>On Feb 18, 4:54 am, "cpshah99" <cpsha...@rediffmail.com> wrote: >> Hi VLV >> >> Thank you for replying. But how to find noise variance of theequalised>> signal? >> >> And why it depends for Soft Demapper? >> >> Your help is really appreciated. :) >> >> Thanks again. >> Chintan >> >> >> >> >cpshah99 wrote: >> >> >> Hi >> >> >> In soft demapper, we need either value of SNR or noise variancesigma.>> >> >It depends. >> >> >> Now >> >> in real time we do not know both of these. >> >> >In real time we can now both of these althoigh we generally don'tneed>> it. >> >> >> And lets say that we know variance of white noise but as the signal >> passes >> >> through equaliser, we get filtered noise with different variance.So>> how to >> >> find variance of filtered noise. >> >> >Just like any other noise. >> >> >> Help in this respect will be greatly appreciated. >> >> >How much is the great appreciation? >> >> >VLV > >I'm assuming you're trying to design a soft-in, soft-out demapper that >operates in the presence of AWGN. If you can make the assumption that >your input bits are equally likely to be ones and zeros (i.e. you have >a perfect source encoder), then you don't necessarily need to know the >noise variance. If you're working in the metric domain, for example, >your input metrics are going to nominally be something like: > >|z - x_i|^2/No + k > >where z is your channel observation, x_i is the i-th noise-free symbol >location (i.e. one of the positions in your signal constellation), No >is twice the noise variance, and k is an additive constant common to >all of the terms (it comes from the constant term in front of the AWGN >pdf; since the transformation to the metric domain uses a logarithm, >it becomes additive). > >Here's where your goal is important: if you're trying to minimize >sequence error, then you would do min-sum processing on these metrics; >i.e. for each allowable configuration of the demapper, you'll sum up >all of the input metrics corresponding to the inputs for each >configuration, then find the minimum total metric that you then use to >derive your output metrics. Since a multiplicative constant (like No) >commutes with the min operator, it doesn't matter whether you have it >in there or not. Only the relative ordering of the metric values is >important; their values are not. Therefore, you can leave the No value >out and you will still get correct demapping. The same argument allows >you to neglect the additive constant k. > >However, if you're aiming to minimize bit error, you would use min*- >sum processing, where min*(x,y) = -ln(1 + exp(-x) + exp(-y)). >Multiplicative constants do not commute with the min* operator, so in >most cases, you do need to know the SNR, or have a good estimate of it >in order to get acceptable performance. Therefore, in a practical >system where it might be difficult to know Eb/No, using MAP sequence >detection with min-sum processing is more reasonable. > >Jason >HI Jason Thank you very much. You have understood my question. What I have is recorded signals from real time experiment where the channels is time and freq varying. Now I have downconverted this received signal to baseband. And I have passed this baseband signal thru DFE. Now the output of DFE is soft QPSK symbol. My aim is to design soft in soft out demapper. I had designed Soft demmaper from Bauch's paper. But in that i had used noise variance. Please explain me once again if you can. Thanking you Chintan