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*To*: STDS-802-3-10GBT@LISTSERV.IEEE.ORG*Subject*: [10GBT] Models for Calculating System Margin*From*: Bijit Halder <bijit@PLATONETWORKS.COM>*Date*: Tue, 1 Mar 2005 12:21:29 -0500*Approved-By*: bradley.booth@INTEL.COM*Organization*: Plato Networks*Reply-To*: Bijit Halder <bijit@PLATONETWORKS.COM>*Sender*: "IEEE P802.3an Reflector" <stds-802-3-10gbt@IEEE.ORG>*User-Agent*: Mozilla Thunderbird 1.0 (Windows/20041206)

All, Most of us agree that having a reasonable system margin is essential for a realistic implementation. There are two areas that still need some investigation: the THP and the power back off. In both cases, we need to verify that we have sufficient system margin, and if not, we need to modify our requirements to guarantee required system margin. Though we have reasonable model for channel insertion loss and alien cross talk there are two areas that are still seems to be open to discussion: the model for noise floor, and method of SNR calculations. However, to come to an agreement about the available system margin we need to establish a common understanding about these two issues. To expedite the process it may be useful to discuss the reasonable models on the reflector. 1. The noise floor: Following the models used in most presentations, I have used -104dBm/Hz to model over all noise floor at the receiver for my calculations. However, it seems there are some objections to the -140dBm/Hz noise floor model being too pessimistic. I am open to changing the model as long as we understand the implication on the design and there is a wide agreement. The second problem with the noise floor model is the effect of power back off. I have used an optimistic model for my calculation. I think a more realistic model is required. One possible model may be as follows: noise_floor (in dBm/Hz) = 10*log10( 10^(noise_floor_fixed/10) + 10^(noise_floor_var/10) * 10^(transmit_pow_dBm/10)) where noise_floor_fixed models the noise sources that are independent of transmit power such as back ground noise and quantization noise, noise_floor_var models the noise sources that are dependent on transmit power, and transmit_pow_dBm is the transmit power in dBm. Once we agree on the two constants, we have noise floor model for all cases including power back off and transmit power variation. 2. The SNR calculation approach is also seems to be open. I have used bandwidth of 400MHz for my calculation without folding and without any receiver filter. I think a more reasonable approach is to consider a receiver filter. I would ask the interested people to suggest what are the reasonable assumptions for calculating the SNR. These assumptions must come from realistic implementation issues and not the need to get certain SNR. To reach any agreement, we should at the least, clearly specify all the assumptions for the calculations so that others can verify the results. As for required margin, I would leave it up to the group. If the group is comfortable with 1dB system margin, I have no argument. However, we should be honest and tell that to the group so that they can make an informed decision. We should keep in mind that if we are serious about 100m operation, we should be careful about specifying a system that can operate over 100m under real conditions. Thanks. -bijit

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