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*To*: STDS-802-3-10GBT@listserv.ieee.org*Subject*: [10GBT] Summary of issues with PAM12*From*: sailesh rao <sailesh_rao@HOTMAIL.COM>*Date*: Sat, 14 Aug 2004 15:56:16 -0400*Reply-To*: "IEEE P802.3an" <STDS-802-3-10GBT@listserv.ieee.org>*Sender*: stds-802-3-10gbt@IEEE.ORG

Hi All, Here’s a summary of the issues we’ve discussed with regard to the PAM12 proposal. I hope this summary will aid in addressing these issues in a systematic manner. 1. Emissions For the same peak-to-peak transmit launch voltage, the transmit PSD for the PAM12 system is 0.62dB to 1.05dB higher in the passband than that for the PAM8 system. The lower limit, 0.62dB, occurs due to the lower symbol rate for PAM12, i.e., 10*log10(Symbol_rate(PAM8)/Symbol_rate(PAM12)) = 10*log10(952.381/825) = 0.62dB The upper limit of 1.05dB occurs because of the additional 0.43dB excursion of the PAM12 transmit PSD as a function of its THP coefficients, as discussed in http://grouper.ieee.org/groups/802/3/10GBT/email/msg00938.html . In response to this calculation, Jose Tellado pointed out that if we use a formulaic approach for the transmit filtering (e.g., 3rd order Butterworth LPF with 3dB point at fs/4 for both proposals), then the “EMI PSD” for PAM8 sticks out at higher frequencies. (Ref. http://grouper.ieee.org/groups/802/3/10GBT/email/msg00872.html ). This is correct, but this argument doesn’t apply if we don’t use formulaic approaches for the transmit filtering for emissions control. Any transmit low pass filter that is used for emissions control reduces the SNR margin of the receiver. A reasonable goal is to keep the SNR margin loss to be around 0.5dB. Let’s say we choose such a transmit filtering scheme for the PAM12 system. Then, if we use the exact same transmit filter, with the exact same 3dB point for the PAM8 transmitter, the SNR margin loss for PAM8 will be about 0.6dB. In return for this extra 0.1dB margin loss, the PAM8 EMI PSD will be strictly lower than that for PAM12, by the aforementioned 0.62dB to 1.05dB. 2. Separation of Levels (Susceptibility) For the same peak-to-peak transmit launch voltage, the separation of levels in the PAM12 system is 3.93dB lower than the separation of levels in the PAM8 system. This is because there are 11 level separations occurring in the PAM12 system while there are 7 level separations occurring in the PAM8 system. Consequently, the 0m susceptibility advantage for PAM8 over PAM12 can be calculated as 20*log10(11/7) = 3.93dB In rao_1_0704.pdf, we further showed that this susceptibility advantage remains in favor of the system with the reduced number of levels, i.e., PAM8, even over a 100m cable, though it reduces to about 2.0dB due to the higher attenuation of the cable at the higher symbol rate. In response to these calculations, Jose Tellado pointed out that the above calculation ignores the whitening effect of the Tomlinson Harashima precoding filter (ref. http://grouper.ieee.org/groups/802/3/10GBT/email/msg00914.html ). He is correct, but there are two issues to consider in relation to this argument. a) Even if the THP whitens the levels perfectly, the 0m susceptibility advantage for PAM8 over PAM12 reduces to 20*log10(12/8) = 3.52dB, which only subtracts 0.41dB from the susceptibility advantages calculated above. b)The PAM12 transmit PSD varies by about 0.43dB as a function of the THP filter coefficients (ref. http://grouper.ieee.org/groups/802/3/10GBT/email/msg00938.html ). If the peak-to-peak voltage for PAM12 is reduced by 0.43dB to counter this variation, then all the calculations done in slides 29-52 of rao_1_0704.pdf are not only valid, but should be considered to be optimistic in favor of PAM12 by about 0.02dB. 3. PAM12 Constellation The bit-to-symbol mapping of the PAM12 system as described in slide 24 of powell_1_0704.pdf results in a 2D constellation for PAM12 as shown on slide 9 of dabiri_1_0304.pdf. This 2D constellation consists of a 12X12 array in which a 4X4 hole is carved out in the center of the constellation. With respect to the optimum 2D constellation for a PAM12 system, this scheme results in at least a 1.1dB SNR margin loss for PAM12 (ref. http://grouper.ieee.org/groups/802/3/10GBT/email/msg00938.html ). The hole in the center of the PAM12 constellation has a couple of important consequences: a) The transmit PSD of PAM12 varies by 0.43dB as a function of the THP filter coefficients, which are controlled by the link partner. b) To calculate worst-case SNR margins, one would need to assume that the alien NEXT and all impairments are operating with a 0.43dB higher transmit power than the received signal. This reduces the calculated SNR margins by around 0.4dB. There has been no response to this issue. 4. Framing Complexity The PAM12 system uses a data framing scheme that encompasses 52,833 data bits that are interspersed with back channel information bits and coded over 33 LDPC blocks. In contrast, the PAM8 system uses a data framing scheme that covers exactly one LDPC coded block. (ref. http://grouper.ieee.org/groups/802/3/10GBT/email/msg00945.html ). There has been no response to this issue. 5. Fixed Patterns The PAM12 system uses a sequence of fixed patterns transmitted once every 4us to indicate the start of a data frame. However, once the receiver has linked up correctly with the transmitter, it receives no additional information through the fixed patterns. The receiver should be able to count and know exactly where the fixed patterns should occur unless it has lost synchronization, which in a sophisticated system like 10GBASE-T would definitely mean loss of link. (ref. http://grouper.ieee.org/groups/802/3/10GBT/email/msg00933.html ). There has been no response to this issue. Regards, Sailesh Rao. srao@phyten.com _________________________________________________________________ Don’t just search. Find. Check out the new MSN Search! http://search.msn.click-url.com/go/onm00200636ave/direct/01/

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