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Re: PAM-5, what are your BERs ?

Rich Taborek wrote:

> Ed,
> Sean is simply stating that FEC can be used to provide a significant improvement
> in "effective" BER over "raw" BER.
> .....
> 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 .....
> .....
> It is possible to add FEC to MultiLevel signaling WITHOUT increasing the line
> rate.....
> Rich


Somethink in the math does not add up: I think that
you cannot have it all: both PAM-5 and include 8b/10b
and include FEC and still transmit at the 1.25 Gbaud
using 800 psec symbols.

The raw data consists of 8 bits at 1.25 Gbaud that come
from the MAC layer. These 8 bits represent 2^8 = 256
possible symbols. Using four PAM-5 transmitters at
1.25 Gbaud the number of possible 4-dimensional symbols
is 5^4 = 625.

Since we have 625 possible 4-dimensional symbols
in one baud period and we only need 256 symbols for
the raw data, we have more than twice the number of
points in the constellation than we really need for raw

    625 > 2 * 256 = 512

It is this fact that enables to add trellis coding and
6 dB coding gain using FEC.

Now, it you also want to add the 8b/10b you would
now have 2^10 "raw" data = 1,024 points. You do not
have so many points in the 4-dimensional PAM-5
constellation (5^4 = 625). If you want to include
the 8b/10b coding you will have to increase the
symbols rate above 1.25 Gbaud.


Jaime E. Kardontchik
Micro Linear
San Jose, CA 95131
email: kardontchik.jaime@xxxxxxxxxxx