Re: PAM-5, what are your BERs ?
Not that I object to your analysis; however...
I believe that the MLT-3 used in FDDI-TPPMD and 100BASE-TX can also be
considered multi-level. If you agree, then it can serve as an example of a
multilevel coding scheme signalled over copper with a BER of 10e-12. I
also believe that the 8B6T signalling scheme in 100BASE-T4 is multi-level;
however, I don't know what it's BER ended up being.
JR
At 04:36 PM 2/27/00 -0800, Jaime Kardontchik wrote:
>
>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
>
>
>