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Author Topic: Virtual noise?  (Read 1205 times)

andyfitter

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Re: Virtual noise?
« Reply #15 on: January 27, 2018, 01:14:33 AM »

I agree, that’s a great explanation. It was kind of where I was in my head having muddled through the docs, but the way you laid it out was perfect.

Much appreciated.

Andy.
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Weaver

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Re: Virtual noise?
« Reply #16 on: January 27, 2018, 04:08:29 AM »

Got it. So needs SRA to make sense.
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kitz

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Re: Virtual noise?
« Reply #17 on: January 27, 2018, 11:22:29 AM »

Doesnt need SRA, just used that as an example where it may be useful. 

It's more useful with Low Power Standby Mode.   Refer to this post I made a few years ago about L2 power saving

Quote
A White paper by Texas Instruments states that there are serious concerns about low power modes and how it can and does introduce Time Varying Crosstalk and for this reason all the major ISPs around the globe agreed that L2 mode has serious effects on FEXT. The ITU study group concluded that it should NOT be a VDSL2 specification.

It's not like vectoring which cancels out effects of x-talk.
Virtual Noise cancels out the 3dB <-> 6dB swings when crosstalk disturbers go on and off line.   
It does this by artificially recreating the crosstalk environment even when the disturber is offline, by mimicking effects FeXT affected tones and ensuring those tones dont ever drop below 'x' dB compared to other tones.

Its not a cure for FEXT, in fact it does the opposite by ensuring the line constantly has the affects of FEXT regardless if the disturber is on or off line.
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Black Sheep

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Re: Virtual noise?
« Reply #18 on: January 27, 2018, 12:04:27 PM »

Lovely piece of feedback, Lesley ...... many thanks for the layman's interpretation.  :)
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Weaver

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Re: Virtual noise?
« Reply #19 on: January 27, 2018, 06:31:05 PM »

I'm with you about low power mode. That's a very good point, that goodness someone realised that. Road to hell and something about good intentions and all so on.

I was referring to the part in your helpful explanation where you talked about increasing speeds on the basis of a false impression gained when disturbers had temporarily gone away. So that would require Sra so I assumed, otherwise it would not be possible to increase speeds unless i have misunderstood. (Well there is the scenario of resynching getting an increased speed because of that only because disturbers have temporarily gone away and then your system keeps going round a long-term loop like that again and again.)

It does make me wonder about my own situation with the Dlinks where the initial reported downstream SNRM does not match the supposed target SNRM for downstream (but this does not apply to upstream).
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Weaver

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Re: Virtual noise?
« Reply #20 on: January 27, 2018, 06:33:55 PM »

btw i didn't understand the term ‘noise mask’. This is some kind of frequency-domain function applied to the actual observed noise PSD?
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ejs

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Re: Virtual noise?
« Reply #21 on: January 27, 2018, 07:31:19 PM »

I think what happens is that the noise level for a particular tone, used to determine how many bits to allocate to it, is the maximum of either the actual noise, or the virtual noise configured for that tone.
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Weaver

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Re: Virtual noise?
« Reply #22 on: January 28, 2018, 02:28:03 AM »

Ejs, surely that has to be correct.
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konrado5

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Re: Virtual noise?
« Reply #23 on: January 29, 2018, 11:01:38 PM »

I've came across the "virtual noise" term a few years earlier and read about it in some technical documents. I've understood that virtual noise is fictional predefined at DSLAM noise causing the lower SNR per tones despite shortage of real noise. For example it can be predefined that virtual noise is 120 dBm/Hz per all tones. :)
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konrado5

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Re: Virtual noise?
« Reply #24 on: January 29, 2018, 11:13:41 PM »

I've looked at G992.3 specifiaction and found.
Quote from: G992.3
Configuration parameter TXREFVN defines the transmitter-referred virtual noise PSD to be used in
determining the SNR margin.
(p. 72)
I've also found.
Quote from: G992.3
The
received virtual noise PSD as defined in SNRM_MODE = 2 shall not be taken into account in
SNR(f).
(p. 108)

So the virtual noise artifically lower SNR margin but it doesn't lower SNR per tones.
http://www.itu.int/rec/T-REC-G.992.3/en
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