Re: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency Offset Frequency
Jim,
Bear in mind that any implementer is free to choose components with *better*
precision, tolerances etc. when manufacturing their products - it is called
product differentiation. The standard specifies the minimum that has to be
met. So if I were a product designer and wanted to develop a product (CNU)
that supports IEEE1588v2, I would certainly use more precise oscillators.
However, in cases of products that do not need to meet such tight
requirements, 100ppm oscillators would do just fine.
Does that make sense? I know it is a mantra we repeat over and over again,
but we are supposed to develop a standard and not a product spec.
Regards
Marek Hajduczenia, PhD
Network Architect, Principal Engineer
Bright House Networks
Office +1-813-295-5644
Cell +1-813-465-0669
From: Jim Farmer [mailto:jofarmer@xxxxxxxxxxxxxx]
Sent: March 13, 2014 9:41 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: Re: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency Offset
Frequency
100 ppm seems to be about where most inexpensive oscillators are. I have
not measured any, but my experience is that they will likely not be any
better than that. It is reasonable to specify something tighter, especially
at the CLT. For cable TV headend equipment, we used to specify 25 ppm and
we beat that pretty easily. But it did take a somewhat more "complex"
crystal and an individual adjustment, both of which add
that-of-which-we-are-not-to-speak. And we didn't have to cover the outdoor
temperature range. If a computer gains or looses 1 minute in a week
(between synchronization to a time server), then its oscillator is in error
by 99 ppm. And that is at room temperature. If you have two independent
oscillators, one on each end, you have to conservatively assume that they
are off indifferent directions, which could double Steve's estimate. But
you can specify a much better oscillator, just that you have to exchange,
uh, "complexity" for accuracy. Probably a good thing to do at the CLT.
This presupposes that we will not have to support IEEE 1588 through the
system - that requires very tight frequency tolerances. Or we would have to
re-specify for 1588 - will likely take different hardware anyway from what I
know (which is not all that much).
jim
Please ignore all the changes in font in my message
- my email client does that just to mess with my head.
Jim Farmer, K4BSE
Mobile 678-640-0860
jofarmer@xxxxxxxxxxxxxx (spam blocked)
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"A people that values its privileges above its principles soon loses both."
Dwight D. Eisenhower, Inaugural Address, January 20, 1953
On 3/13/2014 6:48 PM, Shellhammer, Steve wrote:
Marek,
In that case the frequency error would be around +/- 100 kHz
(for 1 GHz RF) and so there would be even larger frequency uncertainty. So
it would be even more important for the PLC preamble to have some short
training fields that can be used to disambiguate the correct subcarrier.
Is 100 PPM what you would expect for the CNU? I do not think
we have specified the oscillator accuracy yet for the CLT or the CNU. Maybe
something we should figure out. My preference is for a low-cost oscillator
in the CNU and maybe a more accurate one in the CLT, where the cost may be
less of an issue.
Regards,
Steve
From: Marek Hajduczenia [mailto:marek.hajduczenia@xxxxxxxxx]
Sent: Wednesday, March 12, 2014 6:00 PM
To: Shellhammer, Steve; STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: RE: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency Offset
Frequency
Steve,
Once you consider that we might have to work with 100ppm oscillators in
CNUs, the resulting value gets 5 times larger. Is that a big problem ?
Regards
Marek Hajduczenia, PhD
Network Architect, Principal Engineer
Bright House Networks
Office +1-813-295-5644
Cell +1-813-465-0669
From: Shellhammer, Steve [mailto:sshellha@xxxxxxxxxxxxxxxx]
Sent: March 12, 2014 8:02 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency Offset
Frequency
EPoC Group,
I was thinking about the PLC preamble and the initial
frequency error due to use of a low-cost and low-accuracy oscillator in the
CNU. In 802.11 they have a way of dealing with initial frequency offset due
to low-accuracy oscillators. I was wondering if this make sense in EPoC.
I did a few calculations below. I also attached the Word document since I
was not sure what the email reflector would do to the equations.
I would be interested in knowing if this approach used in
802.11 would be useful in EPoC.
Any comments would be appreciated.
Thanks,
Steve
-----------
PLC Preamble Frequency Calculations
Assume low-cost crystal with 20 ppm oscillator in CNU. Assume accuracy of
oscillator in CLT is much better so we will ignore that oscillator error.
Assume the maximum carrier frequency around 1 GHz. Could be a little higher
but for these calculations this is good enough.
Frequency Error
Δf=±fc×20×10-6=±109×20×10-6=±20×< m:e>103=±20 kHz
Initial frequency error at CNU can be up to 20 kHz.
PHY supports subcarrier spacing of 25 kHz and 50 kHz. The worst case
situation from a frequency error perspective is the 25 kHz PHY.
For a 25 kHz PHY the initial frequency error is up to one subcarrier on each
side. So there are three possible tones that represent the middle tone when
first acquiring the PLC. This would triple the acquisition time, since the
CNU would need to search over three times as many cases.
In the 802.11 OFDM PHY the preamble includes several short training fields
(STFs) where only one out of every four tones is used, which disambiguates
the subcarrier selection due to frequency offset.
Does this make sense for EPoC?
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