Turbo codes for multi-level modulation

I understand this now. The only remaining question is that in the following
reference, how is gamma_(i,j) in section 6.1 derived, when it is equated to
V(x_k)X(i,j) + R(Z_k)Z(i,j).

http://csee.wvu.edu/~mvalenti/documents/valenti01.pdf

This gamma equation seems to be a correlation of LLRs with 0 and 1. How is
this expression derived?	 

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