how can i have APP decoder in m.file without using simulink?

i have a systme including multi users coeded by Convolutional code and then transmitted in the channel.

in the receiver we estimate desired signal by 2 estimator, firstly each user's signal is estimated based on Log-likelihood and then it should pass from an APP decoder based on used convolutional code and the updated output LLR back to first estimator and this process is repeated foe several times as iterative decoder.

To put in nutshell, i want to know how i could write code in m.file to have an APP decoder without using APP detector block in simulink.