Re: PAM-5, what are your BERs ?
Ed,
Sean is simply stating that FEC can be used to provide a significant improvement
in "effective" BER over "raw" BER.
Kamran Azadet and Mark Yu of Lucent have presented an excellent overview of FEC
to the HSSG in July '99:
http://grouper.ieee.org/groups/802/3/10G_study/public/july99/azadet_1_0799.pdf
You point out that FEC requires a higher line rate, which mitigates the benefits
of FEC. This is true for a traditional Binary signaled system.
However, for a MultiLevel system such as PAM5, an additional level can easily
accomodate the FEC overhead. Several HSSG PAM5 proposals as well as 1000BASE-T
utilize FEC in this fashion. As an example, my 5 Gbaud PAM5 proposal used 4 of
the levels to respresent 2 bits and the 5th level to represent FEC as well as
special characters. In terms mapping, one 8B/10B code-group could be mapped to 4
PAM5 symbols creating a PAM5x4 "code-group" symbol. Note that the PAM5x4 symbol
period is 800 ps, and is used to convey 10 bits and that the PAM5 symbol period
is 200 ps = 5Gbaud. You can find more detail on thge latter example in:
http://grouper.ieee.org/groups/802/3/tutorial/july99/mastut.pdf
It is possible to add FEC to MultiLevel signaling WITHOUT increasing the line
rate. However, adding extra levels does further decrease SNR. The tradeoff then
becomes FEC complexity and decreased SNR vs. effective BER gain. I like the way
this tradeoff leans to the left :-)
Best Regards,
Rich
--
Ed Grivna wrote:
>
> Hi Sean,
>
> unfortuantely, FEC has additional drawbacks beyond just latency. the
> primary concern here would be that you need to ratchet-up the signaling
> rate even faster to include the overhead of sending the FEC bits.
> Depending on the amount of redundancy in the FEC information,
> you may need to increase the signalling rate as much as 5-to-10%.
>
> Regards,
>
> Ed Grivna
> Cypress Semiconductor
>
> >
> > Hello,
> >
> > With regards to the BER problem, are we open to the possible use of forward
> > error correction to correct for bit errors in the transmission stream.
> > Currently in T1X1.5, there is a submission for inband FEC based on a BCH-3
> > code that turns a 1x10**-7 BER into a corrected BER of 1x10**-17. Prehaps a
> > BCH-1 code could be used?
> >
> > Using FEC would introduce the need for some additional bits within the frame
> > format and it would probably introduce some latency, but it would introduce
> > some coding gain.
> >
> > Cheers,
> > Sean
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