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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

 



 

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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