How RA Code Output Extrinsic Probability....

Hi all....

I do simulate about jointly demodulation and decoding now.
The channel encoder is a RA encoder, the iteration between demodutor
and decoder.
I see many papers, but i don't know how RA ouput a soft probability to
modulator.
(I use the Dr.Jilei Hou's RA decoding Schedule.)

When first iteration, In demodulator :
channel outputs and priori probabilities from decoder input to the
demodulator
They are used to calculate soft outputs.
Finally the soft-outputs substrate the priori probabilities and become
soft-inputs to the decoder.

In decoder:
The RA receive the soft-outputs form demodulator and become inputs as
priori probabilities.
The RA decoder uses the Dr.Hui's Schedule(Variable Nodes fisrt update,
then Check Nodes).
When a iteration is performed in RA decoder, how do i ouput
soft-outputs to demodulatoe.

Case1:
I try to calculate the soft-outputs by add the soft-outputs from two
check nodes.
(one parity node is connected to two check nodes)

Case2:
I try to calculate the soft-outputs by add the soft-outputs from two
check nodes.
And substrate the priori probabilities...(I think it's reasonable)

where soft values are all in LLRs.

In Case1, the BER simulation decrease very slowly when iteration
between demodulator and decoder increases; In Case2, the BER is very
bad....


Someone knows what's happen, Help!!
Please tell me, Thx...

Chasel Weng