX silicon easily achieve BERs of 1e-12 over 170+ meters of twisted pair cable, and the eye is very much closed in this case. Not to mention that near-end crosstalk (NEXT) makes matters significantly worse. Also, in 1000BaseT, if the only impairment was ISI, and Echo, NEXT and FEXT were absent, BERs of 1e-12 could be easily achieved, even with a 5-level symbol constellation.
>
> An open eye is a requirement for systems that use simple lock-and-slice techniques, where little or no processing of the signal is done, other than maybe amplification, and a PLL locks to the data clock and slices the data. In contrast, in a DSP-based system, the combination of linear feedforward equalizers and DFEs using simple LMS adaptation can easily deal with "closed eyes". Even with a blind start-up.
>
> The implementation of a DSP-based system (including a 6-bit ADC) at 5GHz in CMOS is a different matter altogether, and I am eagerly awaiting Oscar's presentation at the March meeting.
>
> Regards,
>
> Vivek
>
> *---------------------------------------
> *
> * Vivek Telang
> * Cicada Semiconductor Inc.
> * 901 MoPac Expressway South
> * Building One, Suite 540
> * Austin, Texas 78746
> *
> * 512-327-3500 x114 voice
> * 512-327-3550 fax
> * vivek@xxxxxxxxxxxxxxx
> * http://www.cicada-semi.com
> *
> *---------------------------------------
>
> -----Original Message-----
> From: Jaime Kardontchik [SMTP:kardontchik.jaime@xxxxxxxxxxx]
> Sent: Sunday, February 27, 2000 6:37 PM
> To: stds-802-3-hssg@xxxxxxxx
> Subject: PAM-5, what are your BERs ?
>
> Hello 10G'ers,
>
> Edward Chang was right on target.
> He wrote on Feb 23:
>
> > In the past, the multiple voltage-level coding
> > was adopted by two LAN standards, ATM and
> > Ethernet. Both of them were twisted-pair
> > applications, and the BER were 10^(-10).
> > I proposed BER of 10^(-12) in both working
> > groups to be consistent with the LAN optical
> > links' BER; however, for some reason, they
> > remain 10^(-10), officially in both standards.
>
> What was the reason ?
>
> Multilevel voltage coding was not the reason.
> On the contrary, it was a remedy. The real reason
> was the strong ISI at the maximum link lengths
> that these Standards wanted to support.
> Multi-level voltage coding was adopted to
> lower the baud rate or the frequency content
> of the signal in order to make the ISI smaller
> and get a better BER.
>
> However, the remaining ISI in these Copper links
> remained high enough that the eye at the
> input of the receiver remained still
> completely closed. It is this closed eye that
> limited the achievable BER in the Copper media
> to 10^(-10). Post-equalizers, no matter how
> sophisticated they were, are not able to
> completely reverse the effects of eye closening
> on the achievable BER.
>
> ---> Conclusion ? In order to get a BER of
> 10^(-12) you have to have a clear
> open eye at the input of the receiver.
> The maximum supported link length
> is set accordingly to meet this basic
> condition: open eye.
>
> Now we can go to the basic two PAM-5 proposals:
>
> 1) PAM-5 serial at 5 Gbaud
>
> Using 1300 nm lasers lasers at 5 Gbaud the
> optical eye is completely closed already at
> ~170 meters due to heavy ISI (installed MMF,
> 500 MHz*km bandwidth).
>
> Oscar claimed a support of 500 meters of
> installed MMF and pointed out that
> DFEs (equalizers) have been successfuly used
> in 100 and 1000 Mbps Copper networks.
> However, the specs and experience of the 100 Mbps
> and 1000 Mbps links over Copper - where the eye
> at the input of the receiver is completely
> closed at the target link length due to ISI -
> put the achievable BERs around 10^(-10) only.
> Even using sophisticated equalizers.
>
> What is the experimental support that a BER
> of 10^(-12) could be achieved when a strong ISI
> closes completely the eye at the input of the
> receiver ? I think that none and that the
> experience points to the contrary. Why then
> could an equalizer running at 5 GHz achieve
> here what sophisticated equalizers running
> 40 times slower (125 MHz) were not able to achieve
> in the Copper Ethernets ? Parallel processing
> could enable perhaps to meet the timing
> constraints of the design by running multiple
> equalizers at a lower clock, but will not
> eliminate the basic limitation on achievable
> BERs once the eye at the input of the receiver
> is already completely closed.
