Re: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency Offset Frequency
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<mailto: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
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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
[cid:image001.png@01CF3ED3.B5032EE0]
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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