Hello all,

I am reading DVB-S2 standard. There is a sentence about LDPC, "a
powerful FEC system based on LDPC (Low-Density Parity Check) codes
concatenated with BCH codes,
allowing Quasi-Error-Free operation at about 0,7dB to 1 dB from the
Shannon limit, depending on the transmission mode (AWGN channel,
modulation constrained Shannon limit)".

So what's "allow Quasi-Error-Free operation"? Thanks!

Regards,
Davy

In the DVB context "quasi error free" (QEF) means a level of errors
that is defined as the "threshold" of where the system is working and
where it is not.   The "threshold of visibility" is another term
sometimes used  referring to a rate of errors that just becomes
noticeable to a TV viewer.


Specifically it is less than one uncorrected error event per transport
stream hour which corresponds to a coded BER of about 1E-10 to 1E-11.
In my experience, the average viewer will not notice errors at a
significantly higher BER  so the DVB definition is somewhat
conservative.


Bottom line, QEF is a somewhat arbitrary low BER value that is used for
evaluating the performance of the system, i.e. the C/N required to
obtain QEF operation is the threshold.

Look around in the other ETSI / ETR documents and you can find the
formal definition of QEF.

Mark