>
> Let us see now the case PAM-5 at 1.25 Gbaud:
>
> 2) PAM-5 4-WDM at 1.25 Gbaud
>
> Let us compare this approach to two other well
> known on-off approaches: 1 GbE and 4-WDM at
> 3.125 Gbaud using the 8b/10b coding. Let us
> assume again 1300 nm lasers and installed
> 500 MHz*km MMF.
>
> The figures below (I hope will not get distorted
> during transmission) show the power levels of
> the three systems. For 1 GbE I assumed no ISI.
> For PAM-5 I assumed 400 meters link length
> (there is no ISI up to this distance, ISI= 0 dB).
> For 4-WDM at 3.125 Gbaud I assumed 300 meters link
> length and about 3 dB optical loss due to ISI.
>
> | |
> | |
> | | 1 GbE, 1.25 Gbaud, no ISI
> | |
> | |
>
> | | | | |
> | | | | |
> | | | | | PAM-5 4-WDM at 1.25 Gbaud
> | | | | | no ISI.
> | | | | |
>
> |**** ****|
> |**** ****|
> |**** ****| 8b/10b 4-WDM at 3.125 Gbaud
> |**** ****| 3 dB ISI loss
> |**** ****|
>
> ( the asterisks denote closening of the eye due
> to ISI)
>
> 2a) 1 GbE
>
> By definition, the optical power difference
> between the '0' and '1' levels in the 1 GbE
> case is 1:
>
> optical signal power = 10*log(1) = 0 dB
>
> optical SNR = 0 dB (reference)
>
> And we get a BER of 10^(-12).
>
> 2b) PAM-5 4-WDM at 1.25 Gbaud
>
> In the PAM-5 case, notice that using the
> "open fiber control" method that I described
> in a previous email, we get the same launched
> power per channel as in the 1 GbE Ethernet.
> However, in PAM-5 the power difference
> between levels is 0.25 (this is the well
> known 6 dB optical power penalty loss of
> the PAM-5 modulation). Let us now add
> the 6 dB electrical coding gain provided
> by the Viterbi decoding: the effective distance
> between levels doubles. Summarizing, in PAM-5
> 4-WDM at 1.25 Gbaud, the effective optical
> signal power difference between levels is:
>
> effec optical signal power diff =
> 10*log(2*0.25) = - 3 dB
>
> where the factor 2 inside the log comes
> from the coding gain.
>
> The noise power at the input of the
> receiver is the same as in 1 GbE because
> we use the same baud rate. Hence,
>
> effec optical SNR = - 3 dB
>
> This is not bad compared to the 0 dB of 1 GbE.
> Furthermore, notice that we could even bring
> back the SNR for PAM5 to 0 dB (the 1 GbE
> reference) if a new laser safety proposal
> to move the maximum safe power from -4 dBm
> to -1 dBm is accepted. See P. Kolesar et al
> presentation in the next March meeting.
>
> 2c) 8b/10b 4-WDM at 3.125 Gbaud
>
> In this case the eye is half closed due
> to ISI. Hence the optical power difference
> is:
>
> optical signal power = 10*log(0.5)
> = - 3 dB
>
> and the electrical noise power at the input
> of the receiver is larger, since the receiver
> needs more bandwidth:
>
> elec noise power = 10*log[(3.125/1.25)^2]
> = 8 dB
>
> Hence, the optical SNR is
>
> optical SNR = - 3 - 8/2 = -7 dB
>
> This case is clearly worse than the 1 GbE
> case in terms of optical SNR. And the
> Task Force considers that this system
> can achieve the needed 10^(-12) BER.
>
> Summarizing:
>
> PAM-5 4-WDM at 1.25 Gbaud has no
> ISI, a clear and wide open eye at
> the input of the receiver, an optical SNR
> only slightly less than the SNR of present
> 1 GbE transceivers and better than
> the proposed 8b/10b 4-WDM at 3.125G
> transceivers.
>
> Hence, it is a good choice if one
> wants to reach the required BER of
> 10^(-12).
>
> Note: the above analysis, based on simple
> static SNRs, is not a substitute for a
> much more careful analysis regarding
> the viabibility of any PAM-5 approach.
> However, a simple back-of-the-envelope
> analysis is very useful to make a quick
> comparison between different PAM-5 proposals
> in order to find out which PAM-5 architecture,
>
> a) one working under strong ISI conditions
> that close the optical eye completely; or
>
> b) another using a lower baud rate
> and working with minimal ISI so that
> the optical eye at the input of the
> receiver is widely open,
>
> which one has more chance to meet the required
> BER of 10^(-12), and discard accordingly the
> one that does not have a chance.
>
> Jaime
>
> Jaime E. Kardontchik
> Micro Linear
> San Jose, CA 95131
> email: kardontchik.jaime@xxxxxxxxxxx