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*To*: STDS-802-3-10GBT@LISTSERV.IEEE.ORG*Subject*: Re: [10GBT] Issues with solarsep_varlen7a.m*From*: sailesh rao <sailesh_rao@HOTMAIL.COM>*Date*: Thu, 8 Jul 2004 15:23:14 -0400*Reply-To*: "IEEE P802.3an" <STDS-802-3-10GBT@LISTSERV.IEEE.ORG>*Sender*: stds-802-3-10gbt@IEEE.ORG

George, Since we are dealing with an alien crosstalk dominated noise environment with its random phase variations, I didn't consider the possibility of an analog matched filter prior to the sampler. Hiroshi (takatori_1_0504.pdf) did not consider 2.5bit PAM as one of the uncoded alternatives. I have no issues with Gottfried's analysis (ungerboeck_1_0504.pdf) and I'm aware that his results were pessimistic for the 100m Cat-6 2.5bit PAM case because of the large background noise (-135dBm/Hz) used in the analysis. However, I'm not expecting such large background noise in a practical 10GBASE-T receiver. Regards, Sailesh Rao. >From: George Zimmerman <gzimmerman@SOLARFLARE.COM> >Reply-To: "IEEE P802.3an" <STDS-802-3-10GBT@LISTSERV.IEEE.ORG> >To: STDS-802-3-10GBT@LISTSERV.IEEE.ORG >Subject: Re: [10GBT] Issues with solarsep_varlen7a.m >Date: Thu, 8 Jul 2004 09:24:49 -0700 > >Glen - >Thanks for a good description of the key difference. The optimal DFE >result includes optimization of filtering prior to baud sampling as well >as any baud spaced FFE. As you point out, the folded SNR result is NOT, >as Sailesh asserts, limited to T/2 spaced FFE systems (that is just one >way of implementing the optimality for the first fold). It is well >known in practice that the SNR on limiting cases can be optimized by >tuning the front end filter. > >On the other hand, the MMSE analysis that Sailesh is assuming assumes NO >front end filtering prior to baud sampling, and, therefore is inherently >pessimistic. Such a design would suffer from aliased noise. Any real >system would have a filtering prior to baud sampling. If we were to >assume a front-end filter function, why not assume an optimal design to >get best performance? This leads us right back to the optimum folded >SNR relation of the DFE found in the code. > >As you also point out, the argument that Sailesh is making is a small >one. If I eliminate the folding entirely, the absolute DFE SNR results >change between approximately 0.1 and 0.2 dB, and the relative values (2 >vs. 2.5 vs. 3 bits/baud/pair PAM) change by 0.1 dB. > >Sailesh - >I hope this answers your folded SNR question. On your question on the >March presentation, I'm not sure which parameters you have in question, >but if you give me a call, I'll be happy to look up whatever information >you're missing. The SNR comparison is generated as margin relative to >capacity, which is scaled relative to 6.02 dB/bit/baud/pair. This can >be found in lines 470-494 of the code, not the section you were >commenting on. The channel model used is stated in the presentation. >Previous emails with Samir on the reflector have clarified the >command-line parameters that correspond to our now-agreed Models 1 2 & >3. (careful - I recall the first part of the exchange had a sign error). >A result similar to those from March can be found on Slide 5 of >mclellan_1_0504, except this one is now for our agreed channel model #1. > > >I will also point out that the optimality of the lower baud rates is for >both the longer channels, and that it has been independently presented >in takatori_1_0504 and Ungerboeck_1_0504, and, at the latter analysis >(Ungerboeck) comes at the problem from a completely different >perspective. >-george > >-----Original Message----- >From: stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On >Behalf Of Glenn Golden >Sent: Thursday, July 08, 2004 8:02 AM >To: STDS-802-3-10GBT@listserv.ieee.org >Subject: Re: [10GBT] Issues with solarsep_varlen7a.m > > > > > Sailesh Rao <saileshrao@OPTONLINE.NET> writes: > > > > > > The folded SNR calculations in lines 443, 453 and 463 are not right. > > > > > > If f1 and f2 are mirror frequencies about fs/2, the formula being > > > used > > > > > is > > > > > > S/N = ABS(S1/N1) + ABS(S2/N2) ; > > > > > > However, the actual SNR at the folded frequency would be > > > > > > S/N = ABS(S1+S2)/ABS(N1+N2) > > > > > > where S1, S2, N1, and N2 are complex phasors. Therefore, in the > > > context of folding, the actual PSD of the signal becomes relevant, > > > whereas the original Salz formula for the optimum DFE SNR is > > independent of the PSD. > > > > > > > George Zimmerman <gzimmerman@SOLARFLARE.COM> writes: > > > > > > On the folded SNR calculation, however, you are incorrect. The > > > optimum DFE is based on a folded SNR which is the sum of the SNRs, > > > not > > > > > the sum of the signal over the sum of the noise. You can check > > > either > > > > > Salz, or for a more direct representation, please check Pottie & > > > Eyuboglu, JSAC, August 1991, equation 6. > > > > > > >The Salz DFE analysis assumes a prefilter prior to the baud sampler. >Following optimization (in AWGN) the prefilter turns out to be >equivalent to the cascade of a channel matched filter followed by >a one-sided synchronous tapped delay line. The matched filter's phase >(conjugate to channel) ensures that the net transfer function >(channel*MF) >lies on the positive real axis prior to the baud sampler. Thus, all >folding translates (f0+k/T, k = -inf ... inf) add unidirectionally, >eliminating the effects of channel phase. It is only because of this >phase alignment that the optimized integrand involves the sum-of-SNRs, >and not sum-of-signal/sum-of-noise. (The same holds for a DFE with >fractionally spaced FFF.) > >But for a synchronous DFE in the absence of a matched filter -- probably >the system of interest to most of us -- no special phase alignment of >the translates can be assumed, and the relevant folding expression (for >flat AWSS noise with variance N0) is > > abs(SUM H(f0+k/T)) ** 2 / N0 , > k > >H(f) being the net transfer function from the Tx to the Rx baud sampler >input. Except for a missing "**2", this is essentially as Sailesh >indicated. > >The bottom line is that without a MF or fractionally spaced FFF, the >value of the summation depends on the channel and front-end phases at >the translate frequencies, which is the point I believe Sailesh was >making. >The sum-of-SNRs folding is an upper bound. Thus, the solarsep code >yields >optimistic results, unless the assumed system model includes a >fractionally >spaced or MF front end. > >For our channel, as long as the rolloff is smooth, the 'optimism' will >not >be very large, because even if translates are completely out of phase, >the >in-band translate magnitudes dominate. Similarly if the front-end rolls >off reasonably above 1/(2Fs). But if there are large ripples near >1/(2Fs) >and shallow front-end rolloff, then significant dips in the folded >spectrum >can be introduced which could result in non-negligible MSE differential >between the solarsep method and a more realistic (synchronous, no MF) >evaluation. > >Glenn Golden >Principal Engineer >Teranetics, Inc. _________________________________________________________________ Get fast, reliable Internet access with MSN 9 Dial-up – now 2 months FREE! http://join.msn.click-url.com/go/onm00200361ave/direct/01/

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