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*To*: STDS-802-3-10GBT@LISTSERV.IEEE.ORG*Subject*: [10GBT] Bit-to-Symbol Mapping Losses in the PAM12 proposal*From*: sailesh rao <sailesh_rao@HOTMAIL.COM>*Date*: Wed, 28 Jul 2004 10:24:25 -0400*Reply-To*: "IEEE P802.3an" <STDS-802-3-10GBT@LISTSERV.IEEE.ORG>*Sender*: stds-802-3-10gbt@IEEE.ORG

Hi All, The bit-to-symbol mapping on slide 24 of powell_1_0704.pdf results in a 2D constellation shown on slide 9 of dabiri_1_0304.pdf. I took the liberty of snipping off that slide and attaching it herewith in order to ease this technical discussion. The problem with this mapping is two-fold. Firstly, it results in an inefficient use of the 12 PAM levels available for the encoding since it eliminates 16 points in the center of the 2D constellation. There are many ways to calculate this inefficieny - loss of capacity in the 0-412.5MHz frequency band used in the PAM12 proposal, reduction in SNR with respect to a hypothetical, optimal PAM12 system, etc. For the purposes of our technical discussion, I will use the latter measure. A simple calculation of the back-channel overhead, the 64B/65B overhead, the LPDC coding overhead and the 10Gb/s data rate, shows that in an "optimally encoded" PAM12 system, the modulation rate should have been approximately 780Ms/s. From the DFE SNR curves output by solarsep_varlen7a.m code, the optimal DFE SNR for Model 3 at this modulation rate shows that there is a 1.1dB additional SNR margin available in this optimal PAM12 system over the proposed practical 825Ms/s PAM12 system. (As an aside, a PAM8 system with the same overhead as in the PAM12 proposal would operate at a symbol rate of ~950Ms/s and I have an e-mail in to cabling experts on this task force to confirm that we can handle cabling environmental variations with such an overhead. This "low overhead" PAM8 system is easy to implement with a simple framing scheme and would result in an additional 1dB SNR margin for PAM8.) In addition to the 1.1dB bit-to-symbol mapping loss in the PAM12 proposal, there is an additional problem that arises due to the proposed 2D constellation. If the THP filter is loaded with ZERO coefficients, then the average symbol amplitude in the 2D constellation is sqrt(53)=7.28. If the THP filter is fully utilized, the PAM12 constellation will smear to cause an uniform distribution from 0-12, which results in an average symbol amplitude of sqrt(1/3)*12=6.928. The difference in power between these two extremes is 0.43dB, a fact that was also pointed out on slide 10 of zimmerman_1_0504.pdf. Furthermore, the results presented in jones_1_0504.pdf show that there are perfectly valid receivers possible for the PAM12 proposal that use ZERO THP coefficients at all line lengths. The main effect of this transmit power variation is that emissions compliance measurements would then become a function of the link partner's receiver structure. Finally, a parting thought which is an important consideration for my continued opposition to the PAM12 proposal. If this task force ever approves a constellation with a hole in the center, 10GBASE-T will become the butt of ribald jokes in the larger community of communications experts who understand these inefficiencies. While this may not be a technical issue, I consider it a fate worse than death for those of us who have reputations to maintain in this community. Regards, Sailesh Rao. srao@phyten.com _________________________________________________________________ FREE pop-up blocking with the new MSN Toolbar – get it now! http://toolbar.msn.click-url.com/go/onm00200415ave/direct/01/

**Follow-Ups**:**Re: [10GBT] Bit-to-Symbol Mapping Losses in the PAM12 proposal***From:*Sterling Vaden <sterlingv@bellsouth.net>

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